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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
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
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 Community’s Human Potential Programunder contractHPRN-CT-2002-00316, HPRN-CT-2002-00305McDonald Observator
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