128 research outputs found
A Survey of z ~ 6 Quasars in the Sloan Digital Sky Survey Deep Stripe. II. Discovery of Six Quasars at z AB>21
We present the discovery of six new quasars at z ~ 6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about 2 mag fainter than the luminous z ~ 6 quasars found in the SDSS main survey and 1 mag fainter than the quasars reported in Paper I. Four of them comprise a complete flux-limited sample at 21 < z_(AB) < 21.8 over an effective area of 195 deg^2. The other two quasars are fainter than z_(AB) = 22 and are not part of the complete sample. The quasar luminosity function at z ~ 6 is well described as a single power law Φ(L_(1450))α L^β_(1450) over the luminosity range –28 < M_(1450) < –25. The best-fitting slope β varies from –2.6 to –3.1, depending on the quasar samples used, with a statistical error of 0.3-0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lyα emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth timescales for these faint quasars at early epochs
Luminous Lyman-alpha Emitters with Very Blue UV-continuum Slopes at Redshift 5.7 <= z <= 6.6
We study six luminous Lyman-alpha emitters (LAEs) with very blue rest-frame
UV continua at . These LAEs have previous HST and Spitzer
IRAC observations. Combining our newly acquired HST images, we find that their
UV-continuum slopes are in a range of . Unlike
previous, tentative detections of in photometrically
selected, low-luminosity galaxies, our LAEs are spectroscopically confirmed and
luminous ( mag). We model their broadband spectral energy
distributions (SEDs), and find that two galaxies can be
well fitted with young and dust-free stellar populations. However, it becomes
increasingly difficult to fit bluer galaxies. We explore further
interpretations by including non-zero LyC escape fraction , very
low metallicities, and/or AGN contributions. Assuming ,
we achieve the bluest slopes when nebular emission is
considered. This can nearly explain the SEDs of two galaxies with
and --2.9 (). Larger
values and very low metallicities are not favored by the strong nebular line
emission (evidenced by the IRAC flux) or the observed (IRAC 1 - IRAC 2) color.
Finally, we find that the galaxy can potentially be well
explained by the combination of a very young population with a high () and an old, dusty population. We are not able to produce two
galaxies. Future deep spectroscopic observations are
needed to fully understand these galaxies.Comment: Published in ApJ on 2020 Feb 1; Authors' version (9 pages); See
published version at
https://iopscience.iop.org/article/10.3847/1538-4357/ab64e
The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
We present the discovery of nine quasars at identified in the Sloan
Digital Sky Survey (SDSS) imaging data. This completes our survey of
quasars in the SDSS footprint. Our final sample consists of 52 quasars at
, including 29 quasars with mag selected from
11,240 deg of the SDSS single-epoch imaging survey (the main survey), 10
quasars with selected from 4223 deg of the SDSS
overlap regions (regions with two or more imaging scans), and 13 quasars down
to mag from the 277 deg in Stripe 82. They span a
wide luminosity range of . This well-defined sample
is used to derive the quasar luminosity function (QLF) at . After
combining our SDSS sample with two faint ( mag) quasars from
the literature, we obtain the parameters for a double power-law fit to the QLF.
The bright-end slope of the QLF is well constrained to be
. Due to the small number of low-luminosity quasars, the
faint-end slope and the characteristic magnitude are
less well constrained, with and
mag. The spatial density of luminous quasars,
parametrized as , drops rapidly
from to 6, with . Based on our fitted QLF and assuming
an IGM clumping factor of , we find that the observed quasar population
cannot provide enough photons to ionize the IGM at \%
confidence. Quasars may still provide a significant fraction of the required
photons, although much larger samples of faint quasars are needed for more
stringent constraints on the quasar contribution to reionization.Comment: 20 pages, 12 figures, Accepted for publication in The Astrophysical
Journa
Modeling the Dust Properties of z ~ 6 Quasars with ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree
The detection of large quantities of dust in z ~ 6 quasars by infrared and
radio surveys presents puzzles for the formation and evolution of dust in these
early systems. Previously (Li et al. 2007), we showed that luminous quasars at
z > 6 can form through hierarchical mergers of gas-rich galaxies. Here, we
calculate the dust properties of simulated quasars and their progenitors using
a three-dimensional Monte Carlo radiative transfer code, ART^2 --
All-wavelength Radiative Transfer with Adaptive Refinement Tree. ART^2
incorporates a radiative equilibrium algorithm for dust emission, an adaptive
grid for inhomogeneous density, a multiphase model for the ISM, and a
supernova-origin dust model. We reproduce the SED and dust properties of SDSS
J1148+5251, and find that the infrared emission are closely associated with the
formation and evolution of the quasar host. The system evolves from a cold to a
warm ULIRG owing to heating and feedback from stars and AGN. Furthermore, the
AGN has significant implications for the interpretation of observation of the
hosts. Our results suggest that vigorous star formation in merging progenitors
is necessary to reproduce the observed dust properties of z~6 quasars,
supporting a merger-driven origin for luminous quasars at high redshifts and
the starburst-to-quasar evolutionary hypothesis. (Abridged)Comment: 26 pages, 22 figures, accepted by ApJ. Version with full resolution
images is available at
http://www.cfa.harvard.edu/~yxli/ARTDUST/astroph0706.3706.pd
A Survey of z~6 Quasars in the SDSS Deep Stripe. II. Discovery of Six Quasars at z_{AB}>21
We present the discovery of six new quasars at z~6 selected from the Sloan
Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by
repeatedly scanning a stripe along the celestial equator. The six quasars are
about two magnitudes fainter than the luminous z~6 quasars found in the SDSS
main survey and one magnitude fainter than the quasars reported in Paper I
(Jiang et al. 2008). Four of them comprise a complete flux-limited sample at
21<z_AB<21.8 over an effective area of 195 deg^2. The other two quasars are
fainter than z_AB=22 and are not part of the complete sample. The quasar
luminosity function at z~6 is well described as a single power law
\Phi(L_{1450}) \propto L_{1450}^{\beta} over the luminosity range
-28<M_{1450}<-25. The best-fitting slope \beta varies from -2.6 to -3.1,
depending on the quasar samples used, with a statistical error of 0.3-0.4.
