682 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
Near Ultraviolet sources in the Great Observatories Origins Deep Survey Fields
We present an Ultraviolet (UV) selected sample of 268 objects in the two
fields of the Great Observatories Origins Deep Survey (GOODS). We used the
parallel observations taken with WFPC2 in the U--band (F300W) which covered 88%
of the GOODS fields to identify sources and selected only objects with
GOODS/ACS counterparts. Spectroscopic redshifts for 95 of these sources are
available and we have used the multiwavelength GOODS data to estimate
photometric redshifts for the others. Most of the objects are between
0.2<z<0.8. We used the spectral types obtained by the photometric redshift
fitting to identify the starburst galaxies. We have also visually checked all
objects and looked for tidal effects and nearby companions. We find that (i)
45% of the UV-selected galaxies are starbursts, (ii) nearly 75% of the
starbursts have tidal tails or show some peculiarity typical of interactions or
mergers, (iii) ~50% have companions within an area of 5x5 arcsec. The
UV-selected sample has an average rest-frame M_B=--19.9 +- 0.1. The bluest
objects in the sample (U-B < 0.2 and B-V < 0.1) are at 1.1<z<1.9 and have
peculiar morphologies that resembles either tadpoles, chains, or double-clump
galaxies. Starbursts with tadpole or clump morphology at z=0.8-1.3 have sizes
comparable to LBGs and compact Ultraviolet-luminous galaxies (UVLGs).Comment: 23 pages, 13 figures, accepted by the Astronomical Journa
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
Recovering the properties of high redshift galaxies with different JWST broad-band filters
Imaging with the James Webb Space Telescope (JWST) will allow for observing
the bulk of distant galaxies at the epoch of reionisation. The recovery of
their properties, such as age, color excess E(B-V), specific star formation
rate (sSFR) and stellar mass, will mostly rely on spectral energy distribution
fitting, based on the data provided by JWST's two imager cameras, namely the
Near Infrared Camera (NIRCam) and the Mid Infrared Imager (MIRI). In this work
we analyze the effect of choosing different combinations of NIRCam and MIRI
broad-band filters, from 0.6 {\mu}m to 7.7 {\mu}m, on the recovery of these
galaxy properties. We performed our tests on a sample of 1542 simulated
galaxies, with known input properties, at z=7-10. We found that, with only 8
NIRCam broad-bands, we can recover the galaxy age within 0.1 Gyr and the color
excess within 0.06 mag for 70% of the galaxies. Besides, the stellar masses and
sSFR are recovered within 0.2 and 0.3 dex, respectively, at z=7-9. Instead, at
z=10, no NIRCam band traces purely the {\lambda}> 4000 {\AA} regime and the
percentage of outliers in stellar mass (sSFR) increases by > 20% (> 90%), in
comparison to z=9. The MIRI F560W and F770W bands are crucial to improve the
stellar mass and the sSFR estimation at z=10. When nebular emission lines are
present, deriving correct galaxy properties is challenging, at any redshift and
with any band combination. In particular, the stellar mass is systematically
overestimated in up to 0.3 dex on average with NIRCam data alone and including
MIRI observations improves only marginally the estimation.Comment: 21 pages, 11 figures, 4 tables. Accepted for publication at the ApJ
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