| Annu. Rev. Astron. Astrophys. 1991. 29:
499-541
Copyright © 1991 by Annual Reviews. All rights reserved
|
3.1 Pencil-Beam Surveys
In trying to balance the requirements of depth, completeness, and
limited telescope time, pencil-beam surveys are the most economical. The
selection of the parameters associated with a pencil-beam survey, such
as the beam cross section, is often tied to instrumental constraints,
e.g. the size of a spectrograph's aperture plate. In order to overcome
the effect of small-scale fluctuations in the galaxian distribution, one
would thus ideally choose the cross section of a pencil-beam survey (at
its nominal redshift depth) to be comparable to or larger than
r0. The
sampling density can then be set by matching the associated sampling
error with that arising from the clustering properties and the chosen
geometry of the sampled region.
A survey spread over many separate fields can afford to sample deeply
in the redshift direction while also covering a large solid angle. Thus
a combination of pencil-beam surveys can economically yield information
on very large-scale features in (r). On the other hand, topological
details that could be investigated with a wide-angle survey tend to be
lost. In addition, the approach may complicate the definition of the
survey's selection function; for example, a space dependence is
introduced by coupling the number of redshifts obtainable per field to
the multiplexing factor of a multiobject spectrograph when the survey
field size is comparable to that of the spectrograph. The special niche
for discrete field surveys of the pencil-beam variety is in the
trailblazing determination of the rough, large-scale properties of the
galaxian distribution, ideally followed by more detailed studies over
wider solid angles.
KOSS
In a series of joint papers, Kirshner, Oemler, and Schechter - later
joined by Schechtman (hence KOSS) - , approached the observational study
of large-scale structure by sampling almost completely to a fairly deep
magnitude limit several widely spaced fields, each about one square
degree in size. At the mean depth of their survey, one pencil-beam field
is somewhat narrower than r0, so the structure seen in
each tends to be
dominated by the small-scale fluctuations in the galaxian distribution.
Nonetheless their survey showed considerable deviations from homogeneity
on very large scales. In 1981, on the basis of 133 redshifts in three
fields ~ 35° apart, KOSS reported the discovery of a large void in
Boötes, centered at cz ~ 15,000 km s-1 and extending
over 6000 km s-1 in the radial direction (see also
KOSS 1983).
In 1987, a more elaborate
analysis confirmed the existence and size of the void (although not as
extreme a density contrast as initially reported); they sampled 283
small fields - each 15' on a side - spread between
13h30m and 16h30m,
+30° to +70°, to mlim ~ 17.
KOSS (1990)
are currently extending their
discrete sampling approach in a survey that covers the two galactic caps
to a redshift of 0.2, to include eventually a total of 10,000 to 20,000
redshifts. The strategy combines the advantages of a multifiber
spectrograph with the large field of view of the Las Campanas 2.5-m
duPont telescope. They are surveying 200 fields, each 1.5° x
3° in size,
covering about 25% of the sky between 10h15m and
15h15m, 0° to - 18°,
for the northern galactic hemisphere, and between
21h00m and 04h30m,
-30° to -48°, for the southern one. They are obtaining their own
photometric and astrometric catalog by means of a CCD drift survey. A
60-fiber spectrograph can yield 40 to 45 redshifts of galaxies with
magnitudes 16.0 < R < 17.3, per 2h exposure, and about
4 fields per
night can be observed. By summer 1990, they obtained about 2000 redshifts.
DURHAM-AAT-SAAO
Peterson et al (1986)
surveyed five high-galactic-latitude UK Schmidt
Telescope (UKST) fields, each with an average area of 14
deg2, widely
separated between the equator and - 50°. To a mean limiting magnitude of
about bJ ~ 17.0, each field contains about 70
galaxies. This first
sample, observed with the Anglo-Australian Telescope (AAT), is commonly
referred to as the Durham-AAT redshift survey (DARS). Comprising
redshifts of 329 galaxies, it also benefits from high-quality
photometric data, obtained digitally on UKST IIIa-J plates. This effort
was recently complemented by that of
Metcalfe et al (1989),
who selected
a sample of about 750 galaxies in nine UKST fields, to a depth
comparable to that of the earlier DARS, and measured the redshift of
every third galaxy in the sample for a total of 264 redshifts. These
observations, carried out at the 1.9-m telescope of the South African
Astronomical Observatory (SAAO), are referred to as the Durham-SAAO
sample. A notable parallel effort, also at the SAAO, concentrates on the
southern sky survey field 349, in which
Parker et al (1986)
reported 107 redshifts to bJ ~ 16.5. These studies have
concentrated on the
determination of the statistical properties of the galaxian
distribution, namely the two-point correlation function
(Shanks et al 1983,
1989),
the galaxy luminosity function
(Efstathiou et al
1988), and
applications of the cosmic virial theorem
(Hale-Sutton et al
1989).
