80 research outputs found

    Effect of radiative transfer on damped Lyman-alpha and Lyman limit systems in cosmological SPH simulations

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    We study the effect of local stellar radiation and UVB on the physical properties of DLAs and LLSs at z=3 using cosmological SPH simulations. We post-process our simulations with the ART code for radiative transfer of local stellar radiation and UVB. We find that the DLA and LLS cross sections are significantly reduced by the UVB, whereas the local stellar radiation does not affect them very much except in the low-mass halos. This is because clumpy high-density clouds near young star clusters effectively absorb most of the ionizing photons from young stars. We also find that the UVB model with a simple density threshold for self-shielding effect can reproduce the observed column density distribution function of DLAs and LLSs very well, and we validate this model by direct radiative transfer calculations of stellar radiation and UVB with high angular resolution. We show that, with a self-shielding treatment, the DLAs have an extended distribution around star-forming regions typically on ~ 10-30 kpc scales, and LLSs are surrounding DLAs on ~ 30-60 kpc scales. Our simulations suggest that the median properties of DLA host haloes are: Mh = 2.4*10^10 Msun, SFR = 0.3 Msun/yr, M* = 2.4*10^8 Msun, and Z/Zsun = 0.1. About 30 per cent of DLAs are hosted by haloes having SFR = 1 - 20 Msun/yr, which is the typical SFR range for LBGs. More than half of DLAs are hosted by the LBGs that are fainter than the current observational limit. Our results suggest that fractional contribution to LLSs from lower mass haloes is greater than for DLAs. Therefore the median values of LLS host haloes are somewhat lower with Mh = 9.6*10^9 Msun, SFR = 0.06 Msun/yr, M* = 6.5*10^7 Msun and Z/Zsun = 0.08. About 80 per cent of total LLS cross section are hosted by haloes with SFR < 1 Msun/yr, hence most LLSs are associated with low-mass halos with faint LBGs below the current detection limit.Comment: 18 pages, 12 figures, accepted for publication in MNRA

    The role of stellar relaxation in the formation and evolution of the first massive black holes

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    We present calculations on the formation of massive black holes with 10^5 Msun at z > 6 that can be the seeds of supermassive black holes at z > 6. Under the assumption of compact star cluster formation in merging galaxies, star clusters in haloes of 10^8 ~ 10^9 Msun can undergo rapid core-collapse leading to the formation of very massive stars (VMSs) with ~1000 Msun which directly collapse into black holes with similar masses. Star clusters in halos of > 10^9 Msun experience type-II supernovae before the formation of VMSs due to long core-collapse time scales. We also model the subsequent growth of black holes via accretion of residual stars in clusters. 2-body relaxation efficiently re-fills the loss cones of stellar orbits at larger radii and resonant relaxation at small radii is the main driver for accretion of stars onto black holes. As a result, more than ninety percent of stars in the initial cluster are swallowed by the central black holes before z=6. Using dark matter merger trees we derive black hole mass functions at z=6-20. The mass function ranges from 10^3 to 10^5 Msun at z ~ 4*10^8 Msun at z ~ 20 successfully leads to the formation of >~ 10^5 Msun BHs by z >~ 10 which can be the potential seeds of supermassive black holes seen today.Comment: 11 pages, 10 figures, accepted for publication in MNRA

    The formation of globular clusters with top-heavy initial mass functions

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    We study the formation of globular clusters in massive compact clouds with the low-metallicity of Z=10βˆ’3Β ZβŠ™Z=10^{-3}~Z_{\odot} by performing three-dimensional radiative-hydrodynamics simulations. Considering the uncertainty of the initial mass function (IMF) of stars formed in low-metallicity and high-density clouds, we investigate the impacts of the IMF on the cloud condition for the GC formation with the range of the power-law index of IMF as Ξ³=1βˆ’2.35\gamma = 1-2.35. We find that the threshold surface density (Ξ£thr\Sigma_{\rm thr}) for the GC formation increases from 800Β MβŠ™β€…β€Špcβˆ’2800~M_{\odot} \; {\rm pc^{-2}} at Ξ³=2.35\gamma = 2.35 to 1600Β MβŠ™β€…β€Špcβˆ’21600~M_{\odot}\; {\rm pc^{-2}} at Ξ³=1.5\gamma = 1.5 in the cases of clouds with Mcl=106Β MβŠ™M_{\rm cl} = 10^6~M_{\odot} because the emissivity of ionizing photons per stellar mass increases as Ξ³\gamma decreases. For Ξ³<1.5\gamma < 1.5, Ξ£thr\Sigma_{\rm thr} saturates with ∼2000Β MβŠ™β€…β€Špcβˆ’2\sim 2000~M_{\odot}\; {\rm pc^{-2}} that is quite rare and observed only in local starburst galaxies due to e.g., merger processes. Thus, we suggest that formation sites of low-metallicity GCs could be limited only in the very high-surface density regions. We also find that Ξ£thr\Sigma_{\rm thr} can be modelled by a power-law function with the cloud mass (MclM_{\rm cl}) and the emissivity of ionizing photons (sβˆ—s_*) as ∝Mclβˆ’1/5sβˆ—2/5\propto M_{\rm cl}^{-1/5} s_{*}^{2/5}. Based on the relation between the power-law slope of IMF and Ξ£thr\Sigma_{\rm thr}, future observations with e.g., the James Webb Space Telescope can allow us to constrain the IMF of GCs.Comment: 9 pages, 6 figures, accepted for publication in MNRA
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