8,517 research outputs found
Prospects for Redshifted 21-cm observations of quasar HII regions
The introduction of low-frequency radio arrays over the coming decade is
expected to revolutionize the study of the reionization epoch. Observation of
the contrast in redshifted 21cm emission between a large HII region and the
surrounding neutral IGM will be the simplest and most easily interpreted
signature. We find that an instrument like the planned Mileura Widefield Array
Low-Frequency Demonstrator (LFD) will be able to obtain good signal to noise on
HII regions around the most luminous quasars, and determine some gross
geometric properties, e.g. whether the HII region is spherical or conical. A
hypothetical follow-up instrument with 10 times the collecting area of the LFD
(MWA-5000) will be capable of mapping the detailed geometry of HII regions,
while SKA will be capable of detecting very narrow spectral features as well as
the sharpness of the HII region boundary. The MWA-5000 will discover
serendipitous HII regions in widefield observations. We estimate the number of
HII regions which are expected to be generated by quasars. Assuming a late
reionization at z~6 we find that there should be several tens of quasar HII
regions larger than 4Mpc at z~6-8 per field of view. Identification of HII
regions in forthcoming 21cm surveys can guide a search for bright galaxies in
the middle of these regions. Most of the discovered galaxies would be the
massive hosts of dormant quasars that left behind fossil HII cavities that
persisted long after the quasar emission ended, owing to the long recombination
time of intergalactic hydrogen. A snap-shot survey of candidate HII regions
selected in redshifted 21cm image cubes may prove to be the most efficient
method for finding very high redshift quasars and galaxies.Comment: 14 pages, 8 figures. Submitted to Ap
A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6
We present the discovery of three new quasars at z>6 in 1300 deg^2 of SDSS
imaging data, J114816.64+525150.3 (z=6.43), J104845.05+463718.3 (z=6.23) and
J163033.90+401209.6 (z=6.05). The first two objects have weak Ly alpha emission
lines; their redshifts are determined from the positions of the Lyman break.
They are only accurate to 0.05 and could be affected by the presence of broad
absorption line systems. The last object has a Ly alpha strength more typical
of lower redshift quasars. Based on a sample of six quasars at z>5.7 that cover
2870 deg^2 presented in this paper and in Paper I, we estimate the comoving
density of luminous quasars at z 6 and M_{1450} < -26.8 to be (8 +/-
3)x10^{-10} Mpc^{-3} (for H_0 = 50 km/s/Mpc, Omega = 1). HST imaging of two
z>5.7 quasars and high-resolution ground-based images (seeing 0.4'') of three
additional z>5.7 quasars show that none of them is gravitationally lensed. The
luminosity distribution of the high-redshfit quasar sample suggests the bright
end slope of the quasar luminosity function at z 6 is shallower than Psi
L^{-3.5} (2-sigma), consistent with the absence of strongly lensed objects.Comment: AJ in press (Apr 2003), 26 pages, 9 figure
Four quasars above redshift 6 discovered by the Canada-France High-z Quasar Survey
The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed
to locate quasars during the epoch of reionization. In this paper we present
the discovery of the first four CFHQS quasars at redshift greater than 6,
including the most distant known quasar, CFHQS J2329-0301 at z=6.43. We
describe the observational method used to identify the quasars and present
optical, infrared, and millimeter photometry and optical and near-infrared
spectroscopy. We investigate the dust properties of these quasars finding an
unusual dust extinction curve for one quasar and a high far-infrared luminosity
due to dust emission for another. The mean millimeter continuum flux for CFHQS
quasars is substantially lower than that for SDSS quasars at the same redshift,
likely due to a correlation with quasar UV luminosity. For two quasars with
sufficiently high signal-to-noise optical spectra, we use the spectra to
investigate the ionization state of hydrogen at z>5. For CFHQS J1509-1749 at
