3,453 research outputs found
UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors
We present ultraviolet (UV) integrated colors of 44 Galactic globular
clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV
and NUV bands. This data-base is the largest homogeneous catalog of UV colors
ever published for stellar systems in our Galaxy. The proximity of GGCs makes
it possible to resolve many individual stars even with the somewhat low spatial
resolution of GALEX. This allows us to determine how the integrated UV colors
are driven by hot stellar populations, primarily horizontal branch stars and
their progeny. The UV colors are found to be correlated with various parameters
commonly used to define the horizontal branch morphology. We also investigate
how the UV colors vary with parameters like metallicity, age, helium abundance
and concentration. We find for the first time that GCs associated with the
Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs
with the same metallicity. Finally, we speculate about the presence of an
interesting trend, suggesting that the UV color of GCs may be correlated with
the mass of the host galaxy, in the sense that more massive galaxies possess
bluer clusters.Comment: Accepted for publication by The Astronomical Journal. 36 pages, 9
figures, 1 tabl
Hot Populations in M87 Globular Clusters
We have obtained HST/STIS far- and near-UV photometry of globular clusters in
four fields in the gE galaxy M87. To a limit of m(FUV) = 25 we detect a total
of 66 globular clusters (GCs) in common with the deep HST optical-band study of
Kundu et al. (1999). Despite strong overlap in V- and I-band properties, the
M87 GCs have UV/optical properties that are distinct from clusters in the Milky
Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot HB
populations than do Milky Way analogues. Cluster mass is probably not a factor
in these distinctions. The most metal-rich M87 GCs in our sample are near Z_sun
and overlap the local E galaxy sample in estimated Mg_2 line indices.
Nonetheless, the clusters produce much more UV light at a given Mg_2, being up
to 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appear
to represent a transition between Milky Way-type clusters and E galaxies. The
differences are in the correct sense if the clusters are significantly older
than the E galaxies. Comparisons with Galactic open clusters indicate that the
hot stars lie on the extreme horizontal branch, rather than being blue
stragglers, and that the EHB becomes well populated for ages > 5 Gyr. We find
that 43 of our UV detections have no optical-band counterparts. Most appear to
be UV-bright background galaxies, seen through M87. Eleven NUV variable sources
detected at only one epoch in the central field are probably classical novae.
[Abridged]Comment: 70 pages, 25 figures (including 4 jpgs), 7 tables. To appear in AJ.
Full resolution version available at
http://www.astro.virginia.edu/~rwo/m87/m87-hotpops.pd
Scalable Mining of Common Routes in Mobile Communication Network Traffic Data
A probabilistic method for inferring common routes from mobile communication network traffic data is presented. Besides providing mobility information, valuable in a multitude of application areas, the method has the dual purpose of enabling efficient coarse-graining as well as anonymisation by mapping individual sequences onto common routes. The approach is to represent spatial trajectories by Cell ID sequences that are grouped into routes using locality-sensitive hashing and graph clustering. The method is demonstrated to be scalable, and to accurately group sequences using an evaluation set of GPS tagged data
UV Bright Globular Clusters in M87: More Evidence for Super-helium-rich Stellar Populations?
We study the UV and optical properties of 38 massive GCs in M87, imaged using
the HST/STIS and WFPC2 instruments. The majority of these GCs appear extremely
bright in the FUV - roughly a magnitude brighter than their Galactic
counterparts with similar metallicities. The observed FUV flux is several times
larger than predictions of canonical old stellar population models. These
canonical models, which assume a fiducial helium enrichment parameter, dY/dZ=2,
are able to reproduce the observed FUV fluxes only if ages are 3--5 Gyr larger
than the WMAP age of the Universe, although the same models fit the UV
photometry of Galactic and M31 GCs for ages less than the WMAP age. A similar
discrepancy is found between the mass-weighted and UV-luminosity weighted ages
of the massive Galactic GC omega Cen, whose CMD (including peculiar features on
its well-populated horizontal branch) can be accurately reproduced by invoking
a small super-He-rich (dY/dZ ~ 90) stellar component. By comparison to omega
Cen, we propose that the majority of M87 GCs in our sample contain strong
signatures of similarly minor super-He-rich sub-components. Although we cannot
prove or disprove the extreme helium scenario at the moment, we show that the
same phenomenon that causes the extended horizontal branch of omega Cen
explains the UV brightness of our sample. If this is indeed due to the extreme
helium, this study would be the first to find its signatures in extragalactic
objects.Comment: 19 pages, 10 figures, accepted for publication in MNRA
The F-GAMMA program: Multi-frequency study of Active Galactic Nuclei in the Fermi era. Program description and the first 2.5 years of monitoring
To fully exploit the scientific potential of the Fermi mission, we initiated
the F-GAMMA program. Between 2007 and 2015 it was the prime provider of
complementary multi-frequency monitoring in the radio regime. We quantify the
radio variability of gamma-ray blazars. We investigate its dependence on source
class and examine whether the radio variability is related to the gamma-ray
loudness. Finally, we assess the validity of a putative correlation between the
two bands. The F-GAMMA monitored monthly a sample of about 60 sources at up to
twelve radio frequencies between 2.64 and 228.39 GHz. We perform a time series
analysis on the first 2.5-year dataset to obtain variability parameters. A
maximum likelihood analysis is used to assess the significance of a correlation
between radio and gamma-ray fluxes. We present light curves and spectra
(coherent within ten days) obtained with the Effelsberg 100-m and IRAM 30-m
telescopes. All sources are variable across all frequency bands with amplitudes
increasing with frequency up to rest frame frequencies of around 60 - 80 GHz as
expected by shock-in-jet models. Compared to FSRQs, BL Lacs show systematically
lower variability amplitudes, brightness temperatures and Doppler factors at
lower frequencies, while the difference vanishes towards higher ones. The time
scales appear similar for the two classes. The distribution of spectral indices
appears flatter or more inverted at higher frequencies for BL Lacs. Evolving
synchrotron self-absorbed components can naturally account for the observed
spectral variability. We find that the Fermi-detected sources show larger
variability amplitudes as well as brightness temperatures and Doppler factors,
than non-detected ones. Flux densities at 86.2 and 142.3 GHz correlate with 1
GeV fluxes at a significance level better than 3sigma, implying that gamma rays
are produced very close to the mm-band emission region.Comment: Accepted for publication in section 4. Extragalactic astronomy of
Astronomy and Astrophysics (18 pages, 9 figures
X-ray Absorption Spectroscopy Study of the Effect of Rh Doping in Sr\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e4\u3c/sub\u3e
We investigate the effect of Rh doping in Sr2IrO4 using X-ray absorption spectroscopy (XAS). We observed appearance of new electron-addition states with increasing Rh concentration (x in Sr2Ir1−xRhxO4) in accordance with the concept of hole doping. The intensity of the hole-induced state is however weak, suggesting weakness of charge transfer (CT) effect and Mott insulating ground states. Also, Ir Jeff = 1/2 upper Hubbard band shifts to lower energy as x increases up to x = 0.23. Combined with optical spectroscopy, these results suggest a hybridisation-related mechanism, in which Rh doping can weaken the (Ir Jeff = 1/2)–(O 2p) orbital hybridisation in the in-planar Rh-O-Ir bond networks
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