120 research outputs found
Mass Distribution in the Central Few Parsecs of Our Galaxy
We estimate the enclosed mass profile in the central 10 pc of the Milky Way
by analyzing the infrared photometry and the velocity observations of
dynamically relaxed stellar population in the Galactic center. HST/NICMOS and
Gemini Adaptive Optics images in the archive are used to obtain the number
density profile, and proper motion and radial velocity data were compiled from
the literature to find the velocity dispersion profile assuming a spherical
symmetry and velocity isotropy. From these data, we calculate the enclosed mass
and density profiles in the central 10 pc of the Galaxy using the Jeans
equation. Our improved estimates can better describe the exact evolution of the
molecular clouds and star clusters falling down to the Galactic center, and
constrain the star formation history of the inner part of the Galaxy.Comment: To appear in the Journal of The Korean Astronomical Society, vol. 42,
p. 17 (2009
Reddening Behaviors of Galaxies in the SDSS Photometric System
We analyze the behaviors of reddening vectors in the SDSS photometric system
for galaxies of different morphologies, ages, and redshifts. As seen in other
photometric systems, the dependence of reddening on the spectral energy
distribution (SED) and the nonlinearity of reddening are likewise
non-negligible for the SDSS system if extinction is significant (~> 1 mag).
These behaviors are most significant for the g filter, which has the largest
bandwidth-to-central wavelength ratio among SDSS filters. The SDSS colors
involving adjacent filters show greater SED-dependence and nonlinearity. A
procedure for calculating the correct amount of extinction from an observed
color excess is provided. The relative extinctions between (i.e., the
extinction law for) SDSS filters given by Schlegel et al., which were
calculated with an older version of filter response functions, would
underestimate the amount of extinction in most cases by ~5 to 10 % (maximum ~20
%). We recommend A/A_{5500} values of 1.574, 1.191, 0.876, 0.671 & 0.486 for
the u, g, r, i, & z filters, respectively, as a representative extinction law
for the SDSS galaxies with a small extinction (i.e., for cases where the
nonlinearity and SED-dependence of the reddening is not important). The
dependence of reddening on redshift at low extinction is the largest for colors
involving the g filter as well, which is due to the Balmer break.Comment: Published in PASP, 119, 1449 (Dec. 2007
A Second-order bias model for the Logarithmic Halo Mass Density
We present an analytic model for the local bias of dark matter halos in a
LCDM universe. The model uses the halo mass density instead of the halo number
density and is searched for various halo mass cuts, smoothing lengths, and
redshift epoches. We find that, when the logarithmic density is used, the
second-order polynomial can fit the numerical relation between the halo mass
distribution and the underlying matter distribution extremely well. In this
model the logarithm of the dark matter density is expanded in terms of log halo
mass density to the second order. The model remains excellent for all halo mass
cuts (from M_{cut}=3\times10^{11}3\times10^{12}h^{-1}M_{\odot}R=5h^{-1}50h^{-1}$Mpc), and redshift ranges
(from z=0 to 1.0) considered in this study. The stochastic term in the relation
is found not entirely random, but a part of the term can be determined by the
magnitude of the shear tensor.Comment: 8 pages, 7 figures, accepted for publication on Ap
Low-End Mass Function of the Arches Cluster
The initial mass function (IMF) of the Arches cluster, which was formed a few
million years ago in the harsh environment of the Galactic center (GC), has
long been a target of interest to those who study the GC and the theory of star
formation. The distinct star-forming conditions in the GC might have caused the
cluster to have a shallower slope or an elevated lower mass cutoff in its IMF.
But its mass function has been revealed only down to 1-2 Msun (the lower limit
of resolved stars), and the low- end mass function of the Arches is still
unknown. To estimate the unresolved part of the Arches mass function, we have
devised a novel photometric method that involves the histogram of pixel
intensities in the observed image, which contains information on the
unresolved, faint stars. By comparing the pixel intensity histograms (PIHs) of
numerous artificial images constructed from model IMFs with the observed PIH,
we find that the best-fit model IMF for the Arches cluster has a cutoff mass
less than or similar to 0.1 Msun and a shape very close to that of the Kroupa
MF. Our findings imply that the IMF of the Arches cluster is similar to those
found in the Galactic disk.Comment: Accepted for publication in MNRA
Alternating Direction Implicit Method for Two-Dimensional Fokker-Planck Equation of Dense Spherical Stellar Systems
The Fokker-Planck (FP) model is one of the commonly used methods for studies
of the dynamical evolution of dense spherical stellar systems such as globular
clusters and galactic nuclei. The FP model is numerically stable in most cases,
but we find that it encounters numerical difficulties rather often when the
effects of tidal shocks are included in two-dimensional (energy and angular
momentum space) version of the FP model or when the initial condition is
extreme (e.g., a very large cluster mass and a small cluster radius). To avoid
such a problem, we have developed a new integration scheme for a
two-dimensional FP equation by adopting an Alternating Direction Implicit (ADI)
method given in the Douglas-Rachford split form. We find that our ADI method
reduces the computing time by a factor of ~2 compared to the fully implicit
method, and resolves problems of numerical instability.Comment: Published in J. Korean Astron. Soc., 40, 91 (2007
Topology of Luminous Red Galaxies from the Sloan Digital Sky Survey
We present measurements of the genus topology of luminous red galaxies (LRGs)
from the Sloan Digital Sky Survey (SDSS) Data Release 7 catalog, with
unprecedented statistical significance. To estimate the uncertainties in the
measured genus, we construct 81 mock SDSS LRG surveys along the past light cone
from the Horizon Run 3, one of the largest N-body simulations to date that
evolved 7210^3 particles in a 10815 Mpc/h size box. After carefully modeling
and removing all known systematic effects due to finite pixel size, survey
boundary, radial and angular selection functions, shot noise and galaxy
biasing, we find the observed genus amplitude to reach 272 at 22 Mpc/h
smoothing scale with an uncertainty of 4.2%; the estimated error fully
incorporates cosmic variance. This is the most accurate constraint of the genus
amplitude to date, which significantly improves on our previous results. In
particular, the shape of the genus curve agrees very well with the mean
topology of the SDSS LRG mock surveys in the LCDM universe. However, comparison
with simulations also shows small deviations of the observed genus curve from
the theoretical expectation for Gaussian initial conditions. While these
discrepancies are mainly driven by known systematic effects such as those of
shot noise and redshift-space distortions, they do contain important
cosmological information on the physical effects connected with galaxy
formation, gravitational evolution and primordial non-Gaussianity. We address
here the key role played by systematics on the genus curve, and show how to
accurately correct for their effects to recover the topology of the underlying
matter. In a forthcoming paper, we provide an interpretation of those
deviations in the context of the local model of non-Gaussianity.Comment: 23 pages, 18 figures. APJ Supplement Series 201
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