Below, is a summary of the my past scientific projects:

  • Impact of baryonic disks on the shape and density profile of SIDM halos: 

N-body simulations of SIDM models suggest dark matter density profiles are shallow in the inner region, and halo shape are more spherical than CDM (e.g. see Vogelsberger et al. 2012 and Peter et al. 2013). However, these simulations ignore the effect of baryons. In Sameie et al. 2018a, we studied the evolution of shape and density profiles of SIDM halos when contribution of baryons to the total gravitational potential is not negligible. We found the halo experiences two distinct phases of evolution. First, it undergoes core-expansion: Self-interaction creates some isothermal sphere with constant velocity dispersion and flat density profile at the center of the halo. In the second phase, which is called core-contraction, the core turns into cuspy density profile and velocity dispersion profile gets mild negative gradient. In principle, this could results in density profiles even cuspier than their CDM+baryons counterparts. The shape of the halos are also affected during these two phase of evolution. During core-expansion phase, the shape becomes triaxial and the halo shape follows the shape of the baryons. On the other hand, over the core-contraction phase halo shape restores its sphericity. Increasing dark matter self-interaction and/or baryonic concentration speeds up the transition from core-expansion to core-contraction (Fig. 1). Furthermore, we simulated a Milky Way-like galactic system (composed of halo, stellar disk, and gaseous bulge) to model the Galaxy. We concluded that an SIDM Milky Way halo, compared to halos in CDM+Hydrodynamical simulations, is more easily accommodated into halo shape observational constraints (Fig. 2).


Figure 1. Top: Dark matter density profiles for our Milky Way-sized halo with different constant cross section and baryonic concentrations. dashed lines are SIDM-only predictions and dot-dashed is contribution from baryonic disks. Bottom: Corresponding velocity dispersion profiles .
Figure 2. Minor-to-major axis ratio for our SIDM Milky Way halo compared to different observational data point from tidal stream analysis. Black and red regions are predictions from CDM-only and CDM+Hydrodynamical simulations.















  • The effect of dark matter-dark radiation on the abundance of the halos-a Press-Schechter approach:

Dark matter could interact with dark radiation (for a complete discussion see this paper and references therein). This results in acoustic waves in matter power spectrum which itself suppresses dark matter halo abundance. In Sameie et al. 2018b (in preparation) we studied this suppression in halo abundance using the analytical models. We carefully calibrated the Press-Schechter model to the simulation data points for benchmark dark matter models in Vogelsberger et al. 2016. We also ran cosmological simulations for the same dark matter models with improved mass resolution, and showed that the analytical model accurately predicts depletion of the halos in the mass scales smaller than the minimum halo mass used in the calibration analysis. We showed that calibration is robust over wide range of redshifts once the model is calibrated to z=0 simulationsWe further computed the prediction of these dark matter models for cumulative halo abundance, and compared them with the observed cumulative galaxy number density (Menci et al. 2016). We conclude that dark matter model with kinetic decoupling temperature ≤ 0.5 keV are outside of 68% confidence level (horizontal dashed line in Fig. 3). Finally, we performed “abundance matching” analysis, and computed stellar-halo mass relation using observed stellar mass function at z=4 (Song et al. 2016). Interestingly, stronger suppression in matter power spectrum leads to less massive dark matter halos (or more massive galaxies) which can be interpreted as higher star formation efficiency in these dark matter models (Fig. 4).

Figure 3. Cumulative number density for different dark matter models in our work at z=6, compared to number density of observed galaxies (Menci et al. 2016, Livermore et al. 2016). We exclude mass scales bellow 10^8 solar mass since these halos had lost most of their baryons due to background UV radiation.
Figure 4. Stellar-halo mass relation for dark matter models in this work at z=4. We compare our results with previous studies (Behroozi et al. 2013, Moster et al. 2013). Red shaded area is mass scales where we extrapolated the Schechter function.