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Register a new userSimulations of the formation and evolution of isolated dwarf galaxies
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We present new fully self-consistent models of the formation and evolution of isolated dwarf galaxies. We have used the publicly available N-body/SPH code HYDRA, to which we have added a set of star formation criteria, and prescriptions for chemical enrichment (taking into account contributions from both SNIa and SNII), supernova feedback, and gas cooling. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. The star-formation histories of the model galaxies exhibit burst-like behaviour. These bursts are a consequence of the blow-out and subsequent in-fall of gas. The amount of gas that leaves the galaxy for good is found to be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol . For the least massive models, however, this is over 80 per cent of their initial gas mass. The local fluctuations in gas density are strong enough to trigger star-bursts in the massive models, or to inhibit anything more than small residual star formation for the less massive models. Between these star-bursts there can be time intervals of several Gyrs. We have compared model predictions with available data for the relations between luminosity and surface brightness profile, half-light radius, central velocity dispersion, broad band colour (B-V) and metallicity, as well as the location relative to the fundamental plane. The properties of the model dwarf galaxies agree quite well with those of observed dwarf galaxies.
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Using a suite of simulations (Governato et al. 2010) which successfully produce bulgeless (dwarf) disk galaxies, we provide an analysis of their associated cold interstellar media (ISM) and stellar chemical abundance patterns. A preliminary comparison with observations is undertaken, in order to assess whether the properties of the cold gas and chemistry of the stellar components are recovered successfully. To this end, we have extracted the radial and vertical gas density profiles, neutral hydrogen velocity dispersion, and the power spectrum of structure within the ISM. We complement this analysis of the cold gas with a brief examination of the simulations metallicity distribution functions and the distribution of alpha-elements-to-iron.
Simulating dwarf galaxy halos in a reionizing Universe puts severe constraints on the sub-grid model employed in the simulations. Using the same sub-grid model that works for simulations without a UV-background (UVB) results in gas poor galaxies that stop forming stars very early on, except for halos with high masses. This is in strong disagreement with observed galaxies, which are gas rich and star forming down to a much lower mass range. To resolve this discrepancy, we ran a large suite of isolated dwarf galaxy simulations to explore a wide variety of sub-grid models and parameters, including timing and strength of the UVB, strength of the stellar feedback, and metallicity dependent Pop III feedback. We compared these simulations to observed dwarf galaxies by means of the baryonic Tully-Fisher relation (BTFR), which links the baryonic content of a galaxy to the observationally determined strength of its gravitational potential. We found that the results are robust to changes in the UVB. The strength of the stellar feedback shifts the results on the BTFR, but does not help to form gas rich galaxies at late redshifts. Only by including Pop III feedback are we able to produce galaxies that lie on the observational BTFR and that have neutral gas and ongoing star formation at redshift zero.
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.
Like massive galaxies, dwarf galaxies are expected to undergo major mergers with other dwarfs. However, the end state of these mergers and the role that merging plays in regulating dwarf star formation is uncertain. Using imaging from the Hyper Suprime-Cam Subaru Strategic program, we construct a sample of dwarf-dwarf mergers and examine the star formation and host properties of the merging systems. These galaxies are selected via an automated detection algorithm from a sample of 6875 spectroscopically selected isolated dwarf galaxies at $z<0.12$ and $log(M_star/M_odot)<9.6$ from the Galaxy and Mass Assembly (GAMA) and Sloan Digital Sky Survey (SDSS) spectroscopic campaigns. We find a total tidal feature detection fraction of 3.29% (6.1% when considering only galaxies at $z<0.05$). The tidal feature detection fraction rises strongly as a function of star formation activity; 15%-20% of galaxies with extremely high H$alpha$ equivalent width (H$alpha$ EW > 250 Angstrom) show signs of tidal debris. Galaxies that host tidal debris are also systematically bluer than the average galaxy at fixed stellar mass. These findings extend the observed dwarf-dwarf merger sequence with a significant sample of dwarf galaxies, indicating that star formation triggered in mergers between dwarf galaxies continues after coalescence.
We present the results of a spectroscopic survey of 675 bright (16.5<Bj<18) galaxies in a 6 degree field centred on the Fornax cluster with the FLAIR-II spectrograph on the UK Schmidt Telescope. We measured redshifts for 516 galaxies of which 108 were members of the Fornax Cluster. Nine of these are new cluster members previously misidentified as background galaxies. The cluster dynamics show that the dwarf galaxies are still falling into the cluster whereas the giants are virialised. Our spectral data reveal a higher rate of star formation among the dwarf galaxies than suggested by morphological classification: 35 per cent have H-alpha emission indicative of star formation but only 19 per cent were morphologically classified as late-types. The distribution of scale sizes is consistent with evolutionary processes which transform late-type dwarfs to early-type dwarfs. The fraction of dwarfs with active star formation drops rapidly towards the cluster centre. The star-forming dwarfs are concentrated in the outer regions of the cluster, the most extreme in an infalling subcluster. We estimate gas depletion time scales for 5 dwarfs with detected HI emission: these are long (of order 10 Gyr), indicating that active gas removal must be involved if they are transformed into gas-poor dwarfs as they fall further into the cluster. In agreement with our previous results, we find no compact dwarf elliptical (M32-like) galaxies in the Fornax Cluster.
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