543 research outputs found
A comprehensive Maximum Likelihood analysis of the structural properties of faint Milky Way satellites
We derive the structural parameters of the recently discovered very low
luminosity Milky Way satellites through a Maximum Likelihood algorithm applied
to SDSS data. For each satellite, even when only a few tens of stars are
available down to the SDSS flux limit, the algorithm yields robust estimates
and errors for the centroid, position angle, ellipticity, exponential
half-light radius and number of member stars. This latter parameter is then
used in conjunction with stellar population models of the satellites to derive
their absolute magnitudes and stellar masses, accounting for `CMD shot-noise'.
We find that faint systems are somewhat more elliptical than initially found
and ascribe that to the previous use of smoothed maps which can be dominated by
the smoothing kernel. As a result, the faintest half of the Milky Way dwarf
galaxies (M_V>-7.5) is significantly (4-sigma) flatter (e=0.47+/-0.03) than its
brightest half (M_V<-7.5, e=0.32+/-0.02). From our best models, we also
investigate whether the seemingly distorted shape of the satellites, often
taken to be a sign of tidal distortion, can be quantified. We find that, except
for tentative evidence of distortion in CVnI and UMaII, these can be completely
accounted for by Poisson scatter in the sparsely sampled systems. We consider
three scenarios that could explain the rather elongated shape of faint
satellites: rotation supported systems, stars following the shape of more
triaxial dark matter subhalos, or elongation due to tidal interaction with the
Milky Way. Although none of these is entirely satisfactory, the last one
appears the least problematic, but warrants much deeper observations to track
evidence of such tidal interaction.Comment: 20 pages, 11 figures, ApJ in press; some typos corrected, magnitude
of BooII corrected (thanks go to Shane Walsh for spotting the erroneous
original value
The Tidal Evolution of Local Group Dwarf Spheroidals
(Abridged) We use N-body simulations to study the evolution of dwarf
spheroidal galaxies (dSphs) driven by galactic tides. We adopt a
cosmologically-motivated model where dSphs are approximated by a King model
embedded within an NFW halo. We find that these NFW-embedded King models are
extraordinarily resilient to tides; the stellar density profile still resembles
a King model even after losing more than 99% of the stars. As tides strip the
galaxy, the stellar luminosity, velocity dispersion, central surface
brightness, and core radius decrease monotonically. Remarkably, we find that
the evolution of these parameters is solely controlled by the total amount of
mass lost from within the luminous radius. Of all parameters, the core radius
is the least affected: after losing 99% of the stars, R_c decreases by just a
factor of ~2. Interestingly, tides tend to make dSphs more dark-matter
dominated because the tightly bound central dark matter ``cusp'' is more
resilient to disruption than the ``cored'' King profile. We examine whether the
extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have
been caused by tidal stripping of once brighter systems. Although dSph tidal
evolutionary tracks parallel the observed scaling relations in the
luminosity-radius plane, they predict too steep a change in velocity dispersion
compared with the observational estimates hitherto reported in the literature.
The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems
like Fornax, Draco, or Sagittarius. Despite spanning four decades in
luminosity, dSphs appear to inhabit halos of comparable peak circular velocity,
lending support to scenarios that envision dwarf spheroidals as able to form
only in halos above a certain mass threshold.Comment: 17 pages, 12 figs., accepted by Ap
Long-term health outcomes after exposure to repeated concussion in elite level: rugby union players
Background: There is continuing concern about effects of concussion in athletes, including risk of the neurodegenerative disease chronic traumatic encephalopathy. However, information on long-term health and wellbeing in former athletes is limited.
Method: Outcome after exposure to repeated brain injury was investigated in 52 retired male Scottish international rugby players (RIRP) and 29 male controls who were similar in age and social deprivation. Assessment included history of playing rugby and traumatic brain injury, general and mental health, life stress, concussion symptoms, cognitive function, disability and markers of chronic stress (allostatic load).
