224 research outputs found
Comoving Space Density and Obscured Fraction of High-Redshift Active Galactic Nuclei in the Subaru/{\it XMM-Newton} Deep Survey
We study the comoving space density of X-ray-selected luminous active
galactic nuclei (AGNs) and the obscured AGN fraction at high redshifts () in the Subaru/{\it XMM-Newton} Deep Survey (SXDS) field. From an X-ray
source catalog with high completeness of optical identification thanks to deep
optical images, we select a sample of 30 AGNs at with intrinsic
(de-absorbed and rest-frame 2--10 keV) luminosities of
erg s detected in the 0.5--2 keV band, consisting of 20 and 10 objects
with spectroscopic and photometric redshifts, respectively. Utilizing the
method, we confirm that the comoving space density of luminous
AGNs decreases with redshift above . When combined with the {\it
Chandra}-COSMOS result of Civano et al.\ (2011), the density decline of AGNs
with erg s is well represented by a power law
of . We also determine the fraction of X-ray obscured
AGNs with cm in the Compton-thin population to be
0.54, by carefully taking into account observational biases
including the effects of photon statistics for each source. This result is
consistent with an independent determination of the type-2 AGN fraction based
on optical properties, for which the fraction is found to be 0.590.09.
Comparing our result with that obtained in the local Universe, we conclude that
the obscured fraction of luminous AGNs increases significantly from to
by a factor of 2.51.1.Comment: 12 pages, 12 figures, 1 table. Accepted for publication in Ap
M/L and Color Evolution for A Deep Sample of M* Cluster Galaxies at z~1: The Formation Epoch and the Tilt of the Fundamental Plane
We have measured velocity dispersions for a sample of 36 galaxies with J <
21.2 or Mr < -20.6 mag in MS1054-03, a massive cluster of galaxies at z = 0.83.
Our data are of uniformly high quality down to our selection limit, our 16-hour
exposures typically yielding errors of only \delta(dispersion)~10% for L* and
fainter galaxies. By combining our measurements with data from the literature,
we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived
sizes, colors and surface brightnesses. This sample is complete for the typical
L* galaxy at z~1, unlike most previous z~1 cluster samples which are complete
only for the massive cluster members (>1e11 M_sun). We find no evidence for a
change in the tilt of the fundamental plane (FP). Nor do we find evidence for
evolution in the slope of the color-dispersion relation and M/L_B-dispersion
relations; measuring evolution at a fixed dispersion should minimize the impact
of size evolution found in other work. The M/L_B at fixed dispersion evolves by
\Delta log10 M/L_B=-0.50 +/- 0.03 between z=0.83 and z=0.02 or d(log10
M/L_B)=-0.60 +/- 0.04 dz, and we find \Delta (U-V)_z=-0.24 +/- 0.02 mag at
fixed dispersion in the rest-frame, matching the expected evolution in M/L_B
within 2.25 standard deviations. The implied formation redshift from both the
color and M/L_B evolution is z*=2.0 +/- 0.2 +/- 0.3 (sys), during the epoch in
which the cosmic star-formation activity peaked, with the systematic
uncertainty showing the dependence of z* on the assumptions we make about the
stellar populations. The lack of evolution in either the tilt of the FP or in
the M/L- and color-dispersion relations imply that the formation epoch depends
weakly on mass, ranging from z*=2.3 +1.3 -0.3 at 300 km/s to z*=1.7 +0.3 -0.2
at 160 km/s and implies that the IMF similarly varies slowly with galaxy mass.Comment: revised; typos corrected, references updated, and other cosmetic
changes to meet ApJ format ApJ accepted, 22 pages in emulate ApJ format, 8
color figures, 1 b/w figur
The Growth of Massive Galaxies Since z=2
We study the growth of massive galaxies from z=2 to the present using data
from the NEWFIRM Medium Band Survey. The sample is selected at a constant
number density of n=2x10^-4 Mpc^-3, so that galaxies at different epochs can be
compared in a meaningful way. We show that the stellar mass of galaxies at this
number density has increased by a factor of ~2 since z=2, following the
relation log(M)=11.45-0.15z. In order to determine at what physical radii this
mass growth occurred we construct very deep stacked rest-frame R-band images at
redshifts z=0.6, 1.1, 1.6, and 2.0. These image stacks of typically 70-80
galaxies enable us to characterize the stellar distribution to surface
brightness limits of ~28.5 mag/arcsec^2. We find that massive galaxies
gradually built up their outer regions over the past 10 Gyr. The mass within a
