154 research outputs found
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
The role of dust in "active" and "passive" low-metallicity star formation
We investigate the role of dust in star formation activity of extremely
metal-poor blue compact dwarf galaxies (BCDs). Observations suggest that star
formation in BCDs occurs in two different regimes: "active" and "passive". The
"active" BCDs host super star clusters (SSCs), and are characterised by compact
size, rich H2 content, large dust optical depth, and high dust temperature; the
"passive" BCDs are more diffuse with cooler dust, and lack SSCs and large
amounts of H2. By treating physical processes concerning formation of stars and
dust, we are able to simultaneously reproduce all the above properties of both
modes of star formation (active and passive). We find that the difference
between the two regimes can be understood through the variation of the
"compactness" of the star-forming region: an "active" mode emerges if the
region is compact (with radius \la 50 pc) and dense (with gas number density
\ga 500 cm). The dust, supplied from Type II supernovae in a compact
star-forming region, effectively reprocesses the heating photons into the
infrared and induces a rapid H2 formation over a period of several Myr. This
explains the high infrared luminosity, high dust temperature, and large H2
content of active BCDs. Moreover, the gas in "active" galaxies cools (\la 300
K) on a few dynamical timescales, producing a "run-away" star formation episode
because of the favourable (cool) conditions. The mild extinction and relatively
low molecular content of passive BCDs can also be explained by the same model
if we assume a diffuse region (with radius \ga 100 pc and gas number density
\la 100 cm). We finally discuss primordial star formation in
high-redshift galaxies in the context of the "active" and "passive" star
formation scenario.Comment: Astronomy and Astrophysics, in press, 16 pages, 8 figure
The influence of Galactic wind upon the star formation histories of Local Group galaxies
We examine the possibility that ram pressure exerted by the galactic wind
from the Galaxy could have stripped gas from the Local Group dwarf galaxies,
thereby affecting their star formation histories. Whether gas stripping occurs
or not depends on the relative magnitudes of two counteracting forces acting on
gas in a dwarf galaxy: ram pressure force by the wind and the gravitational
binding force by the dwarf galaxy itself. We suggest that the galactic wind
could have stripped gas in a dwarf galaxy located within the distance of
kpc
(where is the surface radius and is the total binding
energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of
Gyr, thereby preventing star formation there. Our result based on this Galactic
wind model explains the recent observation that dwarfs located close to the
Galaxy experienced star formation only in the early phase of their lifetimes,
whereas distant dwarfs are still undergoing star formation. The present star
formation in the Large Magellanic Cloud can also be explained through our
Galactic wind model.Comment: 7 pages LaTeX, no figures, to appear in MNRA
The size--density relation of extragalactic HII regions
We investigate the size--density relation in extragalactic HII regions, with
the aim of understanding the role of dust and different physical conditions in
the ionized medium. First, we compiled several observational data sets for
Galactic and extragalactic HII regions and confirm that extragalactic HII
regions follow the same size (D)--density (n) relation as Galactic ones.
Motivated by the inability of static models to explain this, we then modelled
the evolution of the size--density relation of HII regions by considering their
star formation history, the effects of dust, and pressure-driven expansion. The
results are compared with our sample data whose size and density span roughly
six orders of magnitude. We find that the extragalactic size--density relation
does not result from an evolutionary sequence of HII regions but rather
reflects a sequence with different initial gas densities (``density
hierarchy''). Moreover, the size of many HII regions is limited by dust
absorption of ionizing photons, rather than consumption by ionizing neutral
hydrogen. Dust extinction of ionizing photons is particularly severe over the
entire lifetime of compact HII regions with typical gas densities of greater
than 10^3 cm^{-3}. Hence, as long as the number of ionizing photons is used to
trace massive star formation, much star-formation activity could be missed.
Such compact dense environments, the ones most profoundly obscured by dust,
have properties similar to ``maximum--intensity starbursts''. This implies that
submillimeter and infrared wavelengths may be necessary to accurately assess
star formation in these extreme conditions both locally and at high redshift.Comment: 18 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
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