a. Definition of Categories
Within the individual categories there is always some indefinite
range where the noteworthiness of the peculiarities becomes rather
subjective. While objects in this twilight zone of the Catalogue are
undoubtedly incomplete, down to the size limit adopted here we feel
that we have produced a catalogue which, by anyone's standards,
includes all of the most peculiar galaxies.
At the same time, distance-dependent biases do exist in our
selections and these will vary from Category to Category. For
instance, galaxies showing jets and filaments will be lost from the
Catalogue at a larger apparent size than the galaxies having
interacting companions; galaxies with apparent companions will be
selected from a smaller volume of space than apparent doubles,
etc. The reader is therefore warned against making sweeping
comparisons between relative frequencies of peculiarities unless this
very important selection effect is taken into account.
It is important to stress that we did not start this project with a
preconceived set of categories. Rather, we developed the categories
empirically by simply listing all unusual objects from a preliminary
analysis of a set of thirty-six plates. We then noted what categories
these objects themselves defined, and thereby established the
categories listed in Table 1. It
is remarkable, looking back now, to
realize that such a large number of peculiar galaxies could be fitted
into so few categories of peculiarity. In the end, surprisingly few
galaxies needed to be put into Category 22 as
miscellaneous objects.
One of the important uses of the Catalogue is to enable astronomers,
interested in a certain class of objects, to obtain a sufficiently
large list of candidates with similar characteristics as a basis for
further investigation of specific phenomena.
In the photographic section of the Catalogue the objects are ordered
carefully within each Category. It is hoped that this conveys, to some
extent, an empirical continuity of certain characteristics which may
help us understand the cause of some peculiarities. As shown in the
previous table of codes, the few words that describe each Category
furnish a definition of the kinds of objects which comprise each
Category. In addition, there are amplifications of the definitions and
brief comments at the beginning of each Category when it is
introduced. More detailed remarks on the possible significance of the
Categories and their relation to each other are given below.
b. Statistical Frequency of the Various Categories.
We have inspected and counted all galaxies to an estimated size
limit. There is of course some imprecision inherent in this approach
when attempted by visual means. Exposure limits and personal judgement
vary from plate to plate. Additionally, some of our fields are
obscured by galactic absorption. This led to the expected result that
we found only a few galaxies in some fields, and undoubtedly missed
some connections and interactions among those galaxies we did look at
in the obscured fields. Nevertheless, these cases are relatively few
in number and probably do not significantly alter our results. Knowing
the total counts of galaxies in each of our fields, we can give, with
fair accuracy, what percentage of all the galaxies we looked at
actually fall into the various Categories.
In the Appendix, we have listed the number of galaxies that we have
inspected and counted in each of the 537 fields which comprise the
survey area of the Catalogue. It is clear that the counted number of
galaxies varies strongly with the galactic latitude of the field. In
fact, the numbers can be taken as indicative of the average
obscuration for each field. Of course it should be realized that the
obscuration can vary dramatically within a single 6.5 degree by 6.5
degree field and that the counts can also be dramatically affected by
the presence of galaxy clusters within a given field.
At the same time it should be noted that because of the limited
dynamic range of the photographic plates and because we were working
with deep exposures, we were especially sensitive to peculiarities in
the faint outer reaches of most galaxies; the inner disk often being
severely over-exposed. However, in fields obscured by foreground
galactic absorption, galaxies can appear ``ghost-like'', with much lower
apparent surface brightnesses. For these objects at low galactic
latitudes our basis for judging peculiarities would necessarily shift
from the outer to the inner regions. This should be borne in mind by
the reader, since this particular surface-brightness effect makes the
discovery of ``dwarf galaxies'' at low galactic latitudes suspect,
unless there is some additional property that makes the identification
compelling.
The total number of galaxies which are classified into each Category
is obtained by counting the number of times each code number appears
in the positions and descriptions part of the Catalogue (Volume I). As
noted previously, the same galaxy could be classified under more than
one Category. For example, one galaxy could have an interacting
companion and several apparent companions, and some of these
companions in turn could have jets or tails or other peculiarities. By
summing the number of times each Category appears, we obtain the total
number of galaxies exhibiting that particular peculiarity. These
statistics, expressed as percentages, are listed in the final column
of Table 1. For the 6,445
individual entries there are 8,954 classification codings.
A further quantity of interest is a measure of the frequency with
which galaxies in a diameter-limited sample are classified as
peculiar. This statistic can be readily obtained by dividing the
total number of entries in the Catalogue by the total count of
galaxies examined, appropriately scaled for plate overlap. The total
number of galaxies found on each survey plate is given in the
Appendix. These are the raw counts obtained by us as we scanned each
plate. In order to correct these for overlap the numbers listed should
be decreased by a factor of 0.83 determined by us empirically from the
numbers of peculiar galaxies discovered more than once in the course
of the survey. In total we inspected 93,556 galaxies (77,838 when
corrected for overlap) out of which we classified 6,445 as peculiar,
indicating that eight percent of all galaxies looked at are peculiar.
c. The Atlas of Peculiar Galaxies and other Catalogues
In addition to furnishing examples of galaxies which illustrate the
standard classification of types of galaxies, the Hubble Atlas
(Sandage 1961)
presents some well-known examples of peculiar
galaxies. The Hubble Atlas also contains some objects from the
southern hemisphere. Modern systematic surveys of southern hemisphere
galaxies start with
de Vaucouleurs (1956),
who gives types and
descriptions for the listed galaxies. Classification information is
also given in
Evans (1957) and
Shobbrook (1966), while
Sersic (1968)
has published an atlas which includes the most peculiar bright
galaxies in the southern hemisphere.
The most recent morphological classifications of southern galaxies
are given in the Las Campanas Survey
(Dressler and Sandage
1978;
Sandage and Brucato
1979).
Of course, the extensive classification of
both normal and peculiar galaxies is available from the ESO/Uppsala
Survey of the ESO (B) Atlas of the Southern Sky (for recent
references in this series, see
Holmberg et al. (1980)
and the final compilation
by Lauberts (1982).
d. General Introduction to Discussion of Individual Categories of
Peculiarity
The Categories which are empirically assembled in the present
Catalogue point to the existence of various kinds of events and
forces. First, there are the interaction events where two galaxies
are found close to each other and deformed, presumably by
gravitational or other long-range forces. Second, there are apparently
isolated galaxies where the disturbance appears to arise from within
the galaxy itself, possibly due to internal dynamical instabilities,
nuclear activity, or other explosive events including supernovae or
extensive star formation. Third, there are objects which appear to
differ from the norm because they reflect differences in the wider
environment in which they formed and evolved; for instance,
differences noted in two galaxies could be attributed to the effect of
a hot gaseous medium, higher mean velocity dispersion or a rapidly
fluctuating gravitational potential for a galaxy in a cluster, as
compared to a similar object in the field. Peculiarities could then
be ascribed to three main causes, immediate neighbourhood, internal
activity and general environment.