radius of r=5 kpc is nearly constant with redshift whereas the mass at 5-75 kpc
has increased by a factor of ~4 since z=2. Parameterizing the surface
brightness profiles we find that the effective radius and Sersic n parameter
evolve as r_e~(1+z)^-1.3 and n~(1+z)^-1.0 respectively. The data demonstrate
that massive galaxies have grown mostly inside-out, assembling their extended
stellar halos around compact, dense cores with possibly exponential radial
density distributions. Comparing the observed mass evolution to the average
star formation rates of the galaxies we find that the growth is likely
dominated by mergers, as in-situ star formation can only account for ~20% of
the mass build-up from z=2 to z=0. The main uncertainties in this study are
possible redshift-dependent systematic errors in the total stellar masses and
the conversion from light-weighted to mass-weighted radial profiles.Comment: Accepted for publication in the Astrophysical Journal. 26 pages, 13
figures in main tex
BLUE, BLUP and the Kalman filter: some new results
In this contribution, we extend âKalman-filterâ theory by introducing a new BLUEâBLUP recursion of the partitioned measurement and dynamic models. Instead of working with known state-vector means, we relax the model and assume these means to be unknown. The recursive BLUP is derived from first principles, in which a prominent role is played by the modelâs misclosures. As a consequence of the mean state-vector relaxing assumption, the recursion does away with the usual need of having to specify the initial state-vector variance matrix. Next to the recursive BLUP, we introduce, for the same model, the recursive BLUE. This extension is another consequence of assuming the state-vector means unknown. In the standard Kalman filter set-up with known state-vector means, such difference between estimation and prediction does not occur. It is shown how the two intertwined recursions can be combined into one general BLUEâBLUP recursion, the outputs of which produce for every epoch, in parallel, the BLUP for the random state-vector and the BLUE for the mean of the state-vector
Color and stellar population gradients in galaxies. Correlation with mass
We analyze the color gradients (CGs) of ~50000 nearby SDSS galaxies. From
synthetic spectral models based on a simplified star formation recipe, we
derive the mean spectral properties, and explain the observed radial trends of
the color as gradients of the stellar population age and metallicity (Z). The
most massive ETGs (M_* > 10^{11} Msun) have shallow CGs in correspondence of
shallow (negative) Z gradients. In the stellar mass range 10^(10.3-10.5) < M_*
< 10^(11) Msun, the Z gradients reach their minimum of ~ -0.5 dex^{-1}. At M_*
~ 10^{10.3-10.5} Msun, color and Z gradient slopes suddenly change. They turn
out to anti-correlate with the mass, becoming highly positive at the very low
masses. We have also found that age gradients anti-correlate with Z gradients,
as predicted by hierarchical cosmological simulations for ETGs. On the other
side, LTGs have gradients which systematically decrease with mass (and are
always more negative than in ETGs), consistently with the expectation from gas
infall and SN feedback scenarios. Z is found to be the main driver of the trend
of color gradients, especially for LTGs, but age gradients are not negligible
and seem to play a significant role too. We have been able to highlight that
older galaxies have systematically shallower age and Z gradients than younger
ones. Our results for high-mass galaxies are in perfect agreement with
predictions based on the merging scenario, while the evolution of LTGs and
younger and less massive ETGs seems to be mainly driven by infall and SN
feedback. (Abridged)Comment: 20 pages, 16 figures, accepted for publication on MNRAS. This version
includes revisions after the referee's report
The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that
has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of
1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in
the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this
paper we report the i, K and 3.6micron identifications of the 1761 X-ray point
sources. We use the likelihood ratio technique to derive the association of
optical/infrared counterparts for 97% of the X-ray sources. For most of the
remaining 3%, the presence of multiple counterparts or the faintness of the
possible counterpart prevented a unique association. For only 10 X-ray sources
we were not able to associate a counterpart, mostly due to the presence of a
very bright field source close by. Only 2 sources are truly empty fields.
