198 research outputs found
Candidate Type II Quasars at 2 < z < 4.3 in the Sloan Digital Sky Survey III
At low redshifts, dust-obscured quasars often have strong yet narrow
permitted lines in the rest-frame optical and ultraviolet, excited by the
central active nucleus, earning the designation Type II quasars. We present a
sample of 145 candidate Type II quasars at redshifts between 2 and 4.3,
encompassing the epoch at which quasar activity peaked in the universe. These
objects, selected from the quasar sample of the Baryon Oscillation
Spectroscopic Survey of the Sloan Digital Sky Survey III, are characterized by
weak continuum in the rest-frame ultraviolet (typical continuum magnitude of i
\approx 22) and strong lines of CIV and Ly \alpha, with Full Width at Half
Maximum less than 2000 kms-1. The continuum magnitudes correspond to an
absolute magnitude of -23 or brighter at redshift 3, too bright to be due
exclusively to the host galaxies of these objects. Roughly one third of the
objects are detected in the shorter-wavelength bands of the WISE survey; the
spectral energy distributions (SEDs) of these objects appear to be intermediate
between classic Type I and Type II quasars seen at lower redshift. Five objects
are detected at rest frame 6\mu m by Spitzer, implying bolometric luminosities
of several times 10^46 erg s-1. We have obtained polarization measurements for
two objects; they are roughly 3% polarized. We suggest that these objects are
luminous quasars, with modest dust extinction (A_V ~ 0.5 mag), whose
ultraviolet continuum also includes a substantial scattering contribution.
Alternatively, the line of sight to the central engines of these objects may be
partially obscured by optically thick material.Comment: 26 pages, 13 figures, 10 tables, 4 machine readable tables. Accepted
for publication in MNRA
Stellar Mass--Gas-phase Metallicity Relation at : A Power Law with Increasing Scatter toward the Low-mass Regime
We present the stellar mass ()--gas-phase metallicity relation (MZR)
and its scatter at intermediate redshifts () for 1381 field
galaxies collected from deep spectroscopic surveys. The star formation rate
(SFR) and color at a given of this magnitude-limited ( AB)
sample are representative of normal star-forming galaxies. For masses below
, our sample of 237 galaxies is 10 times larger than those
in previous studies beyond the local universe. This huge gain in sample size
enables superior constraints on the MZR and its scatter in the low-mass regime.
We find a power-law MZR at :
. Our MZR
shows good agreement with others measured at similar redshifts in the
literature in the intermediate and massive regimes, but is shallower than the
extrapolation of the MZRs of others to masses below . The SFR
dependence of the MZR in our sample is weaker than that found for local
galaxies (known as the Fundamental Metallicity Relation). Compared to a variety
of theoretical models, the slope of our MZR for low-mass galaxies agrees well
with predictions incorporating supernova energy-driven winds. Being robust
against currently uncertain metallicity calibrations, the scatter of the MZR
serves as a powerful diagnostic of the stochastic history of gas accretion, gas
recycling, and star formation of low-mass galaxies. Our major result is that
the scatter of our MZR increases as decreases. Our result implies that
either the scatter of the baryonic accretion rate or the scatter of the
-- relation increases as decreases. Moreover, our
measures of scatter at appears consistent with that found for local
galaxies.Comment: 18 pages, 10 figures. Accepted by ApJ. Typos correcte
The Lick AGN Monitoring Project 2011: Reverberation Mapping of Markarian 50
The Lick AGN Monitoring Project 2011 observing campaign was carried out over
the course of 11 weeks in Spring 2011. Here we present the first results from
this program, a measurement of the broad-line reverberation lag in the Seyfert
1 galaxy Mrk 50. Combining our data with supplemental observations obtained
prior to the start of the main observing campaign, our dataset covers a total
duration of 4.5 months. During this time, Mrk 50 was highly variable,
exhibiting a maximum variability amplitude of a factor of 4 in the U-band
continuum and a factor of 2 in the H-beta line. Using standard
cross-correlation techniques, we find that H-beta and H-gamma lag the V-band
continuum by tau_cen = 10.64(-0.93,+0.82) and 8.43(-1.28,+1.30) days,
respectively, while the lag of He II 4686 is unresolved. The H-beta line
exhibits a symmetric velocity-resolved reverberation signature with shorter
lags in the high-velocity wings than in the line core, consistent with an
origin in a broad-line region dominated by orbital motion rather than infall or
outflow. Assuming a virial normalization factor of f=5.25, the virial estimate
of the black hole mass is (3.2+-0.5)*10^7 solar masses. These observations
demonstrate that Mrk 50 is among the most promising nearby active galaxies for
detailed investigations of broad-line region structure and dynamics.Comment: Accepted for publication in ApJ Letters. 6 pages, 4 figure
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at Z approximates 2: A mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism spectroscopy of 28 emission-line galaxies at z approximates 2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The high sensitivity of these grism observations, with > 5-sigma detections of emission lines to f > 2.5 X 10(exp -18( erg/s/ square cm, means that the galaxies in the sample are typically approximately 7 times less massive (median M(star). = 10(exp 9.5)M(solar)) than previously studied z approximates 2 emission-line galaxies. Despite their lower mass, the galaxies have [O-III]/H-Beta ratios which are very similar to previously studied z approximates 2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the [O-III] emission line is more spatially concentrated than the H-Beta emission line with 98.1% confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L(sub [O-III])/L(sub 0.5.10keV) ratio is intermediate between typical z approximates 0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked [O-III] spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei
The Dwarf Galaxy Population at z ⌠0.7: A Catalog of Emission Lines and Redshifts from Deep Keck Observations
