We study the early stage of the formation of seed supermassive black holes via direct collapse in dark matter (DM) halos, in the cosmological context. We perform high-resolution zoom-in simulations of such collapse at high-$z$. Using the adaptive mes
h refinement code ENZO, we resolve the formation and growth of a DM halo, until its virial temperature reaches $sim 10^4$K, atomic cooling turns on, and collapse ensues. We demonstrate that direct collapse proceeds in two stages, although they are not well separated. The first stage is triggered by the onset of atomic cooling, and leads to rapidly increasing accretion rate with radius, from $dot Msim 0.1,M_odot {rm yr^{-1}}$ at the halo virial radius to few $M_odot ,{rm yr^{-1}}$, around the scale radius $R_{rm s}sim 30$pc of the NFW DM density profile. The second stage of the collapse commences when the gas density takes precedence over the DM density. This is associated with the gas decoupling from the DM gravitational potential. The ensuing collapse approximates that of an isothermal sphere with $dot M ( r )sim $const. We confirm that the gas loses its angular momentum through non-axisymmetric perturbations and gravitational torques, to overcome the centrifugal barrier. During the course of the collapse, this angular momentum transfer process happens on nearly all spatial scales, and the angular momentum vector of the gas varies with position and time. Collapsing gas also exhibits supersonic turbulent motions which suppress gas fragmentation, and are characterized by density PDF consisting of a lognormal part and a high-density power law tail.
We use high-resolution zoom-in cosmological simulations of galaxies of Romano-Diaz et al., post-processing them with a panchromatic three-dimensional radiation transfer code to obtain the galaxy UV luminosity function (LF) at z ~ 6-12. The galaxies a
re followed in a rare, heavily overdense region within a ~ 5-sigma density peak, which can host high-z quasars, and in an average density region, down to the stellar mass of M_star ~ 4* 10^7 Msun. We find that the overdense regions evolve at a substantially accelerated pace --- the most massive galaxy has grown to M_star ~ 8.4*10^10 Msun by z = 6.3, contains dust of M_dust~ 4.1*10^8 Msun, and is associated with a very high star formation rate, SFR ~ 745 Msun/yr.The attained SFR-M_star correlation results in the specific SFR slowly increasing with M_star. Most of the UV radiation in massive galaxies is absorbed by the dust, its escape fraction f_esc is low, increasing slowly with time. Galaxies in the average region have less dust, and agree with the observed UV LF. The LF of the overdense region is substantially higher, and contains much brighter galaxies. The massive galaxies are bright in the infrared (IR) due to the dust thermal emission, with L_IR~ 3.7*10^12 Lsun at z = 6.3, while L_IR < 10^11 Lsun for the low-mass galaxies. Therefore, ALMA can probe massive galaxies in the overdense region up to z ~ 10 with a reasonable integration time. The UV spectral properties of disky galaxies depend significantly upon the viewing angle.The stellar and dust masses of the most massive galaxy in the overdense region are comparable to those of the sub-millimetre galaxy (SMG) found by Riechers et al. at z = 6.3, while the modelled SFR and the sub-millimetre flux fall slightly below the observed one. Statistical significance of these similarities and differences will only become clear with the upcoming ALMA observations.
We have explored prevailing modes of galaxy growth for redshifts z ~ 6-14, comparing substantially overdense and normal regions of the universe, using high-resolution zoom-in cosmological simulations. Such rare overdense regions have been projected t
o host high-z quasars. We demonstrate that galaxies in such environments grow predominantly by a smooth accretion from cosmological filaments which dominates the mass input from major, intermediate and minor mergers. We find that by z ~6, the accumulated galaxy mass fraction from mergers falls short by a factor of 10 of the cumulative accretion mass for galaxies in the overdense regions, and by a factor of 5 in the normal environments. Moreover, the rate of the stellar mass input from mergers also lies below that of an in-situ star formation (SF) rate. The fraction of stellar masses in galaxies contributed by mergers in overdense regions is ~12%, and ~33% in the normal regions, at these redshifts. Our median SF rates for ~few X 10^9 Mo galaxies agrees well with the recently estimated rates for z ~ 7 galaxies from Spitzers SURF-UP survey. Finally, we find that the main difference between the normal and overdense regions lies in the amplified growth of massive galaxies in massive dark matter halos. This leads to the formation of >= 10^{10} Mo galaxies due to the ~100-fold increase in mass during the above time period. Such galaxies are basically absent in the normal regions at these redshifts.
