|
|
COSMOS Project Scientific Goals
We propose to carry out a comprehensive study of the rest-frame
optical properties of 1000 galaxies at, including: morphology,
structure, kinematics, dust content, star formation rate (SFR),
metallicity, luminosity and mass functions, clustering, and large
scale-structure. The fundamental questions that we are attempting to
answer include:
- What is the nature of the galaxy population at z>2? What
are their local counterparts?
At z~2 the 1-2.5 microns range translates into rest-frame
~3500-8000Å, the region of the spectrum that has been most widely
studied in nearby galaxies. COSMOS will address these questions by
comparing directly the properties of the high redshift galaxies with
those of the nearby population in the same parameter space,
including half-light radii (Re), surface brightnesses (SBe), emission line
ratios (e.g., [OII]3727/[OIII]5007, [OIII]5007/Hb,
[NII]6584/Ha), SFRs (from
both Ha and [OII]3727 emission), metallicities (from various line
strength indices), dust content (using Balmer decrements), and
internal kinematics (from Ha velocity widths). This procedure avoids
the uncertainties in the calibrations and biases that affect similar
studies of high-redshift galaxies in the rest-frame UV. A similar
approach has been succesfully used to identify unambiguously the local
counterparts of compact emission-line galaxies at z~1. For instance,
Figures 1 and 2 show the diagrams for a sample of galaxies at z=1
(Phillips et al. 1997; Guzmán et al. 1997). These two
diagrams provide a complete description of the global galaxy
structural properties. The high blue luminosities and surface
brightnesses of this z=1 galaxy sample, together with their small sizes
and velocity widths, are consistent with their being low-mass (log M<10),
extreme star-forming systems similar to nearby HII galaxies. This is
also corroborated in Figures 3 and 4, showing that HII galaxies and
1 galaxies have the same emission-line ratios and specific
SFR's. Another important diagnostic diagram is the
luminosity-linewidth relation (Tully-Fisher for spirals, Faber-Jackson
or the 'fundamental plane' for ellipticals and for HII galaxies).
Figure 5 shows the relation for the same galaxy sample at , confirming
the similarity between high redshift galaxies and nearby HII galaxies
(Guzmán et al. 1997). Note that a key new aspect common to
these studies at high redshift, including COSMOS, is to measure galaxy
internal motions, which are not affected by the luminosity evolution
of the stellar population.
Figure 1: SBe vs. MB. Meaning of symbols:
crosses: nearby HII galaxies; dotted lines
indicate the locus occupied by various other types of local galaxies;
the arrow (F) represents the direction of fading.
Figure 2:
Re vs. sigma-velocity width.
Symbols as before; dashed lines represent constant
mass-lines in; the arrows represent the effects of
dissipation (D), mergers (M), stripping (S) and winds (W) on Re and sigma.
We adopt H=50 km/s/Mpc and q=0.05.
- How have galaxies at evolved?
Are they the progenitors of today's quiescent stellar systems?
Galaxies at z>2 are being observed when the universe was 20%
of its current age. Many of them will be truly 'primeval' galaxies,
i.e., galaxies that are most likely undergoing their first episode of
star formation.
Once their counterparts both at lower redshifts and at
the present epoch have been identified, COSMOS will be able to
assess the evolution
of galaxies from the early universe till the present like a
step-by-step movie of galaxy evolution. This sequence will
define 'evolutionary tracks' on the various diagnostic diagrams
discussed above, which can then be compared to model predictions of
galaxy formation and evolution including physical processes such as:
fading of the stellar population, SN-driven galactic winds, mergers,
tidal stripping or galaxy harassment (see Figures 3a and 3b).
This procedure has
been used to identify for the first time a class of extreme star-forming
galaxies at as progenitors of today's population of dwarf
elliptical galaxies (Koo et al. 1995; Guzmán et al. 1996).
Figure 3: MB vs. [OIII]/Hb.
