866 research outputs found
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
Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size
We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey
Data Release 4 to study the systematic effects of specific star formation rate
(SSFR) and galaxy size (as measured by the half light radius, r_h) on the
mass-metallicity relation. We find that galaxies with high SSFR or large r_h
for their stellar mass have systematically lower gas phase-metallicities (by up
to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible
origins for these dependencies, including galactic winds/outflows, abundance
gradients, environment and star formation rate efficiencies.Comment: Accepted by ApJ Letter
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
Pharmacist intervention in primary care to improve outcomes in patients with left ventricular systolic dysfunction
<b>Background</b> Meta-analysis of small trials suggests that pharmacist-led collaborative review and revision of medical treatment may improve outcomes in heart failure.<p></p>
<b>Methods and results</b> We studied patients with left ventricular systolic dysfunction in a cluster-randomized controlled, event driven, trial in primary care. We allocated 87 practices (1090 patients) to pharmacist intervention and 87 practices (1074 patients) to usual care. The intervention was delivered by non-specialist pharmacists working with family doctors to optimize medical treatment. The primary outcome was a composite of death or hospital admission for worsening heart failure. This trial is registered, number ISRCTN70118765. The median follow-up was 4.7 years. At baseline, 86% of patients in both groups were treated with an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. In patients not receiving one or other of these medications, or receiving less than the recommended dose, treatment was started, or the dose increased, in 33.1% of patients in the intervention group and in 18.5% of the usual care group [odds ratio (OR) 2.26, 95% CI 1.64–3.10; P< 0.001]. At baseline, 62% of each group were treated with a β-blocker and the proportions starting or having an increase in the dose were 17.9% in the intervention group and 11.1% in the usual care group (OR 1.76, 95% CI 1.31–2.35; P< 0.001). The primary outcome occurred in 35.8% of patients in the intervention group and 35.4% in the usual care group (hazard ratio 0.97, 95% CI 0.83–1.14; P = 0.72). There was no difference in any secondary outcome.<p></p>
<b>Conclusion</b> A low-intensity, pharmacist-led collaborative intervention in primary care resulted in modest improvements in prescribing of disease-modifying medications but did not improve clinical outcomes in a population that was relatively well treated at baseline
A definitive merger-AGN connection at z~0 with CFIS: mergers have an excess of AGN and AGN hosts are more frequently disturbed
The question of whether galaxy mergers are linked to the triggering of active
galactic nuclei (AGN) continues to be a topic of considerable debate. The issue
can be broken down into two distinct questions: 1) Can galaxy mergers trigger
AGN? 2) Are galaxy mergers the dominant AGN triggering mechanism? A complete
picture of the AGN-merger connection requires that both of these questions are
addressed with the same dataset. In previous work, we have shown that galaxy
mergers selected from the Sloan Digital Sky Survey (SDSS) show an excess of
both optically-selected, and mid-IR colour-selected AGN, demonstrating that the
answer to the first of the above questions is affirmative. Here, we use the
same optical and mid-IR AGN selection to address the second question, by
quantifying the frequency of morphological disturbances in low surface
brightness r-band images from the Canada France Imaging Survey (CFIS). Only ~30
per cent of optical AGN host galaxies are morphologically disturbed, indicating
that recent interactions are not the dominant trigger. However, almost 60 per
cent of mid-IR AGN hosts show signs of visual disturbance, indicating that
interactions play a more significant role in nuclear feeding. Both mid-IR and
optically selected AGN have interacting fractions that are a factor of two
greater than a mass and redshift matched non-AGN control sample, an excess that
increases with both AGN luminosity and host galaxy stellar mass.Comment: Accepted for publication in MNRA
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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