Kyungjin Ahn

We study the effect of starlight from the first stars on the ability of other minihaloes in their neighborhood to form additional stars. The question of what the dynamical consequences were for these target minihaloes, of their exposure to the ionizing and dissociating starlight from the Pop III star requires a full radiation-hydrodynamics calculation. Towards this end, we have performed a series of detailed, 1D, radiation-hydrodynamical simulations. We have varied the distance to the source (and, hence, the flux) and the mass and evolutionary stage of the target haloes to quantify this effect. We find: (1) trapping of the I-front and its transformation from R-type to D-type, preceded by a shock front; (2) photoevaporation of the ionized gas (i.e. all gas originally located outside the trapping radius); (3) formation of an H_2 precursor shell which leads the I-front, stimulated by partial photoionization; and (4) the shock-induced formation of H_2 in the minihalo neutral core when the shock speeds up and partially ionizes the gas. The fate of the neutral core is mostly determined by the response of the core to this shock front, which leads to molecular cooling and collapse that, when compared to the same halo without external radiation, is either: (a) expedited, (b) delayed, (c) unaltered, or (d) reversed or prevented, depending upon the flux (i.e. distance to the source) and the halo mass and evolutionary stage. Roughly speaking, most haloes that were destined to cool, collapse, and form stars in the absence of external radiation are found to do so even when exposed to the first Pop III star in their neighborhood, while those that would not have done so are still not able to. (abridged)

We have derived the first, fully-cosmological, similarity solutions for CDM halo formation in the presence of nongravitational collisionality, which provides an analytical theory of the effect of the self-interacting dark matter (SIDM) hypothesis on halo density profiles. Collisions transport heat inward, which produces a constant-density core, while continuous infall pumps energy into the halo to stabilize the core against gravothermal catastrophe. These solutions improve upon earlier attempts to model the formation and evolution of SIDM halos, offer deeper insight than existing N-body experiments, and yield a more precise determination of the dependence of halo density profile on the value of the CDM self-interaction cross section. Different solutions arise for different values of the dimensionless collisionality parameter Q = s rho_b r_v \~ r_v/l_mfp, where s is the scattering cross section per unit mass, rho_b is the cosmic mean matter density, r_v is halo virial radius and l_mfp is the collision mean free path. The maximum flattening of central density occurs for an intermediate value of Q, Q_th, at which the halo is maximally relaxed to isothermality. The density profiles with constant-density cores preferred by dwarf and LSB rotation curves are best fit by the maximally-flattened (Q=Q_th) solution. If we assume that dwarfs and LSB galaxies formed at their typical collapse epoch in LCDM, then the value of s which makes Q=Q_th is s ~ 200 cm^{2}/g, much higher than previous estimates, s ~ 0.5-5 cm^{2}/g, based on N-body experiments. If s is independent of collision velocity, then the same value s ~ 200 cm^{2}/g would make Q>Q_th for clusters, which typically formed only recently, resulting in relatively less flattening of their central density profile and a smaller core.

The H atoms inside minihalos (i.e. halos with virial temperatures T_vir < 10^4 K, in the mass range roughly from 10^4 M_sun to 10^8 M_sun) during the cosmic dark ages in a LCDM universe produce a redshifted background of collisionally-pumped 21-cm line radiation which can be seen in emission relative to the cosmic microwave background (CMB). Previously, we used semi-analytical calculations of the 21-cm signal from individual halos of different mass and redshift and the evolving mass function of minihalos to predict the mean brightness temperature of this 21-cm background and its angular fluctuations. Here we use high-resolution cosmological N-body and hydrodynamic simulations of structure formation at high redshift (z > 8) to compute the mean brightness temperature of this background from both minihalos and the intergalactic medium (IGM) prior to the onset of Ly-alpha radiative pumping. We find that the 21-cm signal from gas in collapsed, virialized minihalos dominates over that from the diffuse shocked gas in the IGM.

We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions taking account of the collisional interaction of SIDM particles. This interaction results in a thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume an initial mass profile dM/M M^{-eps}, as in the familiar secondary infall model. If eps=1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall terminated by a strong accretion shock. Different solutions arise for different values of the collisionality parameter, Q= sigma rho_b r_s, where sigma is the scattering cross section, rho_b is the cosmic mean density, and r_s is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of r_s. We find two different regimes for these solutions: 1) for Q \leq Q_{th}, the core density decreases and core size increases as Q increases; 2) for Q \geq Q_{th}, the core density increases and core size decreases as Q increases. Our similarity solutions are in agreement with previous N-body simulations of SIDM halos, which correspond to the low-Q regime, if Q=[8.4e-4 - 4.9e-2]Q_{th} (low-Q), or sigma=[0.56-5.6]cm^2/g. As Q=\infty, our similarity solution aquires a central density cusp, in agreement with some simulation results which used an ordinary collisional fluid to approximate the effects of SIDM collisionality. When Q=[18.6-231]Q_{th} or sigma=[1.2e4 - 2.71e4]cm^2/g, for which we find flat-density cores comparable to those of the observationally acceptable low-Q solutions, has not previously been identified. Further study of this regime is warranted.