400 research outputs found
SMASH 1 : A VERY FAINT GLOBULAR CLUSTER DISRUPTING in the OUTER REACHES of the LMC?
We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the Magellanic Stellar History (SMASH), SMASH 1 is a compact (rh=9.1-3.4+5.9pc) and very low luminosity (Mv=-1.0±0.9,Lv=102.3±0.4Lâ ) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of âŒ18.8, i.e., a distance of . Situated at 11.°3 from the LMC in projection, its three-dimensional distance from the Cloud is 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least . Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only âČ19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (Ï”=0.62-0.21+0.17) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.Peer reviewe
The discovery of two extremely low luminosity Milky Way globular clusters
We report the discovery of two extremely low luminosity globular clusters in
the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey
Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory.
The clusters, Koposov 1 and Koposov 2, are located at kpc and
appear to have old stellar populations and luminosities of only
mag. Their observed sizes of pc are well within the expected tidal
limit of 10 pc at that distance. Together with Palomar 1, AM 4 and
Whiting 1, these new clusters are the lowest luminosity globulars orbiting the
Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to
distant branch of the Sagittarius stream. The half-mass relaxation times of
Koposov 1 and 2 are only and Myr respectively (2 orders of
magnitude shorter than the age of the stellar populations), so it would seem
that they have undergone drastic mass segregation. Since they do not appear to
be very concentrated, their evaporation timescales may be as low as . These discoveries show that the structural parameter space of
globular clusters in the Milky Way halo is not yet fully explored. They also
add, through their short remaining survival times, significant direct evidence
for a once much larger population of globular clusters.Comment: accepted for publication in ApJ, minor revision
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (Ό = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy
We report the discovery of an almost complete Einstein ring of diameter 10"
in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data
from the 6m telescope of the Special Astrophysical Observatory reveals that the
deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km/s
and a redshift of 0.444, whilst the source is a star-forming galaxy with a
redshift of 2.379. From its color and luminosity, we conclude that the lens is
an exceptionally massive Luminous Red Galaxy (LRG) with a mass within the
Einstein radius of 5 x 10^12 solar masses. This remarkable system provides a
laboratory for probing the dark matter distribution in LRGs at distances out to
3 effective radii, and studying the properties of high redshift star-forming
galaxies.Comment: ApJ (Letters), in pres
GEMS: Galaxy fitting catalogues and testing parametric galaxy fitting codes
In the context of measuring structure and morphology of intermediate redshift
galaxies with recent HST/ACS surveys, we tune, test, and compare two widely
used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic
models to the light profiles of both simulated and real galaxy data. We find
that fitting accuracy depends sensitively on galaxy profile shape. Exponential
disks are well fit with Sersic models and have small measurement errors,
whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the
large amount of light at large radii. We find that both codes provide reliable
fits and little systematic error, when the effective surface brightness is
above that of the sky. Moreover, both codes return errors that significantly
underestimate the true fitting uncertainties, which are best estimated with
simulations. We find that GIM2D suffers significant systematic errors for
spheroids with close companions owing to the difficulty of effectively masking
out neighboring galaxy light; there appears to be no work around to this
important systematic in GIM2D's current implementation. While this crowding
error affects only a small fraction of galaxies in GEMS, it must be accounted
for in the analysis of deeper cosmological images or of more crowded fields
with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors
because it can simultaneously fit the profiles of multiple companions thereby
deblending their effect on the fit to the galaxy of interest. We find GALFIT's
robustness to nearby companions and factor of >~20 faster runtime speed are
important advantages over GIM2D for analyzing large HST/ACS datasets. Finally
we include our final catalog of fit results for all 41,495 objects detected in
GEMS.Comment: Accepted for publication in ApJS October 2007, v172n2; 25 pages, 16
Figures, 9 Tables; for hi-resolution version, see
http://www.mpia.de/homes/bhaeussl/galaxy_fitting.pdf. For results, catalogues
and files for code-testing, see http://www.mpia.de/GEMS/fitting_paper.htm
Structured star formation in the Magellanic inter-Cloud region
