1,186 research outputs found
Stellar Kinematics of the Andromeda II Dwarf Spheroidal Galaxy
We present kinematical profiles and metallicity for the M31 dwarf spheroidal
(dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy
of 531 red giant branch stars. Our kinematical sample is among the largest for
any M31 satellite and extends out to two effective radii (r_eff = 5.3' = 1.1
kpc). We find a mean systemic velocity of -192.4+-0.5 km/s and an average
velocity dispersion of sigma_v = 7.8+-1.1 km/s. While the rotation velocity
along the major axis of And II is nearly zero (<1 km/s), the rotation along the
minor axis is significant with a maximum rotational velocity of v_max=8.6+-1.8
km/s. We find a kinematical major axis, with a maximum rotational velocity of
v_max=10.9+-2.4 km/s, misaligned by 67 degrees to the isophotal major axis. And
II is thus the first dwarf galaxy with evidence for nearly prolate rotation
with a v_max/sigma_v = 1.1, although given its ellipticity of epsilon = 0.10,
this object may be triaxial. We measured metallicities for a subsample of our
data, finding a mean metallicity of [Fe/H] = -1.39+- 0.03 dex and an internal
metallicity dispersion of 0.72+-0.03 dex. We find a radial metallicity gradient
with metal-rich stars more centrally concentrated, but do not observe a
significant difference in the dynamics of two metallicity populations. And II
is the only known dwarf galaxy to show minor axis rotation making it a unique
system whose existence offers important clues on the processes responsible for
the formation of dSphs.Comment: 14 pages, 10 figures, 4 tables, accepted for publication in Ap
Emergency Operations for Bleeding Duodenal Ulcer:A simple option to consider: Case Report
We report a 46 years-old man who had severe bleeding from a posterior
duodenal ulcer (DU) that was diagnosed but could not be treated endoscopically
in another health facility. He went into shock as he was being admitted to the
casualty at Khartoum North Teaching Hospital (KNTH). His haemoglobin (Hb)
dropped to five gram/dl. He required resuscitation and transfusion of six units of
blood overnight. Emergency surgery was performed. Over sewing (OS) of the ulcer was done and the stenosed first part of duodenum was closed transversely (pyloroplasty). No acid-reducing procedure (ARP) was done. The patient received anti-
helicobacter therapy via the intravenous route preoperatively and continued postoperatively. This was later given orally after he started taking by mouth. He made an uneventful recovery with no recurrence of bleeding and was discharged home one week latter. Endoscopy was done at KNTH six weeks later. This showed complete healing of the ulcer with no evidence of Helicobacter pylori in the biopsies taken. We found simple OS of the bleeding DU together with anti-helicobacter therapy safe, efficient, and not associated with re-bleeding. We discuss the rationale of this simple treatment. We
propose the need for a randomized controlled study comparing it with acid-reducing procedure (ARP) as options in the surgical treatment of bleeding DU. Keywords: Bleeding duodenal ulcer, Emergency surgical operations, Peptic ulcer, Helicobacter pylori, Anti-helicobacter therapy, Sudan Journal of Medical Science Vol. 3 (4) 2008: pp. 339-34
A Megacam Survey of Outer Halo Satellites. IV. Two foreground populations possibly associated with the Monoceros substructure in the direction of NGC2419 and Koposov2
The origin of the Galactic halo stellar structure known as the Monoceros ring
is still under debate. In this work, we study that halo substructure using deep
CFHT wide-field photometry obtained for the globular clusters NGC2419 and
Koposov2, where the presence of Monoceros becomes significant because of their
coincident projected position. Using Sloan Digital Sky Survey photometry and
spectroscopy in the area surrounding these globulars and beyond, where the same
Monoceros population is detected, we conclude that a second feature, not likely
to be associated with Milky Way disk stars along the line-of-sight, is present
as foreground population. Our analysis suggests that the Monoceros ring might
be composed of an old stellar population of age t ~ 9Gyr and a new component ~
4Gyr younger at the same heliocentric distance. Alternatively, this detection
might be associated with a second wrap of Monoceros in that direction of the
sky and also indicate a metallicity spread in the ring. The detection of such a
low-density feature in other sections of this halo substructure will shed light
on its nature.Comment: 10 pages, 10 figures, accepted for publication in Ap
The Baryon Content of Extremely Low Mass Dwarf Galaxies
We investigate the gas content and baryonic Tully-Fisher relationship for
extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >
Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists
of 101 galaxies for which we have obtained follow-up HI observations using the
Arecibo Observatory and Green Bank Telescope. This represents the largest
homogeneous sample of dwarfs at low luminosities with well-measured HI and
optical properties. The sample spans a range of environments, from dense groups
to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,
significantly exceeding that of typical gas-rich galaxies at higher
luminosities. Dwarf galaxies are therefore less efficient at turning gas into
