![]() | Annu. Rev. Astron. Astrophys. 2000. 38:
667-715 Copyright © 2000 by Annual Reviews. All rights reserved |
4.4. Spectroscopic and Photometric Redshifts
The HDF-N and flanking fields have been the focus of some of the most
intensive and complete spectroscopic redshift survey work on faint
galaxies. Essentially all spectroscopic redshifts
in the HDF have come from the W.M. Keck Observatory and LRIS spectrograph
[Cohen et
al. 1996,
Cohen et
al. 2000,
Lowenthal et
al. 1997,
Phillips et
al. 1997,
Steidel et al. 1996,
Steidel et
al. 1999,
Dickinson
1998,
Adelberger et
al. 1998,
Spinrad et
al. 1998,
Weymann et
al. 1998,
Waddington et
al. 1999,
Zepf et al. 1997,
Hu et al. 1998].
The most recent compilation is 92% complete for R < 24 in the
central HDF-N, and also 92% complete for R < 23 in the
flanking fields
[Cohen et
al. 2000].
The median redshift is
z 1
at R = 24. Altogether, including unpublished
redshifts from the Steidel group, more than 700 spectroscopic redshifts
have been measured in the immediate vicinity of the HDF-N, with ~ 150 in
the central HDF (WF and PC) alone. This latter corresponds to
30 redshifts per arcmin2, spanning
0.089 < z < 5.60 (and a few
stars), a density unmatched by any other field galaxy survey.
At the same time, it is sobering to realize
that less than 5% of the galaxies from the
[Williams et al. 1996]
HDF-N catalog have spectroscopic redshifts. The vast majority are
fainter than the spectroscopic limit achievable with present-day
telescopes.
For the HDF-S, the extant spectroscopy is more limited, consisting of
about 250 redshifts in an irregular area surrounding the three HST
fields
[Glazebrook et
al. 2000,
Dennefeld et
al. 2000,
Tresse et al. 1999].
Three galaxies at
2.8 < z < 3.5 have been confirmed by the VLT
([Cristiani
1999]).
Eleven HDF-S ISO sources have redshift measurements from VLT IR spectroscopy
[Rigopoulou
et al. 2000].
The estimation of galaxy redshifts using broad-band colors received a tremendous boost from the HDF observations and followup studies. Indeed, the advent of the HDF marked a transition for photometric redshifts, which evolved from an experimental exercise to a widely used technique. This came about for several reasons. Perhaps foremost, as noted above, the WFPC2 HDF data readily detected galaxies as much as 100 times fainter than the practical spectroscopic limits of 10m telescopes. Therefore, in order to interpret the properties of the vast majority of HDF galaxies, non-spectroscopic techniques for estimating redshifts are necessary. Fortunately, the HDF-N WFPC2 data set offered very-high-quality, four-band photometry from 0.3 to 0.8 µm for thousands of faint galaxies, and the extensive followup observations that came later extended this to other wavelengths and wider fields of view. Rapid dissemination of data and source catalogs from the HDF and followup observations made it relatively easy for any investigator to test their photometric redshift method. Indeed, it should be noted that many photometric redshift studies to date have been tested and calibrated solely using the HDF-N.
Several groups
([Steidel et
al. 1996,
Lowenthal et
al. 1997,
Clements & Couch
1996,
Madau et al. 1996,
Madau et al. 1998])
used two-color selection keyed to the passage of the Lyman limit and Lyman
forest breaks through
the F300W
and F450W passbands to identify galaxy candidates at z > 2. Such
color-selected objects are variously referred to as Lyman-break
galaxies, or UV dropouts. The
[Madau et
al. 1996,
Madau et al. 1998]
studies were
seminal in combining color-selected HDF samples at z ~ 3 and 4
with previous (spectroscopic, non-HDF) studies at z < 2 to
derive a
measure of the global history of cosmic star formation, a goal
subsequently pursued by many other groups using various kinds of HDF
photometric redshifts (see Section 5.5).
More general approaches to photometric redshifts have either fit redshifted spectral templates to the photometric data (e.g., [Gwyn & Hartwick 1996, Mobasher et al. 1996, Lanzetta et al. 1996, Sawicki et al. 1997, Fernandez-Soto et al. 1999, Benítez et al. 1999] for the HDF-N), or have used generalized polynomial fits of redshift vs. multi-band fluxes to a spectroscopic training set (e.g., [Connolly et al. 1997, Wang et al. 1998]). [Cowie & Songaila 1996] and [Connolly et al. 1997] were the first to include IR HDF photometry when deriving photometric redshifts. In principle, this is especially useful in the 1 < z < 2 range where it is difficult to measure redshifts from optical spectroscopy due to the absence of strong, accessible emission or absorption features, and where the strong spectral breaks (e.g. at 4000Å and the 912Å Lyman limit) fall outside the optical passbands. Many other groups have since included IR photometry in their photometric redshift work, using the KPNO 4m JHK images from Dickinson et al. [Fernandez-Soto et al. 1999], or more recently HST NICMOS data [Thompson 1999, Budavari et al. 1999, Yahata et al. 2000].
[Hogg et
al. 1998]
and [Cohen et
al. 2000]
have carried out blind tests
of the accuracy and reliability of HDF photometric redshifts, comparing
photometric predictions by the various groups with spectroscopic
redshifts unknown to those groups when the predictions were made. In
general, all of the photometric techniques are quite successful at
z 1.4,
where the spectroscopic training sets are extensive,
with the best prediction schemes achieving
| zphot - zspec| / (1 +
zspec)
0.05 for
90% of the
galaxies. At z > 1.9 the results are also generally quite
good, with 10 to 15% RMS in
z / (1 +
z) after excluding the worst outliers. The intermediate redshift
range, where photometric redshifts are particularly interesting, has not
yet been tested due to the lack of spectroscopic calibrators.
Photometric redshifts have been used to identify galaxy candidates
in both HDFs at z > 5. Two objects noted by
[Lanzetta et
al. 1996]
and [Fernandez-Soto
et al. 1999]
have been subsequently confirmed
by spectroscopy, with z = 5.34 and 5.60
[Spinrad et
al. 1998,
Weymann et
al. 1998].
With the addition of near-infrared data,
it becomes possible, in principle, to push to still higher redshifts.
Indeed, at z > 6.5, galaxies should have virtually no detectable
optical flux. Candidates at these very large redshifts have been
identified by
[Lanzetta et
al. 1998]
and [Yahata et
al. 2000].
One of these in the HDF-N was readily detected in NICMOS 1.6 µm
images by
[Dickinson et
al. 2000b],
but is not significantly detected
at J110 or at optical wavelengths. Although this is a
plausible candidate for an object at
z 12,
there are also other possible
interpretations. It is curious that several of the proposed z > 10
candidates, including this "J-dropout" object in the HDF-N and
several of the HDF-S/NICMOS candidates proposed by
[Yahata et
al. 2000],
are surprisingly bright (~ 1 µJy) at 2.2
µm. These would be
significantly more luminous than the brightest known Lyman break galaxies
at 2 < z < 5. It remains to be seen whether this
points to some remarkable epoch of bright objects at z > 10,
or whether instead the photometric redshifts for these objects have been
overestimated.