21,318 research outputs found
The Local Group dwarf Leo T: HI on the brink of star formation
We present Giant Meterwave Radio Telescope (GMRT) and Westerbork ynthesis
Radio Telescope (WSRT) observations of the recently discovered Local Group
dwarf galaxy, Leo T. The peak HI column density is measured to be 7x10^20
cm^-2, and the total HI mass is 2.8Xx10^5 Msun, based on a distance of 420 kpc.
Leo T has both cold (~ 500 K) and warm (~ 6000 K) HI at its core, with a global
velocity dispersion of 6.9 km/s, from which we derive a dynamical mass within
the HI radius of 3.3x10^6 Msun, and a mass-to-light ratio of greater than 50.
We calculate the Jeans mass from the radial profiles of the HI column density
and velocity dispersion, and predict that the gas should be globally stable
against star formation. This finding is inconsistent with the half light radius
of Leo T, which extends to 170 pc, and indicates that local conditions must
determine where star formation takes place. Leo T is not only the lowest
luminosity galaxy with on-going star formation discovered to date, it is also
the most dark matter dominated, gas-rich dwarf in the Local Group.Comment: 6 pages, 7 figures, accepted for publication in MNRAS on November
15th 2007, full resolution version at:
http://www.ast.cam.ac.uk/~eryan/leot.pdf . Typographical error in sound speed
equation has led to a new Figure 6 and minor changes to the tex
Spitzer and HHT observations of starless cores: masses and environments
We present Spitzer observations of a sample of 12 starless cores selected to
have prominent 24 micron shadows. The Spitzer images show 8 and 24 micron
shadows and in some cases 70 micron shadows; these spatially resolved
absorption features trace the densest regions of the cores. We have carried out
a 12CO (2-1) and 13CO (2-1) mapping survey of these cores with the Heinrich
Hertz Telescope (HHT). We use the shadow features to derive optical depth maps.
We derive molecular masses for the cores and the surrounding environment; we
find that the 24 micron shadow masses are always greater than or equal to the
molecular masses derived in the same region, a discrepancy likely caused by CO
freeze--out onto dust grains. We combine this sample with two additional cores
that we studied previously to bring the total sample to 14 cores. Using a
simple Jeans mass criterion we find that ~ 2/3 of the cores selected to have
prominent 24 micron shadows are collapsing or near collapse, a result that is
supported by millimeter line observations. Of this subset at least half have
indications of 70 micron shadows. All cores observed to produce absorption
features at 70 micron are close to collapse. We conclude that 24 micron
shadows, and even more so the 70 micron ones, are useful markers of cloud cores
that are approaching collapse.Comment: 41 pages, 28 figures, 5 tables; accepted by Ap
On the assembly of the Milky Way dwarf satellites and their common mass scale
We use a particle tagging technique to dynamically populate the N-body Via
Lactea II high-resolution simulation with stars. The method is calibrated using
the observed luminosity function of Milky Way satellites and the concentration
of their stellar populations, and self-consistently follows the accretion and
disruption of progenitor dwarfs and the build-up of the stellar halo in a
cosmological "live host". Simple prescriptions for assigning stellar
populations to collisionless particles are able to reproduce many properties of
the observed Milky Way halo and its surviving dwarf satellites, like velocity
dispersions, sizes, brightness profiles, metallicities, and spatial
distribution. Our model predicts the existence of approximately 1,850 subhalos
harboring "extremely faint" satellites (with mass-to-light ratios >5,000) lying
beyond the Sloan Digital Sky Survey detection threshold. Of these, about 20 are
"first galaxies", i.e. satellites that formed a stellar mass above 10 Msun
before redshift 9. The ten most luminous satellites (L> 1e6 Lsun) in the
simulation are hosted by subhalos with peak circular velocities today in the
range V_max=10-40 km/s that have shed between 80% and 99% of their dark mass
after being accreted at redshifts 1.7< z <4.6. The satellite maximum circular
velocity and stellar line-of-sight velocity dispersion today follow the
relation V_max=2.2 sigma_los. We apply a standard mass estimation algorithm
based on Jeans modelling of the line-of-sight velocity dispersion profiles to
the simulated dwarf spheroidals, and test the accuracy of this technique. The
inner (within 300 pc) mass-luminosity relation for currently detectable
satellites is nearly flat in our model, in qualitative agreement with the
