69 research outputs found
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
Evolution of the Dark Matter Distribution with 3-D Weak Lensing
We present a direct detection of the growth of large-scale structure, using
weak gravitational lensing and photometric redshift data from the COMBO-17
survey. We use deep R-band imaging of two 0.25 square degree fields, affording
shear estimates for over 52000 galaxies; we combine these with photometric
redshift estimates from our 17 band survey, in order to obtain a 3-D shear
field. We find theoretical models for evolving matter power spectra and
correlation functions, and fit the corresponding shear correlation functions to
the data as a function of redshift. We detect the evolution of the power at the
7.7 sigma level given minimal priors, and measure the rate of evolution for
0<z<1. We also fit correlation functions to our 3-D data as a function of
cosmological parameters sigma_8 and Omega_Lambda. We find joint constraints on
Omega_Lambda and sigma_8, demonstrating an improvement in accuracy by a factor
of 2 over that available from 2D weak lensing for the same area.Comment: 11 pages, 4 figures; submitted to MNRA
Galaxy-galaxy lensing studies from COMBO-17
We study the dark matter halos of galaxies with galaxy-galaxy lensing using
the COMBO-17 survey. This survey offers an unprecedented data set for studying
lens galaxies at z=0.2-0.7 including redshift information and spectral
classification from 17 optical filters for objects brighter than R=24. So far,
redshifts and classification for the lens galaxies have mainly been available
for local surveys like the Sloan Digital Sky Survey (SDSS). Further, redshifts
for the source galaxies have typically not been available at all but had to be
estimated from redshift probability distribution which -- for faint surveys --
even had to be extrapolated.
To study the dark matter halos we parametrize the lens galaxies as singular
isothermal spheres (SIS) or by Navarro-Frenk-White (NFW) profiles. In both
cases we find a dependence of the velocity dispersion or virial radius,
respectively, on lens luminosity and colour. For the SIS model, we are able to
reproduce the Tully-Fisher/Faber-Jackson relation on a scale of 150h^-1kpc. For
the NFW profile we also calculate virial masses, mass-to-light ratios and
rotation velocities.
Finally, we investigate differences between the three survey fields used
here.Comment: 6 pages, 5 figures. To be published in the proceedings of IAU
Symposium No. 225: The Impact of Gravitational Lensing on Cosmology, Y.
Mellier and G. Meylan, ed
The impact of the dark matter-gas interaction on the collapse behaviour of spherical symmetric systems
If the gas in the evolving cosmic halos is dissipating energy (cooling) then
due to the variation of the gravitational potential the dark matter halo also
undergoes a compactification. This is well-known as Adiabatic contraction (AC).
Complementary to the AC we investigate the resulting dynamical behaviour of the
whole system if the backreaction of the AC of DM onto the gas is taken into
account. In order to achieve sufficient high resolution also within the central
halo region, we use a crude fluid approximation for the DM obeying the
adiabatic contraction behaviour. Further, we restrict ourself to spherical
symmetry and vanishing angular momentum of the studied matter configurations.
The computations are done using a first-order Godunov type scheme. Our results
show that the dynamical interaction between gas and DM may lead to significant
shorter collapse times. If the gas cools the dynamical behaviour of the whole
system depends strongly on the shape of the initial density profile. Our
findings indicate that for a certain mass range of halo configurations the
dynamical interaction between gas and DM might be important for the halo
evolution and must be taken into account.Comment: 9 pages, 13 figures, accepted for publication in A&A, added
reference
Towards an understanding of third-order galaxy-galaxy lensing
Third-order galaxy-galaxy lensing (G3L) is a next generation galaxy-galaxy
lensing technique that either measures the excess shear about lens pairs or the
excess shear-shear correlations about lenses. It is clear that these statistics
assess the three-point correlations between galaxy positions and projected
matter density. For future applications of these novel statistics, we aim at a
more intuitive understanding of G3L to isolate the main features that possibly
can be measured. We construct a toy model ("isolated lens model"; ILM) for the
distribution of galaxies and associated matter to determine the measured
quantities of the two G3L correlation functions and traditional galaxy-galaxy
lensing (GGL) in a simplified context. The ILM presumes single lens galaxies to
be embedded inside arbitrary matter haloes that, however, are statistically
independent ("isolated") from any other halo or lens position. In the ILM, the
average mass-to-galaxy number ratio of clusters of any size cannot change. GGL
and galaxy clustering alone cannot distinguish an ILM from any more complex
scenario. The lens-lens-shear correlator in combination with second-order
statistics enables us to detect deviations from a ILM, though. This can be
