86 research outputs found
Physical Properties of Complex C Halo Clouds
Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey
of the tail of Complex C are presented and the halo clouds associated with this
complex cataloged. The properties of the Complex C clouds are compared to
clouds cataloged at the tail of the Magellanic Stream to provide insight into
the origin and destruction mechanism of Complex C. Magellanic Stream and
Complex C clouds show similarities in their mass distributions (slope = -0.7
and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative
of a warm component), which may indicate a common origin and/or physical
process breaking down the clouds. The clouds cataloged at the tail of Complex C
extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to
10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median
pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase
structure in Complex C, possibly due to its low metallicity and inefficient
cooling compared to other halo clouds. From assuming the Complex C clouds are
in pressure equilibrium with a hot halo medium, we find a median halo density
of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a
z-height of ~3 kpc. Using the same argument for the Stream results in a median
halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are
consistent with previous observational constraints and cosmological
simulations. We also assess the derived cloud and halo properties with three
dimensional grid simulations of halo HI clouds and find the temperature is
generally consistent within a factor of 1.5 and the volume densities, pressures
and halo densities are consistent within a factor of 3.Comment: Accepted for publication in AJ. 54 pages, including 6 tables and 16
figure
Constraints on the pMSSM from LAT Observations of Dwarf Spheroidal Galaxies
We examine the ability for the Large Area Telescope (LAT) to constrain
Minimal Supersymmetric Standard Model (MSSM) dark matter through a combined
analysis of Milky Way dwarf spheroidal galaxies. We examine the Lightest
Supersymmetric Particles (LSPs) for a set of ~71k experimentally valid
supersymmetric models derived from the phenomenological-MSSM (pMSSM). We find
that none of these models can be excluded at 95% confidence by the current
analysis; nevertheless, many lie within the predicted reach of future LAT
analyses. With two years of data, we find that the LAT is currently most
sensitive to light LSPs (m_LSP < 50 GeV) annihilating into tau-pairs and
heavier LSPs annihilating into b-bbar. Additionally, we find that future LAT
analyses will be able to probe some LSPs that form a sub-dominant component of
dark matter. We directly compare the LAT results to direct detection
experiments and show the complementarity of these search methods.Comment: 24 pages, 9 figures, submitted to JCA
The GALFA-HI Compact Cloud Catalog
We present a catalog of 1964 isolated, compact neutral hydrogen clouds from
the Galactic Arecibo L-Band Feed Array Survey Data Release One (GALFA-HI DR1).
The clouds were identified by a custom machine-vision algorithm utilizing
Difference of Gaussian kernels to search for clouds smaller than 20'. The
clouds have velocities typically between |VLSR| = 20-400 km/s, linewidths of
2.5-35 km/s, and column densities ranging from 1 - 35 x 10^18 cm^-2. The
distances to the clouds in this catalog may cover several orders of magnitude,
so the masses may range from less than a Solar mass for clouds within the
Galactic disc, to greater than 10^4 Solar Masses for HVCs at the tip of the
Magellanic Stream. To search for trends, we separate the catalog into five
populations based on position, velocity, and linewidth: high velocity clouds
(HVCs); galaxy candidates; cold low velocity clouds (LVCs); warm, low
positive-velocity clouds in the third Galactic Quadrant; and the remaining warm
LVCs. The observed HVCs are found to be associated with previously-identified
HVC complexes. We do not observe a large population of isolated clouds at high
velocities as some models predict. We see evidence for distinct histories at
low velocities in detecting populations of clouds corotating with the Galactic
disc and a set of clouds that is not corotating.Comment: 34 Pages, 9 Figures, published in ApJ (2012, ApJ, 758, 44), this
version has the corrected fluxes and corresponding flux histogram and masse
Small-Scale Structure in the SDSS and LCDM: Isolated L* Galaxies with Bright Satellites
We use a volume-limited spectroscopic sample of isolated galaxies in the
Sloan Digital Sky Survey (SDSS) to investigate the frequency and radial
distribution of luminous (M_r <~ -18.3) satellites like the Large Magellanic
Cloud (LMC) around ~L* Milky Way analogs and compare our results
object-by-object to LCDM predictions based on abundance matching in
simulations. We show that 12% of Milky Way-like galaxies host an LMC-like
satellite within 75 kpc (projected), and 42 % within 250 kpc (projected). This
implies ~10% have a satellite within the distance of the LMC, and ~40% of L*
galaxies host a bright satellite within the virialized extent of their dark
matter halos. Remarkably, the simulation reproduces the observed frequency,
radial dependence, velocity distribution, and luminosity function of observed
secondaries exceptionally well, suggesting that LCDM provides an accurate
reproduction of the observed Universe to galaxies as faint as L~10^9 Lsun on
~50 kpc scales. When stacked, the observed projected pairwise velocity
dispersion of these satellites is sigma~160 km/s, in agreement with
abundance-matching expectations for their host halo masses. Finally, bright
satellites around L* primaries are significantly redder than typical galaxies
in their luminosity range, indicating that environmental quenching is operating
within galaxy-size dark matter halos that typically contain only a single
bright satellite. This redness trend is in stark contrast to the Milky Way's
LMC, which is unusually blue even for a field galaxy. We suggest that the LMC's
discrepant color might be further evidence that it is undergoing a triggered
star-formation event upon first infall.Comment: 14 pages, 11 figures; accepted to Ap
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
Compact HI clouds from the GALFA-HI survey
The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the
entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR),
at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin.
