642 research outputs found
Analysis of the SFR - M* plane at z<3: single fitting versus multi-Gaussian decomposition
The analysis of galaxies on the star formation rate - stellar mass (SFR-M*)
plane is a powerful diagnostic for galaxy evolution at different cosmic times.
We consider a sample of 24463 galaxies from the CANDELS/GOODS-S survey to
conduct a detailed analysis of the SFR-M* relation at redshifts 0.53 over more than three dex in stellar mass. To obtain SFR estimates, we
utilise mid- and far-IR photometry when available, and rest-UV fluxes for all
the other galaxies. We perform our analysis in different redshift bins, with
two different methods: 1) a linear regression fitting of all star-forming
galaxies, defined as those with specific star formation rates , similarly to what is typically done in the
literature; 2) a multi-Gaussian decomposition to identify the galaxy main
sequence (MS), the starburst sequence and the quenched galaxy cloud. We find
that the MS slope becomes flatter when higher stellar mass cuts are adopted,
and that the apparent slope change observed at high masses depends on the SFR
estimation method. In addition, the multi-Gaussian decomposition reveals the
presence of a starburst population which increases towards low stellar masses
and high redshifts. We find that starbursts make up ~5% of all galaxies at
z=0.5-1.0, while they account for ~16% of galaxies at 23 with
log8.25-11.25. We conclude that the dissection of the SFR-M* in
multiple components over a wide range of stellar masses is necessary to
understand the importance of the different modes of star formation through
cosmic time.Comment: 15 pages, 12 figures, 1 table. Accepted for publication in A&A, after
addressing referee report. Main changes with respect to v1: two new
appendixes to investigate the impact of redshift outliers and to test a
two-Gaussian component fit to the sSFR distribution. No conclusion change
Prevalence of X-ray variability in the Chandra Deep Field South
We studied the X-ray variability of sources detected in the Chandra Deep
Field South (Giacconi et al. 2002), nearly all of which are low to moderate z
AGN (Tozzi et al. 2001). We find that 45% of the sources with >100 counts
exhibit significant variability on timescales ranging from a day up to a year.
The fraction of sources found to be variable increases with observed flux,
suggesting that >90% of all AGNs possess intrinsic variability. We also find
that the fraction of variable sources appears to decrease with increasing
intrinsic absorption; a lack of variability in hard, absorbed AGNs could be due
to an increased contribution of reflected X-rays to the total flux. We do not
detect significant spectral variability in the majority (~70%) of our sources.
In half of the remaining 30%, the hardness ratio is anti-correlated with flux,
mimicking the high/soft-low/hard states of galactic sources. The X-ray
variability appears anti-correlated with the luminosity of the sources, in
agreement with previous studies. High redshift sources, however, have larger
variability amplitudes than expected from extrapolations of their low-z
counterparts, suggesting a possible evolution in the accretion rate and/or size
of the X-ray emitting region. Finally, we discuss some effects that may produce
the observed decrease in the fraction of variable sources from z=0.5 out to
z=2.Comment: 24 pages, including 15 figures and 1 table. In press on Ap
Emission-Line Galaxies from the HST PEARS Grism Survey I: The South Fields
We present results of a search for emission-line galaxies in the Southern
Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization
Spectroscopically) grism survey. The PEARS South Fields consist of five ACS
pointings (including the Hubble Ultra Deep Field) with the G800L grism for a
total of 120 orbits, revealing thousands of faint object spectra in the
GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of
objects that are prevalent among the grism spectra. Using a 2-dimensional
detection and extraction procedure, we find 320 emission lines orginating from
226 galaxy "knots'' within 192 individual galaxies. Line identification results
in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field.
We measure emission line fluxes using standard Gaussian fitting techniques. At
the resolution of the grism data, the H-beta and [OIII] doublet are blended.
However, by fitting two Gaussian components to the H-beta and [OIII] features,
we find that many of the PEARS ELGs have high [OIII]/H-beta ratios compared to
other galaxy samples of comparable luminosities. The star-formation rates
(SFRs) of the ELGs are presented, as well as a sample of distinct giant
star-forming regions at z~0.1-0.5 across individual galaxies. We find that the
radial distances of these HII regions in general reside near the galaxies'
optical continuum half-light radii, similar to those of giant HII regions in
local galaxies.Comment: 15 pages, 13 figures; Accepted for publication in A
Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population
We present quantitative morphological analyses of 37 HST/WFPC2 counterparts
of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate:
1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF-
convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration
indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near-
neighbor analyses comparing local environments of X-ray sources versus the
field control sample. Significant nuclear point-source optical components
appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow
range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray
sources (putative type-1 AGN) in our fields. We infer roughly half of the
HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in
the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources.