quantified by a difference signal defined in the paper. We demonstrate with the
ILM that this correlator picks up the excess matter distribution about galaxy
pairs inside clusters. The lens-shear-shear correlator is sensitive to
variations among matter haloes. In principle, it could be devised to constrain
the ellipticities of haloes, without the need for luminous tracers, or maybe
even random halo substructure. [Abridged]Comment: 14 pages, 3 figures, 1 table, accepted by A&A; some
"lens-shear-shear" were falsely "lens-lens-shear
Galaxy-Galaxy Lensing by Non-Spherical Haloes I:Theoretical Considerations
We use Monte Carlo simulations to investigate the theory of galaxy-galaxy
lensing by non-spherical dark matter haloes. The simulations include a careful
accounting of the effects of multiple deflections. In a typical data set where
the mean tangential shear of sources with redshifts zs ~ 0.6 is measured with
respect to the observed symmetry axes of foreground galaxies with redshifts zl
~ 0.3, the signature of anisotropic galaxy-galaxy lensing differs substantially
from the expectation that one would have in the absence of multiple
deflections. The observed ratio of the mean tangential shears, g+/g-, is
strongly suppressed compared to the function that one would measure if the
intrinsic symmetry axes of the foreground galaxies were known. Depending upon
the characteristic masses of the lenses, the observed ratio of the mean
tangential shears may be consistent with an isotropic signal (despite the fact
that the lenses are non-spherical), or it may even be reversed from the
expected signal (i.e., the mean tangential shear for sources close to the
observed minor axes of the lenses may exceed the mean tangential shear for
sources close to the observed major axes of the lenses). These effects are
caused primarily by the fact that the lens galaxies have, themselves, been
lensed and therefore the observed symmetry axes of the lenses differ from their
intrinsic symmetry axes. The effects of lensing of the foreground galaxies on
the observed function g+/g- cannot be eliminated by the rejection of foreground
galaxies with small image ellipticities, nor by focusing the analysis on
sources that are located very close to the observed symmetry axes of the
foreground galaxies. We conclude that any attempt to use a measurement of g+/g-
to constrain the shapes of dark matter galaxy haloes must include Monte Carlo
simulations that take multiple deflections properly into account.Comment: 15 pages, 17 figures, submitted to MNRAS, full manuscript with
high-resolution version of Fig. 4 can be found at
http://firedrake.bu.edu/preprints/preprints.htm
The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey
We present a determination of the optical/UV AGN luminosity function and its
evolution, based on a large sample of faint (R<24) QSOs identified in the
COMBO-17 survey. Using multi-band photometry in 17 filters within 350nm <
lambda_obs < 930nm, we could simultaneously determine photometric redshifts
with an accuracy of sigma_z<0.03 and obtain spectral energy distributions. The
redshift range covered by the sample is 1.2<z<4.8, which implies that even at
z~3, the sample reaches below luminosities corresponding to M_B = -23,
conventionally employed to distinguish between Seyfert galaxies and quasars. We
clearly detect a broad plateau-like maximum of quasar activity around z~2 and
map out the smooth turnover between z~1 and z~4. The shape of the LF is
characterised by some mild curvature, but no sharp `break' is present within
the range of luminosities covered. Using only the COMBO-17 data, the evolving
LF can be adequately described by either a pure density evolution (PDE) or a
pure luminosity evolution (PLE) model. However, the absence of a strong L*-like
feature in the shape of the LF inhibits a robust distinction between these
modes. We present a robust estimate for the integrated UV luminosity generation
by AGN as a function of redshift. We find that the LF continues to rise even at
the lowest luminosities probed by our survey, but that the slope is
sufficiently shallow that the contribution of low-luminosity AGN to the UV
luminosity density is negligible. Although our sample reaches much fainter flux
levels than previous data sets, our results on space densities and LF slopes
are completely consistent with extrapolations from recent major surveys such as
SDSS and 2QZ.Comment: 17 pages, 14 figures, Astronomy & Astrophysics, in print, revised
versio
Deep BVR photometry of the Chandra Deep Field South from the COMBO-17 survey
We report on deep multi-color imaging (R_5-sigma = 26) of the Chandra Deep
Field South, obtained with the Wide Field Imager (WFI) at the MPG/ESO 2.2 m
telescope on La Silla as part of the COMBO-17 survey. As a result we present a
catalog of 63501 objects in a field measuring 31.5' x 30' with astrometry and
BVR photometry. A sample of 37 variable objects is selected from two-epoch
photometry. We try to give interpretations based on color and variation
amplitude.Comment: revised version, major changes(!), reorganized material, submitted to
A&A, 7 page
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