We use a new contiguous imaging survey conducted using the Dark Energy Camera to investigate the distribution and properties of young stellar populations in the Magellanic inter-Cloud region. These young stars are strongly spatially clustered, forming a narrow chain of low-mass associations that trace the densest H I gas in the Magellanic Bridge and extend, in projection, from the SMC to the outer disc of the LMC. The associations in our survey footprint have ages âČ 30 Myr, masses in the range âŒ100-1200 Mâ and very diffuse structures with half-light radii of up to âŒ100 pc. The two most populous are strongly elliptical and aligned to â10°, with the axis joining the centres of the LMC and the SMC. These observations strongly suggest that the young inter-Cloud populations formed in situ, likely due to the compression of gas stripped during the most recent close LMC-SMC encounter. The associations lie at distances intermediate between the two Clouds, and we find no evidence for a substantial distance gradient across the imaged area. Finally, we identify a vast shell of young stars surrounding a central association, that is spatially coincident with a low column density bubble in the H I distribution. The properties of this structure are consistent with a scenario where stellar winds and supernova explosions from massive stars in the central cluster swept up the ambient gas into a shell, triggering a new burst of star formation. This is a prime location for studying stellar feedback in a relatively isolated environment
The Sagittarius Streams in the Southern Galactic Hemisphere
The structure of the Sagittarius stream in the Southern Galactic hemisphere
is analysed with the Sloan Digital Sky Survey Data Release 8. Parallel to the
Sagittarius tidal track, but ~ 10deg away, there is another fainter and more
metal-poor stream. We provide evidence that the two streams follow similar
distance gradients but have distinct morphological properties and stellar
populations. The brighter stream is broader, contains more metal-rich stars and
has a richer colour-magnitude diagram with multiple turn-offs and a prominent
red clump as compared to the fainter stream. Based on the structural properties
and the stellar population mix, the stream configuration is similar to the
Northern "bifurcation". In the region of the South Galactic Cap, there is
overlapping tidal debris from the Cetus Stream, which crosses the Sagittarius
stream. Using both photometric and spectroscopic data, we show that the blue
straggler population belongs mainly to Sagittarius and the blue horizontal
branch stars belong mainly to the Cetus stream in this confused location in the
halo.Comment: accepted to ApJ with minor changes (3 tables added). All-sky color
maps and the movie from the press-release are available as auxiliary files on
arXiv:1111.7042 (and from
http://www.ast.cam.ac.uk/~koposov/files/sgr2011_pr/
Exposing Sgr tidal debris behind the Galactic disc with M giants selected in WISEâ©2MASS
We show that a combination of infrared photometry from WISE and 2MASS surveys can yield highly pure samples of M giant stars. We take advantage of the new WISEn2MASS M giant selection to trace the Sagittarius (Sgr) trailing tail behind the Galactic disc in the direction of the anticentre. The M giant candidates selected via broad-band photometry are confirmed spectroscopically using AAOmega on the Anglo-Australian Telescope in three fields around the extremity of the Sgr trailing tail in the Southern Galactic hemisphere.We demonstrate that at the Sgr longitude (n-ary logical and) â = 204Ă°, the line-of-sight velocities of the trailing tail starts to deviate from the track of the Law & Majewski model, confirming the prediction of Belokurov et al. This discovery serves to substantiate the measurement of low differential orbital precession of the Sgr stream which in turn may imply diminished dark matter content within 100 kpc
Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1
We present results from spectroscopic observations with the Michigan/Magellan
Fiber System (M2FS) of stellar targets along the line of sight to the
newly-discovered `ultrafaint' stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru
1). Based on simultaneous estimates of line-of-sight velocity and
stellar-atmospheric parameters, we identify 8 and 7 stars as probable members
of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to
resolve an internal velocity dispersion of km s
about a mean of km s (solar rest frame), and to
estimate a mean metallicity of [Fe/H]= . These results
place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies,
suggesting a dominant dark matter component with dynamical mass
enclosed within the central
pc, and dynamical mass-to-light ratio
. For Gru 1 we estimate a mean velocity of
km s and a mean metallicity of
[Fe/H]=, but our sample does not resolve Gru 1's
velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic
phase space suggest that their orbits are among the most energetic within
distance kpc. Moreover, their proximity to each other in this space
arises naturally if both objects are trailing the Large Magellanic Cloud.Comment: replaced with ApJ-accepted version, all spectra and data products
(including samples from posterior PDFs) are available at
http://www.andrew.cmu.edu/user/mgwalker/tuc2gru1_dataproducts.tar.g
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