The discovery of a substantial population of LSB galaxies at z = 0 is relevant to a number of cosmological issues. These issues are enumerated below; those issues that are only briefly discussed here are covered in more detail in Impey and Bothun (1997).
QSO absorption lines: Increasing the number density of
galaxies is a very good first step towards understanding the origin of
QSO absorption lines. In addition, the discovery of Malin 1, which
has 105 kpc2 of gaseous cross section at a level
of Nh
3 x 1019 cm-2 is a helpful addition to the zoology
of objects that might produce damped Ly
systems.
Large Scale Structure (LSS): To date, most studies of
LSS are based on redshift surveys of optically selected samples,
which, by definition, do not contain many LSB galaxies. If LSB
galaxies are better tracers of the mass distribution, then
the linear biasing factor between mass and light has a surface brightness
dependence. This would be an ugly complication to the various
sophisticated attempts to determine b0.6,
where b is the (scale-independent) linear bias parameter
(see Strauss & Willick 1995). In an
attempt to shed light on this, Bothun
et al. (1985,
1986) performed a redshift survey
of
400 LSB disk
galaxies, primarily selected from the UGC. Few of these galaxies
were in the CFA redshift survey at that time. That data, coupled
with a thorough analysis of the clustering properties of LSB galaxies
by Bothun et al. (1993) and
Mo et al. (1994) provides the
following insights:
(1) | On scales ![]() |
(2) | LSB galaxies generally avoid virialized regions. |
(3) | On scales ![]() |
(4) | Whether in groups or clusters, LSB galaxies are usually near the edge of the galaxy distribution. The results of O'Neil et al. (1997a) show that, in the cluster environment, there is a limiting value of µ 0 which depends on local galaxy density. If one considers that the highest surface brightness galaxies of all, ellipticals, tend to occur in virialized regions then we have the surface brightness extension of the morphology-density relation of Dressler (1980). That is, the local galaxy density limits the surface mass/luminosity density of galaxies. |
(5) | LSB galaxies are not preferentially found in large scale voids. For instance, there are none in either the CFA bubble (Bothun et al. 1992) or the Bootes Void (Aldering et al. 1997). |
The following scenario can be advanced. Given a Gaussian initial
spectrum of density perturbations which will eventually form galaxies,
there should be many more low density perturbations than high density
ones. Many of these low density perturbations are subject to disruption or
assimilation into other perturbations and hence will not produce individual
galaxies. However, if a substantial percentage of them do survive to
produce individual galaxies then we would expect them to dominate the
galaxy population. We tentatively identify the LSB galaxy population
with these 1-2 peaks in
the initial Gaussian perturbation
spectrum. If that is the case, we expect LSB galaxies to (1) have
formed in isolation, and (2) be fair tracers of the mass distribution
on large scales. This isolation on small scales must clearly affect
their evolution since, compared to HSB galaxies, LSB galaxies have
experienced fewer tidal encounters with nearby galaxies over a Hubble
time. Tidal encounters are effective at clumping gas and driving
global star formation. Without this external hammer, LSB galaxies
would continue to evolve slowly and passively.
The Faint End Slope of the Galaxy Luminosity Function:
In recent years, a lot of different values have appeared for the
faint end slope, , of the
Schechter luminosity function.
The canonical value of
-1.25 for HSB field
galaxies can be compared with observations of nearby clusters that show
of
-1.1 (but see
Wilson et al. 1997 for significantly steeper
values for more distant clusters). However,
= -1.9
has been determined for low mass irregular
galaxies in the CFA redshift survey (Marzke et al. 1994). The safest
thing to then conclude is that recent surveys have now produced a variety
of different values of
indicating that its a) uncertain and b) may evolve with time.
In general, corrections for incompleteness to the galaxy luminosity function have been erroneously applied in the past based on apparent magnitude only. This is conceptually incorrect since galaxies are selected on the basis of a combination of surface brightness and luminosity. This is best illustrated in the Virgo and Fornax clusters, where the diffuse and fairly large LSB dwarf were missed in earlier surveys but detected using the Malin method. The integrated luminosity of these galaxies is brighter then many of the Binggeli et al. (1984) dwarfs due to their large scale length. The surveys in Virgo (Impey et al. 1988) and Fornax (Bothun et al. 1991) revealed LSB galaxies that were up to 3 magnitudes brighter than the nominal magnitude limit of the Binggeli et al. (1984) survey but which were missed by that survey.
