The effect of feedback and reionization on star formation in low-mass dwarf galaxy haloes

Abstract

We simulate the evolution of a 10 9 M dark matter halo in a cosmological setting with an adaptive-mesh refinement code as an analogue to local low luminosity dwarf irregular and dwarf spheroidal galaxies. The primary goal of our study is to investigate the roles of reionization and supernova feedback in determining the star formation histories of low mass dwarf galaxies. We include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation, and a simple model for supernova driven energetic feedback. To better understand the impact of each physical effect, we carry out simulations excluding each major effect in turn. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order of magnitude difference in the final stellar mass of the system. For our full physics run with reionization at z = 9, we find a stellar mass of about 10 5 M at z = 0, and a mass-to-light ratio within the half-light radius of approximately 130 M /L , consistent with observed low-luminosity dwarfs. However, the resulting median stellar metallicity is 0.06 Z , considerably larger than observed systems. In addition, we find star formation is truncated between redshifts 4 and 7, at odds with the observed late time star formation in isolated dwarf systems but in agreement with Milky Way ultrafaint dwarf spheroidals. We investigate the efficacy of energetic feedback in our simple thermal-energy driven feedback scheme, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 M , and a comoving spatial resolution of 11 pc.

Forged in fire: cusps, cores and baryons in low-mass dwarf galaxies

“…Instead, the baryons slowly "evaporate" out. This is in contrast with other studies (Sawala et al 2011;Simpson et al 2013) that show sharper jumps. However, the lower resolution used by Sawala et al (2011) could explain why their evolution is less smooth.…”

“…For the halo 6 Abundance matching results below ∼ 10 6 M stellar mass are extrapomass range presented here, this is particularly impressive, as the integrated stellar mass is suppressed by factors of ∼ 1000 relative to the Universal baryon fraction (upper solid gray line). The smaller points in Figure 2 show results from previous hydrodynamical simulations of dwarf galaxies (Governato et al 2010;Sawala et al 2011;Simpson et al 2013;Munshi et al 2013;Shen et al 2014;Trujillo-Gomez et al 2015). The open points are those that have reported at least mild flattening of the central dark matter cusp in response to feedback effects.…”

“…However, the lower resolution used by Sawala et al (2011) could explain why their evolution is less smooth. Simpson et al (2013) used comparable resolution to this work but studied a lower mass system, 10 9 M , which could explain the much more drastic effect due to the UV background in the gas virial fraction that they found.…”

“…The past several years have proven fruitful in this regard, with many published studies achieving substantial suppression in the conversion of baryons to stars on the scale of dwarf galaxy halos (Governato et al 2010;Sawala et al 2011;Simpson et al 2013;Munshi et al 2013;Governato et al 2015;Trujillo-Gomez et al 2015). As we show below, however, many of these studies have not quite reached the level of suppression that seems to be required by local galaxy counts.…”

Early flattening of dark matter cusps in dwarf spheroidal galaxies

“…A successful formation model should ⋆ E-mail: carlo.nipoti@unibo.it be able to account for the all the properties of dSphs that can be inferred from observations, such as the structure and kinematics of the stellar distribution, metallicity of stars, and star formation history. Several studies have been carried out trying to reproduce at least some of these properties with either cosmological hydrodynamical simulations (Sawala et al 2010;Sawala, Scannapieco, & White 2012;Simpson et al 2013), simulations of isolated halos (Revaz et al 2009;Assmann et al 2013) or semi-analytic models (Li, De Lucia, & Helmi 2010;Font et al 2011;Romano & Starkenburg 2013;Starkenburg et al 2013). Here we focus on the two specific properties of dSphs: the facts that DM dominates the mass budget throughout these galaxies and that the present-day DM profile is not divergent in the central regions.…”