SIDM on FIRE: hydrodynamical self-interacting dark matter simulations of low-mass dwarf galaxies.

We compare a suite of four simulated dwarf galaxies formed in 1010 M☉ haloes of collisionless cold dark matter (CDM) with galaxies simulated in the same haloes with an identical galaxy formation model but a non-zero cross-section for DM self-interactions. These cosmological zoom-in simulations are part of the Feedback In Realistic Environments (fire) project and utilize the fire-2 model for hydrodynamics and galaxy formation physics. We find the stellar masses of the galaxies formed in self-interacting dark matter (SIDM) with σ/m = 1 cm2 g-1 are very similar to those in CDM (spanning M ★ ≈ 105.7-7.0 M☉) and all runs lie on a similar stellar mass-size relation. The logarithmic DM density slope (α = d log ρ/d log r) in the central 250-500 pc remains steeper than α = -0.8 for the CDM-Hydro simulations with stellar mass M ★ ~ 106.6 M☉ and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with α > -0.4. Moreover, the central density profiles predicted in SIDM runs without baryons are similar to the SIDM runs that include fire-2 baryonic physics. Thus, SIDM appears to be much more robust to the inclusion of (potentially uncertain) baryonic physics than CDM on this mass scale, suggesting that SIDM will be easier to falsify than CDM using low-mass galaxies. Our fire simulations predict that galaxies less massive than M ★ ≲ 3 × 106 M☉ provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.

Dwarf galaxy mass estimators versus cosmological simulations.

We use a suite of high-resolution cosmological dwarf galaxy simulations to test the accuracy of commonly used mass estimators from Walker et al. (2009) and Wolf et al. (2010), both of which depend on the observed line-of-sight velocity dispersion and the 2D half-light radius of the galaxy, Re . The simulations are part of the Feedback in Realistic Environments (fire) project and include 12 systems with stellar masses spanning 105­107M⊙ that have structural and kinematic properties similar to those of observed dispersion-supported dwarfs. Both estimators are found to be quite accurate: MWolf∕Mtrue=0.98−0.12+0.19 and MWalker∕Mtrue=1.07−0.15+0.21, with errors reflecting the 68 per cent range over all simulations. The excellent performance of these estimators is remarkable given that they each assume spherical symmetry, a supposition that is broken in our simulated galaxies. Though our dwarfs have negligible rotation support, their 3D stellar distributions are flattened, with short-to-long axis ratios c/a ≃ 0.4­0.7. The median accuracy of the estimators shows no trend with asphericity. Our simulated galaxies have sphericalized stellar profiles in 3D that follow a nearly universal form, one that transitions from a core at small radius to a steep fall-off ∝r−42 at large r; they are well fit by Sérsic profiles in projection. We find that the most important empirical quantity affecting mass estimator accuracy is Re . Determining Re by an analytic fit to the surface density profile produces a better estimated mass than if the half-light radius is determined via direct summation.

Effect of Native Oxide Film on Commercial Magnesium Alloys Substrates and Carbonate Conversion Coating Growth and Corrosion Resistance.

Possible relations between the native oxide film formed spontaneously on the AZ31 and AZ61 magnesium alloy substrates with different surface finish, the chemistry of the outer surface of the conversion coatings that grows after their subsequent immersion on saturated aqueous NaHCO3 solution treatment and the enhancement of corrosion resistance have been studied. The significant increase in the amount of aluminum and carbonate compounds on the surface of the conversion coating formed on the AZ61 substrate in polished condition seems to improve the corrosion resistance in low chloride ion concentration solutions. In contrast, the conversion coatings formed on the AZ31 substrates in polished condition has little effect on their protective properties compared to the respective as-received surface.