803 research outputs found
Antlia B: A faint dwarf galaxy member of the NGC 3109 association
We report the discovery of Antlia B, a faint dwarf galaxy at a projected
distance of 72 kpc from NGC 3109 (15 mag), the primary
galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The
tip of the red giant branch distance to Antlia B is =1.290.10 Mpc,
which is consistent with the distance to NGC 3109. A qualitative analysis
indicates the new dwarf's stellar population has both an old, metal poor red
giant branch (10 Gyr, [Fe/H]2), and a younger blue population
with an age of 200-400 Myr, analogous to the original Antlia dwarf,
another likely satellite of NGC 3109. Antlia B has \ion{H}{1} gas at a velocity
of =376 km s, confirming the association with NGC 3109
(=403 km s). The HI gas mass
(M=2.80.210 M), stellar luminosity
(=9.70.6 mag) and half light radius (=27329 pc) are
all consistent with the properties of dwarf irregular and dwarf spheroidal
galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy.
The discovery of Antlia B is the initial result from a Dark Energy Camera
survey for halo substructure and faint dwarf companions to NGC 3109 with the
goal of comparing observed substructure with expectations from the
+Cold Dark Matter model in the sub-Milky Way regime.Comment: 7 pages, 3 figures. Submitted to ApJ
Limits on thermal variations in a dozen quiescent neutron stars over a decade
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the
origin of the thermal X-ray component may be either release of heat from the
core of the neutron star, or continuing low-level accretion. In general, heat
from the core should be stable on timescales years, while continuing
accretion may produce variations on a range of timescales. While some quiescent
neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal
components on a range of timescales, several others, particularly those in
globular clusters with no detectable nonthermal hard X-rays (fit with a
powerlaw), have shown no measurable variations. Here, we constrain the spectral
variations of 12 low mass X-ray binaries in 3 globular clusters over
years. We find no evidence of variations in 10 cases, with limits on
temperature variations below 11% for the 7 qLMXBs without powerlaw components,
and limits on variations below 20% for 3 other qLMXBs that do show non-thermal
emission. However, in 2 qLMXBs showing powerlaw components in their spectra
(NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in
temperature, suggesting the presence of continuing low-level accretion. This
work adds to the evidence that the thermal X-ray component in quiescent neutron
stars without powerlaw components can be explained by heat deposited in the
core during outbursts. Finally, we also investigate the correlation between
hydrogen column density (N) and optical extinction (A) using our sample
and current models of interstellar X-ray absorption, finding .Comment: 16 pages, 5 figures, MNRAS, in pres
Hubble Space Telescope Imaging of the Ultra-Compact High Velocity Cloud AGC 226067: A stripped remnant in the Virgo Cluster
We analyze the optical counterpart to the ultra-compact high velocity cloud
AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS)
on the Hubble Space Telescope. The color magnitude diagram of the main body of
AGC 226067 reveals an exclusively young stellar population, with an age of
7--50 Myr, and is consistent with a metallicity of [Fe/H]0.3 as
previous work has measured via HII region spectroscopy. Additionally, the color
magnitude diagram is consistent with a distance of 17 Mpc,
suggesting an association with the Virgo cluster. A secondary stellar system
located 1.6' (8 kpc) away in projection has a similar stellar
population. The lack of an old red giant branch (5 Gyr) is contrasted
with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw
J122147+132853), and the total diffuse light from AGC~226067 is consistent with
the luminosity function of the resolved 7--50 Myr stellar population. The
main body of AGC~226067 has a =11.30.3, or
=5.41.310 given the stellar population.
We searched 20 deg of imaging data adjacent to AGC~226067 in the Virgo
Cluster, and found two similar stellar systems dominated by a blue stellar
population, far from any massive galaxy counterpart -- if this population has
similar star formation properties as AGC~226067, it implies 0.1
yr in Virgo intracluster star formation. Given its unusual
stellar population, AGC~226067 is likely a stripped remnant and is plausibly
the result of compressed gas from the ram pressure stripped M86 subgroup
(350 kpc away in projection) as it falls into the Virgo Cluster.Comment: 9 pages, 4 figures, 1 table. Accepted by the Astrophysical Journa
GPCRs through the keyhole: the role of protein flexibility in ligand binding to β-adrenoceptors
G protein-coupled receptors (GPCRs) are proteins of pharmaceutical importance, with over 30% of all drugs in clinical use targeting them. Increasing numbers of X-ray crystal (XRC) structures of GPCRs offer a wealth of data relating to ligand binding. For the β-adrenoceptors (β-ARs), XRC structures are available for human β2- and turkey β1-subtypes, in complexes with a range of ligands. While these structures provide insight into the origins of ligand structure-activity relationships (SARs), questions remain. The ligands in all published complexed XRC structures lack extensive substitution, with no obvious way the ligand-binding site can accommodate β1-AR-selective antagonists with extended side-chains para- to the common aryloxypropanolamine pharmacophore. Using standard computational docking tools with such ligands generally returns poses that fail to explain known SARs. Application of our Active Site Pressurisation modelling method to β-AR XRC structures and homology models, however, reveals a dynamic area in the ligand-binding pocket that, through minor changes in amino acid side chain orientations, opens a fissure between transmembrane helices H4 and H5, exposing intra-membrane space. This fissure, which we term the “keyhole”, is ideally located to accommodate extended moieties present in many high-affinity β1-AR-selective ligands, allowing the rest of the ligand structure to adopt a canonical pose in the orthosteric binding site. We propose the keyhole may be a feature of both β1- and β2-ARs, but that subtle structural differences exist between the two, contributing to subtype-selectivity. This has consequences for the rational design of future generations of subtype-selective ligands for these therapeutically important targets
X-ray and radio observations of central black holes in nearby low-mass early-type galaxies: Preliminary evidence for low Eddington fractions
We present new radio and X-ray observations of two nearby ( Mpc)
low-mass early-type galaxies with dynamically-confirmed central black holes:
NGC 5102 and NGC 205. NGC 5102 shows a weak nuclear X-ray source and has no
core radio emission. However, for the first time we demonstrate that it shows
luminous extended radio continuum emission in low-resolution, low-frequency ( GHz) data, consistent with jet lobes on scales pc formed from
past accretion and jet activity. By contrast, in new, extremely deep,
strictly-simultaneous Very Large Array and Chandra observations, no radio or
X-ray emission is detected from the black hole in NGC 205. We consider these
measurements and upper limits in the context of the few other low-mass
early-type galaxies with dynamically-confirmed black holes, and show that the
mean ratio of bolometric to Eddington luminosity in this sample is only
. These Eddington
ratios are lower than typical in a comparison sample of more massive early-type
galaxies, though this conclusion is quite tentative due to our small sample of
low-mass galaxies and potential biases in the comparison sample. This
preliminary result is in mild tension with previous work using less sensitive
observations of more distant galaxies, which predict higher X-ray luminosities
than we observe for low-mass galaxies. If it is confirmed that central black
holes in low-mass galaxies typically have low Eddington ratios, this presents a
challenge to measuring the occupation fraction of central black holes with
standard optical emission line, X-ray, or radio surveys.Comment: 13 pages, 4 figures, 3 tables. Accepted for publication in Ap
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