3,527 research outputs found
QSO environments at intermediate redshifts
We have made a survey of quasar environments at 0.5 < z < 0.8, using a sample
of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our
observations include images of background control fields to provide a good
determination of the field galaxy counts. About 10 per cent of the quasars
appear to live in rich clusters, whereas approximately 45 per cent live in
environments similar to that of field galaxies.
The richness of galaxies within a 0.5 Mpc radius around the radio-quiet
quasars is found to be indistinguishable from the richness around the
radio-loud quasars, corresponding on average to groups or poorer clusters of
galaxies. Comparing the galaxy richness in the radio-loud quasar fields with
quasar fields in the literature, we find no evidence of an evolution in the
environment with epoch. Instead, a weak, but significant correlation between
quasar radio luminosity and environmental richness is present. It is thus
possible that the environments of quasars, at least the powerful ones, do not
evolve much between the present epoch and z \approx 0.8.Comment: 6 pages, 4 figures, to appear in the proc. of the workshop `QSO hosts
and their environments', IAA, Granada 10-12 Jan, 200
Introduction and Expression of a Rabbit β-globin Gene in Mouse Fibroblasts
The cloned chromosomal rabbit ß-globin gene has been introduced into mouse fibroblasts by DNA-mediated gene transfer (transformation). In this report, we examine the expression of the rabbit gene in six independent transformants that contain from 1 to 20 copies of the cloned globin gene. Rabbit globin transcripts were detected in two of these transformants at steady-state concentrations of 5 and 2 copies per cell. The globin transcripts from one cell line are polyadenylylated and migrate as 9S RNA on methylmercury gels. These transcripts reflect correct processing of the two intervening sequences but lack 48 ± 5 nucleotides present at the 5' terminus of rabbit erythrocyte globin mRNA
Increasing the Efficacy of Oncolytic Adenovirus Vectors
Oncolytic adenovirus (Ad) vectors present a new modality to treat cancer. These vectors attack tumors via replicating in and killing cancer cells. Upon completion of the vector replication cycle, the infected tumor cell lyses and releases progeny virions that are capable of infecting neighboring tumor cells. Repeated cycles of vector replication and cell lysis can destroy the tumor. Numerous Ad vectors have been generated and tested, some of them reaching human clinical trials. In 2005, the first oncolytic Ad was approved for the treatment of head-and-neck cancer by the Chinese FDA. Oncolytic Ads have been proven to be safe, with no serious adverse effects reported even when high doses of the vector were injected intravenously. The vectors demonstrated modest anti-tumor effect when applied as a single agent; their efficacy improved when they were combined with another modality. The efficacy of oncolytic Ads can be improved using various approaches, including vector design, delivery techniques, and ancillary treatment, which will be discussed in this review
Quasar 3C 298: a test-case for meteoritic nanodiamond 3.5 µm emission
Aims. We calculate the dust emission expected at 3.43 and 3.53 µm if meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the far-UV break observed in quasars.
Methods. We integrate the UV flux that hydrogenated nanodiamonds must absorb to reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond surfaces, we analyse the radiative energy budget and derive theoretically the IR emission profiles expected for possible C-H surface stretch modes of the diamonds.
Results. Using as test case a spectrum of 3C 298 provided by the Spitzer Observatory, we do not find evidence of these emission bands.
Conclusions. While diamonds without surface adsorbates remain a viable candidate for explaining the far-UV break observed in quasars, hydrogenated nanodiamonds appear to be ruled out, as they would give rise to IR emission bands, which have not been observed so far
Radio-quiet quasar environments at 0.5 < z < 0.8
We have quantified the galaxy environments around a sample of 0.5 < z < 0.8
radio-quiet quasars using the amplitude of the spatial galaxy--quasar
correlation function, B_gq. The quasars exist in a wide variety of
environments, some sources are located in clusters as rich as Abell class 1--2
clusters, whereas others exist in environments comparable to the field. We find
that on average, the quasars prefer poorer clusters of approximately Abell
class 0, which suggests that quasars are biased tracers of mass compared to
galaxies.
The mean B_gq for the sample is found to be indistinguishable from the mean
amplitude for a sample of radio-loud quasars matched in redshift and optical
luminosity. These observations are consistent with recent studies of the hosts
of radio-quiet quasars at low to intermediate redshifts, and suggest that the
mechanism for the production of powerful radio jets in radio-loud quasars is
controlled by processes deep within the active galactic nucleus itself, and is
unrelated to the nature of the hosts or their environments.Comment: 19 pages, 8 figures, accepted for publication in MNRA
Genome-Wide Mapping of in Vivo Protein-DNA Interactions
In vivo protein-DNA interactions connect each transcription factor with its direct targets to form a gene network scaffold. To map these protein-DNA interactions comprehensively across entire mammalian genomes, we developed a large-scale chromatin immunoprecipitation assay (ChIPSeq) based on direct ultrahigh-throughput DNA sequencing. This sequence census method was then used to map in vivo binding of the neuron-restrictive silencer factor (NRSF; also known as REST, for repressor element–1 silencing transcription factor) to 1946 locations in the human genome. The data display sharp resolution of binding position [±50 base pairs (bp)], which facilitated our finding motifs and allowed us to identify noncanonical NRSF-binding motifs. These ChIPSeq data also have high sensitivity and specificity [ROC (receiver operator characteristic) area ≥ 0.96] and statistical confidence (P <10^(–4)), properties that were important for inferring new candidate interactions. These include key transcription factors in the gene network that regulates pancreatic islet cell development
Black hole mass and variability in quasars
We report on a study that finds a positive correlation between black hole
mass and variability amplitude in quasars. Roughly 100 quasars at z<0.75 were
selected by matching objects from the QUEST1 Variability Survey with
broad-lined objects from the Sloan Digital Sky Survey. Black hole masses were
estimated with the virial method using the broad Hbeta line, and variability
was characterized from the QUEST1 light curves. The correlation between black
hole mass and variability amplitude is significant at the 99% level or better
and does not appear to be caused by obvious selection effects inherent to
flux-limited samples. It is most evident for rest frame time lags of the order
a few months up to the QUEST1 maximum temporal resolution of about 2 years. The
correlation between black hole mass and variability amplitude means that the
more massive black holes have larger percentage flux variations. Over 2-3
orders of magnitude in black hole mass, the amplitude increases by
approximately 0.2 mag. A likely explanation for the correlation is that the
more massive black holes are starving and produce larger flux variations
because they do not have a steady inflow of gaseous fuel. Assuming that the
variability arises from changes in the accretion rate Li & Cao [8] show that
flux variations similar to those observed are expected as a consequence of the
more massive black holes having cooler accretion disks.Comment: 4 pages, to be published in the proceedings of the "2nd Kolkata
conference on observational evidence for black holes in the Universe", Feb
10-15, 2008, Kolkata, Indi
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