"common mass scale" found in Milky Way dwarfs. We do, however, predict a weak,
but significant positive correlation for these objects: M_300 ~L^{0.088 \pm
0.024}.Comment: 14 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Nova-like Cataclysmic Variables in the Infrared
Novalike cataclysmic variables have persistently high mass transfer rates and prominent steady state accretion disks. We present an analysis of infrared observations of twelve novalikes obtained from the Two Micron All Sky Survey, the Spitzer Space Telescope, and the Wide-field Infrared Survey Explorer All Sky Survey. The presence of an infrared excess at >3-5 microns over the expectation of a theoretical steady state accretion disk is ubiquitous in our sample. The strength of the infrared excess is not correlated with orbital period, but shows a statistically significant correlation (but shallow trend) with system inclination that might be partially (but not completely) linked to the increasing view of the cooler outer accretion disk and disk rim at higher inclinations. We discuss the possible origin of the infrared excess in terms of emission from bremsstrahlung or circumbinary dust, with either mechanism facilitated by the mass outflows (e.g., disk wind/corona, accretion stream overflow, and so on) present in novalikes. Our comparison of the relative advantages and disadvantages of either mechanism for explaining the observations suggests that the situation is rather ambiguous, largely circumstantial, and in need of stricter observational constraints.Peer reviewe
Hierarchical Fragmentation and Jet-like Outflows in IRDC G28.34+0.06, a Growing Massive Protostar Cluster
We present Submillimeter Array (SMA) \lambda = 0.88mm observations of an
infrared dark cloud (IRDC) G28.34+0.06. Located in the quiescent southern part
of the G28.34 cloud, the region of interest is a massive (\,\msun)
molecular clump P1 with a luminosity of \lsun, where our previous
SMA observations at 1.3mm have revealed a string of five dust cores of 22-64
\msun\ along the 1 pc IR-dark filament. The cores are well aligned at a
position angle of 48 degrees and regularly spaced at an average projected
separation of 0.16 pc. The new high-resolution, high-sensitivity 0.88\,mm image
further resolves the five cores into ten compact condensations of 1.4-10.6
\msun, with sizes a few thousands AU. The spatial structure at clump (
pc) and core ( pc) scales indicates a hierarchical fragmentation.
While the clump fragmentation is consistent with a cylindrical collapse, the
observed fragment masses are much larger than the expected thermal Jeans
masses. All the cores are driving CO(3-2) outflows up to 38 km/s, majority of
which are bipolar, jet-like outflows. The moderate luminosity of the P1 clump
sets a limit on the mass of protostars of 3-7 \msun. Because of the large
reservoir of dense molecular gas in the immediate medium and ongoing accretion
as evident by the jet-like outflows, we speculate that P1 will grow and
eventually form a massive star cluster. This study provides a first glimpse of
massive, clustered star formation that currently undergoes through an
intermediate-mass stage.Comment: 24 pages, 4 figures, 4 tables, accepted to Ap
The origin of fast molecular outflows in quasars: molecule formation in AGN-driven galactic winds
We explore the origin of fast molecular outflows that have been observed in
Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is
difficult to create such an outflow by accelerating existing molecular clouds
in the host galaxy, as the clouds will be destroyed before they can reach the
high velocities that are observed. In this work, we consider an alternative
scenario where molecules form in-situ within the AGN outflow. We present a
series of hydro-chemical simulations of an isotropic AGN wind interacting with
a uniform medium. We follow the time-dependent chemistry of 157 species,
including 20 molecules, to determine whether molecules can form rapidly enough
to produce the observed molecular outflows. We find H outflow rates up to
140 M yr, which is sensitive to density, AGN luminosity, and
metallicity. We compute emission and absorption lines of CO, OH and warm (a few
hundred K) H from the simulations in post-processing. The CO-derived
outflow rates and OH absorption strengths at solar metallicity agree with
observations, although the maximum line of sight velocities from the model CO
spectra are a factor 2 lower than is observed. We derive a CO (1-0) to
H conversion factor of = 0.13 M (K km
s pc), 6 times lower than is commonly assumed in observations
of such systems. We find strong emission from the mid-infrared lines of H.
