315,827 research outputs found
The VIMOS VLT Deep Survey: The build-up of the colour-density relation
We investigate the redshift and luminosity evolution of the galaxy
colour-density relation using the data from the First Epoch VIMOS-VLT Deep
Survey (VVDS). The size (6582 galaxies), depth (I_AB<=24) and redshift sampling
rate of the survey enable us to reconstruct the 3D galaxy environment on
relatively local scales (R=5 Mpc) up to z~1.5. Particular attention has been
devoted to calibrate a density reconstruction scheme, which factors out survey
selection effects and reproduces in an unbiased way the underlying `real'
galaxy environment. While at lower redshift we confirm the existence of a steep
colour-density relation, with the fraction of the reddest(/bluest) galaxies of
the same luminosity increasing(/decreasing) as a function of density, this
trend progressively disappears in the highest redshift bins investigated. The
rest frame u*-g' colour-magnitude diagram shows a bimodal pattern in both low
and high density environments up to z~1.5. We find that the bimodal
distribution is not universal but strongly depends upon environment: at lower
redshifts the colour-magnitude diagrams in low and high density regions are
significantly different while the progressive weakening of the colour-density
relation causes the two bimodal distributions to nearly mirror each other in
the highest redshift bin investigated. Both the colour-density and the
colour-magnitude-density relations appear to be a transient, cumulative product
of genetic and environmental factors operating over at least a period of 9 Gyr.
These findings support an evolutionary scenario in which star formation/gas
depletion processes are accelerated in more luminous objects and in high
density environments: star formation activity is shifting with cosmic time
towards lower luminosity (downsizing), and out of high density environments.Comment: 17 pages, 10 figures, figures added, accepted by A&
Measuring Galaxy Environments with Deep Redshift Surveys
We study the applicability of several galaxy environment measures
(n^th-nearest-neighbor distance, counts in an aperture, and Voronoi volume)
within deep redshift surveys. Mock galaxy catalogs are employed to mimic
representative photometric and spectroscopic surveys at high redshift (z ~ 1).
We investigate the effects of survey edges, redshift precision, redshift-space
distortions, and target selection upon each environment measure. We find that
even optimistic photometric redshift errors (\sigma_z = 0.02) smear out the
line-of-sight galaxy distribution irretrievably on small scales; this
significantly limits the application of photometric redshift surveys to
environment studies. Edges and holes in a survey field dramatically affect the
estimation of environment, with the impact of edge effects depending upon the
adopted environment measure. These edge effects considerably limit the
usefulness of smaller survey fields (e.g. the GOODS fields) for studies of
galaxy environment. In even the poorest groups and clusters, redshift-space
distortions limit the effectiveness of each environment statistic; measuring
density in projection (e.g. using counts in a cylindrical aperture or a
projected n^th-nearest-neighbor distance measure) significantly improves the
accuracy of measures in such over-dense environments. For the DEEP2 Galaxy
Redshift Survey, we conclude that among the environment estimators tested the
projected n^th-nearest-neighbor distance measure provides the most accurate
estimate of local galaxy density over a continuous and broad range of scales.Comment: 17 pages including 16 figures, accepted to Ap
Compact groups from the Millennium Simulations: I. Their Nature and the completeness of the Hickson sample
We identify compact groups of galaxies (CGs) within mock galaxy catalogues
from the Millennium Simulation at z=0 with three semi-analytic models (SAMs) of
galaxy formation. CGs are identified using the same 2D criteria as those
visually applied by Hickson (1982) to his CGs (HCGs), but with a brightest
galaxy magnitude limit, and the blending of close projected pairs. Half of the
mock CGs identified in projection contain at least 4 accordant velocities
(mvCGs), versus 70% for HCGs. In comparison to mvCGs, the HCGs are only 8%
complete at distances < 9000 km/s, missing the CGs with small angular sizes, a
strongly dominant galaxy, and (for one SAM) the CGs that are fainter and those
with lower surface brightness. We explore different ways to determine the
fraction of physically dense groups. Binding energy criteria turn out to be
inapplicable given the segregation between galaxies and dark matter particles.
We rely instead on the combination of the 3D length of the CGs (maximum real
space galaxy separation) and their elongation along the line-of-sight (ratio of
maximum line-of-sight to maximum projected separations), restricting ourselves
in both cases to smallest quartets within the CGs. We find that between 64% and
80% (depending on the SAM) of the mvCGs have 3D lengths shorter than 200 kpc/h,
between 71% and 80% have line-of-sight elongations less than 2, while between
59% and 76% have either 3D lengths shorter than 100 kpc/h or both lengths
shorter than 200 kpc/h and elongations smaller than 2. Therefore, chance
alignments (CAs) of galaxies concern at most 40% of the mvCGs. These CAs are
mostly produced from larger host groups, but a few have galaxies extending a
few Mpc beyond the host group. The mvCGs built with the Hickson selection with
(without) the close projected pair blending criterion have 10% higher (lower)
fractions of physically dense systems.Comment: 19 pages, 10 figures. Accepted for publication in MNRAS. The new
version has a different title and a different author list. The analysis of
the nature of compact groups has been thoroughly modified, while the
subsections on compact group properties and correlations have been omitted
and should appear elsewher
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