Of course, the assigning of probable causes for peculiarity can only
be tentative when one is dealing with the appearance alone. What
appears as a single peculiar galaxy, for example, might be an
accidental superposition of two galaxies along the line of sight. In
other instances an apparently isolated but disturbed galaxy might be
the physical merging of two systems. Alternatively, what appears to be
a gravitational interaction might be enhanced internal activity (such
as the fueling of an active nucleus) only triggered, but not caused,
by the proximity of another galaxy. Or internal activity may be
simultaneously occurring in two or more galaxies which are only
coincidentally close together. Correlation and cause are rarely easy
to disentangle in astronomy. As a starting point, however, the
Categories which most suggest certain causes are listed below:
Like the Atlas which preceded it, the present Catalogue poses
intriguing questions as to whether some of the different Categories
are physically related to each other, and if so, how. The following
detailed discussion of each Category is meant to high-light some of
the important peculiarities in galaxies and suggest ways in which
possible relationships between the Categories might help us to make
progress towards explaining these systems.
Category 1: Galaxies With Interacting
Companions
The first examples of this Category in Volume II are spirals with
well-defined spiral arms. The reason for starting with these cases is
to insure that the effects of the interaction on the arms can be well
seen. The experiments which are thus presented to us deal with the
phenomenon of spiral structure. The nature and cause of spiral arms
has long been debated, with the density wave theories of Lin
(Lin and Shu 1964),
the magnetic tube concepts of
Piddington (1970),
the ejection track theories of
Ambartsumian (1958), Arp
(1963,
1969,
1978),
and Oort (1975),
and most recently, the self-propagating star
formation theories of
Mueller and Arnett
(1976),
all attempting to
explain spiral structure. One way of making progress on this problem
is to examine spiral arms under physical stress. Of particular
interest in this regard are new cases shown here where the spiral arm
appears retarded in rotation by the nearby companion as illustrated by
AM 2011-705 and AM 2203-281. In the present Catalogue, there is also
one case where the spiral arm seems to flow around the companion
(i.e., AM 0244-302).
Later in Category 1, we encounter sub-category
1b, also illustrated
Volume II. Here the companions are interacting strongly with the whole
body of the main galaxy. They furnish some of the most certain
physical interactions and the companions might be considered in the
role of test probes. As we go still later into this category we
encounter companions relatively more widely separated and central
galaxies relatively less disturbed. Among these later sub-categories,
we encounter some very unusual shapes in the companions themselves
(e. g., AM 0036-432). Some of these suggest that the companions are
internally active in their own right, although that internal activity
may or may not have been triggered by an interaction.
By the end of the category, the evidence for interaction is more
subtle. This arrangement emphasizes the important point that the
present category is perfectly continuous with
Category 8: Galaxies with Apparent
Companions. Whatever the important considerations in
Category 8 are, be they initial conditions,
evolution, or interaction, some of the same forces may be acting here.
Category 2: Interacting Doubles
The continuity of this Category with the preceding is made empirically
clear by considering examples of galaxies with interacting companions;
when the companion is larger than approximately half the size of the
main galaxy, we classed it as an interacting double. It is by no means
certain, however, that all such systems arise from the same causes or
initial conditions as those in Category 1. It
is interesting to note
that some galaxies with small companions can appear to be just as
violently disturbed as those interacting with galaxies of much larger
size. This raises questions about how the interaction takes place: Is
it strictly a gravitational interaction, primarily between stars or do
plasma, gas, dust or magnetic fields also play a role? It also raises
questions concerning time-scales and frequencies: Over what fraction
of their life-times are interacting galaxies in contact? And what
fraction of all galaxies interact at all?
By consulting the statistics in
Table 1, it is seen that this
Category is the most frequently used classification in the whole
Catalogue. There are simply a very large number of galaxies
encountered where the two galaxies are roughly the same size, a few
diameters apart, and apparently disturbed. The forms of these
disturbances are endlessly varied and Volume II attempts to give a
feeling for these forms.
The E + E sub-category, which leads off the interacting doubles,
contains examples of systems where galaxies without much apparent gas
and dust are interacting. There the forms of the interaction suggest
primarily gravitational perturbations of stars in orbits. Undoubtedly,
many of these cases can be treated by the classic precepts of
gravitational interaction, as most extensively studied by
Toomre and Toomre
(1972).
The E + S sub-category, which follows, shows spirals in the
gravitational field of a companion E. Presumably here we can see
spirals deform in a purely gravitational perturbing field. This
sub-category is interesting also from the standpoint that E galaxies
have a tendency to occur in clusters and groups with other E or
lenticular galaxies. The question is, how do these E + S associations
come about? There is also a puzzling suggestion in the literature
that, when ellipticals and spirals are in physical pairs, the spirals
have systematically higher redshifts
(Tifft 1972;
Tifft and Tarenghi 1975;
Jaakkola 1971,
1973;
Arp 1976a;
Sulentic 1977).
The final photographic sub-category shows a selection of S + S
interactions. These are usually the most spectacular because
deviations from equilibrium can be most easily seen in spirals. Also,
in many cases, the gas and dust, which have stronger interactive
properties than stars, are involved. Study of this sub-category should
yield some new information on the physical differences between
different types of spirals. Moreover, the relation between this
interacting sub-set of spirals and the apparently non-interacting
double spirals in Category 23 needs to be
clarified. This is
especially interesting in light of the strong correlation of Hubble
type
(Page 1975,
Noerdlinger 1979)
and colour
(Tomov 1978)
between pairs of spirals in binary systems. This correlation could be an
initial condition of the system, but it is also possible that in these
cases the Hubble type and colours are determined by the on-going
interaction rather than the internal dynamics of the individual
galaxies
(Madore 1980).
A whole sub-category of interacting doubles is not included in
Category 2. It consists of those interacting
doubles which are
apparent companions of spirals
(Sulentic, Arp and di
Tullio 1978).
This sub-category is illustrated in
Category 8. There are also
examples of interacting doubles in
Category 6: Ring Galaxies, and in
other Categories as well.
In general, there remains an enormous amount of observational work
to be done on these interacting doubles. Large numbers of accurate
redshifts need to be obtained. Morphologies need to be specified more
precisely. Sizes and apparent magnitudes need to be measured. Perhaps
new detailed information on this large Category of doubles in the
present Catalogue will help to interpret the data which has already
been gathered on the Karachentsev doubles in the northern hemisphere
(Karachentsev 1974,
1980;
Peterson 1979;
Tifft 1980)
and those of
Schweizer (1979)
in the south.
