70 research outputs found
Discovery of a new Galactic bona fide luminous blue variable with Spitzer
We report the discovery of a circular mid-infrared shell around the emission-line star Wray 16-
137 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of
Wray 16-137 with the Southern African Large Telescope revealed a rich emission spectrum
typical of the classical luminous blue variables (LBVs) like P Cygni. Subsequent spectroscopic
and photometric observations showed drastic changes in the spectrum and brightness during
the last three years, meaning that Wray 16-137 currently undergoes an S Dor-like outburst.
Namely, we found that the star has brightened by ≈1 mag in the V and Ic bands, while its
spectrum became dominated by Fe II lines. Taken together, our observations unambiguously
show that Wray 16-137 is a new member of the family of Galactic bona fide LBVs
Dwarf galaxies beyond our doorstep: the Centaurus A group
The study of dwarf galaxies in groups is a powerful tool for investigating
galaxy evolution, chemical enrichment and environmental effects on these
objects. Here we present results obtained for dwarf galaxies in the Centaurus A
complex, a dense nearby (~4 Mpc) group that contains two giant galaxies and
about 30 dwarf companions of different morphologies and stellar contents. We
use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive
physical properties and evolutionary histories from the resolved stellar
populations of these dwarf galaxies. In particular, for early-type dwarfs we
are able to construct metallicity distribution functions, find population
gradients and quantify the intermediate-age star formation episodes. For
late-type dwarfs, we compute recent (~1 Gyr) star formation histories and study
their stellar distribution. We then compare these results with properties of
the dwarfs in our Milky Way and in other groups. Our work will ultimately lead
to a better understanding of the evolution of dwarf galaxies.Comment: 6 pages, 5 figures; to appear in the proceedings of the conference "A
Universe of dwarf galaxies" (Lyon, June 14-18, 2010
Evidence for multiple populations in intermediate age cluster Lindsay 1 in the SMC
Lindsay 1 is an intermediate age (≈8 Gyr) massive cluster in the Small Magellanic Cloud (SMC). Using VLT FORS2 spectra of 16 probable cluster members on the lower RGB of the cluster, we measure CN and CH band strengths (at ≃ 3883 and 4300 Å respectively), along with carbon and nitrogen abundances and find that a sub-population of stars has significant nitrogen enrichment. A lack of spread in carbon abundances excludes evolutionary mixing as the source of this enrichment, so we conclude that this is evidence of multiple populations. Therefore, L1 is the youngest cluster to show such variations, implying that the process triggering the onset of multiple populations must operate until at least redshift ∼1
A Low Latitude Halo Stream around the Milky Way
We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180 deg to l = 227 deg; the ring could encircle the Galaxy. The low Galactic latitude structure is at a fairly constant distance of kpc from the Galactic Center above the Galactic plane, and has kpc in the region sampled below the Galactic plane. The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 deg of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b) = (198,-27), 22 km/s at (l,b) = (225, 28), 30 km/s at (l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure co-rotates with the Galactic disk stars at km/s. The narrow measured velocity dispersion is inconsistent with power law spheroid or thick disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l,b) = (165, -55) degrees. We interpret our measurements as evidence for a tidally disrupted satellite of to solar masses which rings the Galaxy
Observed Properties of Dark Matter: dynamical studies of dSph galaxies
The Milky Way satellite dwarf spheroidal (dSph) galaxies are the smallest
dark matter dominated systems in the universe. We have underway dynamical
studies of the dSph to quantify the shortest scale lengths on which Dark Matter
is distributed, the range of Dark Matter central densities, and the density
profile(s) of DM on small scales. Current results suggest some surprises: the
central DM density profile is typically cored, not cusped, with scale sizes
never less than a few hundred pc; the central densities are typically 10-20
GeV/cc; no galaxy is found with a dark mass halo less massive than ~5.10^7
M_sun. We are discovering many more dSphs, which we are analysing to test the
generality of these results.Comment: Presented at UCLA Dark Matter 2006 conference, March 2006. In press
in Nuclear Physics
Kinematic modelling of the Milky Way using the RAVE and GCS stellar surveys
We investigate the kinematic parameters of the Milky Way disc using the RAVE
and GCS stellar surveys. We do this by fitting a kinematic model to the data
taking the selection function of the data into account. For stars in the GCS we
use all phase-space coordinates, but for RAVE stars we use only . Using MCMC technique, we investigate the full posterior distributions
of the parameters given the data. We investigate the `age-velocity dispersion'
relation for the three kinematic components
(), the radial dependence of the velocity
dispersions, the Solar peculiar motion (), the
circular speed at the Sun and the fall of mean azimuthal motion with
height above the mid-plane. We confirm that the Besan\c{c}on-style Gaussian
model accurately fits the GCS data, but fails to match the details of the more
spatially extended RAVE survey. In particular, the Shu distribution function
(DF) handles non-circular orbits more accurately and provides a better fit to
the kinematic data. The Gaussian distribution function not only fits the data
poorly but systematically underestimates the fall of velocity dispersion with
radius. We find that correlations exist between a number of parameters, which
highlights the importance of doing joint fits. The large size of the RAVE
survey, allows us to get precise values for most parameters. However, large
systematic uncertainties remain, especially in and . We
find that, for an extended sample of stars, is underestimated by as
much as if the vertical dependence of the mean azimuthal motion is
neglected. Using a simple model for vertical dependence of kinematics, we find
that it is possible to match the Sgr A* proper motion without any need for
being larger than that estimated locally by surveys like GCS.Comment: 27 pages, 13 figures, accepted for publication in Ap
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Finding, Characterizing, and Classifying Variable Sources in Multi-epoch Sky Surveys: QSOs and RR Lyrae in PS1 3Ï€ data
In area and depth, the Pan-STARRS1 (PS1) 3π survey is unique among many-epoch, multi-band surveys and has enormous potential for the all-sky identification of variable sources. PS1 has observed the sky typically seven times in each of its five bands (grizy) over 3.5 years, but unlike SDSS, not simultaneously across the bands. Here we develop a new approach for quantifying statistical properties of non-simultaneous, sparse, multi-color light curves through light curve structure functions, effectively turning PS1 into a ~35-epoch survey. We use this approach to estimate variability amplitudes and timescales (ωr, τ) for all point sources brighter than rP1 = 21.5 mag in the survey. With PS1 data on SDSS Stripe 82 as "ground truth," we use a Random Forest Classifier to identify QSOs and RR Lyrae based on their variability and their mean PS1 and WISE colors. We find that, aside from the Galactic plane, QSO and RR Lyrae samples of purity ~75% and completeness ~92% can be selected. On this basis we have identified a sample of ~1,000,000 QSO candidates, as well as an unprecedentedly large and deep sample of ~150,000 RR Lyrae candidates with distances from ~10 to ~120 kpc. Within the Draco dwarf spheroidal, we demonstrate a distance precision of 6% for RR Lyrae candidates. We provide a catalog of all likely variable point sources and likely QSOs in PS1, a total of 25.8 × 106 sources
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