40 research outputs found
Black hole mass measurement using molecular gas kinematics: what ALMA can do
We study the limits of the spatial and velocity resolution of radio
interferometry to infer the mass of supermassive black holes (SMBHs) in
galactic centres using the kinematics of circum-nuclear molecular gas, by
considering the shapes of the galaxy surface brightness profile,
signal-to-noise ratios (S/Ns) of the position-velocity diagram (PVD) and
systematic errors due to the spatial and velocity structure of the molecular
gas. We argue that for fixed galaxy stellar mass and SMBH mass, the spatial and
velocity scale that need to be resolved increase and decrease, respectively,
with decreasing \sersic\ index of the galaxy surface brightness profile. We
validate our arguments using simulated PVDs for varying beam size and velocity
channel width. Furthermore, we consider the systematic effects on the inference
of the SMBH mass by simulating PVDs including the spatial and velocity
structure of the molecular gas, which demonstrates that their impacts are not
significant for a PVD with good S/N unless the spatial and velocity scale
associated with the systematic effects are comparable to or larger than the
angular resolution and velocity channel width of the PVD from pure circular
motion. Also, we caution that a bias in a galaxy surface brightness profile
owing to the poor resolution of a galaxy photometric image can largely bias the
SMBH mass by an order of magnitude. This study shows the promise and the limit
of ALMA observations for measuring SMBH mass using molecular gas kinematics and
provides a useful technical justification for an ALMA proposal with the science
goal of measuring SMBH mass.Comment: MNRAS publishe
Spinning Nanoparticles Impacted by C-shock: Implications for Radio-millimeter Emission from Star-forming Regions
We investigate the impact of anomalous microwave emission (AME) on the
radio-millimeter spectral energy distribution for three typical interstellar
medium (ISM) conditions surrounding star-forming regions -- cold neutral
medium, warm neutral medium, and photodissociation region -- by comparing the
emissivities of three major contributors: free-free, thermal dust emission, and
AME. In particular, for spinning nanoparticles (i.e., potential carriers of
AME), we consider a known grain destruction mechanism due to a centrifugal
force from spin-up processes caused by collisions between dust grains and
supersonic neutral streams in a magnetized shock (C-shock). We demonstrate
that, if the ISM in a magnetic field is impacted by a C-shock developed by a
supernova explosion in the early phase of massive star-formation (
Myr), AME can be significantly or almost entirely suppressed relative to
free-free and thermal dust continuum emission if the grain tensile strength is
small enough. This study may shed light on explaining the rare observations of
AME from extragalactic star-forming regions preferentially observed from
massive star clusters and suggest a scenario of "the rise and fall of AME" in
accordance with the temporal evolution of star-forming regions.Comment: 20 pages, 8 figures, Accepted for publication in Ap
New insight on galaxy structure from GALPHAT I. Motivation, methodology, and benchmarks for Sersic models
We introduce a new galaxy image decomposition tool, GALPHAT (GALaxy
PHotometric ATtributes), to provide full posterior probability distributions
and reliable confidence intervals for all model parameters. GALPHAT is designed
to yield a high speed and accurate likelihood computation, using grid
interpolation and Fourier rotation. We benchmark this approach using an
ensemble of simulated Sersic model galaxies over a wide range of observational
conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the PSF
and the image size, and errors in the assumed PSF; and a range of structural
parameters: the half-light radius and the Sersic index . We
characterise the strength of parameter covariance in Sersic model, which
increases with S/N and , and the results strongly motivate the need for the
full posterior probability distribution in galaxy morphology analyses and later
inferences.
The test results for simulated galaxies successfully demonstrate that, with a
careful choice of Markov chain Monte Carlo algorithms and fast model image
generation, GALPHAT is a powerful analysis tool for reliably inferring
morphological parameters from a large ensemble of galaxies over a wide range of
different observational conditions. (abridged)Comment: Submitted to MNRAS. The submitted version with high resolution
figures can be downloaded from
http://www.astro.umass.edu/~iyoon/GALPHAT/galphat1.pd
A remarkably simple and accurate method for computing the Bayes Factor from a Markov chain Monte Carlo Simulation of the Posterior Distribution in high dimension
Weinberg (2012) described a constructive algorithm for computing the marginal
likelihood, Z, from a Markov chain simulation of the posterior distribution.
