The onset and regulation of star formation in the lowest mass dark matter halos
Date
2022-09-02
Authors
Pereira-Wilson, Matthew
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Abstract
We use the APOSTLE suite of cosmological simulations to examine the role of the cosmic ionizing background in regulating star formation (SF) in low-mass LCDM halos. In agreement with earlier work, we find that, after reionization, SF can only proceed in halos whose mass exceeds a redshift-dependent ``critical'' virial mass determined by the structure of LCDM halos and the thermal pressure of UV-heated gas. This critical mass increases from Mcrit~10^8 Msun at z~11$ to ~10^9.7 Msun at z=0, roughly following the average mass growth history of halos in that mass range. This implies that most halos above or below critical at present have remained so since early times. In particular, the halos of most galaxies today were already above-critical (and thus forming stars) at high redshift, providing a simple explanation for the ubiquitous presence of ancient stellar populations in dwarfs, regardless of luminosity. It also implies that Mcrit today represents a ``threshold'' mass below which the fraction of ``dark'' halos increases steeply. Sub-critical halos may still host luminous galaxies if they were above-critical at some point in the past. SF ceases if a halo falls into the sub-critical regime; depending on each halo's accretion history this can occur over a wide range of times, explaining why SF in many dwarfs seems to continue well past the reionization epoch. It also suggests a tantalizing explanation for the episodic nature of SF in some dwarfs, which, in this interpretation, would be linked to temporary halo excursions above and below the critical boundary. In the simulations, Mcrit(z) cleanly separates star-forming from non-star-forming systems at all redshifts, indicating that the ionizing UV background, and not stellar feedback, is what regulates the beginning and the end of SF in the faintest dwarfs. Galaxies in sub-critical halos should make up a sizable population of faint field dwarfs, distinct from those in more massive halos because of their lack of ongoing star formation. Although few such galaxies are known at present, the discovery of this population would provide strong support for our results.
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Keywords
dark matter, galaxies: formation, galaxies: evolution, galaxies: dwarf, galaxies: kinematics and dynamics, globular clusters: general