Figure 8 (adapted from Figure 10 in Impey et al. 1988) illustrates
the basic point for the Virgo cluster.
When these "missing galaxies" are properly accounted for, there is
a significant increase in the faint end slope of the luminosity function
relative to that which is obtained simply by applying a correction
for incompleteness based on apparent flux. The correlation between
surface brightness and magnitude that seems to hold for dwarf
elliptical galaxies (e.g.,
Sandage et al. 1985;
Binggeli et al. 1985;
Caldwell and Bothun 1987;
Bothun et al. 1989) has been
shown to break down below µ 0
24.5 (e.g.,
Bothun et al. 1991)
indicating that corrections for apparent flux do not mimic the
more proper correction based on surface brightness selection effects
(see also Sprayberry et al.
1997; Impey and Bothun 1997).
Combining the Virgo and Fornax cluster sample of dwarfs yields
=
-1.55 ± 0.05 after these proper corrections are made.
Such a steepening indicates that LSB galaxies dominate numerically in
clusters. If stellar/baryonic M / L increases with decreasing
surface brightness for this
cluster population then they contain a significant fraction of the baryonic
material in clusters (which might exacerbate the fb
problem in clusters).
It is interesting to speculate that cluster LSB galaxies might represent
the results of a phase of intense baryonic blow-out at higher redshifts.
These relic galaxies then might be the source of the gas seen in clusters
as well as the enrichment of that gas. In the case of the Virgo cluster,
there are approximately twice as many dwarfs as brighter galaxies.
![]() Figure 8. The luminosity function of the Virgo cluster galaxies showing the difference between corrections based on apparent magnitude from those based on surface brightness selection effects. The solid circles/squares give the raw counts and error bars as a function of apparent magnitude. The filled circles are the result of applying an incompleteness correction based only on apparent magnitude. The open circles are the result of applying a correction based on surface brightness. These galaxies are missed not because of reduced apparent flux, but because too much of their flux is below the night sky background. As a result, the correction correction for incompleteness based on surface brightness considerations sets in at a substantially brighter magnitude than corrections based only on apparent flux. These are the galaxies in Virgo that were discovered by Impey et al. 1988. |
The APM survey of Impey et al. (1996) allows the contribution of LSB
galaxies to the field galaxy luminosity function to be estimated.
This has been done in detail by
Sprayberry et al. (1997)
who find a faint end slope of =
-1.42 compared to
=
-1.09 for all disk types in the CFA redshift survey. It is too soon to
tell if there is a significant difference between the cluster and
field faint end slopes but in both cases, the discovery of LSB
galaxies in both environments yields the same result: the faint
end slope steepens.
The Faint Blue Galaxy Connection: One immediate result
from various deep surveys of galaxies (e.g.,
Lilly et al. 1991,
Lin et al. 1997,
Odewahn et al. 1996,
Driver et al. 1996,
Glazebrook et al. 1995) was
the discovery of an apparent excess, relative to local observations,
of faint galaxies with blue colors. These enigmatic faint blue galaxies
(FBGs) are the subject of much research. Since the FBG population
apparently does not exist at z = 0, reasons for their absence from
galaxy catalogs of the local universe must be found. Two possible
approaches involve either a very large merger rate since z 0.7
or rapid fading of this FBG population
(Efstathiou et al. 1991;
Colless et al. 1990). This
faded population is potentially an important
constituent of the total baryonic content of the universe (see
Cowie 1991;
Bouwens and Silk 1996). However,
arguments presented above suggest that these faded remnants have not yet
been detected in large numbers (see also
Dalcanton 1993).
A number of explanations for the FBG ``problem'' have a natural linkage with LSB galaxies at z = 0. Some of the possible explanations are the following:
If optically selected LSB galaxies do have normal UV fluxes, then this population can become conspicuous in faint galaxy surveys when this light is redshifted into the ground based filter set. This would occur, in general, over the redshift range 0.5-1. It is possible that some of the FBGs are in fact LSB galaxies! For example, the median luminosity of the FBGs in Lilly et al. (1991) is identical to the LSB sample of McGaugh & Bothun (1994) and both samples are dominated by late type galaxies which are weakly clustered.
