All about baryons: revisiting SIDM predictions at small halo masses

We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both $Lambda$ dominated, Cold (CDM) and Self--Interacting (SIDM) dark matter models. The SIDM model adopts a constant cross section of 2 $cm^{2}/g$, a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter cores. Remarkably, SIDM fails to significantly lower the central dark matter density at halo peak velocities V$_{max}$ $<$ 30 Km/s. This is due to the fact that the central regions of very low--mass field halos have relatively low central velocity dispersion and densities, leading to time scales for SIDM collisions greater than a Hubble time. CDM halos with V$_{max}$ $<$ 30 km/s have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm2/g SIDM cross section, the DM content of very low mass CDM and SIDM halos differs by no more than a factor of two within 100-200pc. At larger halo masses ($sim$ 10$^{10}$ solar masses), the introduction of baryonic processes creates field dwarf galaxies with dark matter cores and central DM$+$baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity dependent cross section that rapidly increases to values larger than 2 $cm^{2}/g$ for halos with V$_{max}$ < 25-30 Km/s needs to be introduced.

Cuspy No More: How Outflows Affect the Central Dark Matter and Baryon Distribution in Lambda CDM Galaxies

We examine the evolution of the inner dark matter (DM) and baryonic density profile of a new sample of simulated field galaxies using fully cosmological, Lambda CDM, high resolution SPH + N-Body simulations. These simulations include explicit H2 and metal cooling, star formation (SF) and supernovae (SNe) driven gas outflows. Starting at high redshift, rapid, repeated gas outflows following bursty SF transfer energy to the DM component and significantly flatten the originally `cuspy central DM mass profile of galaxies with present day stellar masses in the 10^4.5 -- 10^9.8 Msolar range. At z=0, the central slope of the DM density profile of our galaxies (measured between 0.3 and 0.7 kpc from their centre) is well fitted by rhoDM propto r^alpha with alpha simeq -0.5 + 0.35 log_10(Mstar/10^8Msolar) where Mstar is the stellar mass of the galaxy and 4 < log_10 Mstar < 9.4. These values imply DM profiles flatter than those obtained in DM--only simulations and in close agreement with those inferred in galaxies from the THINGS and LITTLE THINGS survey. Only in very small halos, where by z=0 star formation has converted less than ~ 0.03% of the original baryon abundance into stars, outflows do not flatten the original cuspy DM profile out to radii resolved by our simulations. The mass (DM and baryonic) measured within the inner 500 pc of each simulated galaxy remains nearly constant over four orders of magnitudes in stellar mass for Mstar 10^9 Msolar. This finding is consistent with estimates for faint Local Group dwarfs and field galaxies. These results address one of the outstanding problems faced by the CDM model, namely the strong discrepancy between the original predictions of cuspy DM profiles and the shallower central DM distribution observed in galaxies.

Forming a Large Disc Galaxy from a z<1 Major Merger

Using high resolution SPH simulations in a fully cosmological Lambda CDM context we study the formation of a bright disk dominated galaxy that originates from a wet major merger at z=0.8. The progenitors of the disk galaxy are themselves disk galaxies that formed from early major mergers between galaxies with blue colors. A substantial thin stellar disk grows rapidly following the last major merger and the present day properties of the final remnant are typical of early type spiral galaxies, with an i band B/D ~0.65, a disk scale length of 7.2 kpc, g-r = 0.5 mag, an HI line width (W_{20}/2) of 238 km/sec and total magnitude i = -22.4. The key ingredients for the formation of a dominant stellar disk component after a major merger are: i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, ii) supernova feedback that is able to partially suppress star formation during mergers and iii) relative fading of the spheroidal component. The gas fraction of the progenitors disks does not exceed 25% at z<3, emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of disks and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disk galaxies by the high likelihood of interactions and mergers for galaxy sized halos at relatively low z.