93 research outputs found
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
The stellar masses of 25000 galaxies at 0.2<z<1.0 estimated by the COMBO-17 survey
We present an analysis of stellar mass estimates for a sample of 25000
galaxies from the COMBO-17 survey over the interval 0.2<z<1.0. We have
developed, implemented, and tested a new method of estimating stellar
mass-to-light ratios, which relies on redshift and spectral energy distribution
(SED) classification from 5 broadband and 12 medium band filters. We find that
the majority (>60%) of massive galaxies with M_* > 10^{11} solar masses at all
z<1 are non-star-forming; blue star-forming galaxies dominate at lower masses.
We have used these mass estimates to explore the evolution of the stellar mass
function since z=1. We find that the total stellar mass density of the universe
has roughly doubled since z~1. Our measurements are consistent with other
measurements of the growth of stellar mass with cosmic time and with estimates
of the time evolution of the cosmic star formation rate. Intriguingly, the
integrated stellar mass of blue galaxies with young stars has not significantly
changed since z~1, even though these galaxies host the majority of the star
formation: instead, the growth of the total stellar mass density is dominated
by the growth of the total mass in the largely passive galaxies on the red
sequence.Comment: Astronomy and Astrophysics in press. 15 pages, 12 figure
Nearly 5000 Distant Early-Type Galaxies in COMBO-17: a Red Sequence and its Evolution since z~1
We present the rest-frame colors and luminosities of ~25000 m_R<24 galaxies
in the redshift range 0.2<z<1.1, drawn from 0.78 square degrees of the COMBO-17
survey. We find that the rest-frame color distribution of these galaxies is
bimodal at all redshifts out to z~1. This bimodality permits a
model-independent definition of red, early-type galaxies and blue, late-type
galaxies at any given redshift. The colors of the blue peak become redder
towards the present day, and the number density of blue luminous galaxies has
dropped strongly since z~1. Focusing on the red galaxies, we find that they
populate a color-magnitude relation. Such red sequences have been identified in
galaxy cluster environments, but our data show that such a sequence exists over
this redshift range even when averaging over all environments. The mean color
of the red galaxy sequence evolves with redshift in a way that is consistent
with the aging of an ancient stellar population. The rest-frame B-band
luminosity density in red galaxies evolves only mildly with redshift in a
Lambda-dominated cold dark matter universe. Accounting for the change in
stellar mass-to-light ratio implied by the redshift evolution in red galaxy
colors, the COMBO-17 data indicate an increase in stellar mass on the red
sequence by a factor of two since z~1. The largest source of uncertainty is
large-scale structure, implying that considerably larger surveys are necessary
to further refine this result. We explore mechanisms that may drive this
evolution in the red galaxy population, finding that both galaxy merging and
truncation of star formation in some fraction of the blue, star-forming
population are required to fully explain the properties of these galaxies.Comment: To appear in the Astrophysical Journal 20 June 2004. 16 pages, 6
embedded figures. Substantial revision of photometric redshifts and extensive
minor changes to the paper throughout: conclusions unchange
Star Formation and the Growth of Stellar Mass
Recent observations have demonstrated a significant growth in the integrated
stellar mass of the red sequence since z=1, dominated by a steadily increasing
number of galaxies with stellar masses M* < 10^11 M_sun. In this paper, we use
the COMBO-17 photometric redshift survey in conjunction with deep Spitzer 24
micron data to explore the relationship between star formation and the growth
of stellar mass. We calculate `star formation rate functions' in four different
redshift slices, splitting also into contributions from the red sequence and
blue cloud for the first time. We find that the growth of stellar mass since
z=1 is consistent with the integrated star formation rate. Yet, most of the
stars formed are in blue cloud galaxies. If the stellar mass already in, and
formed in, z<1 blue cloud galaxies were to stay in the blue cloud the total
stellar mass in blue galaxies would be dramatically overproduced. We explore
the expected evolution of stellar mass functions, finding that in this picture
the number of massive M* > 3x10^10 M_sun blue galaxies would also be
overproduced; i.e., most of the new stars formed in blue cloud galaxies are in
the massive galaxies. We explore a simple truncation scenario in which these
`extra' blue galaxies have their star formation suppressed by an unspecified
mechanism or mechanisms; simple cessation of star formation in these extra blue
galaxies is approximately sufficient to build up the red sequence at M*<10^11
M_sun.Comment: 9 Pages; ApJ in pres
GEMS: Galaxy Evolution from Morphologies and SEDs
GEMS, Galaxy Evolution from Morphologies and SEDs, is a large-area (800
arcmin2) two-color (F606W and F850LP) imaging survey with the Advanced Camera
for Surveys on HST. Centered on the Chandra Deep Field South, it covers an area
of ~28'x28', or about 120 Hubble Deep Field areas, to a depth of
m_AB(F606W)=28.3 (5sigma and m_AB(F850LP)=27.1 (5sigma) for compact sources. In
its central ~1/4, GEMS incorporates ACS imaging from the GOODS project.