Throughout the examples of interacting galaxies runs the question of
whether the galaxies are suffering temporary encounters or are
merging, fissioning, or ejecting. Considerable study will be needed to
decide eventually whether one or more of these processes is working,
and if there is more than one, whether any one of them is dominant.
One statistic of importance, which is derived in the present
Catalogue, is the percentage of all galaxies that are interacting
doubles. This must be an important number in constructing theories of
how the interactions occur in terms of initial and present conditions.
Moreover, in the multiply interacting systems, we frequently encounter
linear arrangements. This surprising feature of the Catalogue implies
that something more than random encounters of galaxies is taking
place. Comparisons with similar samples at higher redshift will be
especially interesting.
Categories 3 through
5: Interacting Triples to Quintets
When three galaxies fall within a few diameters of each other on the
sky and are disturbed, they have been classed as an interacting
triple. When the number of apparently interacting galaxies is greater,
they have been classified as interacting quartets and quintets. It is
significant that no unequivocal examples of six, or more, interacting
galaxies of roughly equal size were encountered in our study. Any
candidates for interacting galaxies in greater number than five will
have to be sought in the present Catalogue in
Categories 17 and 18,
Chains and Groups of Galaxies. Sometimes there are smaller galaxies
present in interacting systems, which would take the group into a
higher order interacting classification if they were considered of
dynamical importance in the group. The classification here, however,
has to be made on the subjective impression as to what the main
components of the group are.
Considering that the interacting double category is by far the
largest category in the Catalogue, it is perhaps surprising that there
are so few interacting triples. The reduction in numbers as we proceed
to interacting quartets and higher orders, is even more
drastic. Theories explaining the frequency of interacting doubles
should also lead to predictions of the numbers of interacting triples,
quadruples and higher-order interactions; the present data should help
to select between such theories.
Among the triples, there are two sub-categories distinguished in
Volume II: the first, where three galaxies of any type which are
roughly the same size seem to be interacting; and the second, where
there is rather striking alignment. This tendency towards alignment
persists also into the quartets and quintets: AM 1303-372, which
consists of a group almost in contact, shows linear elongation. In the
northern hemisphere, interacting or disturbed groups have already been
shown to have a tendency to be aligned
(Arp 1973).
In the well-known
Stephan's Quintet, for example, the four high-redshift members are
aligned very well, extending even to a fifth high-redshift member to
the east. The tendency for the groupings of disturbed galaxies to
align is clearly related to
Category 17: Chains, because many of the
galaxies in chains are also disturbed.
In some groups of disturbed galaxies ring forms appear. Since rings
are one of the most striking non-equilibrium forms found, the ring
classification generally took precedence over other
Categories. Therefore, some apparently interacting groups can also be
found in Category 6: Ring Galaxies, which will
now be discussed.
Any galaxy with a well-defined ring around it, and any galaxies
apparently related to this form, have been included in this
Category. We have tried to exclude galaxies where spiral arms have
become wound up tightly into a circle or near circle (they are in
Category 10: Galaxies with Peculiar Spiral Arms)
but there is some
question as to whether the latter forms are related. There may be some
overlap or there may simply be an ambiguity in the classifications.
There have been three major attempts to explain the origin of ring
galaxies
(Freeman and de
Vaucouleurs 1974;
Theys and Spiegel 1976;
Lynds and Toomre 1976).
All of these approaches involve the collision
of a cloud or galaxy with another galaxy in a special way, usually
through the center of the galaxy which becomes the ring galaxy. The
resulting, nearly symmetrical disturbance is proposed to account for
the formation of the ring. Naturally, the presence of nearby
companions is of great importance to these kinds of theories, and the
present examples of ring galaxies with companions furnish many cases
where redshifts, magnitudes, and spectra can be obtained in order to
check these theories. Conversely, it will be very interesting to
uncover examples where there are no apparent companions in the
vicinity of the ring. Some examples which deserve further study are
shown in Volume II under this Category. Perhaps they have an
independent origin.
The first sub-category starts with examples of empty rings, that is
rings lacking any apparent nucleus. These are particularly interesting
because it seems difficult, although by no means impossible (see
Theys and Spiegel 1976),
to conceive of a collision which would remove a
nucleus entirely. Alternatively one could postulate that the target
galaxy had no dominant nucleus to begin with. A further alternative is
to postulate that the radial perturbation which formed the ring was
caused by internal activity of the nucleus (Saslaw, private
communication 1978; Williams, in preparation 1985). Later in this
sub-category, we encounter some galaxies with nuclei which are quite
off-center, while toward the end of this sub-category the nuclei
become fairly well centered. In the examples showing moderately
off-center nuclei, we encounter the spectacular rings such as
AM 0644-741 and
AM 1006-380 and the remarkable ``Cartwheel'' (previous
discussions of some of these objects can be found in
Graham (1974),
Fosbury and Hawarden
(1977),
Few, Madore and Arp
(1980)).
Morphologically, the objects toward the end of these
illustrations lead more or less continuously into the second
sub-category.
The second sub-category has rings with centrally located
nuclei. Some resemble spiral galaxies in which the arms have become
wound up by differential rotation, as arms would unless sustained by
whatever the spiral arm generating mechanism is. From the pictures
available to us, we put objects into the present Category if their
rings are more or less unbroken circles with no hint of spiral
connections to the central nucleus. The existence of this
sub-category, however, raises the interesting question of whether some
rings can originate from the winding up of spiral arms or luminous
ejected filaments by differential rotation.
Finally, there is a sub-category consisting predominantly of barred
spirals which have a high-surface-brightness inner ring and a faint
outer ring as well. These ``bulls-eye'' shapes, to our knowledge, have
received no analytical attention to date.
Kormendy (1979)
discusses
some ring-form galaxies which may have some relation to ``bull's-eye''
forms as well as to rings in spirals in general.
One rather interesting object in this Ring Galaxy Category, is
AM 0226-320. It represents the best example of
either a prolate galaxy
with a fine, well-delineated equatorial ring, or an edge-on galaxy
with a ring circling the poles. For further discussion of this class
of objects and references to previous work see
Schweizer, Whitmore
and Rubin (1983).
Category 7: Galaxies with Jets
We consider jets here as straight or nearly straight linear features.
Because of the importance and relative rarity of jets, classification
into this Category tended to override classification into other
Categories. This Category perforce includes many objects which may
only possibly have jets. Accidentally superposed companions or stars
can fool the eye, so higher resolution or photography to lower surface
brightness will be needed to confirm whether many of these cases are
actually jets or not. In general, any linear feature that might be a
jet was included in this Category, with the expectation that further
confirmatory work would be done.
It should be remembered that one of the most famous cases of jets
from a galaxy, NGC 1097, was discovered initially as an uncertain
possibility on a very deep Schmidt plate, and only confirmed after
special efforts
(Wolstencroft and
Zealey 1975).
