Hidenobu Yajima

Escape fraction of ionizing photons from high-redshift galaxies in cosmological SPH simulations

Combing the three-dimensional radiative transfer (RT) calculation and cosmological smoothed particle hydrodynamics (SPH) simulations, we study the escape fraction of ionizing photons (f esc ) of high-redshift galaxies at z = 3-6. Our simulations cover the halo mass range of M h = 10 9 -10 12 M ⊙ . We post-process several hundred simulated galaxies with the Authentic Radiative Transfer (ART) code to study the halo mass dependence of f esc . In this paper, we restrict ourselves to the transfer of stellar radiation from local stellar population in each dark matter halo. We find that the average f esc steeply decreases as the halo mass increases, with a large scatter for the lower-mass haloes. The low-mass haloes with M h ∼ 10 9 M ⊙ have large values of f esc (with an average of ∼0.4), whereas the massive haloes with M h ∼ 10 11 M ⊙ show small values of f esc (with an average of ∼0.07). This is because in our simulations, the massive haloes show more clumpy structure in gas distribution, and the star-forming regions are embedded inside these clumps, making it more difficult for the ionizing photons to escape. On the other hand, in low-mass haloes, there are often conical regions of highly ionized gas due to the shifted location of young star clusters from the centre of dark matter halo, which allows the ionizing photons to escape more easily than in the high-mass haloes. By counting the number of escaped ionizing photons, we show that the star-forming galaxies can ionize the intergalactic medium at z = 3-6. The main contributor to the ionizing photons is the haloes with M h ≲ 10 10 M ⊙ owing to their high f esc . The large dispersion in f esc suggests that there may be various sizes of H II bubbles around the haloes even with the same mass in the early stages of reionization. We also examine the effect of UV background radiation field on f esc using simple, four different treatments of UV background.

Escape of Lyα and continuum photons from star-forming galaxies

A large number of high-redshift galaxies have been discovered via their narrow-band Lya line or broad-band continuum colors in recent years. The nature of the escaping process of photons from these early galaxies is crucial to understanding galaxy evolution and the cosmic reionization. Here, we investigate the escape of Lya, non-ionizing UV-continuum (l = 1300 - 1600 angstrom in rest frame), and ionizing photons (l 6, but they can maintain the ionization of IGM at z ~ 0 - 5.

The escape of ionizing photons from supernova-dominated primordial galaxies

In order to assess the contribution of Lyman break galaxies (LBGs) and Lyman α emitters (LAEs) at redshifts 3 6. That implies that additional ionization sources may be required at z > 6.

ART2: coupling Lyα line and multi-wavelength continuum radiative transfer

Narrow-band Lya line and broad-band continuum have played important roles in the discovery of high-redshift galaxies in recent years. Hence, it is crucial to study the radiative transfer of both Lya and continuum photons in the context of galaxy formation and evolution in order to understand the nature of distant galaxies. Here, we present a three-dimensional Monte Carlo radiative transfer code, All-wavelength Radiative Transfer with Adaptive Refinement Tree (ART^2), which couples Lya line and multi-wavelength continuum, for the study of panchromatic properties of galaxies and interstellar medium. This code is based on the original version of Li et al., and features three essential modules: continuum emission from X-ray to radio, Lya emission from both recombination and collisional excitation, and ionization of neutral hydrogen. The coupling of these three modules, together with an adaptive refinement grid, enables a self-consistent and accurate calculation of the Lya properties. As an example, we apply ART^2 to a cosmological simulation that includes both star formation and black hole growth, and study in detail a sample of massive galaxies at redshifts z=3.1 - 10.2. We find that these galaxies are Lya emitters (LAEs), whose Lya emission traces the dense gas region, and that their Lya lines show a shape characteristic of gas inflow. Furthermore, the Lya properties, including photon escape fraction, emergent luminosity, and equivalent width, change with time and environment. Our results suggest that LAEs evolve with redshift, and that early LAEs such as the most distant one detected at z ~ 8.6 may be dwarf galaxies with a high star formation rate fueled by infall of cold gas, and a low Lya escape fraction.

Were Progenitors of Local L* Galaxies Lyα Emitters at High Redshift?

The Lya emission has been observed from galaxies over a redshift span z ~ 0 - 8.6. However, the evolution of high-redshift Lya emitters (LAEs), and the link between these populations and local galaxies, remain poorly understood. Here, we investigate the Lya properties of progenitors of a local L* galaxy by combining cosmological hydrodynamic simulations with three-dimensional radiative transfer calculations using the new ART^2 code. We find that the main progenitor (the most massive one) of a Milky Way-like galaxy has a number of Lya properties close to those of observed LAEs at z ~ 2 - 6, but most of the fainter ones appear to fall below the detection limits of current surveys. The Lya photon escape fraction depends sensitively on a number of physical properties of the galaxy, such as mass, star formation rate, and metallicity, as well as galaxy morphology and orientation. Moreover, we find that high-redshift LAEs show blue-shifted Lya line profiles characteristic of gas inflow, and that the Lya emission by excitation cooling increases with redshift, and becomes dominant at z > 6. Our results suggest that some observed LAEs at z ~ 2-6 with luminosity of L_Lya ~ 10^{42-43} ergs/s may be similar to the main progenitor of the Milky Way at high redshift, and that they may evolve into present-day L* galaxies.

