166 research outputs found
The ensemble photometric variability of over quasars in the Dark Energy Camera Legacy Survey and the Sloan Digital Sky Survey
We present the ensemble variability analysis results of quasars using the
Dark Energy Camera Legacy Survey (DECaLS) and the Sloan Digital Sky Survey
(SDSS) quasar catalogs. Our dataset includes 119,305 quasars with redshifts up
to 4.89. Combining the two datasets provides a 15-year baseline and permits
analysis of the long timescale variability. Adopting a power-law form for the
variability structure function, , we use the
multi-dimensional parametric fitting to explore the relationships between the
quasar variability amplitude and a wide variety of quasar properties, including
redshift (positive), bolometric luminosity (negative), rest-frame wavelength
(negative), and black hole mass (uncertain). We also find that can be
also expressed as a function of redshift (negative), bolometric luminosity
(positive), rest-frame wavelength (positive), and black hole mass (positive).
Tests of the fitting significance with the bootstrap method show that, even
with such a large quasar sample, some correlations are marginally significant.
The typical value of for the entire dataset is ,
consistent with the results in previous studies on both the quasar ensemble
variability and the structure function. A significantly negative correlation
between the variability amplitude and the Eddington ratio is found, which may
be explained as an effect of accretion disk instability.Comment: 13 pages, 8 figures, 4 tables, accepted for publication in Ap
The Extremely Luminous Quasar Survey (ELQS) in the SDSS footprint I.: Infrared Based Candidate Selection
Studies of the most luminous quasars at high redshift directly probe the
evolution of the most massive black holes in the early Universe and their
connection to massive galaxy formation. However, extremely luminous quasars at
high redshift are very rare objects. Only wide area surveys have a chance to
constrain their population. The Sloan Digital Sky Survey (SDSS) has so far
provided the most widely adopted measurements of the quasar luminosity function
(QLF) at . However, a careful re-examination of the SDSS quasar sample
revealed that the SDSS quasar selection is in fact missing a significant
fraction of quasars at the brightest end. We have identified the
purely optical color selection of SDSS, where quasars at these redshifts are
strongly contaminated by late-type dwarfs, and the spectroscopic incompleteness
of the SDSS footprint as the main reasons. Therefore we have designed the
Extremely Luminous Quasar Survey (ELQS), based on a novel near-infrared JKW2
color cut using WISE AllWISE and 2MASS all-sky photometry, to yield high
completeness for very bright () quasars in the redshift
range of . It effectively uses random forest machine-learning
algorithms on SDSS and WISE photometry for quasar-star classification and
photometric redshift estimation. The ELQS will spectroscopically follow-up
new quasar candidates in an area of in the
SDSS footprint, to obtain a well-defined and complete quasars sample for an
accurate measurement of the bright-end quasar luminosity function at . In this paper we present the quasar selection algorithm and the
quasar candidate catalog.Comment: 16 pages, 8 figures, 9 tables; ApJ in pres
Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions
We present the discovery of eight quasars at z~6 identified in the Sloan
Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have
some overlap with each other, leading to repeat observations over an area
spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap
regions provide a unique dataset that allows us to select high-redshift quasars
more than 0.5 mag fainter in the z band than those found with the SDSS
single-epoch data. Our quasar candidates were first selected as i-band dropout
objects in the SDSS imaging database. We then carried out a series of follow-up
observations in the optical and near-IR to improve photometry, remove
contaminants, and identify quasars. The eight quasars reported here were
discovered in a pilot study utilizing the overlap regions at high galactic
latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a
flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag,
the typical magnitude limit of z~6 quasars used for the SDSS single-epoch
images. In addition, we recover eight previously known quasars at z~6 that are
located in the overlap regions. These results validate our procedure for
selecting quasar candidates from the overlap regions and confirming them with
follow-up observations, and provide guidance to a future systematic survey over
all SDSS imaging regions with repeat observations.Comment: AJ in press (8 pages
Close companions to two high-redshift quasars
We report the serendipitous discoveries of companion galaxies to two
high-redshift quasars. SDSS J025617.7+001904 is a z=4.79 quasar included in our
recent survey of faint quasars in the SDSS Stripe 82 region. The initial MMT
slit spectroscopy shows excess Lyman alpha emission extending well beyond the
quasar's light profile. Further imaging and spectroscopy with LBT/MODS1
confirms the presence of a bright galaxy (i_AB = 23.6) located 2arcsec (12 kpc
projected) from the quasar with strong Lyman alpha emission (EW_0 ~ 100Ang) at
the redshift of the quasar, as well as faint continuum. The second quasar,
CFHQS J005006.6+344522 (z=6.25), is included in our recent HST SNAP survey of
z~6 quasars searching for evidence of gravitational lensing. Deep imaging with
ACS and WFC3 confirms an optical dropout ~4.5 mag fainter than the quasar
(Y_AB=25) at a separation of 0.9 arcsec. The red i_775-Y_105 color of the
galaxy and its proximity to the quasar (5 kpc projected if at the quasar
redshift) strongly favor an association with the quasar. Although it is much
fainter than the quasar it is remarkably bright when compared to field galaxies
at this redshift, while showing no evidence for lensing. Both systems may
represent late-stage mergers of two massive galaxies, with the observed light
for one dominated by powerful ongoing star formation and for the other by rapid
black hole growth. Observations of close companions are rare; if major mergers
are primarily responsible for high-redshift quasar fueling then the phase when
progenitor galaxies can be observed as bright companions is relatively short.Comment: 13 pages, 12 figures, accepted for publication in A
Constraining C iii] Emission in a Sample of Five Luminous z = 5.7 Galaxies
Recent observations have suggested that the CIII] emission
lines could be alternative diagnostic lines for galaxies in the reionization
epoch. We use the F128N narrowband filter on the Hubble Space Telescope's
() Wide Field Camera 3 (WFC3) to search for CIII] emission in a
sample of five galaxies at z = 5.7 in the Subaru Deep Field and the
Subaru/XMM-Newton Deep Field. Using the F128N narrowband imaging, together with
the broadband imaging, we do not detect CIII] emission for the five galaxies
with ranging from 24.10 -- 27.00 in our sample. For the brightest
galaxy J132416.13+274411.6 in our sample (z = 5.70, ),
which has a significantly higher signal to noise, we report a CIII] flux of
, which places a
stringent 3- upper limit of $\mathrm{erg\
s^{-1}\ cm^{-2}}\rm\sigma\mathrm{2.55\times10^{-18}\ erg\ s^{-1}\ cm^{-2}}\rm\sigma>$ 5.70 exhibit a wide range
of distribution. Our strong limits on CIII] emission could be used as a guide
for future observations in the reionization epoch
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