Such information-stacked plates taken with the CTIO 4m reflector,
eventually revealed a total of 4 jets emerging from the nucleus of
this galaxy
(Arp 1976b;
Lorre 1978).
The picture of NGC 1097 =
AM 0244-302, shown in Volume II, is from a standard
IIIa-J survey plate
and the print here does not show even the brightest jet.
In general, counterparts to high-surface-brightness jets known in
the northern hemisphere, such as in M87 and in 3C273, were not found
in our southern hemisphere survey. Because the deep exposure IIIa-J
plates we surveyed were saturated over a moderately large fraction of
the central galaxy images, there may be some such jets that we have
missed in interior regions. But results from the ESO Quick Blue
Survey, which was made using shorter exposures, turned up no
outstanding new cases
(Holmberg et al. 1980),
so we can conclude that
such strong jets are very rare and unusual (see also the Proceedings
of Second ESO/ESA Workshop, Munich, 18-19 Feb. 1981, ESA Sp-162, March
1981).
Remarks on some of the individual candidates for jets which we have
illustrated, are made in the introduction to the category in Volume
II. We should note that
Category 15: Galaxies with Tails, Loops of
Material or Debris, may be allied closely in some cases with
Category 7: Galaxies with Jets. If we consider
an initially straight jet
deforming under the action of rotation, then some tails or loops could
have been jets. Conversely, it could be that some filaments are being
viewed in the plane of their orbits, and therefore appear like
straight jets.
Category 8: Galaxies with Apparent Companions
The galaxies that are nearest in space to us, and that we therefore
know the most about, are almost all grouped together in units of one
large, dominant galaxy, and a retinue of smaller companions. Examples
are the Local Group (dominated by M31) and the M81 and M101 groups. It
is important to find out how general a phenomenon this kind of
grouping is over large regions of the universe. The present Catalogue,
including all of the sky south of declination -22 degrees, presents
extensive lists of the most conspicuous, apparent groupings. These are
the cases most likely to turn out to be physical associations, when
more detailed observations become available.
The forms of the companions, and their distribution around the
central galaxy, present many puzzling problems. Holmberg
(1940,
1969)
shows that fainter galaxies, many of them presumably physical
companions, tend to concentrate in the vicinity of the minor axes of
large spiral galaxies. Later,
Sulentic, Arp, and de
Tullio (1978)
showed that disturbed or non-equilibrium galaxies group preferentially
around large spirals. A striking local example of this phenomenon is
seen in the two closest companions to M81, (i.e., M82 and
NGC 3077).
Lynds and Sandage (1963),
Sandage (1961),
Bertola (1981b), and
Arp (1980)
established the extremely peculiar nature of these
companions. Arp
(1981a,
1981b,
1982)
has shown that companion
galaxies, including many of the ones shown in Volume II, tend to have
early spectral types and a high incidence of emission. What causes
these peculiarities: Is it the interaction of the companions with the
main galaxy? Or is there some more fundamental reason for the
differences in the companions? Are they formed at a later epoch than
the main galaxy? Or are they formed in some basically different
process?
Some clues to these puzzles may be obtained by further studies of
this Category. It is the one which is illustrated with the largest
number of examples in Volume II, and the reason for this is to supply
observers with many more candidate groups from which they can select
new cases for study. Of particular interest is the tendency for many
companions to be interacting doubles, as shown in the third
sub-category. What is the connection of interacting double companions
to Category 2: Interacting Doubles? Are some
of the interacting
doubles in Category 2 companions to larger
galaxies which are
projected some appreciable distance away on the sky?
In the second sub-category, a number of examples are shown where the
companions are aligned across the central galaxy. If this is a
significant trend, it may be telling us something about the origin or
dynamical evolution of the companions and their orbits. Whatever the
reasons for the alignment, it is tempting to suppose that there is a
common explanation for the alignments observed in the interacting
triples, quartets, and quintets, and also, for the alignments observed
in Category 17: Chains.
In the Atlas of Peculiar
Galaxies
(Arp 1966),
the M51-type galaxies
were called ``Companions on the ends of spiral arms.'' They made up one
of the most important sections of the original Atlas, and were ordered
from the smallest to the largest companions. The purpose was to show a
sequence of forms. One interpretation of this sequence was made by
Arp (1969)
in which he postulated that compact bodies were ejected from
the nucleus of the parent galaxy. During their passage outward these
compact objects formed the spiral arms in the underlying galaxy and
then evolved into larger, companion-sized galaxies.
Subsequently
Toomre and Toomre (1972)
published calculations which
showed that certain kinds of encounters between galaxies could
gravitationally draw out filaments of stars which would connect them.
The Toomres' simulations modelled some of the observed objects so well
that there could be little doubt that such a mechanism was acting to
some degree. One of the cases most successfully modelled was M51. In
recognition of that analysis and its most famous prototype, we renamed
the Category, M51-Types.
It is not necessarily true, however, that in every case companions
on the ends of spirals arms have to be the result of gravitational
encounters (note particularly the effects of small companions; see
discussion by Vorontsov-Velyaminov
(1957,
1975a,
1975b)).
Is a companion originating in the nucleus of a galaxy distinguishable
dynamically from a collisional encounter where the companion passes
very close to the nucleus? Again, reliable masses and densities of the
objects involved are needed to make further analysis. We present many
new cases of M51-types, where the properties of a large number of
members of this Category can be studied.
Finally, there is the question of the spiral arms themselves. The
implication from the Toomre work was that the spiral arms were caused
by the encounter (although, in the theoretical calculations, there
tended to be appreciable differences between the tidal arm and the
counter-tidal arm which are not generally typical of the
observations). Nevertheless, if some spiral arms were formed by
encounters, there is an obvious temptation to relate all spiral
formation to encounters with companions; otherwise there would have to
be two or more separate mechanisms for the formation of spiral arms!
The all-collision origin for spiral arms is contra-indicated, however,
by the existence of spiral galaxies which do not have any
companions. It is, of course, difficult to exclude completely the
possibility that there might always be companions around a galaxy,
postulating that they are of sufficiently high mass-to-light ratio so
that they are not conspicuous optically. But we are disinclined to
invoke the unobservable too soon.
So we return to the question of the nature of spiral arms touched
upon briefly in Category 1: Are spiral arms of
such a nature that they
are only formed by gravitational encounters? Are they even generally
formed by gravitational encounters? Are they formed by perturbations
of an internal origin? Or are there a variety of physical effects all
of which can lead to the formation of spiral arms? The further study
of the nature of spirals will be assisted by examples in the next two
Categories. There we illustrate both the variety of arms that can be
found, and the conditions under which they exist.