The Durham effort has been particularly effective in the study of
high-redshift objects, as we discuss below.
GALAXIES AT HIGH REDSHIFT (z > 0.1)
The study of high-redshift galaxies aims at several goals,
mainly: (a) the approximation of an elusive fair sample, whereby
reliable mean properties of the universe can be obtained; (b) the
understanding of the evolution of galaxies and of the field luminosity
function; and (c) a knowledge of the evolution of clusters and their
galaxian content. Recent reviews that emphasize particularly aspect b
can be found in
Ellis (1987) and
Koo (1988).
Because of the low fluxes,
redshift surveys employ different strategies for distant galaxies than
for nearby ones. At high z, spectroscopic observations can be time
consuming (at z ~ 0.5, a 200 km s-1 quality redshift
requires the better
part of a night's observing on a 4-m class telescope); the surface
density of targets, however, is high, and the use of multiobject
spectrographs is well-suited for the task. Alternatively, large
numbers of galaxies can be surveyed photometrically, and the broad band
spectral information can be used as an indicator of distance, a
photometric red shift.
Photometric redshifts
The technique, first applied by
Baum (1962),
is well reviewed by
Koo (1985).
Observations in many-color passbands yield an estimate of
redshift that relies critically on assumptions of each galaxy's stellar
population and evolutionary state. Evolutionary models, most notably
those of Bruzual
(1983 and
refs. therein), need to be adopted, and
redshifts are derived by best fits of the model spectral energy
distributions to the data. Of particular importance to these studies is
the identification of the 4000-Å break, a blend of the Ca II H and K and
other metal lines whose cosmological potential is discussed by
Dressler & Schectman
(1987).
Koo (1985)
finds that two thirds of his photometric
redshifts agree with spectroscopic values with an accuracy of ±0.04
in z for z 0.35 and
±0.06 to z 0.6.
Loh & Spillar (1986
and refs. therein) have attempted to apply the
results of their photometric survey to place constraints on cosmological
parameters. They obtained six-band photometry for a sample of about 1000
galaxies with a median z 0.5 within a pencil beam of 24 µsr, and
derived photometric redshifts by best-fitting the location of the 4000-Å
break. The number of galaxies observed is essentially the product of the
galaxy luminosity function and the volume sampled, and it can be
expressed in terms of the mean luminosity density (*) and the density
parameter . Somewhat
controversially,
Loh & Spillar (1986)
derived a
relatively high value of = 0.9
± 0.3 for a cosmological constant = 0.
While the photometric technique of estimating redshifts is an economic
one, it remains uncertain, mainly because of evolutionary effects that
are themselves the object of study. Photometric techniques allow the
probing of faint magnitude regimes (mB ~ 23), and as
evolutionary
effects become better understood, they might provide a very fruitful
tool for the study of large-scale structure.
Spectroscopic redshifts
Using a multifiber spectrograph at the AAT,
Broadhurst et al
(1988)
extended an approach similar to that used in DARS to survey 200 galaxies
of 20.0 < bj < 21.5, in five fields of 200 to 400
arcmin2 each, at a
mean redshift of 0.22.
Also at the AAT,
Colless et al (1990)
used a
multislit spectrograph to probe even deeper (21 < bj <
22.5), obtaining
149 redshifts in three fields. With a multislit arrangement at the Kitt
Peak 4-m telescope,
Koo & Kron (1988)
surveyed three fields near the
North Galactic Pole; their survey currently includes 401 redshifts (Kron
1990, personal communication). These two complementary efforts have been
combined to produce a remarkable view out to z ~ 0.5 in the direction of
the two galactic poles
(Broadhurst et al
1990).
The most striking result
is the appearance of a strong feature in the clustering at scales of
128h-1 Mpc. Ongoing efforts will extend the scope of
the Durham-AAT and
Kitt Peak probes to encompass as many as 30 or 40 pencil beams in both
hemispheres, distributed over two patches (each several degrees wide,
with one in each hemisphere) for a total of some 1000 redshifts. This
combination approach of both pencil beam and sparse sampling emphasizes
sensitivity to large transverse structures, since at z ~ 0.15, the
pencil beam separation is ~ 10-60 Mpc. The other important thrust of
these surveys is the understanding of the evolution of the field
luminosity function, as briefly discussed in the last section.