717 research outputs found
Two-size approximation: a simple way of treating the evolution of grain size distribution in galaxies
Full calculations of the evolution of grain size distribution in galaxies are
in general computationally heavy. In this paper, we propose a simple model of
dust enrichment in a galaxy with a simplified treatment of grain size
distribution by imposing a `two-size approximation'; that is, all the grain
population is represented by small (grain radius a < 0.03 micron) and large (a
> 0.03 micron) grains. We include in the model dust supply from stellar ejecta,
destruction in supernova shocks, dust growth by accretion, grain growth by
coagulation and grain disruption by shattering, considering how these processes
work on the small and large grains. We show that this simple framework
reproduces the main features found in full calculations of grain size
distributions as follows. The dust enrichment starts with the supply of large
grains from stars. At a metallicity level referred to as the critical
metallicity of accretion, the abundance of the small grains formed by
shattering becomes large enough to rapidly increase the grain abundance by
accretion. Associated with this epoch, the mass ratio of the small grains to
the large grains reaches the maximum. After that, this ratio converges to the
value determined by the balance between shattering and coagulation, and the
dust-to-metal ratio is determined by the balance between accretion and shock
destruction. With a Monte Carlo simulation, we demonstrate that the simplicity
of our model has an advantage in predicting statistical properties. We also
show some applications for predicting observational dust properties such as
extinction curves.Comment: 14 pages, 12 figures, accepted for publication in MNRA
Observational Test of Environmental Effects on The Local Group Dwarf Spheroidal Galaxies
In this paper, we examine whether tidal forces exerted by the Galaxy or M31
have an influence on the Local Group dwarf spheroidal galaxies (dSphs) which
are their companions. We focus on the surface brightness profiles of the dSphs,
especially their core radii because it is suggested based on the numerical
simulations that tidal disturbance can make core radii extended. We examine the
correlation for the dSphs between the distances from their parent galaxy (the
Galaxy or M31) and the compactnesses of their surface brightness profiles by
using a parameter ``C'' defined newly in this paper. Consequently, we find no
significant correlation. We make some remarks on the origin of this result by
considering three possible scenarios; tidal picture, dark matter picture, and
heterogeneity of the group of dSphs, each of which has been often discussed to
understand fundamental properties and formation processes of dSphs.Comment: 14 pages LaTeX, 2 PostScript figures, to appear in ApJ Letter
Molecular hydrogen in damped Ly-alpha systems: clues to interstellar physics at high-redshift
In order to interpret H2 (molecular hydrogen) quasar absorption line
observations of damped Ly-alpha systems (DLAs) and sub-DLAs, we model their H2
abundance as a function of dust-to-gas ratio, including H2 self-shielding and
dust extinction against dissociating photons. Then, we constrain the physical
state of gas by using H2 data. Using H2 excitation data for DLA with H2
detections, we derive a gas density 1.5 < log n [cm^-3] < 2.5, temperature 1.5
< log T [K] < 3, and internal UV radiation field (in units of the Galactic
value) 0.5 < log \chi < 1.5. We then find that the observed relation between
molecular fraction and dust-to-gas ratio of the sample is naturally explained
by the above conditions. However, it is still possible that H2 deficient DLAs
and sub-DLAs with H2 fractions less than ~ 10^-6 are in a more diffuse and
warmer state. The efficient photodissociation by the internal UV radiation
field explains the extremely small H2 fraction (< 10^-6) observed for \kappa <
1/30 (\kappa is the dust-to-gas ratio in units of the Galactic value); H2
self-shielding causes a rapid increase and the large variations of H2 abundance
for \kappa > 1/30. We finally propose an independent method to estimate the
star formation rates of DLAs from H2 abundances; such rates are then critically
compared with those derived from other proposed methods. The implications for
the contribution of DLAs to the cosmic star formation history are briefly
discussed.Comment: 15 pages, 5 figures, accepted for publication in MNRA
- …