![]() | Annu. Rev. Astron. Astrophys. 2006. 44:
xxx-xxx Copyright © 2006 by Annual Reviews. All rights reserved |
Immediate precursors for cluster ETGs in the local universe were first identified with a population of blue, star-forming galaxies whose fraction in clusters increases very rapidly with redshift (Butcher & Oemler 1978, 1984, a trend known as the Butcher-Oemler effect), and therefore most such blue, star-forming galaxies had to subside to passive ETGs by z ~ 0. Dressler el al. (1997) and Fasano et al. (2000) showed that in clusters up to z ~ 0.5 the fraction of true ellipticals is fairly constant, while the fraction of (star-forming) spirals rapidly increases at the expense of the S0's. This was interpreted as evidence that galaxies in clusters that were star-forming spirals at z ~ 0.5, have changed their morphology to become passively evolving S0's by z = 0, a transformation that may account for much of the Butcher-Oemler effect. In clusters in the same redshift range, a sizable population of ETGs with strong Balmer absorption lines was also identified by Dressler & Gunn (1983), hence called K+A galaxies after the appearance of their spectrum. These ETGs were recognized as poststarburst galaxies, as further documented by Dressler et al. (1999, 2004) and Poggianti et al. (1999).
Thus, the metamorphosis from z ~ 0.5 to z = 0 of a fraction of
cluster galaxies from star forming to passive is well documented by
these studies. However, these changes seem to affect more spirals and
S0's than true ellipticals, and if the bulk of star formation in ETGs
took place at very high redshift, then we need to look further out to
catch them in formation. From the low-redshift studies we have learned
that ETGs are massive, highly clustered, and had to form the bulk of
their stars at z
1.5-3 (depending
on mass and environment)
within a short time interval. Indeed, fast star formation is required
by the
-overabundance,
and by the mere high formation
redshift. At z ~ 3 the universe is only ~ 2 Gyr old, hence
forming ~ 1011
M
of
stars in one object between z = 5 and
z = 3 (
t
~ 1 Gyr) requires a star-formation rate (SFR) of
100
M
yr-1. Altogether, possible precursors of massive ETGs at
low z could be searched among high-z massive, highly
clustered, starburst galaxies with very high SFRs
(
100
M
yr-1). The rest of this section is dedicated to mentioning
the main results in searching for such objects.
Lyman break galaxies (LBGs) were the first ubiquitous population of
galaxies to be identified at z ~ 3, and their SFRs often in excess
of ~ 100
M
yr-1, plus their relatively small size (Re
~ 1-3 kpc), made them natural precursors to local bulges and ETGs (e.g.,
Giavalisco, Steidel, &
Macchetto 1996,
Steidel et al. 1996,
Giavalisco 2002,
but see also
Giavalisco et al. 1995
for an even earlier attempt to identify a z = 3.4 precursor to
ETGs). Recently,
Adelberger et al. (2005)
show that the 3D correlation length
(ro) of LBGs is such to match that of local lesser
spheroids (M
1011
M
) when
the secular increase of ro is
taken into account. However, Adelberger et al. note that the most
massive and most rapidly star forming galaxies at high redshifts are
likely to be lost by the Lyman break selection. From
Figure 2 we see
that local ETGs with M > 1011
M
include almost 50% of the mass
in this kind of galaxies, and ~ 1/4 of the total stellar mass at
z = 0. Therefore, looking at the starforming precursors of the most
massive ETGs refers to a major component of the whole galaxy
population. Hence the search was not limited to the LBG technique.
Other obvious candidates are the ultraluminous infrared galaxies
(ULIRG,
Sanders et al. 1988,
Genzel & Cesarsky
2000),
detected in mm or sub-mm surveys (e.g.,
Ivison et al. 1998,
Lilly et al. 1999),
a class defined for their infrared luminosity (8-1000 µm)
exceeding ~ 1012
L, and
whose typical SFRs
(
200
M
yr-1) well
qualify them for being precursors to massive spheroids, as does their
very high (stellar) density ensuring that they would "land" on the
fundamental plane as star formation subsides
(Kormendy & Sanders
1992,
Doyon et al. 1994).
ULIRGs as ETG in formation are also advocated by
Genzel et al. (2001),
who from the resolved kinematics for 12 of them
argue that typical ULIRGs are likely precursors to intermediate-mass
ETGs rather than to giant ellipticals. However, the internal
kinematics of one ULIRG at z = 2.8 indicates a mass
3 ×
1011
M
(Genzel et al. 2003)
making it a likely precursor to a very massive ETG.
Blain et al. (2004)
note that
submillimeter-selected galaxies at z = 2-3 appear to be more strongly
clustered than LBGs at the same redshifts, which makes them more
attractive candidates than LBGs for being the progenitors of the most
massive ETGs. Still, their space density falls short by a factor of
~ 10 compared to passive EROs at z ~ 1, and they could be the
main precursors to EROs only if their duty cycle is very short.
Nevertheless,
Chapman et al. (2005)
argue that the sub-mm galaxies may
well be the dominant site of massive star formation at z = 2-3, once
more making them excellent candidates for being ETG in formation.
