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$^{-3}$). 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$^{-3}$). 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 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

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 $R_{c}\simeq 120(r_{s}/1 {kpc})^{3/2} ({\cal E}_{b}/10^{50} {erg})^{-1/2}$ kpc (where $r_{s}$ is the surface radius and ${\cal E}_{b}$ 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