752 research outputs found
Physical association between the high-affinity IgG receptor (Fc gamma RI) and the gamma subunit of the high-affinity IgE receptor (Fc epsilon RI gamma).
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
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
Structure and Dynamics of the Globular Cluster Palomar 13
We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of Ï = 2.2 ± 0.4 km s^(â1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is Ï = 0.7^(+0.6)_(â0.5) km s^(â1). Combining our DEIMOS data with literature values, our final velocity dispersion is Ï = 0.4^(+0.4)_( â0.3) km s^(â1). We determine a spectroscopic metallicity of [Fe/H] = â1.6 ± 0.1 dex, placing a 1Ï upper limit of Ï_([Fe/H]) ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V = â2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (ÎŁ α r^η, η = â2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M_(1/2) = 1.3^(+2:7)_(â1.3) Ă 10^3 M_â and a mass-to-light ratio of M/L_V = 2.4^(+5.0)_(â2.4) M_â/L_â. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics
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
A MegaCam Survey of Outer Halo Satellites. VI: The Spatially Resolved Star Formation History of the Carina Dwarf Spheroidal Galaxy
We present the spatially resolved star formation history (SFH) of the Carina
dwarf spheroidal galaxy, obtained from deep, wide-field g,r imaging and a
metallicity distribution from the literature. Our photometry covers
deg, reaching up to times the half-light radius of Carina with a
completeness higher than at , more than one magnitude fainter
than the oldest turnoff. This is the first time a combination of depth and
coverage of this quality has been used to derive the SFH of Carina, enabling us
to trace its different populations with unprecedented accuracy. We find that
Carina's SFH consists of two episodes well separated by a star formation
temporal gap. These episodes occurred at old ( Gyr) and intermediate
(- Gyr) ages. Our measurements show that the old episode comprises the
majority of the population, accounting for of the stellar mass
within times the King tidal radius, while the total stellar mass derived
for Carina is , and the stellar
mass-to-light ratio . The SFH derived is consistent with no recent
star formation which hints that the observed blue plume is due to blue
stragglers. We conclude that the SFH of Carina evolved independently of the
tidal field of the Milky Way, since the frequency and duration of its star
formation events do not correlate with its orbital parameters. This result is
supported by the age/metallicity relation observed in Carina, and the gradients
calculated indicating that outer regions are older and more metal poor.Comment: Accepted in ApJ (22 pages, 13 figures
A MegaCam Survey of Outer Halo Satellites. VII. A Single S\'ersic Index v/s Effective Radius Relation for Milky Way Outer Halo Satellites
In this work we use structural properties of Milky Way's outer halo () satellites (dwarf spheroidal galaxies, ultra-faint dwarf
galaxies and globular clusters) derived from deep, wide-field and homogeneous
data, to present evidence of a correlation in the S\'ersic index v/s effective
radius plane followed by a large fraction of outer halo globular clusters and
satellite dwarf galaxies. We show that this correlation can be entirely
reproduced by fitting empirical relations in the central surface brightness v/s
absolute magnitude and S\'ersic index v/s absolute magnitude parameter spaces,
and by assuming the existence of two types of outer halo globular clusters: one
of high surface brightness (HSB group), with properties similar to inner halo
clusters; and another of low surface brightness (LSB group), which share
characteristics with dwarf spheroidal and ultra-faint dwarf galaxies. Given the
similarities of LSB clusters with dwarf spheroidal and ultra-faint dwarf
galaxies, we discuss the possibility that outer halo clusters also originated
inside dark matter halos and that tidal forces from different galaxy host's
potentials are responsible for the different properties between HSB and LSB
clusters.Comment: 20 pages, 9 figures, 3 table
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