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Register a new userNumerical simulations of expanding supershells in dwarf irregular galaxies. II. Formation of giant HI rings
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We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg I. Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of 10^54 ergs. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast K<100 in the HI column density, whereas single hypernova explosions form rings with K<10. Only multiple SNe can reproduce both the size (diameter ~1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the HI ring in Hoolmberg I. High velocity clouds create HI rings that are much smaller in size (< 0.8 kpc) and contrast (K < 4.5) than seen in Holmberg I. We construct model position-velocity (pV) diagrams and find that they can be used to distinguish among different HI ring formation mechanisms. The observed pV-diagrams of Holmberg I are best reproduced by multiple SNe. We conclude that the giant HI ring in Holmberg I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated HI image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated HI image shows a ring of roughly constant HI column density surrounding a deep central depression, whereas in considerably inclined galaxies (i > 45 deg) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.
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We considered the regions of triggered star formation inside kpc-sized HI supershells in three dwarf galaxies: IC 1613, IC 2574 and Holmberg II. The ionized and neutral gas morphology and kinematics were studied based on our observations with scanning Fabry-Perot interferometer at the SAO RAS 6-m telescope and 21 cm archival data of THINGS and LITTLE THINGS surveys. The qualitative analysis of the observational data performed in order to highlight the two questions: why the star formation occurred very locally in the supershells, and how the ongoing star formation in HI supershells rims influence its evolution? During the investigation we discovered the phenomenon never observed before in galaxies IC 2574 and Holmberg II: we found faint giant (kpc-sized) ionized shells in H-alpha and [SII]6717,6731 lines inside the supergiant HI shells.
To study the star formation and feedback mechanism, we simulate the evolution of an isolated dwarf irregular galaxy (dIrr) in a fixed dark matter halo, similar in size to WLM, using a new stellar feedback scheme. We use the new version of our original N-body/smoothed particle chemodynamics code, GCD+, which adopts improved hydrodynamics, metal diffusion between the gas particles and new modelling of star formation and stellar wind and supernovae (SNe) feedback. Comparing the simulations with and without stellar feedback effects, we demonstrate that the collisions of bubbles produced by strong feedback can induce star formation in a more widely spread area. We also demonstrate that the metallicity in star forming regions is kept low due to the mixing of the metal-rich bubbles and the metal-poor inter-stellar medium. Our simulations also suggest that the bubble-induced star formation leads to many counter-rotating stars. The bubble-induced star formation could be a dominant mechanism to maintain star formation in dIrrs, which is different from larger spiral galaxies where the non-axisymmetric structures, such as spiral arms, are a main driver of star formation.
The results of UBV and H alpha imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fit by composite stellar populations which have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities without a major starburst episode. The derived gas depletion timescales are long, typically ~20 Gyr. These results indicate that dwarf irregular galaxies will be able to continue with their slow, but constant, star formation activity for at least another Hubble time. The sample of isolated dIs is compared to a sample of star bursting dwarf galaxies taken from the literature. The star bursting dwarf galaxies have many similar properties; the main difference between these two types of gas-rich dwarf galaxies is that the current star formation is concentrated in the center of the star bursting systems while it is much more distributed in the quiescent dIs. This results in pronounced color gradients for the starbursting dwarf galaxies, while the majority of the quiescent dwarf irregular galaxies have minor or non-existent color gradients. Thus, the combination of low current star formation rates, blue colors, and the lack of significant color gradients indicates that star formation percolates slowly across the disk of normal dwarf galaxies in a quasi-continuous manner.
We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ~10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ~10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ~2). The star formation rate increased dramatically ~6-8 Gyr ago (z ~ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M(HI)/M(stellar), dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.
Conditions for the fragmentation of expanding shells due to gravitational instability are discussed. The self-similar analytical solution is compared with the results of 3-dimensional computer simulations for the expansion into homogeneous media. For realistic galactic disks we show that the formation of fragments is influenced by the amount of energy supply from the final number of young stars in an OB association, the value of the sound speed, the stratification and density of the ambient medium, the galactic differential rotation and the gravitational force perpendicular to the galactic plane. The typical size of gravitationally unstable shells is 1 kpc for an ambient gas density n=1 cm^-3. In thick disk galaxies the fragmentation occurs in nearly the whole shell while in thin disks it is restricted to the galactic equator. Unstable fragments may become molecular and trigger the formation of molecular clouds, and finally new star formation. We conclude that in dwarf galaxies the star formation may propagate in all directions turning the system into a star- burst. Contrary to that, the star formation in spiral galaxies propagates only in some directions in a thin strip near the symmetry plane, basically at the tips of the expanding shell.
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