We present a catalog of spectroscopically measured redshifts over
and emission line fluxes for 1440 galaxies. The majority (65\%) of the
galaxies come from the HALO7D survey, with the remainder from the DEEPwinds
program. This catalog includes redshifts for 646 dwarf galaxies with
. 810 catalog galaxies did not have previously
published spectroscopic redshifts, including 454 dwarf galaxies. HALO7D used
the DEIMOS spectrograph on the Keck II telescope to take very deep (up to 32
hours exposure, with a median of 7 hours) optical spectroscopy in the
COSMOS, EGS, GOODS-North, and GOODS-South CANDELS fields, and in some areas
outside CANDELS. We compare our redshift results to existing spectroscopic and
photometric redshifts in these fields, finding only a 1\% rate of discrepancy
with other spectroscopic redshifts. We measure a small increase in median
photometric redshift error (from 1.0\% to 1.3\%) and catastrophic outlier rate
(from 3.5\% to 8\%) with decreasing stellar mass. We obtained successful
redshift fits for 75\% of massive galaxies, and demonstrate a similar 70-75\%
successful redshift measurement rate in
galaxies, suggesting similar survey sensitivity in this low-mass range. We
describe the redshift, mass, and color-magnitude distributions of the catalog
galaxies, finding HALO7D galaxies representative of CANDELS galaxies up to
\textit{i}-band magnitudes of 25. The catalogs presented will enable studies of
star formation (SF), the mass-metallicity relation, SF-morphology relations,
and other properties of the dwarf galaxy population.Comment: 23 pages, 19 Figures, updated to version accepted by ApJ
Stellar MassâGas-Phase Metallicity Relation at 0.5 †\u3cem\u3ez\u3c/em\u3e †0.7: A Power Law with Increasing Scatter Toward the Low-Mass Regime
We present the stellar mass (M*)âgas-phase metallicity relation (MZR) and its scatter at intermediate redshifts (0.5 †z †0.7) for for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given M* of this magnitude-limited (R âČ 24 AB) sample are representative of normal star-forming galaxies. For masses below 109 Mâ our sample of 237 galaxies is ~10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at 108 at Mâ \u3c M* \u3c 1011 Mâ: 12 + log(O/H) = (5.83 ± 0.19) + (0.30 ± 0.02) log(M*/Mâ). At 109 Mâ \u3c M* \u3c 1010.5 Mâ, our MZR shows agreement with others measured at similar redshifts in the literature. Our power-law slope is, however, shallower than the extrapolation of the MZRs of others to masses below 109 Mâ. The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the fundamental metallicity relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as M* decreases. Our result implies that either the scatter of the baryonic accretion rate (Ï áč) or the scatter of the M* âMhalo relation (Ï SHMR) increases as M* decreases. Moreover, our measure of scatter at z = 0.7 appears consistent with that found for local galaxies. This lack of redshift evolution constrains models of galaxy evolution to have both Ï áč and Ï SHMR remain unchanged from z = 0.7 to z = 0
Stellar Mass--Gas-phase Metallicity Relation at 0.5 †z †0.7: A Power Law with Increasing Scatter toward the Low-mass Regime
We present the stellar mass (M_*)âgas-phase metallicity relation (MZR) and its scatter at intermediate redshifts (0.5 â©œ z â©œ 0.7) for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given M_* of this magnitude-limited (R ⟠24 AB) sample are representative of normal star-forming galaxies. For masses below 10^9 M_â, our sample of 237 galaxies is ~10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at 10^8 M_â < M_* < 10^(11)M_â: 12 + log(O/H)=(5.83 ± 0.19)+(0.30 ± 0.02)log(M_*/M_â). At 10^9 M_â < M_* < 10^(10.5) M_â, our MZR shows agreement with others measured at similar redshifts in the literature. Our power-law slope is, however, shallower than the extrapolation of the MZRs of others to masses below 10^9 M_â. The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the fundamental metallicity relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as M_* decreases. Our result implies that either the scatter of the baryonic accretion rate (Ï_áč) or the scatter of the M_* M_(halo) relation (Ï_(SHMR)) increases as M_* decreases. Moreover, our measure of scatter at z = 0.7 appears consistent with that found for local galaxies. This lack of redshift evolution constrains models of galaxy evolution to have both Ï_áč and Ï_(SHMR) remain unchanged from z = 0.7 to z = 0
The nature of massive transition galaxies in CANDELS, GAMA and cosmological simulations
We explore observational and theoretical constraints on how galaxies might
transition between the "star-forming main sequence" (SFMS) and varying "degrees
of quiescence" out to . Our analysis is focused on galaxies with stellar
mass , and is enabled by GAMA and CANDELS observations, a
semi-analytic model (SAM) of galaxy formation, and a cosmological
hydrodynamical "zoom in" simulation with momentum-driven AGN feedback. In both
the observations and the SAM, transition galaxies tend to have intermediate
S\'ersic indices, half-light radii, and surface stellar mass densities compared
to star-forming and quiescent galaxies out to . We place an observational
upper limit on the average population transition timescale as a function of
redshift, finding that the average high-redshift galaxy is on a "fast track"
for quenching whereas the average low-redshift galaxy is on a "slow track" for
quenching. We qualitatively identify four physical origin scenarios for
transition galaxies in the SAM: oscillations on the SFMS, slow quenching, fast
quenching, and rejuvenation. Quenching timescales in both the SAM and the
hydrodynamical simulation are not fast enough to reproduce the quiescent
population that we observe at . In the SAM, we do not find a clear-cut
morphological dependence of quenching timescales, but we do predict that the
mean stellar ages, cold gas fractions, SMBH masses, and halo masses of
transition galaxies tend to be intermediate relative to those of star-forming
and quiescent galaxies at .Comment: Re-submitted to MNRAS after referee revisions, the main change is
that the paper has been significantly streamlined and is shorter (25 pages
and 8 figures, without appendices
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