z=6.12, we find significant evolution (beyond a simple extrapolation of lower
redshift data) in the Gunn-Peterson optical depth at z>5.4. The line-of-sight
to this quasar has one of the highest known optical depths at z~5.8. An
analysis of the sizes of the highly-ionized near-zones in the spectra of two
quasars at z=6.12 and z=6.43 suggest the IGM surrounding these quasars was
substantially ionized before these quasars turned on. Together, these
observations point towards an extended reionization process, but we caution
that cosmic variance is still a major limitation in z>6 quasar observations.Comment: 15 pages, 9 figures, AJ, in press, minor changes to previous versio
A slitless spectroscopic survey for quasars near quasars
We present the results of the Quasars near Quasars (QNQ) survey, a CCD-based
slitless spectroscopic survey for faint V<22 quasars at 1.7<z<3.6 on 18
26.2'x33.5' fields centred on bright quasars at 2.76<z<4.69. In total 169
quasar candidates with emission lines were selected from the extracted
flux-calibrated spectra on the basis of well-defined automatic selection
criteria followed by visual inspection and verification. With follow-up
spectroscopy of 81 candidates that were likely to reside at z>1.7 we were able
to confirm 80 new quasars at 0.580<z<3.586 on 16 of our fields. 64 of the newly
discovered quasars are located at z>1.7. The overall high success rate implies
that most of the remaining 88 candidates are quasars as well, although the
majority of them likely resides at z<1.7 on the basis of the observed line
shapes and strengths. Due to the insufficient depth of the input source
catalogues needed for extraction of the slitless spectra our survey is not well
defined in terms of limiting magnitude for faint 2.5<z<3.6 quasars whose Lyman
alpha emission is detectable well beyond V=22, albeit at a continuum S/N<1.
While not useful for characterising the evolving space density of quasars, our
sample provides many new closely spaced quasar sightlines around intensely
studied quasars for further investigations on the three-dimensional
distribution of the intergalactic medium.Comment: 18 pages, 16 figures, accepted by A&A, includes Table 2 from online
material, full paper with appendices containing additional figures and tables
available at http://www.aip.de/People/gworseck/qnqpaper/qnqpaper_final.pd
Discovery of six high-redshift quasars with the Lijiang 2.4m telescope and the Multiple Mirror Telescope
Quasars with redshifts greater than 4 are rare, and can be used to probe the
structure and evolution of the early universe. Here we report the discovery of
six new quasars with -band magnitudes brighter than 19.5 and redshifts
between 2.4 and 4.6 from the YFOSC spectroscopy of the Lijiang 2.4m telescope
in February, 2012. These quasars are in the list of quasar candidates
selected by using our proposed criterion and the photometric redshift
estimations from the SDSS optical and UKIDSS near-IR photometric data. Nine
candidates were observed by YFOSC, and five among six new quasars were
identified as quasars. One of the other three objects was identified as
a star and the other two were unidentified due to the lower signal-to-noise
ratio of their spectra. This is the first time that quasars have been
discovered using a telescope in China. Thanks to the Chinese Telescope Access
Program (TAP), the redshift of 4.6 for one of these quasars was confirmed by
the Multiple Mirror Telescope (MMT) Red Channel spectroscopy. The continuum and
emission line properties of these six quasars, as well as their central black
hole masses and Eddington ratios, were obtained.Comment: 7 pages, 2 figures, published in Research in Astronomy and
Astrophysics (RAA) as a lette
High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data II: The Spring Equatorial Stripe
This is the second paper in a series aimed at finding high-redshift quasars
from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by
the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this
paper, we present 22 high-redshift quasars (z>3.6) discovered from ~250 deg^2
of data in the spring Equatorial Stripe, plus photometry for two previously
known high-redshift quasars in the same region of sky. Our success rate of