Focusing on the redshift range 0.2<=z<=1.1, GEMS provides morphologies and
structural parameters for nearly 10,000 galaxies where redshift estimates,
luminosities and SEDs exist from COMBO-17. At the same time, GEMS contains
detectable host galaxy images for several hundred faint AGN. This paper
provides an overview of the science goals, the experiment design, the data
reduction and the science analysis plan for GEMS.Comment: 24 pages, TeX with 6 eps Figures; to appear in ApJ Supplement. Low
resolution figures only. Full resolution at
http://zwicky.as.arizona.edu/~rix/Misc/GEMS.ps.g
GEMS Survey Data and Catalog
We describe the data reduction and object cataloging for the GEMS survey, a large-area (800 arcmin(2)) two-band (F606W and F850LP) imaging survey with the Advanced Camera for Surveys on the Hubble Space Telescope, centered on the Chandra Deep Field-South.STScI HST-GO-9500.01NASA GO-9500, NAS5-26555, NAG5-13063, NAG5-13102European Communityâs Human Potential Programunder contractHPRN-CT-2002-00316, HPRN-CT-2002-00305McDonald Observator
GEMS: Galaxy fitting catalogues and testing parametric galaxy fitting codes
In the context of measuring structure and morphology of intermediate redshift
galaxies with recent HST/ACS surveys, we tune, test, and compare two widely
used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic
models to the light profiles of both simulated and real galaxy data. We find
that fitting accuracy depends sensitively on galaxy profile shape. Exponential
disks are well fit with Sersic models and have small measurement errors,
whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the
large amount of light at large radii. We find that both codes provide reliable
fits and little systematic error, when the effective surface brightness is
above that of the sky. Moreover, both codes return errors that significantly
underestimate the true fitting uncertainties, which are best estimated with
simulations. We find that GIM2D suffers significant systematic errors for
spheroids with close companions owing to the difficulty of effectively masking
out neighboring galaxy light; there appears to be no work around to this
important systematic in GIM2D's current implementation. While this crowding
error affects only a small fraction of galaxies in GEMS, it must be accounted
for in the analysis of deeper cosmological images or of more crowded fields
with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors
because it can simultaneously fit the profiles of multiple companions thereby
deblending their effect on the fit to the galaxy of interest. We find GALFIT's
robustness to nearby companions and factor of >~20 faster runtime speed are
important advantages over GIM2D for analyzing large HST/ACS datasets. Finally
we include our final catalog of fit results for all 41,495 objects detected in
GEMS.Comment: Accepted for publication in ApJS October 2007, v172n2; 25 pages, 16
Figures, 9 Tables; for hi-resolution version, see
http://www.mpia.de/homes/bhaeussl/galaxy_fitting.pdf. For results, catalogues
and files for code-testing, see http://www.mpia.de/GEMS/fitting_paper.htm
Dry Mergers in GEMS: The Dynamical Evolution of Massive Early-Type Galaxies
We have used the 28'x 28' HST image mosaic from the GEMS (Galaxy Evolution
from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric
redshift survey to constrain the incidence of major mergers between
spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A
set of N-body merger simulations was used to explore the morphological
signatures of such interactions: they are recognizable either as < 5kpc
separation close pairs or because of broad, low surface brightness tidal
features and asymmetries. Data with the depth and resolution of GEMS are
sensitive to dry mergers between galaxies with M_V < -20.5 for z < 0.7; dry
mergers at higher redshifts are not easily recovered in single-orbit HST
imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and
4:1 were found from a sample of 379 red early-type galaxies with M_V < -20.5
and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of
dry merging are visible for ~250Myr and we use this timescale to convert the
observed merger incidence into a rate. On this basis we find that present day
spheroidal galaxies with M_V < -20.5 on average have undergone between 0.5 and
2 major dry mergers since z ~ 0.7. We have compared this result with the
predictions of a Cold Dark Matter based semi-analytic galaxy formation model.
The model reproduces the observed declining major merger fraction of bright
galaxies and the space density of luminous early-type galaxies reasonably well.
The predicted dry merger fraction is consistent with our observational result.
Hence, hierarchical models predict and observations now show that major dry
mergers are an important driver of the evolution of massive early-type galaxies
in recent epochs.Comment: ApJ, in press. The paper has been extensively modified, detailing the
automated+visual selection and dry merger classification. 11 pages emulateapj
with 9 reduced-quality figures. A high quality copy is available at
http://www.mpia-hd.mpg.de/homes/bell/papers/dry.ps.g
An Explanation for the Observed Weak Size Evolution of Disk Galaxies
Surveys of distant galaxies with the Hubble Space Telescope and from the
ground have shown that there is only mild evolution in the relationship between
radial size and stellar mass for galactic disks from z~1 to the present day.
Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky
Survey (SDSS), and high redshift data from the GEMS (Galaxy Evolution from
Morphology and SEDs) survey, we investigate whether this result is consistent
with theoretical expectations within the hierarchical paradigm of structure
formation. The relationship between virial radius and mass for dark matter
halos in the LCDM model evolves by about a factor of two over this interval.
However, N-body simulations have shown that halos of a given mass have less
centrally concentrated mass profiles at high redshift. When we compute the
expected disk size-stellar mass distribution, accounting for this evolution in
the internal structure of dark matter halos and the adiabatic contraction of
the dark matter by the self-gravity of the collapsing baryons, we find that the
predicted evolution in the mean size at fixed stellar mass since z~1 is about
15-20 percent, in good agreement with the observational constraints from GEMS.
At redshift z~2, the model predicts that disks at fixed stellar mass were on
average only 60% as large as they are today. Similarly, we predict that the
rotation velocity at a given stellar mass (essentially the zero-point of the
Tully-Fisher relation) is only about 10 percent larger at z~1 (20 percent at
z~2) than at the present day.Comment: 13 pages, 6 figures, accepted for publication in ApJ. Revised in
response to referee's comments to improve clariry. Results are unchange
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