We reason that, without physical fine-tuning, neither the supermassive black holes (SMBHs) nor the stellar bulges can self-regulate or inter-regulate by driving away already fallen cold gas to produce the observed correlation between them. We suggest
an alternative scenario where the observed mass ratios of the SMBHs to bulges reflect the angular momentum distribution of infallen gas such that the mass reaching the stable accretion disc is a small fraction of that reaching the bulge region, averaged over the cosmological time scales. We test this scenario using high resolution, large-scale cosmological hydrodynamic simulations (without AGN feedback), assuming the angular momentum distribution of gas landing in the bulge region to yield a Mestel disc that is supported by independent simulations resolving the Bondi radii of SMBHs. A mass ratio of $0.1-0.3%$ between the very low angular momentum gas that free-falls to the sub-parsec region to accrete to the SMBH and the overall star formation rate is found. This ratio is found to increase with increasing redshift to within a factor of $sim 2$, suggesting that the SMBH to bulge ratio is nearly redshift independent, with a modest increase with redshift, a testable prediction. Furthermore, the duty cycle of active galactic nuclei (AGN) with high Eddington ratios is expected to increase significantly with redshift. Finally, while SMBHs and bulges are found to coevolve on $sim 30-150$Myr time scales or longer, there is indication that, on shorer time scales, the SMBH accretion rate and star formation may be less correlated.
Simulations have indicated that most of the escaped Lyman continuum photons escape through a minority of solid angles with near complete transparency, with the remaining majority of the solid angles largely opaque, resulting in a very broad and skewe
d probability distribution function (PDF) of the escape fraction when viewed at different angles. Thus, the escape fraction of Lyman continuum photons of a galaxy observed along a line of sight merely represents the properties of the interstellar medium along that line of sight, which may be an ill-representation of true escape fraction of the galaxy averaged over its full sky. Here we study how Lyman continuum photons escape from galaxies at $z=4-6$, utilizing high-resolution large-scale cosmological radiation-hydrodynamic simulations. We compute the PDF of the mean escape fraction ($left<f_{rm esc,1D}right>$) averaged over mock observational samples, as a function of the sample size, compared to the true mean (had you an infinite sample size). We find that, when the sample size is small, the apparent mean skews to the low end. For example, for a true mean of 6.7%, an observational sample of (2,10,50) galaxies at $z=4$ would have have 2.5% probability of obtaining the sample mean lower than $left<f_{rm esc,1D}right>=$(0.007%, 1.8%, 4.1%) and 2.5% probability of obtaining the sample mean being greater than (43%, 18%, 11%). Our simulations suggest that at least $sim$ 100 galaxies should be stacked in order to constrain the true escape fraction within 20% uncertainty.
Halo gas in low-z (z<0.5) >0.1L* galaxies in high-resolution, large-scale cosmological hydrodynamic simulations is examined with respect to three components: (cold, warm, hot) with temperatures equal to (<10^5, 10^{5-6}, >10^6)K, respectively. The wa
rm component is compared, utilizing O VI lambdalambda 1032, 1038 absorption lines, to observations and agreement is found with respect to the galaxy-O VI line correlation, the ratio of O VI line incidence rate in blue to red galaxies and the amount of O VI mass in star-forming galaxies. A detailed account of the sources of warm halo gas (stellar feedback heating, gravitational shock heating and accretion from the intergalactic medium), inflowing and outflowing warm halo gas metallicity disparities and their dependencies on galaxy types and environment is also presented. Having the warm component securely anchored, our simulations make the following additional predictions. First, cold gas is the primary component in inner regions, with its mass comprising 50% of all gas within galacto-centric radius r=(30,150)kpc in (red, blue) galaxies. Second, at r>(30,200)kpc in (red, blue) galaxies the hot component becomes the majority. Third, the warm component is a perpetual minority, with its contribution peaking at ~30% at r=100-300kpc in blue galaxies and never exceeding 5% in red galaxies. The significant amount of cold gas in low-z early-type galaxies found in simulations, in agreement with recent observations (Thom et al.), is intriguing, so is the dominance of hot gas at large radii in blue galaxies.
In the context stellar reionization in the standard cold dark matter model, we analyze observations at z~6 and are able to draw three significant conclusions with respect to star formation and the state of the intergalactic medium (IGM) at z~6. (1) A
n initial stellar mass function (IMF) more efficient, by a factor of 10-20, in producing ionizing photons than the standard Salpeter IMF is required at z~6. This may be achieved by having either (A) a metal-enriched IMF with and a lower mass cutoff of >= 30Msun or (B) 2-4% of stellar mass being Population III massive metal-free stars at z~6. While there is no compelling physical reason or observational evidence to support (A), (B) could be fulfilled plausibly by continued existence of some pockets of uncontaminated, metal-free gas for star formation. (2) The volume-weighted neutral fraction of the IGM of <f_HI>_V~ 10^-4 at z=5.8 inferred from the SDSS observations of QSO absorption spectra provides enough information to ascertain that reionization is basically complete with at most ~0.1-1% of IGM that is un-ionized at z=5.8. (3) Barring some extreme evolution of the IMF, the neutral fraction of the IGM is expected to rise quickly toward high redshift from the point of HII bubble percolation, with the mean neutral fraction of the IGM expected to reach 6-12% at z=6.5, 13-27% at z=7.7 and 22-38% at z=8.8.