We demonstrate that growth of stellar bars in spinning dark matter halos is heavily suppressed in the secular phase of evolution, using numerical simulations of isolated galaxies. In a representative set of models, we show that for values of the cosm
ological spin parameter lambda > 0.03, bar growth (in strength and size) becomes increasingly quenched. Furthermore, slowdown of bar pattern speed weakens substantially with increasing `lambda, until it ceases completely. The terminal structure of bars is affected as well, including extent and shape of their boxy/peanut bulges. The essence of this effect lies in the modified angular momentum exchange between the disk and the halo facilitated by the bar. For the first time we have demonstrated that a dark matter halo can emit and not purely absorb angular momentum. Although the halo as a whole is not found to emit, the net transfer of angular momentum from the disk to the halo is significantly reduced or completely eliminated. The paradigm shift implies that the accepted view that disks serve as sources of angular momentum and halos serve as sinks, must be revised. Halos with lambda > 0.03 are expected to form a substantial fraction, based on lognormal distribution of lambda. Dependence of secular bar evolution on halo spin, therefore, implies profound corollaries for the cosmological evolution of galactic disks.
We use numerical simulations to explore whether direct collapse can lead to the formation of SMBH seeds at high-z. We follow the evolution of gas within DM halos of 2 x 10^8 Mo and 1 kpc. We adopt cosmological density profiles and j-distributions. Ou
r goal is to understand how the collapsing flow overcomes the centrifugal barrier and whether it is subject to fragmentation. We find that the collapse leads either to a central runaway or to off-center fragmentation. A disk-like configuration is formed inside the centrifugal barrier. For more cuspy DM distribution, the gas collapses more and experiences a bar-like perturbation and a central runaway. We have followed this inflow down to ~10^{-4} pc. The flow remains isothermal and the specific angular momentum is efficiently transferred by gravitational torques in a cascade of nested bars. This cascade supports a self-similar, disk-like collapse. In the collapsing phase, virial supersonic turbulence develops and fragmentation is damped. For larger initial DM cores the timescales become longer. In models with more organized initial rotation, a torus forms and appears to be supported by turbulent motions. The evolution depends on the competition between two timescales, corresponding to the onset of the central runaway and off-center fragmentation. For less organized rotation, the torus is greatly weakened, the central accretion timescale is shortened, and off-center fragmentation is suppressed --- triggering the central runaway even in previously `stable models. The resulting SMBH masses lie in the range 2 x 10^4 Mo - 2 x 10^6 Mo, much higher than for Population III remnants. We argue that the above upper limit appears to be more realistic mass. Corollaries of this model include a possible correlation between SMBH and DM halo masses, and similarity between the SMBH and halo mass functions, at time of formation.
Galactic disks can form in asymmetric potentials of the assembling dark matter (DM) halos, giving rise to the first generation of gas-rich bars. Properties of these bars differ from canonical bars analyzed so far. Moreover, rapid disk growth is assoc
iated with the influx of clumpy DM and baryons along the large-scale filaments. Subsequent interactions between this substructure and the disk can trigger generations of bars, which can explain their ubiquity in the Universe. I provide a brief summary of such bar properties and argue that they fit naturally within the broad cosmological context of a hierarchical buildup of structure in the universe.
We study the central dark matter (DM) cusp evolution in cosmological galactic halos. Models with and without baryons (baryons+DM, hereafter BDM model, and pure DM, PDM model, respectively) are advanced from identical initial conditions. The DM cusp p
roperties are contrasted by a direct comparison of pure DM and baryonic models. We find a divergent evolution between the PDM and BDM models within the inner ~10 kpc region. The PDM model forms a R^{-1} cusp as expected, while the DM in the BDM model forms a larger isothermal cusp R^{-2} instead. The isothermal cusp is stable until z~1 when it gradually levels off. This leveling proceeds from inside out and the final density slope is shallower than -1 within the central 3 kpc (i.e., expected size of the R^{-1} cusp), tending to a flat core within ~2 kpc. This effect cannot be explained by a finite resolution of our code which produces only a 5% difference between the gravitationally softened force and the exact Newtonian force of point masses at 1 kpc from the center. Neither is it related to the energy feedback from stellar evolution or angular momentum transfer from the bar. Instead it can be associated with the action of DM+baryon subhalos heating up the cusp region via dynamical friction and forcing the DM in the cusp to flow out and to `cool down. The process described here is not limited to low z and can be efficient at intermediate and even high z.
