52,334 research outputs found
Do Quasars Lens Quasars?
If the unexpectedly high frequency of quasar pairs with very different
component redshifts is due to the lensing of a population of background quasars
by the foreground quasar, typical lens masses must be \sim10^{12}M_{\sun} and
the sum of all such quasar lenses would have to contain times the
closure density of the Universe. It then seems plausible that a very high
fraction of all \sim10^{12} M_{\sun} gravitational lenses with redshifts
contain quasars. Here I propose that these systems have evolved to
form the present population of massive galaxies with M and M
>5\times10^{11} M_{\sun}.Comment: 6 pages, aas style, ams symbols, ApJL (accepted
The Quasar-LBG Two-point Angular Cross-correlation Function at z ~ 4 in the COSMOS Field
In order to investigate the origin of quasars, we estimate the bias factor
for low-luminosity quasars at high redshift for the first time. In this study,
we use the two-point angular cross-correlation function (CCF) for both
low-luminosity quasars at and Lyman-break galaxies
(LBGs). Our sample consists of both 25 low-luminosity quasars (16 objects are
spectroscopically confirmed low-luminosity quasars) in the redshift range
and 835 color-selected LBGs with at
in the COSMOS field. We have made our analysis for the following two
quasar samples; (1) the spectroscopic sample (the 16 quasars confirmed by
spectroscopy), and (2) the total sample (the 25 quasars including 9 quasars
with photometric redshifts). The bias factor for low-luminosity quasars at
is derived by utilizing the quasar-LBG CCF and the LBG
auto-correlation function. We then obtain the upper limits of the bias
factors for low-luminosity quasars, that are 5.63 and 10.50 for the total and
the spectroscopic samples, respectively. These bias factors correspond to the
typical dark matter halo masses, log
and , respectively. This result is not inconsistent with the predicted
bias for quasars which is estimated by the major merger models.Comment: 13 pages, 9 figures, Accepted for publication in Ap
A Sample of Quasars with Strong Nitrogen Emission Lines from the Sloan Digital Sky Survey
We report on 293 quasars with strong NIV] lambda 1486 or NIII] lambda 1750
emission lines (rest-frame equivalent width > 3 \AA) at 1.7 < z < 4.0 selected
from the Sloan Digital Sky Survey (SDSS) Fifth Data Release. These
nitrogen-rich (N-rich) objects comprise ~1.1% of the SDSS quasars. The
comparison between the N-rich quasars and other quasars shows that the two
quasar subsets share many common properties. We also confirm previous results
that N-rich quasars have much stronger Lya and NV lambda 1240 emission lines.
Strong nitrogen emission in all ionization states indicates high overall
nitrogen abundances in these objects. We find evidence that the nitrogen
abundance is closely related to quasar radio properties. The radio-loud
fraction in the NIII]-rich quasars is 26% and in the NIV]-rich quasars is 69%,
significantly higher than ~8% measured in other quasars with similar redshift
and luminosity. Therefore, the high nitrogen abundance in N-rich quasars could
be an indicator of a special quasar evolution stage, in which the radio
activity is also strong.Comment: 8 pages, 4 figures; accepted by ApJ (ApJ June 10, 2008, v680 n1
issue
The radio luminosity, black hole mass and Eddington ratio for quasars from the Sloan Digital Sky Survey
We investigate the \mbh- \sigma_* relation for radio-loud quasars with
redshift in Data Release 3 of the Sloan Digital Sky Survey (SDSS). The
sample consists of 3772 quasars with better model of H and \oiii lines
and available radio luminosity, including 306 radio-loud quasars, 3466
radio-quiet quasars with measured radio luminosity or upper-limit of radio
luminosity (181 radio-quiet quasars with measured radio luminosity). The virial
supermassive black hole mass (\mbh) is calculated from the broad \hb line, the
host stellar velocity dispersion () is traced by the core \oiii
gaseous velocity dispersion, and the radio luminosity and the radio loudness
are derived from the FIRST catalog. Our results are follows: (1) For
radio-quiet quasars, we confirm that there is no obvious deviation from the
\mbh- \sigma_* relation defined in inactive galaxies when \mbh uncertainties
and luminosity bias are concerned. (2) We find that radio-loud quasars deviate
much from the \mbh- \sigma_* relation respect to that for radio-quiet
quasars. This deviation is only partly due to the possible cosmology evolution
of the \mbh- \sigma_* relation and the luminosity bias. (3) The radio
luminosity is proportional to
\mbh^{1.28^{+0.23}_{-0.16}}(\lb/\ledd)^{1.29^{+0.31}_{-0.24}} for radio-quiet
quasars and \mbh^{3.10^{+0.60}_{-0.70}}(\lb/\ledd)^{4.18^{+1.40}_{-1.10}} for
radio-loud quasars. The weaker correlation of the radio luminosity dependence
upon the mass and the Eddington ratio for radio-loud quasars shows that other
physical effects would account for their radio luminosities, such as the black
hole spin.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in ChJA
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