890 research outputs found
Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
In this work we use a sample of 318 radio-quiet quasars (RQQ) to investigate
the dependence of the ratio of optical/UV flux to X-ray flux, alpha_ox, and the
X-ray photon index, Gamma_X, on black hole mass, UV luminosity relative to
Eddington, and X-ray luminosity relative to Eddington. Our sample is drawn from
the SDSS, with X-ray data from ROSAT and Chandra, and optical data mostly from
the SDSS; 153 of these sources have estimates of Gamma_X from Chandra. We
estimate M_BH using standard estimates derived from the Hbeta, Mg II, and C IV
broad emission lines. Our sample spans a broad range in black hole mass (10^6 <
M_BH / M_Sun < 10^10) and redshift (z < 4.8). We find that alpha_ox increases
with increasing M_BH and L_UV / L_Edd, and decreases with increasing L_X /
L_Edd. In addition, we confirm the correlation seen in previous studies between
Gamma_X and M_BH and both L_UV / L_Edd and L_X / L_Edd; however, we also find
evidence that the dependence of Gamma_X of these quantities is not monotonic,
changing sign at M_BH ~ 3 x 10^8 M_Sun. We argue that the alpha_ox correlations
imply that the fraction of bolometric luminosity emitted by the accretion disk,
as compared to the corona, increases with increasing accretion rate relative to
Eddington. In addition, we argue that the Gamma_X trends are caused by a
dependence of X-ray spectral index on accretion rate. We discuss our results
within the context of accretion models with comptonizing corona, and discuss
the implications of the alpha_ox correlations for quasar feedback. To date,
this is the largest study of the dependence of RQQ X-ray parameters on black
hole mass and related quantities, and the first to attempt to correct for the
large statistical uncertainty in the broad line mass estimates.Comment: Accepted by ApJ, 23 pages, 15 figures, emulateapj styl
An epidemiologic study of early biologic effects of benzene in Chinese workers.
Benzene is a recognized hematotoxin and leukemogen, but its mechanisms of action in humans are still uncertain. To provide insight into these processes, we carried out a cross-sectional study of 44 healthy workers currently exposed to benzene (median 8-hr time-weighted average; 31 ppm), and unexposed controls in Shanghai, China. Here we provide an overview of the study results on peripheral blood cells levels and somatic cell mutation frequency measured by the glycophorin A (GPA) gene loss assay and report on peripheral cytokine levels. All peripheral blood cells levels (i.e., total white blood cells, absolute lymphocyte count, platelets, red blood cells, and hemoglobin) were decreased among exposed workers compared to controls, with the exception of the red blood cell mean corpuscular volume, which was higher among exposed subjects. In contrast, peripheral cytokine levels (interleukin-3, interleukin-6, erythropoietin, granulocyte colony-stimulating factor, tissue necrosis factor-alpha) in a subset of the most highly exposed workers (n = 11) were similar to values in controls (n = 11), suggesting that benzene does not affect these growth factor levels in peripheral blood. The GPA assay measures stem cell or precursor erythroid cell mutations expressed in peripheral red blood cells of MN heterozygous subjects, identifying NN variants, which result from loss of the GPA M allele and duplication of the N allele, and N phi variants, which arise from gene inactivation. The NN (but not N phi) GPA variant cell frequency was elevated in the exposed workers compared with controls (mean +/- SD, 13.9 +/- 8.4 mutants per million cells versus 7.4 +/- 5.2 per million cells, (respectively; p = 0.0002), suggesting that benzene produces gene-duplicating but not gene-inactivating mutations at the GPA locus in bone marrow cells of exposed humans. These findings, combined with ongoing analyses of benzene macromolecular adducts and chromosomal aberrations, will provide an opportunity to comprehensively evaluate a wide range of early biologic effects associated with benzene exposure in humans
Association between rotator cuff muscle size and glenoid deformity in primary glenohumeral osteoarthritis
BACKGROUND: Although glenoid morphology has been associated with fatty infiltration of the rotator cuff in arthritic shoulders, the association of rotator cuff muscle area with specific patterns of glenoid wear has not been studied. The purpose of our study was to assess the associations of glenoid deformity in primary glenohumeral osteoarthritis and rotator cuff muscle area.
