861 research outputs found
Discovery of an anomalous Sub Giant Branch in the Color Magnitude Diagram of omega Centauri
Using deep high-resolution multi-band images taken with the Very Large
Telescope and the Hubble Space Telescope, we discovered a new anomalous
sequence in the Color Magnitude Diagram of omega Cen. This feature appears as a
narrow, well-defined Sub Giant Branch (SGB-a), which merges into the Main
Sequence of the dominant cluster population at a magnitude significantly
fainter than the cluster Turn-Off (TO). The simplest hypothesis assumes that
the new feature is the extension of the anomalous Red Giant Branch (RGB-a)
metal-rich population discovered by Lee et al. (1999) and Pancino et al.
(2000). However, under this assumption the interpretation of the SGB-a does not
easily fit into the context of a self-enrichment scenario within omega Cen. In
fact, its TO magnitude, shape and extension are not compatible with a young,
metal-rich population, as required by the self-enrichment process. The TO level
of the SGB-a suggests indeed an age as old as the main cluster population,
further supporting the extra-cluster origin of the most metal rich stars, as
suggested by Ferraro, Bellazzini & Pancino (2002). Only accurate measurements
of radial velocities and metal abundances for a representative sample of stars
will firmly establish whether or not the SGB-a is actually related to the RGB-a
and will finally shed light on the origin of the metal rich population of omega
Cen.Comment: ApJL, in pres
PRIMUS: The Effect of Physical Scale on the Luminosity-Dependence of Galaxy Clustering via Cross-Correlations
We report small-scale clustering measurements from the PRIMUS spectroscopic
redshift survey as a function of color and luminosity. We measure the
real-space cross-correlations between 62,106 primary galaxies with PRIMUS
redshifts and a tracer population of 545,000 photometric galaxies over
redshifts from z=0.2 to z=1. We separately fit a power-law model in redshift
and luminosity to each of three independent color-selected samples of galaxies.
We report clustering amplitudes at fiducial values of z=0.5 and L=1.5 L*. The
clustering of the red galaxies is ~3 times as strong as that of the blue
galaxies and ~1.5 as strong as that of the green galaxies. We also find that
the luminosity dependence of the clustering is strongly dependent on physical
scale, with greater luminosity dependence being found between r=0.0625 Mpc/h
and r=0.25 Mpc/h, compared to the r=0.5 Mpc/h to r=2 Mpc/h range. Moreover,
over a range of two orders of magnitude in luminosity, a single power-law fit
to the luminosity dependence is not sufficient to explain the increase in
clustering at both the bright and faint ends at the smaller scales. We argue
that luminosity-dependent clustering at small scales is a necessary component
of galaxy-halo occupation models for blue, star-forming galaxies as well as for
red, quenched galaxies.Comment: 13 pages, 6 figures, 5 tables; published in ApJ (revised to match
published version
The Mid-IR and X-ray Selected QSO Luminosity Function
We present the J-band luminosity function of 1838 mid-infrared and X-ray
selected AGNs in the redshift range 0<z<5.85. These luminosity functions are
constructed by combining the deep multi-wavelength broad-band observations from
the UV to the mid-IR of the NDWFS Bootes field with the X-ray observations of
the XBootes survey and the spectroscopic observations of the same field by
AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using
modifications of the Stern et al.(2005) criteria, complemented by MIPS 24
microns and X-ray selected AGNs to alleviate the biases of IRAC mid-IR
selection against z~4.5 quasars and AGNs faint with respect to their hosts.
This sample provides an accurate link between low and high redshift AGN
luminosity functions and does not suffer from the usual incompleteness of
optical samples at z~3. We find that the space density of the brightest quasars
strongly decreases from z=3 to z=0, while the space density of faint quasars is
at least flat, and possibly increasing, over the same redshift range. At z>3 we
observe a decrease in the space density of quasars of all brightnesses. We
model the luminosity function by a double power-law and find that its evolution
cannot be described by either pure luminosity or pure density evolution, but
must be a combination of both. Our best-fit model has bright and faint
power-law indices consistent with the low redshift measurements based on the
2QZ and 2SLAQ surveys and it generally agrees with the number of bright quasars
predicted by other LFs at all redshifts. If we construct the QSO luminosity
function using only the IRAC-selected AGNs, we find that the biases inherent to
this selection method significantly modify the behavior of phi*(z) only for z<1
and have no significant impact upon the characteristic magnitude M*_J(z).Comment: Corrected minor typo in equations (4) and (6). Accepted for
publication in The Astrophysical Journal. 56 pages + 6 tables + 16 figure
Dark Matter Halo Models of Stellar Mass-Dependent Galaxy Clustering in PRIMUS+DEEP2 at 0.2<z<1.2
We utilize CDM halo occupation models of galaxy clustering to
investigate the evolving stellar mass dependent clustering of galaxies in the
PRIsm MUlti-object Survey (PRIMUS) and DEEP2 Redshift Survey over the past
eight billion years of cosmic time, between . These clustering
measurements provide new constraints on the connections between dark matter
halo properties and galaxy properties in the context of the evolving
large-scale structure of the universe. Using both an analytic model and a set
of mock galaxy catalogs, we find a strong correlation between central galaxy
stellar mass and dark matter halo mass over the range
-, approximately consistent
with previous observations and theoretical predictions. However, the
stellar-to-halo mass relation (SHMR) and the mass scale where star formation
efficiency reaches a maximum appear to evolve more strongly than predicted by
other models, including models based primarily on abundance-matching
constraints. We find that the fraction of satellite galaxies in haloes of a
given mass decreases significantly from to , partly due to
the fact that haloes at fixed mass are rarer at higher redshift and have lower
abundances. We also find that the ratio, a model parameter
that quantifies the critical mass above which haloes host at least one
satellite, decreases from at to at .
Considering the evolution of the subhalo mass function vis-\`{a}-vis satellite
abundances, this trend has implications for relations between satellite
galaxies and halo substructures and for intracluster mass, which we argue has
grown due to stripped and disrupted satellites between and
.Comment: 17 pages, 9 figures and 4 tables; Astrophysical Journal, publishe
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