Making use of the large number of X-ray sources, we update the "classic locus"
of AGN and define a new locus containing 90% of the AGN in the survey with full
band luminosity >10^42 erg/s. We present the linear fit between the total i
band magnitude and the X-ray flux in the soft and hard band, drawn over 2
orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and
XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we
test its known correlation with redshift and luminosity, and a recently
introduced anti-correlation with the concentration index (C). We find a strong
anti-correlation (though the dispersion is of the order of 0.5 dex) between C
and X/O, computed in the hard band, and that 90% of the obscured AGN in the
sample with morphological information live in galaxies with regular morphology
(bulgy and disky/spiral), suggesting that secular processes govern a
significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5
erg/s.Comment: 21 pages, 17 figures, 4 tables; accepted for publication in ApJS. The
catalog is available at the urls listed in the pape
Connecting stellar mass and star-formation rate to dark matter halo mass out to z ~ 2
We have constructed an extended halo model (EHM) which relates the total
stellar mass and star-formation rate (SFR) to halo mass (M_h). An empirical
relation between the distribution functions of total stellar mass of galaxies
and host halo mass, tuned to match the spatial density of galaxies over 0<z<2
and the clustering properties at z~0, is extended to include two different
scenarios describing the variation of SFR on M_h. We also present new
measurements of the redshift evolution of the average SFR for star-forming
galaxies of different stellar mass up to z=2, using data from the Herschel
Multi-tiered Extragalactic Survey (HerMES) for infrared-bright galaxies.
Combining the EHM with the halo accretion histories from numerical
simulations, we trace the stellar mass growth and star-formation history in
halos spanning a range of masses. We find that: (1) The intensity of the
star-forming activity in halos in the probed mass range has steadily decreased
from z~2 to 0; (2) At a given epoch, halos in the mass range between a few
times 10^{11} M_Sun and a few times 10^{12} M_Sun are the most efficient at
hosting star formation; (3) The peak of SFR density shifts to lower mass halos
over time; (4) Galaxies that are forming stars most actively at z~2 evolve into
quiescent galaxies in today's group environments, strongly supporting previous
claims that the most powerful starbursts at z~2 are progenitors of today's
elliptical galaxies.Comment: 15 pages, 12 figures, accepted for publication in MNRA
Newborn spheroids at high redshift: when and how did the dominant, old stars in today's massive galaxies form?
We study ~330 massive (M* > 10^9.5 MSun), newborn spheroidal galaxies (SGs)
around the epoch of peak star formation (1<z<3), to explore the high-redshift
origin of SGs and gain insight into when and how the old stellar populations
that dominate today's Universe formed. The sample is drawn from the HST/WFC3
Early-Release Science programme, which provides deep 10-filter (0.2 - 1.7
micron) HST imaging over a third of the GOODS-South field. We find that the
star formation episodes that built the SGs likely peaked in the redshift range
2<z<5 (with a median of z~3) and have decay timescales shorter than ~1.5 Gyr.