METHODS: A retrospective study of 370 computed tomographic (CT) scans of osteoarthritic shoulders was performed. Glenoid deformity according to the modified Walch classification was determined, and retroversion, inclination, and humeral-head subluxation were calculated using automated 3-dimensional software. Rotator cuff muscle area was measured on sagittal CT scan reconstructions. A ratio of the area of the posterior rotator cuff muscles to the subscapularis was calculated to approximate axial plane potential force imbalance. Univariate and multivariate analyses to determine associations with glenoid bone deformity and rotator cuff measurements were performed.
RESULTS: Patient age and sex were significantly related to cuff muscle area across glenoid types. Multivariate analysis did not find significant differences in individual rotator cuff cross-sectional areas across glenoid types, with the exception of a larger supraspinatus area in Type-B2 glenoids compared with Type-A glenoids (odds ratio [OR], 1.5; p = 0.04). An increased ratio of the posterior cuff area to the subscapularis area was associated with increased odds of a Type-B2 deformity (OR, 1.3; p = 0.002). Similarly, an increase in this ratio was significantly associated with increased glenoid retroversion (beta = 0.92; p = 0.01) and humeral-head subluxation (beta = 1.48; p = 0.001). Within the Type-B glenoids, only posterior humeral subluxation was related to the ratio of the posterior cuff to the subscapularis (beta = 1.15; p = 0.001).
CONCLUSIONS: Age and sex are significantly associated with cuff muscle area in arthritic shoulders. Asymmetric glenoid wear and humeral-head subluxation in osteoarthritis are associated with asymmetric atrophy within the rotator cuff transverse plane. Increased posterior rotator cuff muscle area compared with anterior rotator cuff muscle area is associated with greater posterior glenoid wear and subluxation. It is unclear if the results are causative or associative; further research is required to clarify the relationship.
LEVEL OF EVIDENCE: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence
Laccase-catalyzed decolorization of the synthetic azo-dye diamond black PV 200 and of some structurally related derivatives
The kinetics of laccase-catalyzed transformation of the azo-dye Diamond Black PV 200 (CI Mordant Black 9) and various related synthesized derivatives were analyzed for dependence on pH and substrate structure. The reaction mixture of Diamond Black PV 200 was analyzed
by HPLC/MS_/MS and it was shown that upon laccase oxidation, reactive chinoid fragments of lower molecular weight were formed. These may further oligomerize as indicated by the appearance of a number of compounds
with increased molecular weight. The pH optimum for the decolorization was pH 5 for Diamond Black PV 200 which did not change significantly when the substitution pattern of its basic structure was varied. Biodegradability, however, was strongly dependent on the structure of the dyes
Evolution of the X-ray Emission of Radio-Quiet Quasars
We report new Chandra observations of seven optically faint, z \sim 4
radio-quiet quasars. We have combined these new observations with previous
Chandra observations of radio-quiet quasars to create a sample of 174 sources.