The concentration indices of the CDFS counterparts are clearly larger on
average than those of the field distribution, at 5-sigma, suggesting that the
strong correlation between central black hole mass and host galaxy properties
(including concentration index) observed in nearby galaxies is already evident
by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved
CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the
frequency of I<23 near neighbors around the CDFS counterparts is not
significantly different from the field sample. These results, combined with
previous similar findings for local samples, suggest that recent merger/
interaction history is not a good indicator of AGN activity over a substantial
range of look-back time.Comment: 30 pages, incl. 8 figures; accepted for publication in the
Astrophysical Journa
Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe
In order to derive the AGN contribution to the cosmological ionizing
emissivity we have selected faint AGN candidates at in the CANDELS
GOODS-South field which is one of the deepest fields with extensive
multiwavelength coverage from Chandra, HST, Spitzer and various groundbased
telescopes. We have adopted a relatively novel criterion. As a first step high
redshift galaxies are selected in the NIR band down to very faint levels
() using reliable photometric redshifts. This corresponds at to
a selection criterion based on the galaxy rest-frame UV flux. AGN candidates
are then picked up from this parent sample if they show X-ray fluxes above a
threshold of cgs (0.5-2 keV). We have found 22 AGN
candidates at and we have derived the first estimate of the UV luminosity
function in the redshift interval and absolute magnitude interval
typical of local Seyfert galaxies. The
faint end of the derived luminosity function is about two/four magnitudes
fainter at than that derived from previous UV surveys. We have then
estimated ionizing emissivities and hydrogen photoionization rates in the same
redshift interval under reasonable assumptions and after discussion of possible
caveats, the most important being the large uncertainties involved in the
estimate of photometric redshift for sources with featureless, almost power-law
SEDs and/or low average escape fraction of ionizing photons from the AGN host
galaxies. We argue that, under reasonable evaluations of possible biases, the
probed AGN population can produce at photoionization rates consistent
with that required to keep highly ionized the intergalactic medium observed in
the Lyman- forest of high redshift QSO spectra, providing an important
contribution to the cosmic reionization.Comment: 15 pages, 8 figures, A&A accepted, updated figure 6, corrected typo
in table 3, updated reference
Analytic Time Delays and H_0 Estimates for Gravitational Lenses
We study gravitational lens time delays for a general family of lensing
potentials, which includes the popular singular isothermal elliptical potential
and singular isothermal elliptical density distribution but allows general
angular structure. Using a novel approach, we show that the time delay can be
cast in a very simple form, depending only on the observed image positions.
Including an external shear changes the time delay proportional to the shear
strength, and varying the radial profile of the potential changes the time
delay approximately linearly. These analytic results can be used to obtain
simple estimates of the time delay and the Hubble constant in observed
gravitational lenses. The naive estimates for four of five time delay lenses
show surprising agreement with each other and with local measurements of H_0;
the complicated Q 0957+561 system is the only outlier. The agreement suggests
that it is reasonable to use simple isothermal lens models to infer H_0,
although it is still important to check this conclusion by examining detailed
models and by measuring more lensing time delays.Comment: 16 pages with 2 embedded figures; submitted to Ap
The Role of Bulge Formation in the Homogenization of Stellar Populations at as revealed by Internal Color Dispersion in CANDELS
We use data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy
Survey to study how the spatial variation in the stellar populations of
galaxies relate to the formation of galaxies at . We use the
Internal Color Dispersion (ICD), measured between the rest-frame UV and optical
bands, which is sensitive to age (and dust attenuation) variations in stellar
populations. The ICD shows a relation with the stellar masses and morphologies
of the galaxies. Galaxies with the largest variation in their stellar
populations as evidenced by high ICD have disk-dominated morphologies (with
S\'{e}rsic indexes ) and stellar masses between . There is a marked decrease in the ICD as the stellar mass and/or
the S\'ersic index increases. By studying the relations between the ICD and
other galaxy properties including sizes, total colors, star-formation rate, and
dust attenuation, we conclude that the largest variations in stellar
populations occur in galaxies where the light from newly, high star-forming
clumps contrasts older stellar disk populations. This phase reaches a peak for
galaxies only with a specific stellar mass range, , and prior to the formation of a substantial bulge/spheroid. In contrast,
galaxies at higher or lower stellar masses, and/or higher S\'{e}rsic index () show reduced ICD values, implying a greater homogeneity of their stellar
populations. This indicates that if a galaxy is to have both a quiescent bulge
along with a star forming disk, typical of Hubble Sequence galaxies, this is
most common for stellar masses and when the
bulge component remains relatively small ().Comment: 15 pages, 14 figure
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