EROs in GOODS:
The sample, morphologies, and X-ray properties of
Extremely Red Object galaxies in the CDF-S field
Authors
L. A. Moustakas (STScI), N. Grogin (JHU), D. M. Alexander (PSU), S. Casertano (STScI), C. Conselice (Caltech), M. Dickinson, H.
Ferguson (STScI), J. P. Gardner (NASA-GSFC), A. M. Koekemoer (STScI), C. Kretchmer (JHU), R. A. Lucas, B. Mobasher (STScI), C.
Papovich (U. Arizona), M. Richardson (STScI), D. Stern (JPL, Caltech), M. Stiavelli (STScI), E. Daddi, A. Renzini, L. da Costa (ESO), S.
Arnouts (LAS, Marseille), B. Vandame (ESO), and the GOODS Team
ABSTRACT
Using collated U'- through Ks-band imaging data in the GOODS-South ACS mosaic (which overlaps with the
Chandra Deep Field-South), we select ''extremely red object'' (ERO) galaxies through optical-minus-infrared colors
that are comparable to, or in excess of, those possible by aged stellar populations at redshifts z=1-2. Using simple
models, we employ color-selection criteria to split the sample of 130 galaxies into old, dusty, and other subsets. We
independently quantify their concentration and asymmetry indices using the high-resolution HST/ACS Z-band data.
The objects are cross-correlated with the 1MS Chandra X-ray sourcelist, and comparisons made between the
''AGN'' and ''non-AGN'' ERO samples, particularly in view of the morphologies. Photometric redshifts for all
objects have been determined using data from up to 13 bands combining ground- and ACS-photometry. Using these
measurements, we attempt to determine the astrophysical parameters that characterize the main types of EROs, and
place them in the context of views of galaxy formation at redshifts z=1-2.
The X-ray ERO sources - By correlating the Giacconi et al (2002) 1MS CDFS X-ray
source catalog against the current full catalog we find 120 matches; the characteristics of
these objects is the topic of, e.g. B. Simmons' poster (see also N. Grogin's talk). Only 7 of
these match the ERO catalog. The X-ray characteristics of these X-ray EROs is broad,
spanning a wide range of hardness ratios. C.f. object 3816 in the montage below, for a purehard-X-ray ERO source. This diversity is indicative of both obscured AGN and pure starburst
activity (or low-level AGN contributions).
Introduction - The scope of GOODS is to probe not only the very highest-redshift objects, but also
galaxies in the ''spectroscopic dessert'' of z=1-3. In that redshift range,building large samples of
galaxies is difficult, and particularly high-SNR spectroscopic work is presently prohibitive. With
GOODS' concerted effort of ultraviolet-through-mid-IR deep imaging and follow-up spectroscopy, it
is possible to tackle the problem of the census and characterization of a significant sample of galaxies
at z>1. The ultimate goal is to empirically infer the way galaxies assembled their mass, resulting in the
''Hubble sequence'' we see at redshifts of z<1.
One of the first steps we are taking is to look ''under the lamp-post'' for massive, bright z=1..2 galaxies.
This is done by using a simple optical-minus-infrared color criterion that does a demonstrably
remarkable job of selecting such galaxies. The literature has bestowed these objects the moniker
''extremely red objects'' (EROs), and these galaxies may have colors of R-K>5.3 -- the threshold
corresponding to the approximate color of an old early-type galaxy at z~1. In fact, this color selection
isolates several different families of galaxies, including the expected old, massive early-type galaxies;
extremely dusty starburst or ''ultraluminous infrared'' galaxies; X-ray-bright galaxies; sub-mm-bright
galaxies; and the occassional (and quite rare) ultra-cool Galactic star.
The GOODS data & photometric redshifts - The catalog we employ is ACS Z-band-
selected (F850LP), based on the first three epochs of the CDF-S observations, matched to the full set
of HST and ground-based bands available. The 50% completeness limit in this catalog is Z=25.3 and
Ks=22.5 (vega). (The infrared data used here is from VLT:ISAAC and NTT:SOFI.) Photometric
redshifts are calculated by a variety of methods, using all aperture-matched bands -- see poster by
Mobasher et al. A 'quality' assessment value is attached to each photometric redshift, which for this
work has not been used to preselect objects, but rather for assessing galaxy samples' quality.
Extremely red galaxies - As shown in the first plot on the right, the selection of ''extremely red''
All but one of these objects SED-type to ''early-type'' galaxies 5 / 7 with ''good'' photometricredshift classification. The morphologies are more diverse, as seen in the montage below. A
stack of the non-detections has not yet been performed, though from e.g. Brusa et al (2002)
and Alexander et al. (2002), it is expected that there may be detections in both hard and soft
bands. This analysis will help distinguish between the AGN and starburst origins of the
observed X-ray emission in different subsets of the EROs, which may be related to their
morphologies and stellar populations.
The spatial distribution of objects in the CDFS GOODS
field. - The density plot (and the points) depict the full ACS-based
sample of galaxies in RA and Dec, showing some surface density
variations. The galaxies with R-K>5.3 are shown as red polygons,
the size scaling by the color. The blue points mark the positions of
the R-K>5.3 X-ray sources, based on the Giacconi et al 2002 catalog.
