18 research outputs found
NICMOS and VLA Observations of the Gravitatonally Lensed Ultraluminous BAL Quasar APM~08279+5255: Detection of a Third Image
We present a suite of observations of the recently identified ultraluminous
BAL quasar APM 08279+5255, taken both in the infra-red with the NICMOS high
resolution camera on board the Hubble Space Telescope, and at 3.5cm with the
Very Large Array. With an inferred luminosity of ~5x10^15 Solar luminosities,
APM 08279+5255 is apparently the most luminous system known. Extant
ground-based images show that APM 08279+5255 is not point-like, but is instead
separated into two components, indicative of gravitational lensing. The much
higher resolution images presented here also reveal two point sources, A and B,
of almost equal brightness (f_B/f_A=0.782 +/- 0.010), separated by 0."378 +/-
0."001, as well as a third, previously unknown, fainter image, C, seen between
the brighter images. While the nature of C is not fully determined, several
lines of evidence point to it being a third gravitationally lensed image of the
quasar, rather than being the lensing galaxy. Simple models which recover the
relative image configuration and brightnesses are presented. While proving to
be substantially amplified, APM 08279+5255 possesses an intrinsic bolometric
luminosity of ~10^14 to 10^15 Solar luminosities and remains amongst the most
luminous objects known.Comment: 21 pages, 5 figures (2 as GIF files); accepted for publication in the
Astronomical Journa
Submillimeter Observations of the Ultraluminous BAL Quasar APM 08279+5255
With an inferred bolometric luminosity of 5\times10^{15}{\rm \lsun}, the
recently identified z=3.87, broad absorption line quasar APM 08279+5255 is
apparently the most luminous object currently known. As half of its prodigious
emission occurs in the infrared, APM 08279+5255 also represents the most
extreme example of an Ultraluminous Infrared Galaxy. Here, we present new
submillimeter observations of this phenomenal object; while indicating that a
vast quantity of dust is present, these data prove to be incompatible with
current models of emission mechanisms and reprocessing in ultraluminous
systems. The influence of gravitational lensing upon these models is considered
and we find that while the emission from the central continuum emitting region
may be significantly enhanced, lensing induced magnification cannot easily
reconcile the models with observations. We conclude that further modeling,
including the effects of any differential magnification is required to explain
the observed emission from APM 08279+5255.Comment: 12 Pages with Two figures. Accepted for publication in the
Astrophysical Journal Letter
APM 08279+5255: an ultraluminous BAL quasar at a redshift z=3.87
We report on the discovery of a highly luminous, broad absorption line quasar
at a redshift of which is positionally coincident, within one
arcsecond, with the IRAS FSC source F08279+5255. A chance alignment of the
quasar and the IRAS source is extremely unlikely and we argue that the optical
and FIR flux are different manifestations of the same object. With an R-band
magnitude of 15.2, and an IRAS 60\mum flux of 0.51\jy, APM 08279+5255 is
(apparently) easily the most intrinsically luminous object known, with
L_{Bol}\sim5\times10^{15}L_{\odot}}. Imaging suggests that gravitational
lensing may play a role in amplifying the intrinsic properties of the system.
The optical spectrum of the quasar clearly reveals the presence of three
potential lensing galaxies, \mg absorption systems at and ,
and a \ly absorption system at . We estimate the total amplification of
the optical component to be , but, due to the larger scale of the
emitting region, would expect the infrared amplification to be significantly
less. Even making the conservative assumption that all wavelengths are
amplified by a factor 40, APM 08279+5255 still possesses a phenomenal
luminosity of \simgt 10^{14L_{\odot}}, indicating that it belongs to a small,
but significant population of high--redshift, hyperluminous objects with
copious infrared emission.Comment: 15 Pages with Four figures. Accepted for publication in the
Astrophysical Journa
Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way
An all high-latitude sky survey for cool carbon giant stars in the Galactic
halo has revealed 75 such stars, of which the majority are new detections. Of
these, more than half are clustered on a Great Circle on the sky which
intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its
proper motion vector, while many of the remainder are outlying Magellanic Cloud
C-stars. A pole-count analysis of the carbon star distribution clearly
indicates that the Great Circle stream we have isolated is statistically
significant, being a 5-6 sigma over-density. These two arguments strongly
support our conclusion that a large fraction of the Halo carbon stars
originated in Sgr. The stream orbits the Galaxy between the present location of
Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star,
at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a
large range in both Galactic R and z distances are probed. That the stream is
observed as a Great Circle indicates that the Galaxy does not exert a
significant torque upon the stream, so the Galactic potential must be nearly
spherical in the regions probed by the stream. We present N-body experiments
simulating this disruption process as a function of the distribution of mass in
the Galactic halo. A likelihood analysis shows that, in the Galactocentric
distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost
spherical. We rule out, at high confidence levels, the possibility that the
Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7.
This result is quite unexpected and contests currently popular galaxy formation
models. (Abridged)Comment: 26 pages, 13 figures (6 in color, 8 chunky due to PS compression),
minor revisions, accepted by Ap
Life Beyond the Solar System: Remotely Detectable Biosignatures
For the first time in human history, we will soon be able to apply to the scientific method to the question "Are We Alone?" The rapid advance of exoplanet discovery, planetary systems science, and telescope technology will soon allow scientists to search for life beyond our Solar System through direct observation of extrasolar planets. This endeavor will occur alongside searches for habitable environments and signs of life within our Solar System. While these searches are thematically related and will inform each other, they will require separate observational techniques. The search for life on exoplanets holds potential through the great diversity of worlds to be explored beyond our Solar System. However, there are also unique challenges related to the relatively limited data this search will obtain on any individual world
The Hepatitis B Virus Ribonuclease H Is Sensitive to Inhibitors of the Human Immunodeficiency Virus Ribonuclease H and Integrase Enzymes
Nucleos(t)ide analog therapy blocks DNA synthesis by the hepatitis B virus (HBV) reverse transcriptase and can control the infection, but treatment is life-long and has high costs and unpredictable long-term side effects. The profound suppression of HBV by the nucleos(t)ide analogs and their ability to cure some patients indicates that they can push HBV to the brink of extinction. Consequently, more patients could be cured by suppressing HBV replication further using a new drug in combination with the nucleos(t)ide analogs. The HBV ribonuclease H (RNAseH) is a logical drug target because it is the second of only two viral enzymes that are essential for viral replication, but it has not been exploited, primarily because it is very difficult to produce active enzyme. To address this difficulty, we expressed HBV genotype D and H RNAseHs in E. coli and enriched the enzymes by nickel-affinity chromatography. HBV RNAseH activity in the enriched lysates was characterized in preparation for drug screening. Twenty-one candidate HBV RNAseH inhibitors were identified using chemical structure-activity analyses based on inhibitors of the HIV RNAseH and integrase. Twelve anti-RNAseH and anti-integrase compounds inhibited the HBV RNAseH at 10 ΞΌM, the best compounds had low micromolar IC50 values against the RNAseH, and one compound inhibited HBV replication in tissue culture at 10 ΞΌM. Recombinant HBV genotype D RNAseH was more sensitive to inhibition than genotype H. This study demonstrates that recombinant HBV RNAseH suitable for low-throughput antiviral drug screening has been produced. The high percentage of compounds developed against the HIV RNAseH and integrase that were active against the HBV RNAseH indicates that the extensive drug design efforts against these HIV enzymes can guide anti-HBV RNAseH drug discovery. Finally, differential inhibition of HBV genotype D and H RNAseHs indicates that viral genetic variability will be a factor during drug development. Β© 2013 Tavis et al
Robust estimation of bacterial cell count from optical density
Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data