Starburst timescales and ages show no trend with stellar mass in the range
10^9.5 < M* < 10^10.5 MSun. However, the timescales show increased scatter
towards lower values ( 10^10.5 MSun, and an age trend becomes
evident in this mass regime: SGs with M* > 10^11.5 MSun are ~2 Gyrs older than
their counterparts with M* < 10^10.5 MSun. Nevertheless, a smooth downsizing
trend with galaxy mass is not observed, and the large scatter in starburst ages
indicate that SGs are not a particularly coeval population. Around half of the
blue SGs appear not to drive their star formation via major mergers, and those
that have experienced a recent major merger, show only modest enhancements
(~40%) in their specific star formation rates. Our empirical study indicates
that processes other than major mergers (e.g. violent disk instability driven
by cold streams and/or minor mergers) likely play a dominant role in building
SGs, and creating a significant fraction of the old stellar populations that
dominate today's Universe.Comment: MNRAS in pres
The bimodality of the 10k zCOSMOS-bright galaxies up to z ~ 1: a new statistical and portable classification based on the optical galaxy properties
Our goal is to develop a new and reliable statistical method to classify
galaxies from large surveys. We probe the reliability of the method by
comparing it with a three-dimensional classification cube, using the same set
of spectral, photometric and morphological parameters.We applied two different
methods of classification to a sample of galaxies extracted from the zCOSMOS
redshift survey, in the redshift range 0.5 < z < 1.3. The first method is the
combination of three independent classification schemes, while the second
method exploits an entirely new approach based on statistical analyses like
Principal Component Analysis (PCA) and Unsupervised Fuzzy Partition (UFP)
clustering method. The PCA+UFP method has been applied also to a lower redshift
sample (z < 0.5), exploiting the same set of data but the spectral ones,
replaced by the equivalent width of H. The comparison between the two
methods shows fairly good agreement on the definition on the two main clusters,
the early-type and the late-type galaxies ones. Our PCA-UFP method of
classification is robust, flexible and capable of identifying the two main
populations of galaxies as well as the intermediate population. The
intermediate galaxy population shows many of the properties of the green valley
galaxies, and constitutes a more coherent and homogeneous population. The
fairly large redshift range of the studied sample allows us to behold the
downsizing effect: galaxies with masses of the order of Msun
mainly are found in transition from the late type to the early type group at
, while galaxies with lower masses - of the order of Msun -
are in transition at later epochs; galaxies with Msun did not
begin their transition yet, while galaxies with very large masses ( Msun) mostly completed their transition before .Comment: 16 pages, 14 figures, accepted for publication in A&
Improved constraints on the expansion rate of the Universe up to z~1.1 from the spectroscopic evolution of cosmic chronometers
We present new improved constraints on the Hubble parameter H(z) in the
redshift range 0.15 < z < 1.1, obtained from the differential spectroscopic
evolution of early-type galaxies as a function of redshift. We extract a large
sample of early-type galaxies (\sim11000) from several spectroscopic surveys,
spanning almost 8 billion years of cosmic lookback time (0.15 < z < 1.42). We
select the most massive, red elliptical galaxies, passively evolving and
without signature of ongoing star formation. Those galaxies can be used as
standard cosmic chronometers, as firstly proposed by Jimenez & Loeb (2002),
whose differential age evolution as a function of cosmic time directly probes
H(z). We analyze the 4000 {\AA} break (D4000) as a function of redshift, use
stellar population synthesis models to theoretically calibrate the dependence
of the differential age evolution on the differential D4000, and estimate the
Hubble parameter taking into account both statistical and systematical errors.
We provide 8 new measurements of H(z) (see Tab. 4), and determine its change in
H(z) to a precision of 5-12% mapping homogeneously the redshift range up to z
\sim 1.1; for the first time, we place a constraint on H(z) at z \neq 0 with a
precision comparable with the one achieved for the Hubble constant (about 5-6%
at z \sim 0.2), and covered a redshift range (0.5 < z < 0.8) which is crucial
to distinguish many different quintessence cosmologies. These measurements have
been tested to best match a \Lambda CDM model, clearly providing a
statistically robust indication that the Universe is undergoing an accelerated
expansion. This method shows the potentiality to open a new avenue in constrain
a variety of alternative cosmologies, especially when future surveys (e.g.
Euclid) will open the possibility to extend it up to z \sim 2.Comment: 34 pages, 15 figures, 6 tables, published in JCAP. It is a companion
to Moresco et al. (2012b, http://arxiv.org/abs/1201.6658) and Jimenez et al.
(2012, http://arxiv.org/abs/1201.3608). The H(z) data can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
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