Category 10: Galaxies with Peculiar Spiral
Arms
This Category deals with galaxies where the major peculiarity is in
the spiral arms and is to be distinguished from
Category 12: Peculiar Disks, where any arm
peculiarity seems to be just a general
consequence of the disturbance of the main body of the galaxy.
The largest sub-category illustrated here is that of one-armed
spirals. In one portion of this sub-category, one arm is simply much
stronger than the other arm; but, in other cases, there seems to be
truly only a single arm. It would seem very difficult, in the density-
wave theory, for spiral arms to be formed by an asymmetrical density
perturbation which extended on only one side of the nucleus. If these
cases, on further examination, turn out to be truly one-armed spirals,
they would seem to pose a formidable challenge for any theory of
spiral-arm formation. The stability and persistence of the phenomenon
in any one example is, of course, an open question.
In Volume II, examples of spirals are given both with very narrow
arms and with very broad arms. It is possible that by studying the
different conditions in the disks of these galaxies, some clue as to
the determining causes of spiral-arm width may emerge.
Another puzzling phenomenon, of which three examples are given in
the photographs, is that of barred spirals where the arms are double
as they come off one end of the bar. In some cases, they remain
closely parallel for a considerable distance.
Finally, there is a sub-category of what are essentially elliptical
galaxies with spiral arms around them. Since these spiral arms
(rings?) are usually only discovered with deep photography, there may
be further examples of this sub-category just below the level of usual
detection. (It should be noted that these features are not the
plateaus in surface brightness or ``ringing'' structures originally
described in the
Atlas of Peculiar Galaxies
(Arp 1966)
and later popularized by
Malin and Carter (1980)).
This is a rather amusing Category to classify because the
terminology ``E galaxy with spiral arms'' would seem to be a
contradiction in terms. Accordingly, we have placed such objects in
Category 10: Galaxies with Peculiar Spiral Arms
by using the rationale
that if arm regeneration were to stop in a spiral, the arms would grow
old, fade, and possibly wind up. We then might be seeing examples of
large-bulge systems whose arm formation has been discontinued within
the last few hundred million years. The spectrum of the nucleus of
AM 0148-472
(Arp 1981a),
for example, shows an old stellar population;
but the nuclear-bulge population of an early-type spiral could not
easily be distinguished from that of an E galaxy. Detailed surface
photometry and/or kinematic studies will help to decide between the
elliptical versus bulge hypotheses. But these objects possibly
represent transitional systems between elliptical and spiral galaxies,
and are therefore very interesting in their own right.
Category 11: Three-Armed Spirals and
Multiple-Armed Spirals
Three-armed spirals are a surprising Category to encounter because the
majority of spiral galaxies appear to have some kind of two-armed,
grand design, while the remainder have highly bifurcated or patchy
arms. The density-wave theory, as mentioned in the discussion of
one-armed spirals, usually gives symmetric density perturbations that
are long-lived. Radio ejection is usually in the form of jet and
counter jet, or lobe and counter lobe. There are, however, examples of
short-lived minor density asymmetries and of triple ejections. In any
case, as shown in Volume II we find a number of spiral galaxies with
three, fairly equal arms, often symmetrically arranged. We expect they
should be important tests for theories of spiral structure and in fact
some work in the direction of modal analysis of spiral arms, from the
point of density wave theory, has already beens begun (e.g.,
Haas 1983).
It is to be noted that often the clearest examples of three-armed
spirals are the more distant objects where resolution does not reveal
details of the arms, but rather emphasizes their general aspects. Some
of these galaxies are illustrated in the photographs, as well as cases
where more resolution in the arms is possible.
One commonly noticed property of spiral arms is their tendency to
bifurcate somewhere along their length. We show some cases here where
the bifurcation is so strong that the galaxy has the appearance of
being a three-armed spiral. This may have some implications for the
interpretation of all three-armed spirals and undoubtedly makes the
challenge of supplying a general mechanism for spiral-arm formation
even more difficult.
We also encounter some multiple-armed spirals and give some examples
of these in Volume II. They may or may not form a significant
transition to flocculent-arm spirals. The case of
AM 0545-341 is shown
here as an example of the latter kind of spiral. The subject of grand
design and patchy-armed spirals is treated in
Gerola and Seiden (1978),
while the role of dust in spiral arms is discussed in
Elmegreen (1980a),
and the subject of spurs on the arms of spiral galaxies is treated in
Elmegreen (1980b).
Finally, we should note that spiral arms usually remain fairly
distinct, but sometimes they can apparently interact or even cross
over each other. In this Catalogue AM 0102-274 is an example where
some arms seem to cross each other and seem almost braided or
intertwined. Other northern hemisphere examples of galaxies with
crossing arms are discussed in
Arp (1969).
This Category is meant to illustrate spirals with peculiarities other
than in their arms. The classification is made when the main body of
the spiral shows unusual asymmetries or global distortions. There are
no obvious sub-categories. The pecularities tend to be quite
individual and not readily allied to other kinds of peculiarities. An
example is the rectangular nucleus of AM 0902-680, similar to the
peculiar northern hemisphere galaxy NGC 128
(Bertola and
Cappacioli 1977).
The significance of this Category may lie in illuminating what
happens to the arms of a galaxy when the main body of the system is
disturbed. If the inner regions of the galaxy become irregular,
presumably there are irregular gravitational perturbations which are
transmitted outward and should effect the spiral arms if the arms are
a result of gravitational density waves. By studying this correlation
(or lack of correlation) between the behaviour of the spiral arms and
the structure of the interior, some essential properties of spiral
galaxies may eventually be derived.
It is also true that strong disturbances are more apparent in spiral
galaxies than ellipticals because of the gas, dust and rotation in
spirals. Some of the mysterious internal activity which takes place in
the centers of galaxies, therefore, can best be studied in the
Catalogue examples, given here, of peculiar spirals.
Even on a survey taken under the uniform conditions of the SRC
Southern Schmidt Survey, inevitable differences in registered density
of sky background make it difficult to estimate surface brightness of
galaxies by inspection. Occasionally, however, the surface brightness
of an object becomes so high that it burns out an area of the
image. In this case, galaxies can be recognized if they are either a)
bigger in diameter than stars, b) appear like bright stellar objects
without diffraction spikes, or c) if there is a slight fuzz around the
edge of a burned-out image. The above definition of compact objects
also fits, rather well, one class of plate defects which can be
encountered; therefore, particular effort has been made to check that
our candidates are real, by assuring that they are present on more
than one plate.
In general, we have not listed faint images which are only suspected
of being non-stellar. Such faint objects generally comprise the
interesting class of compact objects and groups of compact objects
investigated by
Zwicky (1971) and
Shakbazian (1978).