Note the clear clustering of the EROs. Furthermore, only a very
small fraction of the X-ray sources in this field coincide with EROs:
merely 7 / 120. This is a much smaller ratio than found by other
work to date.
Morphologies and X-ray contours of X-ray EROs in CDF-S - The right column
shows 5x5'' ACS Z-band images of each of the 7 ERO X-ray sources in our sample. The left
column shows 20x20'' ACS Z-band images of the same objects (as indicated by the lines),
with 1MS Chandra contours overlaid, in 3-sigma intervals.
galaxies is purely photometric (above a certain magnitude limit). This early work is based on a (largely
complete) subset of all possible ''extremely red'' objects: we require that there is a detection in all three of
the Z, R, and Ks-bands, and that R-K>5.3. Most of these have Ks<21. The cyan-colored points reflect
'good' photometric redshifts; therefore about half the ERO sample has reliable photo-zs. We also have
several spectroscopic redshifts from the GOODS VLT:VIMOS campaigns. As indicated in the second
plot on the right, this selects galaxies that are predominantly (typed as) early-type galaxies, and with a
very tight redshift distribution with a median redshift of z~1.2. Of the 130 objects in this sample of
EROs, virtually all are classified as ''early-type'' galaxies, with little or no star formation (SED-typing <~
2). Though a passable majority of EROs are morphologically ''early-type'' as well (as shown below), the
high-resolution ACS data clearly show a generous set of late-type galaxies, mergers, and others. (Our
sample also includes an ultracool Galactic star candidate). The spatial clustering of the resulting set of
EROs is strongly evident, consistent with recent work (e.g. Firth et al 2002; Daddi et al. 2001).
Quantifying galaxy morphologies - One of the key data products from the GOODS project is quantitative measures
of the morphologies of all galaxies for which it is feasible. To that end, several independent approaches are being taken,
including GALFIT (Peng et al), GIM2D (Simard et al), and the CAS (concentration, asymmetry, clumpiness) morphology
system (Conselice et al 2002). The first two are parametric in that some combination of specific functional forms for the
surface brightness profiles of galaxies are assumed. The last method is empirical, in the sense that the measured
''concentration'' and ''asymmetry'' are determined directly from the data, so that one can immediately see similarities or
differences between a sample of galaxies and a control sample.
''CAS'' morphologies - All nearby major galaxy types fall in well defined corners of a volume created out to the
concentration and asymmetry parameters. Early types are those with a high concentration, low asymmetry and low
clumpiness values. Early type spirals have a lower light concentration, higher asymmetries and high clumpiness values.
Later type disks on average have even lower concentrations and slightly higher asymmetries and clumpiness values.
Mergers are galaxies which have high asymmetry values. These features can broadly be seen in the top-panel plot on the
left, which plots asymmetry vs (the inverse of) concentration (such that the more concentrated objects are towards the left).
The green points are those found to be early-types based on their SED-fitting; and the blue points are late-types. The earlytypes fall in the lower-left part of the plot, where ellipticals are morphologically expected to reside, and conversely for the
late-types. This provides an excellent control sample. See the poster by R. Lucas for a detailed explanation of the CAS
definitions and applications of this technique.
ERO morphologies - In the two CAS plots on the left, the red stars show the EROs with CAS classification. More than
half fall very tightly in the ''early-type'' locus, while the remaining fraction span a broad range of morphologies. The top
panel is made using all possible objects, whereas the lower one is restricted to a (generous) redshift range appropriate for this
ERO sample: z=0.8..1.6. The general behavior of the 'control' sample is qualitatively similar (including the locus spanned by
the SED-typed galaxies), though there is more scatter. Six examples of the EROs are labelled in both plots by ID number,
and shown below.
CONCLUSIONS
We have constructed a sample of ~130 EROs with precision
multiband photometry, good photometric redshifts, and with exquisite
rest-optical spatial resolution.
Though nearly all have SED-types close to ''early-type'' galaxies, only
about half have early-type morphologies; the rest span a large range.
Seven EROs are detected in the Chandra X-ray map, but reveal a
mixture of star-formation and AGN activity.
Work on spatially-resolved ACS colors, X-ray stacking of the full
ERO set, and (later in 2003) SIRTF infrared observations will provide
more clues for the nature and role of massive galaxies at z>1.
Examples of ACS Z-band morphologies of CDF-S color-selected ERO galaxies - The galaxies shown here correspond to the
ones marked in the CAS plots above. In the legend, in addition to the RZK magnitudes and colors, are given the photometric
redshift, its 'quality' measurement (>0.95 is 'good'), and the SED-based typing. Values of two or less are ''early-type''; larger values
are for 'late-type'. All but a few of the 130 EROs in our sample are SED-typed as early-types. This is at least in part indicative of
the inevitable limitations of the assumed templates; but simultaneously reflects the need for a more careful study of their intrinsic
SEDs and stellar populations. The inset plot shows the photometry-based SED.