3,204 research outputs found
Gas infall towards Sgr A* from the clumpy circumnuclear disk
We present the first large-scale mosaic performed with the Submillimeter
Array (SMA) in the Galactic center. We have produced a 25-pointing mosaic,
covering a ~2' x 2' area around Sgr A*. We have detected emission from two
high-density molecular tracers, HCN(4-3) and CS(7-6), the latter never before
reported in this region. The data have an angular resolution of 4.6" x 3.1",
and the spectral window coverage is from -180 km/s to 1490 km/s for HCN(4-3)
and from -1605 km/s to 129 km/s for CS(7-6). Both molecular tracers present a
very clumpy distribution along the circumnuclear disk (CND), and are detected
with a high signal-to-noise ratio in the southern part of the CND, while they
are weaker towards the northern part. Assuming that the clumps are as close to
the Galactic center as their projected distances, they are still dense enough
to be gravitationally stable against the tidal shear produced by the
supermassive black hole. Therefore, the CND is a non-transient structure. This
geometrical distribution of both tracers suggests that the southern part of the
CND is denser than the northern part. Also, by comparing the HCN(4-3) results
with HCN(1-0) results we can see that the northern and the southern parts of
the CND have different excitation levels, with the southern part warmer than
the northern. Finally, we compare our results with those obtained with the
detection of NH3, which traces the warmer and less dense material detected in
the inner cavity of the CND. We suggest that we are detecting the origin point
where a portion of the CND becomes destabilized and approaches the dynamical
center of the Milky Way, possibly being impacted by the southern streamer and
heated on its way inwards.Comment: 35 pages, 25 figures, 2 tables, accepted for publication in ApJ,
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Cyclic AMP Regulation of Protein Lysine Acetylation in Mycobacterium Tuberculosis
Protein lysine acetylation networks can regulate central processes such as carbon metabolism and gene expression in bacteria. In Escherichia coli, cyclic-AMP (cAMP) regulates protein lysine acetyltransferase (PAT) activity at the transcriptional level, but in Mycobacterium tuberculosis, fusion of a cyclic-nucleotide binding domain to a Gcn5-like PAT domain enables direct cAMP control of protein acetylation. Here we describe the allosteric activation mechanism of M. tuberculosis PAT. The crystal structures of the auto-inhibited and cAMP-activated PAT reveal that cAMP binds to a cryptic site in the regulatory domain over 32 Å from the catalytic site. An extensive conformational rearrangement relieves auto-inhibition by a substrate-mimicking lid that covers the protein-substrate binding surface. A steric double latch couples the domains by harnessing a classic, cAMP-mediated, conformational switch. The structures suggest general features that enable the evolution of long-range communication between linked domains
Sloan Digital Sky Survey III Photometric Quasar Clustering: Probing the Initial Conditions of the Universe using the Largest Volume
The Sloan Digital Sky Survey has surveyed 14,555 square degrees of the sky,
and delivered over a trillion pixels of imaging data. We present the
large-scale clustering of 1.6 million quasars between z = 0.5 and z = 2.5 that
have been classified from this imaging, representing the highest density of
quasars ever studied for clustering measurements. This data set spans ~11,000
square degrees and probes a volume of 80(Gpc/h)^3. In principle, such a large
volume and medium density of tracers should facilitate high-precision
cosmological constraints. We measure the angular clustering of photometrically
classified quasars using an optimal quadratic estimator in four redshift slices
with an accuracy of ~25% over a bin width of l ~10 - 15 on scales corresponding
to matter-radiation equality and larger (l ~ 2 - 30). Observational systematics
can strongly bias clustering measurements on large scales, which can mimic
cosmologically relevant signals such as deviations from Gaussianity in the
spectrum of primordial perturbations. We account for systematics by employing a
new method recently proposed by Agarwal et al. (2014) to the clustering of
photometrically classified quasars. We carefully apply our methodology to
mitigate known observational systematics and further remove angular bins that
are contaminated by unknown systematics. Combining quasar data with the
photometric luminous red galaxy (LRG) sample of Ross et al. (2011) and Ho et
al. (2012), and marginalizing over all bias and shot noise-like parameters, we
obtain a constraint on local primordial non-Gaussianity of fNL = -113+/-154
(1\sigma error). [Abridged]Comment: 35 pages, 15 figure
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