Our objects are
more like the brighter objects catalogued by Zwicky. Some
investigators
(Massey 1975;
Kormendy 1977;
Fairall 1978)
claim that
compact galaxies are not different from ordinary galaxies. From our
experience with the brighter compacts, this seems difficult to accept
because, for the same diameter, our compact galaxies have surface
brightnesses which burn out or saturate the plate, whereas the vast
majority of galaxies do not have this effect. In any case, the present
Catalogue lists galaxies that represent the largest,
highest-surface-brightness compact galaxies encountered in the
southern hemisphere.
An initial investigation of the spectroscopic properties of
thirty-one compact galaxies, selected from U.K. Schmidt Survey plates,
is given in
Allen et al. (1978)
and also
Martin, Penfold and
Glass (1978).
Spectroscopy of southern compact and bright-nucleus galaxies
has been reported by Fairall
(1977,
1979,
1980,
1981,
1983,
1984).
Galaxies with high-surface-brightness ``hot spots'' in their
nuclei have been catalogued by
Sersic and Pastoriza
(1965),
but in general these are not a sub-set of our compact galaxy Category which
usually implicates the entire galaxy. Some factors which might be
involved in producing the high surface brightness are: strong gaseous
emission lines, high mass density, extensive non-thermal emission, or
recent, high-mass star formation. Like most extragalactic objects, the
present compact objects tend to be associated with other
galaxies. Further investigations will have to be made to determine
more accurately if these apparent associations are real, and what they
might tell us about the cause of the high surface brightness. It would
be particularly interesting to answer the inverse question: Are there
any compact galaxies which are truly isolated in space?
Category 14: Galaxies with Prominent or Unusual
Dust Absorption
Dust absorption is usually most conspicuous in edge-on spirals. In
general, this kind of object has not been catalogued here, unless the
dust appears to deviate from a flat, planar distribution. When
dissipative particles like dust accumulate in a thin rotational plane,
as is presumed to be the case where rotating spirals are concerned,
the phenomenon is not especially a curiosity. But when we encounter
irregular or asymmetric absorption features, we have good reason to
believe that something must have perturbed them. Moreover, when we see
filaments or tubes of dust, such as in the famous northern hemisphere
galaxy NGC 4753, the puzzling question arises as to what mechanism
could have formed such features. As in the case of stellar
configurations, it is also true that non-equilibrium dust features
should be differentially wound up, twisted, or dissipated by the
normal motions within a galaxy. Therefore their presence would appear
to indicate recent formation.
Another kind of object, included in
Category 14, are galaxies where
the dust marks an unexpected orientation of the rotation plane. Most
elongated galaxies have been assumed to be oblate spheroids, by
analogy to the many flattened spirals seen at varying degrees of
edge-on projection. A few rare cases of elliptical galaxies are known,
however, where the dust indicates a rotational plane that suggests
that the object is a prolate spheroid. One famous case is that of the
giant elliptical radio galaxy, NGC 5128 Cen A. Some additional cases
like this are noted in the present Catalogue. The possibility of
triaxial shapes for galaxies has also received recent theoretical and
observational attention; an overview of work on the three-dimensional
shapes of elliptical galaxies can be found in Bertola
(1981a,
1981b).
Category 15: Galaxies with Tails, Loops of
Material or Debris
Galaxies with luminous filaments which are more or less straight have
been classified into
Category 7: Galaxies with Jets. Luminous features
which are appreciably curved have been called tails and when there are
any signs of re-entrant behaviour, that is arms returning in the
direction from which they originated, they have been called loops.
These latter types are put into the present
Category 15: Galaxies with Tails, Loops of
Material or Debris.
Many doubles, and galaxies with interacting companions, show tails
or loops. Presumably this is because of their mutual gravitational
interaction, as can be seen in
Categories 1 through
5. However, there
is a category of galaxies whose members appear single, but also show
tails or loops. This is an important Category because it implies that
if these galaxies are truly single, then internal activity is a good
candidate for producing the phenomenon. In this regard it is useful to
consider that an initially straight jet could drift, fall back or be
perturbed by other objects in the vicinity, and, with time, evolve
into a tail or loop.
Of course there is always the possibility that what looks like a
single galaxy may, in fact, be a merged, double galaxy. This could
possibly enable the tails and loops to be accounted for with the usual
gravitational scenario. In the Catalogue, if there was any chance that
the filament could be emanating from a single galaxy, the object was
put into the present Category.
Category 16: Irregular or Disturbed Galaxies
In this Category, as in the previous one, we have tried to select
galaxies that have a good chance of being only one entity. But in this
case we require more than exterior peculiar features, like arms or
filaments. Rather we require that the entire galaxy must be
asymmetrical, irregular, or of an unusual nature. Again the
implication is that single irregular galaxies may have been subjected
to irregular conditions of formation or that the more disturbed cases
may be reacting to some recent internal perturbation.
Single galaxies are defined here as galaxies which have no
comparably-sized galaxy in the immediate neighbourhood which appears
to be affecting them. These
Category 16 galaxies can, however, be
companions to larger galaxies. The prototype for a
Category 16: Irregular or Disturbed Galaxies,
is the famous northern hemisphere
irregular, M82 (see The Hubble Atlas,
Sandage 1961;
Atlas of Peculiar Galaxies,
No. 337,
Arp 1966).
An interpretation of M82 is that it is
not its proximity to M81, but rather its own internal activity, which
accounts for its fractured, asymmetrical appearance. As emphasized in
previous Catagories of this Catalogue and elsewhere
(Arp 1973,
1982;
Sulentic, Arp and di
Tullio 1978),
companion galaxies tend often to occur in non-equilibrium forms.
As a future step in the study of the various disturbed categories in
this Catalogue, it will be of interest to investigate more
quantitatively the relative occurrence of disturbances, as a function
of the galaxy's proximity to larger galaxies, as a function of
membership in groups, in clusters, etc. Similarly, it will be of
importance to know whether any disturbed galaxies are completely
isolated in space. While such a study was beyond the scope of this
investigation, some feeling for the trends can be had by consulting
the relative frequencies of the various Categories as listed in
Table 1, since the classification
codes themselves (as well as the comments
in the main Catalogue) do incorporate some statement as to the local
environment defined by other nearby galaxies.
For some investigations this is possibly one of the most significant
and least expected categories in the Catalogue. It was noted some time
ago that chaining or linear alignments of galaxies appeared to be a
frequent occurrence
(Arp 1968;
Oort 1970).
But until we started to
list such configurations for the present Catalogue it was not realized
just how prevalent these a chains of galaxies were. As
Table 1 shows,
they are almost as frequent as apparently unaligned groups of four or
more galaxies. These groups of unaligned galaxies serve as a control
in the sense that we assume four galaxies in a group will be equally
recognizable, whether they are aligned or not aligned. Although we
feel that the tendency for chaining is very striking visually, we must
leave for future statistical analysis the problem of deriving the
exact frequency and accuracy of alignment which is in excess of that
expected from random arrangements of unaligned groups. Of course, some
opinion has already been expressed that chains are accidental
projections without significance
(Turner and Sargent
1973).
