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Register a new userSimulating the Formation of the Local Galaxy Population
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H. Mathis
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We simulate the formation and evolution of the local galaxy population starting from initial conditions with a smoothed linear density field which matches that derived from the IRAS 1.2 Jy galaxy survey. Our simulations track the formation and evolution of all dark matter haloes more massive than 10e+11 solar masses out to a distance of 8000 km/s from the Milky Way. We implement prescriptions similar to those of Kauffmann et al. (1999) to follow the assembly and evolution of the galaxies within these haloes. We focus on two variants of the CDM cosmology: an LCDM and a tCDM model. Galaxy formation in each is adjusted to reproduce the I-band Tully-Fisher relation of Giovanelli et al. (1997). We compare the present-day luminosity functions, colours, morphology and spatial distribution of our simulated galaxies with those of the real local population, in particular with the Updated Zwicky Catalog, with the IRAS PSCz redshift survey, and with individual local clusters such as Coma, Virgo and Perseus. We also use the simulations to study the clustering bias between the dark matter and galaxies of differing type. Although some significant discrepancies remain, our simulations recover the observed intrinsic properties and the observed spatial distribution of local galaxies reasonably well. They can thus be used to calibrate methods which use the observed local galaxy population to estimate the cosmic density parameter or to draw conclusions about the mechanisms of galaxy formation. To facilitate such work, we publically release our z=0 galaxy catalogues, together with the underlying mass distribution.
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In this review I will describe a number of recent advances in extragalactic astronomy. First of all I will describe our current best estimates of the star formation history of the Universe. Then I will describe measurements of local galaxies and their stellar populations, concentrating on measurements of the luminosity functions and stellar population compositions of the different kinds of galaxies. Finally, I will investigate the relationship between these two sets of results. The ultimate aim is to tell at what stage in the history of the Universe the different stars seen in the local galaxies formed. At present much is known but there are significant uncertainties and I will highlight some prospects for the future.
We present a suite of 15 cosmological zoom-in simulations of isolated dark matter halos, all with masses of $M_{rm halo} approx 10^{10},{rm M}_odot$ at $z=0$, in order to understand the relationship between halo assembly, galaxy formation, and feedbacks effects on the central density structure in dwarf galaxies. These simulations are part of the Feedback in Realistic Environments (FIRE) project and are performed at extremely high resolution. The resultant galaxies have stellar masses that are consistent with rough abundance matching estimates, coinciding with the faintest galaxies that can be seen beyond the virial radius of the Milky Way ($M_star/{rm M}_odotapprox 10^5-10^7$). This non-negligible spread in stellar mass at $z=0$ in halos within a narrow range of virial masses is strongly correlated with central halo density or maximum circular velocity $V_{rm max}$. Much of this dependence of $M_star$ on a second parameter (beyond $M_{rm halo}$) is a direct consequence of the $M_{rm halo}sim10^{10},{rm M}_odot$ mass scale coinciding with the threshold for strong reionization suppression: the densest, earliest-forming halos remain above the UV-suppression scale throughout their histories while late-forming systems fall below the UV-suppression scale over longer periods and form fewer stars as a result. In fact, the latest-forming, lowest-concentration halo in our suite fails to form any stars. Halos that form galaxies with $M_stargtrsim2times10^{6},{rm M}_odot$ have reduced central densities relative to dark-matter-only simulations, and the radial extent of the density modifications is well-approximated by the galaxy half-mass radius $r_{1/2}$. This apparent stellar mass threshold of $M_star approx 2times 10^{6} approx 2times 10^{-4} ,M_{rm halo}$ is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in LCDM.
We have made the first detailed study of the high-frequency radio-source population in the local universe, using a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey identified with galaxies from the 6dF Galaxy Survey (6dFGS). The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity, allowing us to make the first measurement of the local radio luminosity function at 20 GHz. Our sample includes some classical FR-1 and FR-2 radio galaxies, but most of the AT20G-6dFGS galaxies host compact (FR-0) radio AGN which appear lack extended radio emission even at lower frequencies. Most of these FR-0 sources show no evidence for relativistic beaming, and the FR-0 class appears to be a mixed population which includes young Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio galaxies. We see a strong dichotomy in the Wide-field Infrared Survey Explorer (WISE) mid-infrared colours of the host galaxies of FR-1 and FR-2 radio sources, with the FR-1 systems found almost exclusively in WISE `early-type galaxies and the FR-2 radio sources in WISE `late-type galaxies. The host galaxies of the flat- and steep-spectrum radio sources have a similar distribution in both K--band luminosity and WISE colours, though galaxies with flat-spectrum sources are more likely to show weak emission lines in their optical spectra. We conclude that these flat-spectrum and steep-spectrum radio sources mainly represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.
The star formation history of the dE NGC 185, together with its spatial variations, has been investigated using new ground-based $H_alpha$ and $BVI$ photometry, and synthetic color--magnitude diagrams (CMDs). We find that the bulk of the stars were formed in NGC 185 at an early epoch of its evolution. After that, the star formation proceeded at a low rate until the recent past, the age of the most recent traces of star formation activity detected in the galaxy being some 100 Myr. The star formation rate, $psi(t)$ for old and intermediate ages shows a gradient in the sense of taking smaller values for higher galactocentric radii. Moreover, recent star formation is detected in the central $150 times 90$ pc$^2$ only, where the youngest, 100 Myr old population is found. The luminous blue {it stars} discovered by Baade (1951) in the center of NGC 185 are discussed using new CCD images in $B$ and Baades original photographic plates, reaching the conclusion that most of them are in fact star clusters. A consistent picture arises in which the gas observed in the central region of NGC 185 would have an internal origin. The rate at which evolved stars return gas to the ISM is enough to seed the recent star formation observed in the center of the galaxy and the SN rate is probably low enough to allow the galaxy to retain the gas not used in the new stellar generations.
We study the properties of tidal disruption event (TDE) host galaxies in the context of a catalog of ~500,000 galaxies from the Sloan Digital Sky Survey. We explore whether selection effects can account for the overrepresentation of TDEs in E+A/post-starburst galaxies by creating matched galaxy samples. Accounting for possible selection effects due to black hole (BH) mass, redshift completeness, strong AGN presence, bulge colors, and surface brightness can reduce the apparent overrepresentation of TDEs in E+A host galaxies by a factor of ~4 (from ~$times$100-190 to ~$times$25-48), but cannot fully explain the preference. We find that TDE host galaxies have atypical photometric properties compared to similar, typical galaxies. In particular, TDE host galaxies tend to live in or near the green valley between star-forming and passive galaxies, and have bluer bulge colors ($Delta (g-r) approx 0.3$ mag), lower half-light surface brightnesses (by ~1 mag/arcsec$^2$), higher Sersic indices ($Delta n_{rm g} approx 3$), and higher bulge-to-total-light ratios ($Delta B/T approx 0.5$) than galaxies with matched BH masses. We find that TDE host galaxies appear more centrally concentrated and that all have high galaxy Sersic indices and $B/T$ fractions---on average in the top 10% of galaxies of the same BH mass---suggesting a higher nuclear stellar density. We identify a region in Sersic index and BH mass parameter space that contains ~2% of our reference catalog galaxies but $ge!60%$ of TDE host galaxies. The unique photometric properties of TDE host galaxies may be useful for selecting candidate TDEs for spectroscopic follow-up observations in large transient surveys.
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