Its key point is: the choice of an integration subdomain that eliminates
subvolumes with poor sampling owing to low tail-values of posterior
probability. Conversely, this same idea may be used to choose the subdomain
that optimizes the accuracy of Z. Here, we explore using the simulated
distribution to define a small region of high posterior probability, followed
by a numerical integration of the sample in the selected region using the
volume tessellation algorithm described in Weinberg (2012). Even more promising
is the resampling of this small region followed by a naive Monte Carlo
integration. The new enhanced algorithm is computationally trivial and leads to
a dramatic improvement in accuracy. For example, this application of the new
algorithm to a four-component mixture with random locations in 16 dimensions
yields accurate evaluation of Z with 5% errors. This enables Bayes-factor model
selection for real-world problems that have been infeasible with previous
methods.Comment: 14 pages, 3 figures, submitted to Bayesian Analysi
Comparative study between N-body and Fokker-Planck simulations for rotating star clusters: I. Equal-mass system
We have carried out N-body simulations for rotating star clusters with equal
mass and compared the results with Fokker-Planck models. These two different
approaches are found to produce fairly similar results, although there are some
differences with regard to the detailed aspects. We confirmed the acceleration
of the core collapse of a cluster due to an initial non-zero angular momentum
and found a similar evolutionary trend in the central density and velocity
dispersion in both simulations. The degree of acceleration depends on the
initial angular momentum. Angular momentum is being lost from the cluster due
to the evaporation of stars with a large angular momentum on a relaxation time
scale.Comment: 11 pages, 13 eps figures, accepted for the publication of MNRA
The HI content of dark matter halos at from ALFALFA
We combine information from the clustering of HI galaxies in the 100% data
release of the Arecibo Legacy Fast ALFA survey (ALFALFA), and from the HI
content of optically-selected galaxy groups found in the Sloan Digital Sky
Survey (SDSS) to constrain the relation between halo mass and its average
total HI mass content . We model the abundance and clustering of
neutral hydrogen through a halo-model-based approach, parametrizing the relation as a power law with an exponential mass cutoff. To break the
degeneracy between the amplitude and low-mass cutoff of the
relation, we also include a recent measurement of the cosmic HI abundance from
the .100 sample. We find that all datasets are consistent with a
power-law index and a cutoff halo mass . We compare these results with
predictions from state-of-the-art magneto-hydrodynamical simulations, and find
both to be in good qualitative agreement, although the data favours a
significantly larger cutoff mass that is consistent with the higher cosmic HI
abundance found in simulations. Both data and simulations seem to predict a
similar value for the HI bias () and shot-noise power
() at redshift .Comment: 17 pages, 11 figures. Comments welcom
Why Post-Starburst Galaxies are Now Quiescent
Post-starburst or "E+A" galaxies are rapidly transitioning from star-forming
to quiescence. While the current star formation rate of post-starbursts is
already at the level of early type galaxies, we recently discovered that many
have large CO-traced molecular gas reservoirs consistent with normal star
forming galaxies. These observations raise the question of why these galaxies
have such low star formation rates. Here we present an ALMA search for the
denser gas traced by HCN (1--0) and HCO+ (1--0) in two CO-luminous, quiescent
post-starburst galaxies. Intriguingly, we fail to detect either molecule. The
upper limits are consistent with the low star formation rates and with
early-type galaxies. The HCN/CO luminosity ratio upper limits are low compared
to star-forming and even many early type galaxies. This implied low dense gas
mass fraction explains the low star formation rates relative to the CO-traced
molecular gas and suggests the state of the gas in post-starburst galaxies is
unusual, with some mechanism inhibiting its collapse to denser states. We
conclude that post-starbursts galaxies are now quiescent because little dense
gas is available, in contrast to the significant CO-traced lower density gas
reservoirs that still remain.Comment: accepted for publication in Ap