The other striking aspect of this Category is how frequently the
galaxies in these chains seem to be disturbed or in non-equilibrium
forms. The empirical fact of the disturbance supports the conclusion
that the chains are not accidental configurations, because the chance
of finding successive random galaxies in such disturbed forms is
small. Another result, which bolsters the physical reality of the
chains, is the tendency for interacting triples and quadruples to be
aligned, as was discussed earlier under those Categories. Finally, the
diametric companions of Category 8, many of
which are themselves
non-equilibrium galaxies, form aligned chains in the sense that the
large galaxy is at the center of the chain. In
Arp (1968)
it was shown
that chains of galaxies often originate on either side of radio
galaxies, which tended to be the largest and/or the most active
galaxies in the chain.
The reason why we feel that the general existence of chains of
galaxies, if validated, is so important is because of the following
argument: If chains were formed on the order of ten billion years ago,
their presently indicated redshift dispersions should have dissipated
the chains long ago. So, either the indicated redshift dispersions are
wrong, or the groupings are accidental, or the chains are considerably
younger than ten billion years.
To have galaxies recently formed is a radical departure from
conventional theories of an epoch of galaxy formation in the far past.
As for those chains which contain disturbed galaxies, they could have
been collisionally disturbed by interactions on plunging orbits
through the center. If such a circumstance seems unlikely, we are left
with galaxies which are disturbed because they are internally active;
this in turn may or may not be further evidence for recent formation.
One possible method to form galaxies in chains is to have the
pregalactic material collapse through linear instabilities in the
pregalactic medium
(Oort 1970).
Perhaps magnetic flux tubes could
guide such collapses. However we again encounter the age difficulty,
in that with the measured redshift dispersions the chains would have
to have been formed recently and, as far as we know, we see no
evidence for such recent condensations out of the intergalactic
medium. An alternative is to eject protogalaxies from central parent
galaxies
(Arp 1976a).
But this also would have to be a recent
activity, unless the velocity dispersion along the chain was
incorrectly inferred from the measured redshifts. If the physical
significance of the candidate chains given in this Catalogue is
confirmed, they will perhaps force profound changes on our ideas about
the formation, evolution, and nature of galaxies.
Finally, it should be remarked that there is some evidence
(Arp 1973)
that non-equilibrium chains like that in Stephan's Quintet, for
example, fall close to and appear to be associated with larger nearby,
low-redshift spirals. If this line of argument is valid, it would
mark the non-equilibrium chains as having large components of
non-velocity redshifts.
Objects classified into this Category fit the definition of being four
or more galaxies which are not aligned. Additionally they are galaxies
which stand out conspicuously by virtue of their density contrast with
respect to the surrounding field density of similar sized objects. The
distinction between a large group and a small cluster is somewhat
arbitrary, but is made along the following lines: a cluster is
regarded as a populous, dense grouping of galaxies the membership of
which generally shows a gradation in galaxy sizes from large to small;
a group tends to be a small aggregate of galaxies each more or less
the same size. Of course, some group members may have additional small
companions, but they are not a necessary or dominant component.
We have distinguished three broad types of galaxy aggregates in the
present Catalogue: galaxies with relatively small companions, galaxies
in groups with each member being about the same size, and populous
clusters which have a broad range of sizes. We are making a rough
statement about the luminosity function of galaxies within these
aggregates; saying that there appear to be distinctly different
luminosity functions. The challenge now becomes that of systematizing
this impression and discovering the reasons why different aggregates
of galaxies have different luminosity functions, if such is the
case. Our definitions may of course force these correlations, but one
must then wonder why the missing categories, such as populous,
high-density groupings of low-luminosity galaxies are not found.
Some conspicuous groups of galaxies in the southern hemisphere have
been previously listed by
Klemola (1969)
and by Sersic (1968).
Rose (1976),
Snow (1970),
and Duus and Newall
(1977)
have each catalogued
groups and clusters of galaxies. Some of their poorer clusters appear
in our catalogue under
Category 8: Galaxies with Apparent Companions;
this difference in point of view is interesting to note.
Over the area of the southern sky which we catalogued, we naturally
saw many clusters of galaxies. We have made no systematic attempt to
record all of these since a complete catalogue of southern galaxy
clusters has been made by Abell and Corwin (Clusters of Galaxies in
the Southern Hemisphere, in preparation) and we refer the reader to
that source. However, in the course of inspecting the southern sky,
we did encounter some clusters that were outstanding. In the spirit of
including all very unusual objects in the present Catalogue, we have
listed all these most conspicuous galaxy clusters. A number of these
are clusters dominated by cD galaxies and double, interacting, giant
elliptical galaxies. Not surprisingly, many of the identifications of
Parkes radio sources with objects in our Catalogue are with these
clusters. Some overlap is also found with the previous brief
catalogues published by
Snow (1970),
Rose (1976), and
Duus and Newall (1977).
Category 20: Dwarf Galaxies (Low Surface
Brightness)
The characteristic of low surface brightness is usually taken as a
sufficient condition for low intrinsic luminosity. It is apparently
not a necessary condition for low luminosity because there are
intrinsically faint galaxies known with high surface brightness
(Arp 1965;
Sargent and Searle
1970).
The dwarfs which are large in angular
extent usually reveal a very irregular morphology. It is difficult to
identify specific classes of irregularities, however, and although
such objects are completely listed in the descriptive part of the
Catalogue Volume I, only a few are illustrated in Volume II.
It should be noted at the outset that there is a technical problem
in discovering small, low-surface-brightness objects: many times plate
defects can look very much like real objects, and especially like
dwarf galaxies. We have made every effort to check such images on two
independent plates in order to minimize this error.
The outstanding problem in dwarf galaxies seems to be to explain why
the grand spiral design becomes bore and more disorganized and
irregular as the dwarfs become lower in surface brightness, lower in
luminosity, and presumably lower in mass. The examples of dwarfs given
here, it is hoped, will furnish a varied enough list of objects so
that this problem can begin to be investigated.
Some analyses of dwarfs from U.K. Schmidt Survey plates have already
been made.
Webster et al. (1979)
have searched for dwarfs belonging to
NGC 5128.
Longmore et al. (1982)
have presented a list of optical and
neutral hydrogen data for 151 low-surface-brightness galaxies. An
example of a low-surface-brightness resolved galaxy, which turned out
to be a member of the Local Group, is AM 0640-505, given in
Cannon, Hawarden, and
Tritton (1977)
and now known as the ``Carina
dwarf''. Other possibilities have been cited by
Longmore et at. (1978)
and by
Madore and Arp (1979).