About 40% of the quasars discovered in the SDSS southern survey have very
narrow Lya emission lines, which may indicate small black hole masses and high
Eddington luminosity ratios, and therefore short black hole growth time scales
for these faint quasars at early epochs.Comment: Accepted for publication in A
Light Curve Templates and Galactic Distribution of RR Lyrae Stars from Sloan Digital Sky Survey Stripe 82
We present an improved analysis of halo substructure traced by RR Lyrae stars
in the SDSS stripe 82 region. With the addition of SDSS-II data, a revised
selection method based on new ugriz light curve templates results in a sample
of 483 RR Lyrae stars that is essentially free of contamination. The main
result from our first study persists: the spatial distribution of halo stars at
galactocentric distances 5--100 kpc is highly inhomogeneous. At least 20% of
halo stars within 30 kpc from the Galactic center can be statistically
associated with substructure. We present strong direct evidence, based on both
RR Lyrae stars and main sequence stars, that the halo stellar number density
profile significantly steepens beyond a Galactocentric distance of ~30 kpc, and
a larger fraction of the stars are associated with substructure. By using a
novel method that simultaneously combines data for RR Lyrae and main sequence
stars, and using photometric metallicity estimates for main sequence stars
derived from deep co-added u-band data, we measure the metallicity of the
Sagittarius dSph tidal stream (trailing arm) towards R.A.2h-3h and Dec~0 deg to
be 0.3 dex higher ([Fe/H]=-1.2) than that of surrounding halo field stars.
Together with a similar result for another major halo substructure, the
Monoceros stream, these results support theoretical predictions that an early
forming, smooth inner halo, is metal poor compared to high surface brightness
material that have been accreted onto a later-forming outer halo. The mean
metallicity of stars in the outer halo that are not associated with detectable
clumps may still be more metal-poor than the bulk of inner-halo stars, as has
been argued from other data sets.Comment: Submitted to ApJ, 68 pages, 26 figures, supplemental material (light
curves, templates, animation) can be downloaded from
http://www.astro.washington.edu/bsesar/S82_RRLyr.htm
Cosmic Evolution of Star Formation In SDSS Quasar Hosts Since z=1
We present Spitzer IRS observations of a complete sample of 57 SDSS type-1
quasars at z~1. Aromatic features at 6.2 and/or 7.7 um are detected in about
half of the sample and show profiles similar to those seen in normal galaxies
at both low- and high-redshift, indicating a star-formation origin for the
features. Based on the ratio of aromatic to star-formation IR (SFIR)
luminosities for normal star-forming galaxies at z~1, we have constructed the
SFIR luminosity function (LF) of z~1 quasars. As we found earlier for
low-redshift PG quasars, these z~1 quasars show a flatter SFIR LF than do z~1
field galaxies, implying the quasar host galaxy population has on average a
higher SFR than the field galaxies do. As measured from their SFIR LF,
individual quasar hosts have on average LIRG-level SFRs, which mainly arise in
the circumnuclear regions. By comparing with similar measurements of
low-redshift PG quasars, we find that the comoving SFIR luminosity density in
quasar hosts shows a much larger increase with redshift than that in field
galaxies. The behavior is consistent with pure density evolution since the
average SFR and the average SFR/BH-accretion-rate in quasar hosts show little
evolution with redshift. For individual quasars, we have found a correlation
between the aromatic-based SFR and the luminosity of the nuclear radiation,
consistent with predictions of some theoretical models. We propose that type 1
quasars reside in a distinct galaxy population that shows elliptical morphology
but that harbors a significant fraction of intermediate-age stars and is
experiencing intense circumnuclear star formation.Comment: Accepted for publication in ApJ, 20 pages, 11 figure
Multicolor photometry of 145 of the HII regions in M33
This paper is the first in a series presenting CCD multicolor photometry for
145 HII regions, selected from 369 candidate regions from Boulesteix et al.
(1974), in the nearby spiral galaxy M33. The observations, which covered the
whole area of M33, were carried out by the Beijing Astronomical Observatory
60/90 cm Schmidt Telescope, in 13 intermediate-band filters, covering a range
of wavelength from 3800 to 10000 A. This provides a series of maps which can be
converted to a multicolor map of M33, in pixels of 1.7''*1.7''. Using aperture
photometry we obtain the spectral energy distributions (SEDs) for these HII
regions. We also give their identification charts. Using the relationship
between the BATC intermediate-band system used for the observations and the
UBVRI broad-band system, the magnitudes in the B and V bands are then derived.
Histograms of the magnitudes in V and in B-V are plotted, and the
color-magnitude diagram is also given. The distribution of magnitudes in the V
band shows that the apparent magnitude of almost all the regions is brighter
than 18, corresponding to an absolute magnitude of -6.62 for an assumed
distance modulus of 24.62, which corresponds to a single main sequence O5 star,
while the distribution of color shows that the sample is blue, with a mode
close to -0.05 as would be expected from a range of typical young clusters.Comment: 34 pages, 19 figures including 16 of jpg form, will appear in the
December 2002 issue of A
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