1,476 research outputs found
Support over Cavities of Unknown Depth for an Underground Facility
Rock excavation for a 7,200-square-meter building site in Saudi Arabia intercepted a large cavity underlying approximately half the site. Relocation was impossible; therefore, a grouting program was selected to modify the site to accept the allowable design load of 2.1 mega pascals. Due to the extreme depth of the cavity, the modification was limited to a depth of 25 meters under the building
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
Continuum and Emission-Line Properties of Broad Absorption Line Quasars
We investigate the continuum and emission-line properties of 224 broad
absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital
Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide
quasars. We find that low-ionization BALQSOs (LoBALs) are significantly
reddened as compared to normal quasars, in agreement with previous work.
High-ionization BALQSOs (HiBALs) are also more reddened than the average
nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount
of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is
E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We
find that there are differences in the emission-line properties between the
average HiBAL, LoBAL, and nonBAL quasar. These differences, along with
differences in the absorption line troughs, may be related to intrinsic quasar
properties such as the slope of the intrinsic (unreddened) continuum; more
extreme absorption properties are correlated with bluer intrinsic continua.
Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear
to be drawn from the same parent population as nonBALQSOs when both are
selected by their UV/optical properties. We find that the overall fraction of
traditionally defined BALQSOs, after correcting for color-dependent selection
effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and
shows no significant redshift dependence for 1.7<z<3.45. After a rough
completeness correction for the effects of dust extinction, we find that
approximately one in every six quasars is a BALQSO.Comment: 35 pages, 11 figures (1 color), 1 table; accepted by A
Exploratory Chandra Observations of the Three Highest Redshift Quasars Known
We report on exploratory Chandra observations of the three highest redshift
quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky
Survey. These data, combined with a previous XMM-Newton observation of a z =
5.74 quasar, form a complete set of color-selected, z > 5.7 quasars. X-ray
emission is detected from all of the quasars at levels that indicate that the
X-ray to optical flux ratios of z ~ 6 optically selected quasars are similar to
those of lower redshift quasars. The observations demonstrate that it will be
feasible to obtain quality X-ray spectra of z ~ 6 quasars with current and
future X-ray missions.Comment: 15 pages, ApJL, in press; small revisions to address referee Comment
Clustering Analyses of 300,000 Photometrically Classified Quasars--II. The Excess on Very Small Scales
We study quasar clustering on small scales, modeling clustering amplitudes
using halo-driven dark matter descriptions. From 91 pairs on scales <35 kpc/h,
we detect only a slight excess in quasar clustering over our best-fit
large-scale model. Integrated across all redshifts, the implied quasar bias is
b_Q = 4.21+/-0.98 (b_Q = 3.93+/-0.71) at ~18 kpc/h (~28 kpc/h). Our best-fit
(real-space) power index is ~-2 (i.e., ), implying
steeper halo profiles than currently found in simulations. Alternatively,
quasar binaries with separation <35 kpc/h may trace merging galaxies, with
typical dynamical merger times t_d~(610+/-260)m^{-1/2} Myr/h, for quasars of
host halo mass m x 10^{12} Msolar/h. We find UVX quasars at ~28 kpc/h cluster
>5 times higher at z > 2, than at z < 2, at the level. However, as
the space density of quasars declines as z increases, an excess of quasar
binaries (over expectation) at z > 2 could be consistent with reduced merger
rates at z > 2 for the galaxies forming UVX quasars. Comparing our clustering
at ~28 kpc/h to a \xi(r)=(r/4.8\Mpch)^{-1.53} power-law, we find an upper
limit on any excess of a factor of 4.3+/-1.3, which, noting some caveats,
differs from large excesses recently measured for binary quasars, at
. We speculate that binary quasar surveys that are biased to z > 2
may find inflated clustering excesses when compared to models fit at z < 2. We
provide details of 111 photometrically classified quasar pairs with separations
<0.1'. Spectroscopy of these pairs could significantly constrain quasar
dynamics in merging galaxies.Comment: 12pages, 3 figures, 2 tables; uses amulateapj; accepted to Ap
A Simple Likelihood Method for Quasar Target Selection
We present a new method for quasar target selection using photometric fluxes
and a Bayesian probabilistic approach. For our purposes we target quasars using
Sloan Digital Sky Survey (SDSS) photometry to a magnitude limit of g=22. The
efficiency and completeness of this technique is measured using the Baryon
Oscillation Spectroscopic Survey (BOSS) data, taken in 2010. This technique was
used for the uniformly selected (CORE) sample of targets in BOSS year one
spectroscopy to be realized in the 9th SDSS data release. When targeting at a
density of 40 objects per sq-deg (the BOSS quasar targeting density) the
efficiency of this technique in recovering z>2.2 quasars is 40%. The
completeness compared to all quasars identified in BOSS data is 65%. This paper
also describes possible extensions and improvements for this techniqueComment: Updated to accepted version for publication in the Astrophysical
Journal. 10 pages, 10 figures, 3 table
SDSS J094604.90+183541.8: A Gravitationally Lensed Quasar at z=4.8
We report the discovery of a gravitationally lensed quasar identified
serendipitously in the Sloan Digital Sky Survey (SDSS). The object, SDSS
J094604.90+183541.8, was initially targeted for spectroscopy as a luminous red
galaxy, but the SDSS spectrum has the features of both a z=0.388 galaxy and a
z=4.8 quasar. We have obtained additional imaging that resolves the system into
two quasar images separated by 3.06 arcsec and a bright galaxy that is strongly
blended with one of the quasar images. We confirm spectroscopically that the
two quasar images represent a single lensed source at z=4.8 with a total
magnification of 3.2, and we derive a model for the lensing galaxy. This is the
highest redshift lensed quasar currently known. We examine the issues
surrounding the selection of such an unusual object from existing data and
briefly discuss implications for lensed quasar surveys.Comment: AJ accepted, 9 pages, 6 figures, referee suggestions include
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