The Baryon Cycle of Dwarf Galaxies: Dark, Bursty, Gas-Rich Polluters

Abstract

We present results from a fully cosmological, very high-resolution, ΛCDM "zoom-in" simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M vir = 4.4 × 10 8 − 3.6 × 10 10 M ⊙ . The simulation includes a blastwave scheme for supernova feedback, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background that modifies the ionization and excitation state of the gas. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M vir < 10 9 M ⊙ are devoid of stars, as they never reach the density threshold for star formation of 100 atoms cm −3 . The other four, M vir > 10 9 M ⊙ dwarfs have blue colors, low star formation efficiencies, −4.5 ≤ log M * /M vir ≤ −2.5, high cold gas to stellar mass ratios, 0.2 ≤ M HI /M * ≤ 20, and low stellar metallicities, −2 ≤ [Fe/H] ≤ −1. Their bursty star formation histories are characterized by peak specific star formation rates in excess of 50 − 100 Gyr −1 , far outside the realm of normal, more massive galaxies, and in agreement with observations of extreme emission-line starbursting dwarfs by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey. The median stellar age of the simulated galaxies decreases with decreasing halo mass, with the two M vir ≃ 2 − 3 × 10 9 M ⊙ dwarfs being predominantly young, and the two more massive systems hosting intermediate and older populations. The two cosmologically young dwarfs are lit up by tidal interactions, have compact morphologies, and have metallicities and cold gas fractions similar to the relatively quiescent, extremely metal-deficient dwarf population that includes the recently-discovered Leo P. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a Mpc-size region of the intergalactic medium, but are not sufficient to completely quench star formation activity and are absent in the faintest dwarfs. Within the limited size of the sample, our simulations appear to simultaneously reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of field dwarfs in the Local Volume.

Constraining the Nature of Dark Matter with the Star-formation History of the Faintest Local Group Dwarf Galaxy Satellites

“…Note that the concentrations assigned in the standard procedures are on the high side of those expected in WDM models, so we can instead start by assigning a typical concentration measured in WDM simulations (Schneider 2015), which then yields halos that are 5-10 times heavier than in our default method. This choice also matches the extrapolation of the halo mass-stellar mass relation well (see Shen et al 2014;Behroozi et al 2013). By using a dynamical friction time estimate, we then computed the earliest infall time the galaxy could have, based on their current distance to the MW center.…”

“…Indeed, while the dearth of faint satellite galaxies, hosted by halos with V vir <20 km s −1 , can be explained by the combined effect of reionization, stellar feedback and environmental processes (Bullock et al 2001;Brooks & Zolotov 2014;Di Cintio et al 2014), it has been pointed out that there is still a possible excess of massive satellite halos with V vir >20 km s −1 relative to the number of observed bright dwarf galaxies (BoylanKolchin et al 2011), which also appear to have lower central densities than predicted by CDM. Squelching by reionization cannot provide a simple solution in the latter case since gas will be retained within hosts of this mass (Shen et al 2014), which actually ought to be even more massive before infall (Mayer 2010). Cosmological hydrodynamical simulations, however, suggest that feedback before infall may modify the DM density profiles enough to reduce the central densities of satellites (Brooks & Zolotov 2014;Wheeler et al 2015), while recent simulations embedding high-resolution models of satellites within cosmological MW-sized halos do show that tidal stripping and stirring of satellites with such previously modified central DM profiles may have a strong effect on the resulting satellite population, possibly eliminating the "massive failures" (Tomozeiu et al 2016a(Tomozeiu et al , 2016b).…”

The imprint of reionization on the star formation histories of dwarf galaxies

“…Their simulations, however, evolve dwarfs only until z = 8, and therefore large and uncertain extrapolation is needed to contrast these predictions with observations of dwarfs in the local Universe. Shen et al (2013) evolve a group of dwarf galaxies to z = 0 and report "bursty" and diverse SFHs, but do not explore in detail the role of reionization in the origin of such diversity.…”

Bimodality of low-redshift circumgalactic O vi in non-equilibrium eagle zoom simulations

“…Ford et al 2013Ford et al , 2016Suresh et al 2015a,b) and zooms with cosmological initial conditions (e.g. Stinson et al 2012;Hummels et al 2013;Shen et al 2014;Gutcke et al 2016) have found an intimate link between galaxies and their metal-enriched CGM. Simulations that specifically compared with the COS-Halos survey find lower O VI column densities than observed around star-forming galaxies (Suresh et al 2015b;Ford et al 2016).…”