stars over their lifetimes. The strong environmental dependence of the gas
fraction distribution demonstrates that while internal processes can reduce the
gas fractions to roughly f_gas=0.4, external processes are required to fully
remove gas from a dwarf galaxy. The average rotational velocity of our sample
is vrot=50 km/s. Including more massive galaxies from the literature, we fit a
baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope
compares well with CDM models that assume an equal baryon to dark matter ratio
at all masses. While gas stripping or other processes may modify the baryon to
dark matter ratio for dwarfs in the densest environments, the majority of dwarf
galaxies in our sample have not preferentially lost significant baryonic mass
relative to more massive galaxies.Comment: 33 pages, 8 figures. Accepted to ApJ. Data available at
http://www.ociw.edu/~mgeha/researc
On time dilation in quasar light curves
In this paper we set out to measure time dilation in quasar light curves. In
order to detect the effects of time dilation, sets of light curves from two
monitoring programmes are used to construct Fourier power spectra covering
timescales from 50 days to 28 years. Data from high and low redshift samples
are compared to look for the changes expected from time dilation. The main
result of the paper is that quasar light curves do not show the effects of time
dilation. Several explanations are discussed, including the possibility that
time dilation effects are exactly offset by an increase in timescale of
variation associated with black hole growth, or that the variations are caused
by microlensing in which case time dilation would not be expected.Comment: 8 pages, 5 figures. Accepted for publication in MNRAS. Published
online 9 April 2010
Accurate masses for dispersion-supported galaxies
We derive an accurate mass estimator for dispersion-supported stellar systems
and demonstrate its validity by analyzing resolved line-of-sight velocity data
for globular clusters, dwarf galaxies, and elliptical galaxies. Specifically,
by manipulating the spherical Jeans equation we show that the dynamical mass
enclosed within the 3D deprojected half-light radius r_1/2 can be determined
with only mild assumptions about the spatial variation of the stellar velocity
dispersion anisotropy. We find M_1/2 = 3 \sigma_los^2 r_1/2 / G ~ 4
\sigma_los^2 R_eff / G, where \sigma_los^2 is the luminosity-weighted square of
the line-of-sight velocity dispersion and R_eff is the 2D projected half-light
radius. While deceptively familiar in form, this formula is not the virial
theorem, which cannot be used to determine accurate masses unless the radial
profile of the total mass is known a priori. We utilize this finding to show
that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent
with having formed within a halo of mass approximately 3 x 10^9 M_sun in Lambda
CDM cosmology. The faintest MW dSphs seem to have formed in dark matter halos
that are at least as massive as those of the brightest MW dSphs, despite the
almost five orders of magnitude spread in luminosity. We expand our analysis to
the full range of observed dispersion-supported stellar systems and examine
their I-band mass-to-light ratios (M/L). The M/L vs. M_1/2 relation for
dispersion-supported galaxies follows a U-shape, with a broad minimum near M/L
~ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to
M/L ~ 3,200 for ultra-faint dSphs, and a more shallow rise to M/L ~ 800 for
galaxy cluster spheroids.Comment: 20 pages, 13 figures. Accepted to MNRAS on March 27th, 201
Multiple Chemodynamic Stellar Populations of the Ursa Minor Dwarf Spheroidal Galaxy
We present a Bayesian method to identify multiple (chemodynamic) stellar
populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity,
and positional stellar data without the assumption of spherical symmetry. We
apply this method to a new Keck/DEIMOS spectroscopic survey of the Ursa Minor
(UMi) dSph. We identify 892 likely members, making this the largest UMi sample
with line-of-sight velocity and metallicity measurements. Our Bayesian method
detects two distinct chemodynamic populations with high significance
(). The metal-rich () population is
kinematically colder (radial velocity dispersion of ) and more centrally concentrated than the metal-poor () and kinematically hotter population (). Furthermore, we apply the same analysis to
an independent MMT/Hectochelle data set and confirm the existence of two
chemodynamic populations in UMi. In both data sets, the metal-rich population
is significantly flattened () and the metal-poor
population is closer to spherical (). Despite
the presence of two populations, we are unable to robustly estimate the slope
of the dynamical mass profile. We found hints for prolate rotation of order
in the MMT data set, but further observations
are required to verify this. The flattened metal-rich population invalidates
assumptions built into simple dynamical mass estimators, so we computed new
astrophysical dark matter annihilation (J) and decay profiles based on the
rounder, hotter metal-poor population and inferred
for the Keck
data set. Our results paint a more complex picture of the evolution of Ursa
Minor than previously discussed.Comment: 20 pages, 11 figures, data included. Comments welcome. Accepted to
MNRA
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