1,779 research outputs found
FIRST-2MASS Red Quasars: Transitional Objects Emerging from the Dust
We present a sample of 120 dust-reddened quasars identified by matching radio
sources detected at 1.4 GHz in the FIRST survey with the near-infrared 2MASS
catalog and color-selecting red sources. Optical and/or near-infrared
spectroscopy provide broad wavelength sampling of their spectral energy
distributions that we use to determine their reddening, characterized by
E(B-V). We demonstrate that the reddening in these quasars is best-described by
SMC-like dust. This sample spans a wide range in redshift and reddening (0.1 <
z < 3, 0.1 < E(B-V) < 1.5), which we use to investigate the possible
correlation of luminosity with reddening. At every redshift, dust-reddened
quasars are intrinsically the most luminous quasars. We interpret this result
in the context of merger-driven quasar/galaxy co-evolution where these reddened
quasars are revealing an emergent phase during which the heavily obscured
quasar is shedding its cocoon of dust prior to becoming a "normal" blue quasar.
When correcting for extinction, we find that, depending on how the parent
population is defined, these red quasars make up < 15-20% of the luminous
quasar population. We estimate, based on the fraction of objects in this phase,
that its duration is 15-20% as long as the unobscured, blue quasar phase.Comment: 21 pages, 17 figures plus a spectral atlas. Accepted for publication
in the Astrophysical Journa
Quasar Clustering from SDSS DR5: Dependences on Physical Properties
Using a homogenous sample of 38,208 quasars with a sky coverage of drawn from the SDSS Data Release Five quasar catalog, we study the
dependence of quasar clustering on luminosity, virial black hole mass, quasar
color, and radio loudness. At , quasar clustering depends weakly on
luminosity and virial black hole mass, with typical uncertainty levels for the measured correlation lengths. These weak dependences are
consistent with models in which substantial scatter between quasar luminosity,
virial black hole mass and the host dark matter halo mass has diluted any
clustering difference, where halo mass is assumed to be the relevant quantity
that best correlates with clustering strength. However, the most luminous and
most massive quasars are more strongly clustered (at the level)
than the remainder of the sample, which we attribute to the rapid increase of
the bias factor at the high-mass end of host halos. We do not observe a strong
dependence of clustering strength on quasar colors within our sample. On the
other hand, radio-loud quasars are more strongly clustered than are radio-quiet
quasars matched in redshift and optical luminosity (or virial black hole mass),
consistent with local observations of radio galaxies and radio-loud type 2 AGN.
Thus radio-loud quasars reside in more massive and denser environments in the
biased halo clustering picture. Using the Sheth et al.(2001) formula for the
linear halo bias, the estimated host halo mass for radio-loud quasars is , compared to for
radio-quiet quasar hosts at .Comment: Updated version; accepted for publication in Ap
Black Hole Mass Estimates Based on CIV are Consistent with Those Based on the Balmer Lines
Using a sample of high-redshift lensed quasars from the CASTLES project with
observed-frame ultraviolet or optical and near-infrared spectra, we have
searched for possible biases between supermassive black hole (BH) mass
estimates based on the CIV, Halpha and Hbeta broad emission lines. Our sample
is based upon that of Greene, Peng & Ludwig, expanded with new near-IR
spectroscopic observations, consistently analyzed high S/N optical spectra, and
consistent continuum luminosity estimates at 5100A. We find that BH mass
estimates based on the FWHM of CIV show a systematic offset with respect to
those obtained from the line dispersion, sigma_l, of the same emission line,
but not with those obtained from the FWHM of Halpha and Hbeta. The magnitude of
the offset depends on the treatment of the HeII and FeII emission blended with
CIV, but there is little scatter for any fixed measurement prescription. While
we otherwise find no systematic offsets between CIV and Balmer line mass
estimates, we do find that the residuals between them are strongly correlated
with the ratio of the UV and optical continuum luminosities. Removing this
dependency reduces the scatter between the UV- and optical-based BH mass
estimates by a factor of approximately 2, from roughly 0.35 to 0.18 dex. The
dispersion is smallest when comparing the CIV sigma_l mass estimate, after
removing the offset from the FWHM estimates, and either Balmer line mass
estimate. The correlation with the continuum slope is likely due to a
combination of reddening, host contamination and object-dependent SED shapes.
When we add additional heterogeneous measurements from the literature, the
results are unchanged.Comment: Accepted for publication in The Astrophysical Journal. 37 text pages
+ 8 tables + 23 figures. Updated with comments by the referee and with a
expanded discussion on literature data including new observation
Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4
The clustering of quasars on small scales yields fundamental constraints on
models of quasar evolution and the buildup of supermassive black holes. This
paper describes the first systematic survey to discover high redshift binary
quasars. Using color-selection and photometric redshift techniques, we searched
8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed
them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24
of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse
separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects
known by an order of magnitude. Eight members of this sample are very close
pairs with R_{\perp} 3.5.
The completeness and efficiency of our well-defined selection algorithm are
quantified using simulated photometry and we find that our sample is ~ 50%
complete. Our companion paper uses this knowledge to make the first measurement
of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars.
High redshift binaries constitute exponentially rare coincidences of two
extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one
close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the
analogs of superclusters, in the early Universe.Comment: Submitted to Ap
Defining and Detecting Crossover-Interference Mutants in Yeast
The analysis of crossover interference in many creatures is complicated by the presence of two kinds of crossovers, interfering and noninterfering. In such creatures, the values of the traditional indicators of interference are subject not only to the strength of interference but also to the relative frequencies of crossing over contributed by the two kinds. We formalize the relationship among these variables and illustrate the possibilities and limitations of classical interference analysis with meiotic tetrad data from wild-type Saccharomyces cerevisiae and from mlh1 and ndj1 mutants
Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV
A search for new physics is presented based on an event signature of at least
three jets accompanied by large missing transverse momentum, using a data
sample corresponding to an integrated luminosity of 36 inverse picobarns
collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector
at the LHC. No excess of events is observed above the expected standard model
backgrounds, which are all estimated from the data. Exclusion limits are
presented for the constrained minimal supersymmetric extension of the standard
model. Cross section limits are also presented using simplified models with new
particles decaying to an undetected particle and one or two jets
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
Measurement of the Z/gamma* + b-jet cross section in pp collisions at 7 TeV
The production of b jets in association with a Z/gamma* boson is studied
using proton-proton collisions delivered by the LHC at a centre-of-mass energy
of 7 TeV and recorded by the CMS detector. The inclusive cross section for
Z/gamma* + b-jet production is measured in a sample corresponding to an
integrated luminosity of 2.2 inverse femtobarns. The Z/gamma* + b-jet cross
section with Z/gamma* to ll (where ll = ee or mu mu) for events with the
invariant mass 60 < M(ll) < 120 GeV, at least one b jet at the hadron level
with pT > 25 GeV and abs(eta) < 2.1, and a separation between the leptons and
the jets of Delta R > 0.5 is found to be 5.84 +/- 0.08 (stat.) +/- 0.72 (syst.)
+(0.25)/-(0.55) (theory) pb. The kinematic properties of the events are also
studied and found to be in agreement with the predictions made by the MadGraph
event generator with the parton shower and the hadronisation performed by
PYTHIA.Comment: Submitted to the Journal of High Energy Physic
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