These sources have 0.1 < z < 4.7, and 10^{44} ergs s^{-1} < nu L_{nu} (2500
\AA) < 10^{48} ergs s^{-1}. The X-ray detection fraction is 90%. We find that
the X-ray loudness of radio-quiet quasars decreases with UV luminosity and
increases with redshift. The model that is best supported by the data has a
linear dependence of optical-to-X-ray ratio, alpha_{ox}, on cosmic time, and a
quadratic dependence of alpha_{ox} on log L_{UV}, where alpha_{ox} becomes
X-ray quiet more rapidly at higher log L_{UV}. We find no significant evidence
for a relationship between the X-ray photon index, Gamma_X, and the UV
luminosity, and we find marginally significant evidence that the X-ray
continuum flattens with increasing z (2 sigma). The Gamma_X-z anti-correlation
may be the result of X-ray spectral curvature, redshifting of a Compton
reflection component into the observed Chandra band, and/or redshifting of a
soft excess out of the observed Chandra band. Using the results for Gamma_X, we
show that the alpha_{ox}-z relationship is unlikely to be a spurious result
caused by redshifting of the observable X-ray spectral region. A correlation
between alpha_{ox} and z implies evolution of the accretion process. We present
a qualitative comparison of these new results with models for accretion disk
emission.Comment: Accepted by ApJ, 48 pages, 10 figures, 5 table
The structural properties and star formation history of Leo T from deep LBT photometry
We present deep, wide-field g and r photometry of the transition type dwarf
galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The
data confirm the presence of both very young (5
Gyr) stars. We study the structural properties of the old and young stellar
populations by preferentially selecting either population based on their color
and magnitude. The young population is significantly more concentrated than the
old population, with half-light radii of 104+-8 and 148+-16 pc respectively,
and their centers are slightly offset. Approximately 10% of the total stellar
mass is estimated to be represented by the young stellar population. Comparison
of the color-magnitude diagram (CMD) with theoretical isochrones as well as
numerical CMD-fitting suggest that star formation began over 10 Gyr ago and
continued in recent times until at least a few hundred Myr ago. The CMD-fitting
results are indicative of two distinct star formation bursts, with a quiescent
period around 3 Gyr ago, albeit at low significance. The results are consistent
with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the
system. Finally, the data show little if any sign of tidal distortion of Leo T.Comment: 8 pages, 9 figures, some small textual changes, accepted for
publication in the Astrophysical Journa
The discovery of Segue 2: a prototype of the population of satellites of satellites
We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of Mv=â2.5, a half-light radius of 34 pc and a systemic velocity of âŒâ40 km sâ1. Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km sâ1 and the possible stream as âŒ7 km sâ1. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites - survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoc
The LBT Panoramic View on the Recent Star-Formation Activity in IC2574
We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81
group taken with the Large Binocular Telescope in order to study in detail the
recent star-formation history of this galaxy and to constrain the stellar
feedback on its HI gas. We identify the star-forming areas in the galaxy by
removing a smooth disk component from the optical images. We construct
pixel-by-pixel maps of stellar age and stellar mass surface density in these
regions by comparing their observed colors with simple stellar populations
synthesized with STARBURST99. We find that an older burst occurred about 100
Myr ago within the inner 4 kpc and that a younger burst happened in the last 10
Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar
populations residing in the known HI holes of IC2574. Our results indicate
that, even at the remarkable photometric depth of the LBT data, there is no
clear one-to-one association between the observed HI holes and the most recent
bursts of star formation in IC2574. The stellar populations formed during the
younger burst are usually located at the periphery of the HI holes and are seen
to be younger than the holes dynamical age. The kinetic energy of the holes
expansion is found to be on average 10% of the total stellar energy released by
the stellar winds and supernova explosions of the young stellar populations
within the holes. With the help of control apertures distributed across the
galaxy we estimate that the kinetic energy stored in the HI gas in the form of
its local velocity dispersion is about 35% of the total stellar energy.Comment: 16 pages, 14 figures, accepted for publication in Ap
Molecular Hydrogen in the Damped Ly alpha Absorber of Q1331+170
We used HST/STIS to obtain the spectrum of molecular hydrogen associated with
the damped Ly system at toward the quasar
Q1331+170 at . Strong absorption was detected,
with a total column density of .The molecular hydrogen fraction is , which is the
greatest value reported so far in any redshifted damped Ly system. This
results from the combined effect of a relatively high dust-to-gas ratio, a low
gas temperature, and an extremely low ambient UV radiation field. Based on the
observed population of states, we estimate the photo-absorption rate to be
, corresponding to a
local UV radiation field of , where is the UV intensity
at in the solar neighborhood. This is comparable with the
metagalactic UV background intensity at this redshift, and implies an extremely
low star formation rate in the absorber's environment. The observed CO-to-H
column density ratio is ,
which is similar to the value measured for diffuse molecular clouds in the
Galactic ISM. Finally, applying the inferred physical conditions to the
observed C I fine structure excitation (Songaila {\it et al.} 1994), we
estimate the cosmic microwave background temperature to be at , consistent with the predicted value of from the standard cosmology.Comment: Accepted for publication, Astrophysical Journal. Abstract abbreviate
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