Results: The estimated number of concussions in RIRP averaged 14 (median=7; IQR 5-40). Performance was poorer in RIRP than controls on a test of verbal learning (p=0.022) and of fine co-ordination of the dominant hand (p=0.038) and not significantly different on other cognitive tests (p>0.05). There were no significant associations between number of concussions and performance on cognitive tests. Other than a higher incidence of cardiovascular disease in controls, no group differences were detected in general or mental health or estimates of allostatic load. In RIRP, persisting symptoms attributed to concussion were more common if reporting more than nine concussions (p=0.028), although these symptoms were not perceived to affect social or work functioning.
Conclusions: Despite a high number of concussions in RIRP, differences in mental health, social or work functioning were not found late after injury. Subtle group differences were detected on two cognitive tests, the cause of which is uncertain. Prospective group comparison studies on representative cohorts are required
The Andromeda Stream
The existence of a stream of tidally stripped stars from the Sagittarius
Dwarf galaxy demonstrates that the Milky Way is still in the process of
accreting mass. More recently, an extensive stream of stars has been uncovered
in the halo of the Andromeda galaxy (M31), revealing that it too is
cannibalizing a small companion. This paper reports the recent observations of
this stream, determining it spatial and kinematic properties, and tracing its
three-dimensional structure, as well as describing future observations and what
we may learn about the Andromeda galaxy from this giant tidal stream.Comment: 3 Pages. Refereed contribution to the 5th Galacto Chemodynamics
conference held in Swinburne, July 2003. Accepted for publication in PAS
A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII
We present the first spectroscopic analysis of the faint M31 satellite
galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data
for two further faint companions, And IX and AndXII. By combining data obtained
using the DEIMOS spectrograph mounted on the Keck II telescope with deep
photometry from the Suprime-Cam instrument on Subaru, we have calculated global
properties for the dwarfs, such as systemic velocities, metallicites and
half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both
photometrically and spectroscopically, from their stacked spectrum), and as
such, they continue to follow the luminosity-metallicity relationship
established with brighter dwarfs. We are unable to resolve a dispersion for And
XI due to small sample size and low S/N, but we set a one sigma upper limit of
sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity
dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and
derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4
(+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss
each satellite in the context of the Mateo relations for dwarf spheroidal
galaxies, and the Universal halo profiles established for Milky Way dwarfs
(Walker et al. 2009). For both galaxies, this sees them fall below the
Universal halo profiles of Walker et al. (2009). When combined with the
findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites
are twice as extended (in terms of both half-light and tidal radii) as their
Milky Way counterparts, these results suggest that the satellite population of
the Andromeda system could inhabit halos that are significantly different from
those of the Milky Way in terms of their central densities (abridged).Comment: 26 pages, 18 figures, MNRAS submitte
Towards Precision Photometry with Extremely Large Telescopes: the Double Subgiant Branch of NGC 1851
The Extremely Large Telescopes currently under construction have a collecting
area that is an order of magnitude larger than the present largest optical
telescopes. For seeing-limited observations the performance will scale as the
collecting area but, with the successful use of adaptive optics, for many
applications it will scale as (where is the diameter of the primary
mirror). Central to the success of the ELTs, therefore, is the successful use
of multi-conjugate adaptive optics (MCAO) that applies a high degree correction
over a field of view larger than the few arcseconds that limits classical
adaptive optics systems. In this letter, we report on the analysis of crowded
field images taken on the central region of the Galactic globular cluster NGC
1851 in band using GeMS at the Gemini South telescope, the only
science-grade MCAO system in operation. We use this cluster as a benchmark to
verify the ability to achieve precise near-infrared photometry by presenting
the deepest photometry in crowded fields ever obtained from the ground.
We construct a colour-magnitude diagram in combination with the F606W band from
HST/ACS. As well as detecting the "knee" in the lower main sequence at
, we also detect the double subgiant branch of NGC 1851, that
demonstrates the high photometric accuracy of GeMS in crowded fields.Comment: Accepted for publication in ApJL (3 Sep 2015). A version of the paper
with high-res images is available at
http://www.astro.uvic.ca/~alan/ms_arxiv_hr.pd
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