ART^2 : Coupling Lyman-alpha Line and Multi-wavelength Continuum Radiative Transfer

Narrow-band Lya line and broad-band continuum have played important roles in the discovery of high-redshift galaxies in recent years. Hence, it is crucial to study the radiative transfer of both Lya and continuum photons in the context of galaxy formation and evolution in order to understand the nature of distant galaxies. Here, we present a three-dimensional Monte Carlo radiative transfer code, All-wavelength Radiative Transfer with Adaptive Refinement Tree (ART^2), which couples Lya line and multi-wavelength continuum, for the study of panchromatic properties of galaxies and interstellar medium. This code is based on the original version of Li et al., and features three essential modules: continuum emission from X-ray to radio, Lya emission from both recombination and collisional excitation, and ionization of neutral hydrogen. The coupling of these three modules, together with an adaptive refinement grid, enables a self-consistent and accurate calculation of the Lya properties. As an example, we apply ART^2 to a cosmological simulation that includes both star formation and black hole growth, and study in detail a sample of massive galaxies at redshifts z=3.1 - 10.2. We find that these galaxies are Lya emitters (LAEs), whose Lya emission traces the dense gas region, and that their Lya lines show a shape characteristic of gas inflow. Furthermore, the Lya properties, including photon escape fraction, emergent luminosity, and equivalent width, change with time and environment. Our results suggest that LAEs evolve with redshift, and that early LAEs such as the most distant one detected at z ~ 8.6 may be dwarf galaxies with a high star formation rate fueled by infall of cold gas, and a low Lya escape fraction.

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

We study the effect of local stellar radiation and ultraviolet background (UVB) radiation on the physical properties of damped Lyα systems (DLAs) and Lyman limit systems (LLSs) at z = 3 using cosmological smoothed particle hydrodynamics simulations. We post-process our simulations with the authentic radiation transfer (ART) code for radiative transfer of local stellar radiation and UVB radiation. We find that the DLA and LLS cross-sections are significantly reduced by the UVB radiation, whereas the local stellar radiation does not affect them very much except in the low-mass haloes. This is because the 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 the 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 radiation 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. The DLA gas is less extended than the virial radius of the halo, and LLSs are distributed over the similar scale to the virial radius of the host halo. Our simulations suggest that the median properties of DLA host haloes are Mh = 2.4 × 1010 M⊙, SFR = 0.3 M⊙ yr−1, M★ = 2.4 × 108 M⊙ and Z/Z⊙ = 0.1. About 30 per cent of DLAs are hosted by haloes having SFR = 1–20 M⊙ yr−1, which is the typical star formation rate (SFR) range for Lyman break galaxies (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 × 109 M⊙, SFR = 0.06 M⊙ yr−1, M★ = 6.5 × 107 M⊙ and Z/Z⊙ = 0.08. About 80 per cent of total LLS cross-section are hosted by haloes with SFR ≲ 1 M⊙ yr−1, hence most LLSs are associated with low-mass haloes with faint LBGs below the current detection limit.

Observational properties of simulated galaxies in overdense and average regions at redshifts z ≃ 6–12

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 are 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.

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

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.

EXTENDED Lyα EMISSION FROM INTERACTING GALAXIES AT HIGH REDSHIFTS

Recent observations have discovered a population of extended Lyα sources, dubbed Lyα blobs (LABs), at high redshift z � 2 6.6. These LABs typically have a luminosity of L � 10 42 10 44 erg s −1 , and a size of tens of kiloparsecs, with some giant ones reaching up to D � 100 kpc. However, the origin of these LABs is not well understood. In this paper, we investigate a merger model for the formation of LABs by studying Lyα emission from interacting galaxies at high redshifts by means of a combination of hydrodynamics simulations with three-dimensional radiative transfer calculations. Our galaxy simulations focus on a set of binary major mergers of galaxies with a mass range of 3 7 × 10 12 M⊙ in the redshift range of z � 3 7, and we use the newly improved ART 2 code to perform the radiative transfer calculations which couple multi-wavelength continuum, ionization of hydrogen, and Lyα line emission. We find that intense star formation and enhanced cooling induced by gravitational interaction produce strong Lyα emission from these merging galaxies. The Lyα emission appears to be extended due to the extended distribution of sources and gas. During the close encounter of galaxy progenitors when the star formation rate peaks at � 10 3 M⊙ yr −1 , our model produces LABs with luminosity of L � 10 42 10 44 erg s −1 , and size of D � 10 20 kpc at z > 6 and D � 20 50 kpc at z � 3, in broad agreement with observations in the same redshift range. Our results suggest that merging galaxies may produce some typical LABs as observed, but the giant ones may be produced by mergers more massive than those in our model, or a combination of mergers and cold accretion from filaments on a large scale. Subject headings: galaxies: high-redshift – galaxies: formation – galaxies: evolution – radiative transfer – hydrodynamics – methods: numerical