Examples of dwarf galaxies found by ESO
astronomers can be found in the general compilation by
Lauberts (1982)
as well as in selected announcements by
Schuster and West
(1976),
Lausten et at. (1977) and
Cesarsky et at. (1977).
The low-surface-brightness objects of small apparent diameter are
particularly interesting. Is there a minimum absolute diameter below
which dwarf galaxies do not form (or survive)? To answer this,
distances are needed. Since much less morphological detail is visible,
it is not clear from the appearance alone whether any particular
object is a small, extremely low-luminosity object seen fairly nearby
(i.e., loose, inter-galactic star clusters) or whether it is a more
typical low-luminosity extragalactic system seen at greater distances
(i.e., dwarf galaxies at two to ten megaparsecs, say). How many star
clusters are there in the Local Group large enough to be seen as fuzzy
patches, but far enough away to be poorly resolved? The statistics are
poor, but the searches are far from complete. One such object is
AM-1
(Madore and Arp 1979).
Since such star clusters are of particular
interest in filling out the census of objects belonging to the Milky
Way and other Local Group galaxies, they were especially looked for in
the course of our survey. However, the very faintest and smallest
could easily be missed, even in a careful search (e.g., AM-4,
Madore and Arp 1982);
so we are probably complete only to the level of AM-1,
which is fairly bright. Nevertheless, some of the ``dwarfs'' listed in
our Catalogue may prove to be Local Group globular clusters.
Category 21: Stellar Objects with Associated
Nebulosity
Although it became apparent that the vast majority of the members of
this Category which we encountered were simply reflection nebulae (of
which there are a great many of varied form at low galactic latitudes)
we have retained the Category for the few high galactic latitude
objects which may be of future interest to others. Some of these
objects may yet prove to be more than their morphology suggests. One
reflection nebula is illustrated in
Category 22 in Volume II.
The number of objects necessarily classified into the miscellaneous
Category are very few. This is a simple demonstration that the other
twenty-three Categories encompassed most of the peculiar and unusual
aspects of galaxies. It is surprising that so many different peculiar
objects can be classified into so few Categories. Perhaps this does
mean that the subject is less complicated than it could be, and that
only a few fundamental factors are causing the non-equilibrium forms
with which we have to deal.
There are two examples of galaxies with ``ringing'' or stepped,
concentric rings illustrated in the miscellaneous Category in Volume
II. Objects of this kind were first shown in the
Atlas of Peculiar Galaxies
(Arp 1966);
for example, No. 227 through 231. Later,
Malin and Carter (1980)
printed further high-contrast examples of this type
of system. Generally, the rings imply some shocked star formation,
but the cause of shock, whether it be internal explosion, merger, or
encounter, still needs to be explored. It might be argued that these
belong in Category 6: Ring Galaxies or even in
Category 15: Galaxies with Loops, Tails of
Material or Debris, but they are, we believe,
distinct from these Categories, yet sufficiently rare that they do not
demand a separate Category to themselves. Thus their present location
in the Catalogue.
Category 23 and
24: Close Pairs and Triples
Sometimes large galaxies are encountered which are strikingly close
together, relative to comparable sized galaxies in the general
vicinity, and the supposition is strong that they are close together
in space and probably related. If they are large enough for details to
be resolved, and do not show signs of deformation or interaction, this
is interesting in itself: Are they only temporarily close with
insufficient time to effect gravitational perturbation? Or are all
these cases separated much further in the line-of-sight dimension than
in their projection on the sky? In any case, it was felt useful to
have examples of galaxies which seem to belong together, but which are
apparently not perturbed. This Category has also been included to
complete the reporting of all galaxies which, from their distribution
on the sky, appear to be associated together. For groups of galaxies
greater than three in number, one should consult
Categories 17,
18, and 19.
THE CATEGORIES
The basic approach adopted in the compilation of our Catalogue was
that as we scanned survey plates we inspected all galaxies down to
slightly less than 0.2 mm (i.e., about ten seconds of arc at the scale
of the Schmidt plates). Of course the majority of these galaxies were
more or less regular and approximately symmetric. Only when a galaxy
departed substantially from the norm was it catalogued as peculiar. In
addition, because we initially scanned survey plates at low (5x)
magnification we were aware when galaxies occurred in clusters,
groups, and loose associations. When we encountered galaxies which
were unusually close together compared to the average, we catalogued
them as a pair, group, rich cluster, or galaxy with apparent
companions. Of course in not every instance will these galaxies be
physically associated, but the association categories furnish a
homogeneous selection of those most likely to be associated. They can
later be tested statistically or with individual observations.
The present Catalogue was designed to provide some additional
information which has been previously unavailable. Specifically, we
have counted all galaxies in each field down to the same size limit to
which we have catalogued the peculiar galaxies. This means that we can
calculate how frequent certain types of peculiarities are, down to a
given limiting apparent diameter. For example, we can now say with
what absolute frequency we find M51-types, or ring forms, or galaxies
in chains. Similarly, it is also possible to calculate the relative
frequency with which one Category is likely to be found as compared to
another; for instance, how frequently does one encounter interacting
doubles as compared to interacting triples? As noted before, however,
all such intercomparisons should be done with care.
Although the present Catalogue was developed without specific
reference to the earlier
Atlas of Peculiar Galaxies
(Arp 1966),
looking back now we can see many of the present Categories illustrated
in that Atlas. Some Categories
were given slightly different names in
the Atlas. For example, the Atlas had an important class
of objects
called ``Companions on Arms'', which is now
Category 9: M51-Types. But
many Categories turned out to be essentially the same; some examples
from the earlier Atlas being ``galaxies with associated rings'',
``galaxies with jets'', ``disturbed galaxies with interior absorption'',
and ``chains of galaxies''. New categories were able to be added to the
present Catalogue because of the large number of galaxies included and
because we took advantage of the ability to compare surrounding sky
areas. The new Categories include the apparent associations
(Categories 1, 8,
23, 24), and the
galaxies of very high or very low
surface brightness (Categories 13 and
20).
One of the reasons for the interest in peculiar galaxies arises from
the possibility that the perturbation of a galaxy can be treated as a
departure from dynamic equilibrium. Physical properties of the galaxy
can then be derived by analyzing the event as an experiment performed
on the galaxy. This is particularly true if we can assemble sufficient
examples of the phenomenon to separate the various parameters which
differ between systems, as for example the types of initial impulses
and the time since the last disturbance. Perhaps an even more
important question is what causes the peculiarity in the first
place. Is it a disturbance due to an interaction with a nearby object
or is it an internally generated disturbance such as an explosion or
ejection?