The FBGs are a population of star bursting dwarf galaxies
located at modest redshift. This suggestion takes advantage of the
fact that in any GLF with
-1, low mass
dwarf galaxies
dominate the space density. To produce the FBGs, however, these
dwarf galaxies have to be at least an order of magnitude brighter
at modest redshifts which requires a fairly significant star
formation rate. Subsequent heating of the ISM by massive stars
and supernova should be sufficient to heat it beyond the escape
velocity of these low mass systems (see
Silk et al. 1987;
Wyse and Gilmore 1992) and hence
such galaxies have a significant phase of baryonic blowout after
which they fade to very low absolute luminosities and hard to
detect at z = 0. This mechanism gives the universe a channel for
making baryons ``disappear'' with time. This hypothesis has been
investigated in detail by
Phillips and Driver (1995)
and Babul and Ferguson (1996).
The number density of galaxies is not conserved and the FBGs merge with other galaxies. It is difficult to support this hypothesis because (1) the FGBs are already weakly clustered, and (2) the required merging rate is significantly higher than the inferred rate at modest redshift by Patton et al. (1997). The merger idea works best if the FBGs are predominately at higher redshift, where the merger rate is higher owing to the smaller volume of the universe.
The FGBs represent an entirely new population of galaxies -
one defined by a star formation history or an initial mass function
that allows only a limited window of visibility before the galaxies
fade to extremely low surface brightness levels by z = 0. As an
example,
an IMF which forms no stars less than 1 M produces
a galaxy that is destined to have a very bright phase and then
fade into mostly white dwarf remnants at z = 0. A collection of
1011 white dwarfs distributed in a disk with scale length of
2 kpc would have a central surface brightness of µ
0 = 27.5
which would be below the current detection threshold of LSB galaxies.
This situation illustrates the kind of extreme fading which would
be necessary to hide this component from being detected locally.
Interestingly, the latest MACHO results (see
Sutherland et al. 1996/a>)
strongly suggest that halo white dwarfs are the main cause of the
8 observed lensing events towards the LMC. In that sense, the Galactic
halo might be regarded as a LSB galaxy.
The FBGs are an artifact of uncertainties in the determination
of the local galaxy luminosity function. In particular, the faint end
slope has been seriously underestimated from nearby samples. Alternatively,
the local normalization, (0),
of the galaxy luminosity function could
be too low. This would result if, for instance, deep surveys were more
efficient at selecting galaxies than nearby surveys. Driver et al.
(1994a,
b) have explored this possibility
and concluded that the local
normalization may well be seriously underestimated. However, the effect
of increasing the space density at z = 0 can only partially
offset the excess FBG counts. A much larger lever arm is provided by
steepening
.
Sprayberry et al. (1997) have considered the effects of increasing
(due to LSB inclusion)
and increasing
(0) on the
ability for LSB galaxies to explain the FBG excess. The conclude
that the CFA survey has missed about 1 / 3 of the total galaxy population
at z = 0. While this helps to resolve the apparent difference between
the large numbers of FBGs and the local galaxy population, this effect
by itself is not large enough to completely resolve the issue (see also
Ferguson and McGaugh 1995). For
additional assistance we turn to the
Lilly et al. (1995) redshift
survey of
500 faint galaxies. Their
sample has excellent quality control and is fairly free from selection
effects and is primarily aimed at determining the galaxy luminosity
function up to a redshift
1.
Lilly et al. detect a change in the luminosity function of blue
galaxies by approximately one magnitude between z 0.38 and z
0.62, and
another magnitude between z
0.62 and z
0.85. Moreover, many
of these galaxies have been observed with HST in order to measure
characteristic surface brightnesses.
Schade et al. (1995) find
that the disks of these blue galaxies are
1 magnitude higher in
surface brightness at z = 0.8 than z = 0.3. These studies
provide rather strong evidence for luminosity evolution in the FBGs.
In a 15 Gyr old universe, there are approximately 3.3 billion years
between z = 0.85 and z = 0.38. The data indicates that a
typical
FBG would decline in luminosity by a factor of six (e.g., 2 mags) over this
time period. This modest decline is quite consistent with
standard population synthesis models involving a normal IMF which fades
after a significant burst of star formation has occurred to make
these objects visible at higher redshift.
The decline in luminosity is primarily a reflection of the disappearance
of the upper main sequence. By z = 0, these galaxies will certainly
not have faded to levels that preclude their detection, and indeed
many of them are likely to be represented by the z = 0 red LSB
population which has now been discovered.