The survey and characterization of sub-mm galaxies are currently
limited by the modest sensitivity and resolution of existing sub-mm
facilities, hence optical/near-IR selections are still the most
efficient way of identifying large samples of massive star-forming
galaxies at high redshift. For the star-forming BzK-selected
objects,
Daddi et al. (2004)
estimate <SFR> 200
M
yr-1, which is typical of ULIRGs, and most of them are clearly
mergers on ACS images. Indeed, out of 131 non-AGN (i.e., non X-ray
emitter) star-forming BzKs with K < 20 in the GOODS-North field
(Dickinson et al. in preparation), 82% were individually detected
with Spitzer/MIPS at 24 µm
(Daddi et al. 2005a).
Moreover, by stacking the fluxes of the 131 objects ( <z> =
1.9) from radio to X-rays (i.e., VLA 1.4 GHz, SCUBA 850 and 450
µm, MIPS 24 µm, IRAC 8-3.5 µm,
near-IR and optical bands, and Chandra's 0.5-8 keV)
Daddi and colleagues showed that the resulting composite SED is an
excellent match to that of a template ULIRG with LIR
= 1.7 × 1012
L
and
<SFR>
250
M
yr-1, in agreement with
the typical SFRs derived from the extinction-corrected UV flux. Two of
these BzKs have also been detected at 1.2 mm with MAMBO, implying a
SFR ~ 1000
M
yr-1
(Dannerbauer et
al. 2006).
So, the BzK selection
proves to be an excellent way of finding large numbers of ULIRGs at
high redshift, whose space density at z ~ 2 (~ 1-2 ×
10-4 Mpc-3) is about three orders of magnitudes
higher than the local density of ULIRGs, and a factor of 2-3 higher than
that at z = 1. Moreover, the number of star-forming BzKs with
M > 1011
M
is close to that of passive BzKs of similar mass, and added together
nearly match the space density of massive ETGs at z = 0
(Kong et al. 2006).
Hence, it is tantalizing to conclude that as star formation
subsides in star-forming BzKs the number of passive ETGs will approach
their local density. Finally, worth mentioning is that the majority
among samples of J - K > 2.3 DRGs in the Extended
HDF-South field
(Webb et al. 2006)
and GOODS-South field
(Papovich et al. 2006)
have been recently detected at 24 µm with Spitzer/MIPS,
indicating that the
majority of DRGs are likely to be dusty starburst precursors to ETGs,
rather than having already turned into passive ETGs themselves. Moreover,
DRGs appear to be distributed over a very broad and nearly flat redshift
distribution, from less than 1 to over ~ 3.5
(Reddy et al. 2006).
Moderate redshift precursors to local ETGs are not necessarily
star-forming. They may also be less massive ETGs that will merge by
z = 0, an event now called "dry merging". The merger rate since
z ~ 1.2 has been estimated by
Lin et al. (2004)
as part of the DEEP2 survey, concluding that only ~ 9% of present-day
M* galaxies have undergone a major merger
during the corresponding time interval. However,
Bell et al. (2006)
searched for dry merger
candidates over the GEMS field, and based on 7 ETG-ETG pairs estimated
that each present-day ETG with Mv < -20.5 has
undergone 0.5-1
major dry merger since z ~ 0.7. This may be at variance with the
estimate based on the 3D two-point correlation function of local ETGs
in the overwhelming SDSS database. Indeed, each local ETG is found to have
less than 1% probability per Gyr of merging with another ETG, hence
the dry merging rate appears to be "lower, much lower than the rate
at which ETG-hosting DM halos merge with one another"
(Hogg 2006),
at least for z
0.36
(Masjedi et al. 2006).
The history of star formation in ETGs has been deduced from the
properties of their passively evolving stellar populations in low and
high redshifts galaxies, and we know that ETGs and bulges hold at
least ~ 50% of the stellar mass at z = 0. Threfore, it is worth
addressing here one last issue, even if only in a cursory way: is such
an inferred history of star formation consistent with the direct
measurements of the star-formation density and stellar mass density at
high redshifts? Based on the estimate that the bulk of stars in
spheroids formed at z
3, it has been
suggested that at least
~ 30% of all stars (and metals) have formed by z = 3
(Renzini 1999).
This appears to be at least a factor of ~ 3 higher than
the direct estimate based on the HDF-North, according to which only
3%-14% of today's stars were in place by z = 3
(Dickinson et al. 2003).
Data from HDF-South give the higher value 10%-40%
(Fontana et al. 2003),
most likely as a result of cosmic variance
affecting both HDF fields. Based on the ~ 10 times wider field of
the K20 survey, ~ 30% of the stellar mass appears to be in place
by z ~ 2
(Fontana et al. 2004),
but the corrections for incompleteness are large.
Drory et al. (2005)
find that over the ~
200 arcmin2 area of the combined GOODS-South and FORS Deep Field,
~ 50%, ~ 25%, and at least ~ 15% of the mass in stars
is in place, respectively at z = 1, 2, and 3. So, no gross
discrepancy has emerged so far between the mass density at z ~ 3
as directly measured and
as estimated from the fossil evidence at lower redshift. The same
holds for the comparison between the star-formation
densities as a function of redshift, as inferred from the distribution
of stellar ages of ETGs on the one hand, and as directly measured by
observations on the other hand (e.g.,
Madau et al. 1996,
Steidel et al. 1999,
Giavalisco et
al. 2004b).
Errors on both sides are still as
large as a factor of 2 or 3, but this should rapidly improve
thanks to the deep and wide surveys currently under way.