The unprecedented resolution and sensitivity of the GALFA-HI survey have
resulted in the detection of numerous isolated, very compact HI clouds at low
Galactic velocities, which are distinctly separated from the HI disk emission.
In the limited area of ~4600 deg surveyed so far, we have detected 96 of
such compact clouds. The detected clouds are cold with a median T
(the kinetic temperature in the case in which there is no non-thermal
broadening) of 300 K. Moreover, these clouds are quite compact and faint, with
median values of 5 arcmin in angular size, 0.75 K in peak brightness
temperature, and cm in HI column density. Most of the
clouds deviate from Galactic rotation at the 20-30 km/s level, and a
significant fraction show evidence for a multiphase medium and velocity
gradients. No counterparts for these clouds were found in other wavebands. From
the modeling of spatial and velocity distributions of the whole compact cloud
population, we find that the bulk of the compact clouds are related to the
Galactic disk, and their distances are likely to be in the range of 0.1 to a
few kpc. We discuss various possible scenarios for the formation and
maintenance of this cloud population and its significance for Galactic ISM
studies.Comment: Accepted for publication in the Astrophysical Journa
Local Group Dwarf Spheroidals: Correlated Deviations from the Baryonic Tully-Fisher Relation
Local Group dwarf spheroidal satellite galaxies are the faintest
extragalactic stellar systems known. We examine recent data for these objects
in the plane of the Baryonic Tully-Fisher Relation (BTFR). While some dwarf
spheroidals adhere to the BTFR, others deviate substantially. We examine the
residuals from the BTFR and find that they are not random. The residuals
correlate with luminosity, size, metallicity, ellipticity, and susceptibility
of the dwarfs to tidal disruption in the sense that fainter, more elliptical,
and tidally more susceptible dwarfs deviate farther from the BTFR. These
correlations disfavor stochastic processes and suggest a role for tidal
effects. We identify a test to distinguish between the {\Lambda}CDM and MOND
based on the orbits of the dwarf satellites of the Milky Way and how stars are
lost from them.Comment: 16 pages, 7 figures. Submitted to ApJ. Revised in response to referee
repor
Clues from nearby galaxies to a better theory of cosmic evolution
The great advances in the network of cosmological tests show that the
relativistic Big Bang theory is a good description of our expanding universe.
But the properties of nearby galaxies that can be observed in greatest detail
suggest a still better theory would more rapidly gather matter into galaxies
and groups of galaxies. This happens in theoretical ideas now under discussion.Comment: published in Natur
Modeling the Physical Structure of the Low Density Pre-protostellar Core Lynds 1498
Lynds 1498 is a pre-protostellar core (PPC) and was one of the initial
objects toward which molecular depletion and differentiation was detected.
Despite the considerable scrutiny of L1498, there has not been a extensive
study of the density and temperature structure as derived from radiative
transfer modeling of dust continuum observations. We present deep SCUBA
observations of L1498 at 850 and 450 micron, high resolution BEARS maps of the
N2H+ 1-0 transition, CSO observations of the N2H+ 3-2 transition, and GBT
observations of the C3S 4-3 transition. We also present a comparison of derived
properties between L1498 and nearby PPCs that have been observed at
far-infrared and submillimeter wavelengths. We present a more realistic
treatment of PPC heating which varies the strength of the ISRF, Sisrf, and
includes attenuation of the ISRF due to dust grains at the outer radius of the
core, Av. The best-fitted model consists of a Bonner-Ebert sphere with a
central density of 1 - 3 x 10^4 cm-3, R_o ~ 0.29 pc, 0.5 <= Sisrf <= 1, Av ~ 1
mag, and a nearly isothermal temperature profile of ~ 10.5 K for OH8 opacities.
C3S emission shows a central depletion hole while N2H+ emission is centrally
peaked. The observed depletions of C3S and H2CO, the modest N2H+ abundance, and
a central density that is an order of magnitude lower than other modeled PPCs
suggests that L1498 may be a forming PPC. Our derived temperature and density
profile will improve modeling of molecular line observations that will
explicate the core's kinematical and chemical state. (abridged)Comment: 26 pages, 10 figures (2 color figs). Accepted to Ap
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