Local galaxies (Gallego et al. 1997):
DANS=Dwarf Amorphous Nuclear Starbursts;
SBN=Starburst Nuclei; Sy2:=Seyfert 2 galaxies; HII=HII galaxies.
Dashed lines represent the approximate location of spiral galaxies.
Figure 4: Mass vs. SFR/M. Symbols as before (from Guzmán,
Gallego et al. 1997) .
- What is the real evolution of the star formation history
of the universe with look-back time?
It is now widely assumed that the Star Formation Rate (SFR) density of the
Universe had a peak at z~1 and from then, it descended to the local value
(Madau et al. 1996; Figure 5).
However, the interpretation of this figure should be
approached with caution, given the likely differences in the
calibrations for the various SFR tracers used, incompleteness of the
data sets, and uncertainties in the corrections for dust. Indeed, new
estimates of the global SFR rate density at high redshifts differ in
more than a factor among various researches. Most of the
caveats surrounding these estimates can be avoided by
measuring the SFR using Ha
emission, the best SFR tracer. This is the method adopted by Gallego
et al. (1995) to estimate the local value of the global SFR density,
and will also be used by the COHSI project (Aragón-Salamanca et al.\
1998) to derive the value at 1<z<2.
COSMOS will be the
first major survey to measure Ha emission at 2<z<3 and,
combined with similar surveys at lower redshift, will define a
homogenous, unambiguous picture of the star formation history of the
universe from till the present. In addition, since [OII]3727
is also a fair measure of the star formation rate, COSMOS will be able
to provide a well-calibrated measurement of the global star formation
rate to via observations of the [OII]3727 emission down to
2.5 microns.
Figure 5: Evolution of the global SFR density of the
universe with redshift (Glazebrook et al 1998).
- What is the earliest epoch of galaxy formation?
The quest to observe the birth and formation of normal galaxies like our own has
proved to be a very elusive one... till now. The advent of the
10m-class telescopes combined with powerful new instrumentation has
yielded in only two years over 600 high redshift candidates of which
are spectroscopically confirmed galaxies at 2<z<4 (Steidel
et al. 1996; Lowenthal et al. 1997; Steidel et al. 1998).
The record holder
for the galaxy with the highest redshift known goes to a very recent
discovery of an object at z=5.34 by Spinrad et al. (1998).
This is obviously
the beginning of a new era of discovery of primeval galaxies in the
early universe. The new near-infrared spectrographs planned for the
10m-class telescopes will allow systematic searches for primeval
galaxies at z<3.5, by looking for [OII]3727 from emission-line
galaxies, and Ca H+K and 4000Å break from absorption-line objects
over the range 0.9 to 1.8 microns. COSMOS will be able to extend this
search to -and to much higher redshifts via observations of
Ly-a by extending the wavelength coverage to 2.5 microns.
- When did galaxies group into large-scale structures
and how have these structures evolved?
Recent years have seen spectacular progress in
the study of the large scale structure of the universe, which is now
well constrained at the highest redshifts from microwave background
measurements, and at low redshifts from redshift surveys and peculiar
motions measurements. The key piece of information missing is the
measurement of large scale structure at 1<z<6. These
measurements are vital to resolve the degeneracy among current
cosmological models and to directly constrain the mean density, age
and biasing of the universe. Recent, very preliminary studies of
clustering at z~1 suggest that there does seem to be a significant
amount of
large scale structure at high redshifts, and that it may vary strongly
with the galaxy type, indicating that biasing may be important (e.g.,
Neuschafer & Windhorst 1995; Cole et al. 1994;
Wolfe 1993). These studies are based on
direct observations of galaxies through [OII]3727 emission, or,
indirectly, through QSO absorption-line measurements.
The new major redshift surveys will be
able to map the large-scale structure up to z~3.5 via observations
of the [OII]3727 emission-line down to 1.8 microns.
COSMOS will be the only such survey able to extend these studies to z~6.
|
|