identifying high-redshift quasars is 68%. Five of the newly discovered quasars
have redshifts higher than 4.6 (z=4.62, 4.69, 4.70, 4.92 and 5.03). All the
quasars have i* < 20.2 with absolute magnitude -28.8 < M_B < -26.1 (h=0.5,
q_0=0.5). Several of the quasars show unusual emission and absorption features
in their spectra, including an object at z=4.62 without detectable emission
lines, and a Broad Absorption Line (BAL) quasar at z=4.92.Comment: 28 pages, AJ in press (Jan 2000), final version with minor changes;
high resolution finding charts available at
http://www.astro.princeton.edu/~fan/paper/qso2.htm
Early star formation traced by the highest redshift quasars
The iron abundance relative to alpha-elements in the circumnuclear region of
quasars is regarded as a clock of the star formation history and, more
specifically, of the enrichment by SNIa. We investigate the iron abundance in a
sample of 22 quasars in the redshift range 3.0<z<6.4 by measuring their rest
frame UV FeII bump, which is shifted into the near-IR, and by comparing it with
the MgII 2798 flux. The observations were performed with a device that can
obtain near-IR spectra in the range 0.8-2.4 um in one shot, thereby enabling an
optimal removal of the continuum underlying the FeII bump. We detect iron in
all quasars including the highest redshift (z=6.4) quasar currently known. The
uniform observational technique and the wide redshift range allows a reliable
study of the trend of the FeII/MgII ratio with redshift. We find the FeII/MgII
ratio is nearly constant at all redshifts, although there is marginal evidence
for a higher FeII/MgII ratio in the quasars at z~6. If the FeII/MgII ratio
reflects the Fe/alpha abundance, this result suggests that the z~6 quasars have
already undergone a major episode of iron enrichment. We discuss the possible
implications of this finding for the star formation history at z>6. We also
detect a population of weak iron emitters at z~4.5, which are possibly hosted
in systems that evolved more slowly. Alternatively, the trend of the FeII/MgII
ratio at high redshift may reflect significantly different physical conditions
of the circumnuclear gas in such high redshift quasars.Comment: Replaced to match the accepted version (ApJL in press), 5 page
The LAMOST Survey of Background Quasars in the Vicinity of the Andromeda and Triangulum Galaxies -- II. Results from the Commissioning Observations and the Pilot Surveys
We present new quasars discovered in the vicinity of the Andromeda and
Triangulum galaxies with the LAMOST during the 2010 and 2011 observational
seasons. Quasar candidates are selected based on the available SDSS, KPNO 4 m
telescope, XSTPS optical, and WISE near infrared photometric data. We present
509 new quasars discovered in a stripe of ~135 sq. deg from M31 to M33 along
the Giant Stellar Stream in the 2011 pilot survey datasets, and also 17 new
quasars discovered in an area of ~100 sq. deg that covers the central region
and the southeastern halo of M31 in the 2010 commissioning datasets. These 526
new quasars have i magnitudes ranging from 15.5 to 20.0, redshifts from 0.1 to
3.2. They represent a significant increase of the number of identified quasars
in the vicinity of M31 and M33. There are now 26, 62 and 139 known quasars in
this region of the sky with i magnitudes brighter than 17.0, 17.5 and 18.0
respectively, of which 5, 20 and 75 are newly-discovered. These bright quasars
provide an invaluable collection with which to probe the kinematics and
chemistry of the ISM/IGM in the Local Group of galaxies. A total of 93 quasars
are now known with locations within 2.5 deg of M31, of which 73 are newly
discovered. Tens of quasars are now known to be located behind the Giant
Stellar Stream, and hundreds behind the extended halo and its associated
substructures of M31. The much enlarged sample of known quasars in the vicinity
of M31 and M33 can potentially be utilized to construct a perfect astrometric
reference frame to measure the minute PMs of M31 and M33, along with the PMs of
substructures associated with the Local Group of galaxies. Those PMs are some
of the most fundamental properties of the Local Group.Comment: 26 pages, 6 figures, AJ accepte
- …