LOW-MASS GALAXY FORMATION IN COSMOLOGICAL ADAPTIVE MESH REFINEMENT SIMULATIONS: THE EFFECTS OF VARYING THE SUB-GRID PHYSICS PARAMETERS

Abstract

We present numerical simulations aimed at exploring the effects of varying the sub-grid physics parameters on the evolution and the properties of the galaxy formed in a low-mass dark matter halo (~7 times 10^10 Msun/h at redshift z=0). The simulations are run within a cosmological setting with a nominal resolution of 218 pc comoving and are stopped at z = 0.43. In all of our simulations, an extended old/intermediate-age stellar halo and a more compact younger stellar disk are formed. We found that a non negligible fraction of the halo stars are formed in situ in a spheroidal distribution. Changes in the sub-grid physics parameters affect significantly and in a complex way the evolution and properties of the galaxy: (i) Lower threshold densities nsf produce larger stellar effective radii Re, less peaked circular velocity curves V_c(R), and greater amounts of low-density and hot gas in the disk mid-plane; (ii) When stellar feedback is modeled by temporarily switching off radiative cooling in the star forming regions, Re increases (by a factor of ~ 2 in our particular model), the circular velocity curve becomes flatter, and a complex multi-phase gaseous disk structure develops; (iii) A more efficient local conversion of gas mass to stars, measured by a stellar particle mass distribution biased toward larger values, increases the strength of the feedback energy injection -driving outflows and inducing burstier SF histories; iv) If feedback is too strong, gas loss by galactic outflows -which are easier to produce in low-mass galaxies- interrupts SF, whose history becomes episodic. The simulations exhibit two important shortcomings: the baryon fractions are higher, and the specific SF rates are much smaller, than observationally inferred values for redshifts ~ 0.4-1.