The mass of H traced by this infrared emission is within a few per cent of
the total H mass. This H emission may be observable by JWST.Comment: 30 pages, 21 figures (including appendices), resubmitted to MNRAS
following referee's report. Some results have changed from the previous
version, in particular for warm H2 emission (see Figs. 5 and 13
Spherical Jeans analysis for dark matter indirect detection in dwarf spheroidal galaxies - Impact of physical parameters and triaxiality
Dwarf spheroidal (dSph) galaxies are among the most promising targets for the
indirect detection of dark matter (DM) from annihilation and/or decay products.
Empirical estimates of their DM content - and hence the magnitudes of expected
signals - rely on inferences from stellar-kinematic data. However, various
kinematic analyses can give different results and it is not obvious which are
most reliable. Using extensive sets of mock data of various sizes (mimicking
'ultra-faint' and 'classical' dSphs) and an MCMC engine, here we investigate
biases, uncertainties, and limitations of analyses based on parametric
solutions to the spherical Jeans equation. For a variety of functional forms
for the tracer and DM density profiles, as well as the orbital anisotropy
profile, we examine reliability of estimates for the astrophysical J- and
D-factors for annihilation and decay, respectively. For large (N > 1000)
stellar-kinematic samples typical of 'classical' dSphs, errors tend to be
dominated by systematics, which can be reduced through the use of sufficiently
general and flexible functional forms. For small (N < 100) samples typical of
'ultrafaints', statistical uncertainties tend to dominate systematic errors and
flexible models are less necessary. We define an optimal strategy that would
mitigate sensitivity to priors and other aspects of analyses based on the
spherical Jeans equation. We also find that the assumption of spherical
symmetry can bias estimates of J (with the 95% credibility intervals not
encompassing the true J-factor) when the object is mildly triaxial (axis ratios
b/a = 0.8, c/a = 0.6). A concluding table summarises the typical error budget
and biases for the different sample sizes considered.Comment: 21 pages, 20 figures. Minor changes (several clarifications): match
the MNRAS accepted versio
Unified Dark Matter models with fast transition
We investigate the general properties of Unified Dark Matter (UDM) fluid
models where the pressure and the energy density are linked by a barotropic
equation of state (EoS) and the perturbations are adiabatic. The
EoS is assumed to admit a future attractor that acts as an effective
cosmological constant, while asymptotically in the past the pressure is
negligible. UDM models of the dark sector are appealing because they evade the
so-called "coincidence problem" and "predict" what can be interpreted as
, but in general suffer the effects of a non-negligible
Jeans scale that wreak havoc in the evolution of perturbations, causing a large
Integrated Sachs-Wolfe effect and/or changing structure formation at small
scales. Typically, observational constraints are violated, unless the
parameters of the UDM model are tuned to make it indistinguishable from
CDM. Here we show how this problem can be avoided, studying in detail
the functional form of the Jeans scale in adiabatic UDM perturbations and
introducing a class of models with a fast transition between an early
Einstein-de Sitter CDM-like era and a later CDM-like phase. If the
transition is fast enough, these models may exhibit satisfactory structure
formation and CMB fluctuations. To consider a concrete case, we introduce a toy
UDM model and show that it can predict CMB and matter power spectra that are in
agreement with observations for a wide range of parameter values.Comment: 30 pages, 15 figures, JHEP3 style, typos corrected; it matches the
published versio
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