arXiv subjects

Kenji Hasegawa

Publications and source records attributed to Kenji Hasegawa.

At least 37 records · Page 2Linked to original sources

arXiv2018

Stellar mass dependence of the 21-cm signal around the first star and its impact on the global signal

The 21-cm signal in the vicinity of the first stars is expected to reflect properties of the first stars. In this study we pay special attention to tracing the time evolution of the ionizing photons' escape fraction, which affects the distribution of neutral hydrogen, by performing radiation hydrodynamics (RHD) simulations resolving dense gas in a halo. We find that the radial profile of 21-cm differential brightness temperature is quite sensitive to the stellar and halo masses, which reflects the time evolution of the escape fraction. In the case of a less massive star, ionizing photons hardly escape from its host halo due to the absorption by dense halo gas, thus an deep 21-cm absorption feature at just outside the halo lasts a long time. Whereas photons from a massive star well working to heat the ambient intergalactic medium turn out to cause a spatially extended 21-cm emission signature. Although individual signals are found to be undetectable with the Square Kilometre Array, our analysis using the results from the RHD simulations indicates that the properties of the first stars are imprinted on the 21-cm global signal: its amplitude depends not only on the cosmic star formation rate density, but also on the typical mass of the first stars due to the stellar-mass-dependent heating rate. Thus, we suggest that the initial mass function of the first stars is an essential factor in understanding the global signal.

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arXiv2018

SILVERRUSH. VI. A simulation of Ly$α$ emitters in the reionization epoch and a comparison with Subaru Hyper Suprime-Cam survey early data

The survey of Lyman $α$ emitters (LAEs) with Subaru Hyper Suprime-Cam, called SILVERRUSH (Ouchi et al.), is producing massive data of LAEs at $z\gtrsim6$. Here we present LAE simulations to compare the SILVERRUSH data. In 162$^3$ comoving Mpc$^3$ boxes, where numerical radiative transfer calculations of reionization were performed, LAEs have been modeled with physically motivated analytic recipes as a function of halo mass. We have examined $2^3$ models depending on the presence or absence of dispersion of halo Ly$α$ emissivity, dispersion of the halo Ly$α$ optical depth, $τ_α$, and halo mass dependence of $τ_α$. The unique free parameter in our model, a pivot value of $τ_α$, is calibrated so as to reproduce the $z=5.7$ Ly$α$ luminosity function (LF). We compare our model predictions with Ly$α$ LFs at $z=6.6$ and $7.3$, LAE angular auto-correlation functions (ACFs) at $z=5.7$ and $6.6$, and LAE fractions in Lyman break galaxies at $5<z<7$. The Ly$α$ LFs and ACFs are reproduced by multiple models, but the LAE fraction turns out to be the most critical test. The dispersion of $τ_α$ and the halo mass dependence of $τ_α$ are essential to explain all observations reasonably. Therefore, a simple model of one-to-one correspondence between halo mass and Ly$α$ luminosity with a constant Ly$α$ escape fraction has been ruled out. Based on our best model, we present a formula to estimate the intergalactic neutral hydrogen fraction, $x_{\rm HI}$, from the observed Ly$α$ luminosity density at $z\gtrsim6$. We finally obtain $x_{\rm HI}=0.5_{-0.3}^{+0.1}$ as a volume-average at $z=7.3$.

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arXiv2018

SEURAT: SPH scheme extended with ultraviolet line radiative transfer

We present a novel Lyman alpha (Ly$α$) radiative transfer code, SEURAT, where line scatterings are solved adaptively with the resolution of the smoothed particle hydrodynamics (SPH). The radiative transfer method implemented in SEURAT is based on a Monte Carlo algorithm in which the scattering and absorption by dust are also incorporated. We perform standard test calculations to verify the validity of the code; (i) emergent spectra from a static uniform sphere, (ii) emergent spectra from an expanding uniform sphere, and (iii) escape fraction from a dusty slab. Thereby we demonstrate that our code solves the Ly$α$ radiative transfer with sufficient accuracy. We emphasise that SEURAT can treat the transfer of Ly$α$ photons even in highly complex systems that have significantly inhomogeneous density fields. The high adaptivity of SEURAT is desirable to solve the propagation of Ly$α$ photons in the interstellar medium of young star-forming galaxies like Ly$α$ emitters (LAEs). Thus, SEURAT provides a powerful tool to model the emergent spectra of Ly$α$ emission, which can be compared to the observations of LAEs.

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arXiv2018

SILVERRUSH. VII. Subaru/HSC Identifications of 42 Protocluster Candidates at z~6-7 with the Spectroscopic Redshifts up to z=6.574: Implications for Cosmic Reionization

We report fourteen and twenty-eight protocluster candidates at z=5.7 and 6.6 over 14 and 19 deg^2 areas, respectively, selected from 2,230 (259) Lya emitters (LAEs) photometrically (spectroscopically) identified with Subaru/Hyper Suprime-Cam (HSC) deep images (Keck, Subaru, and Magellan spectra and the literature data). Six out of the 42 protocluster candidates include 1-12 spectroscopically confirmed LAEs at redshifts up to z=6.574. By the comparisons with the cosmological Lya radiative transfer (RT) model reproducing LAEs with the reionization effects, we find that more than a half of these protocluster candidates are progenitors of the present-day clusters with a mass of > 10^14 M_sun. We then investigate the correlation between LAE overdensity delta and Lya rest-frame equivalent width EW_Lya^rest, because the cosmological Lya RT model suggests that a slope of EW_Lya^rest-delta relation is steepened towards the epoch of cosmic reionization (EoR), due to the existence of the ionized bubbles around galaxy overdensities easing the escape of Lya emission from the partly neutral intergalactic medium (IGM). The available HSC data suggest that the slope of the EW_Lya^rest-delta correlation does not evolve from the post-reionization epoch z=5.7 to the EoR z=6.6 beyond the moderately large statistical errors. There is a possibility that we would detect the evolution of the EW_Lya^rest - delta relation from z=5.7 to 7.3 by the upcoming HSC observations providing large samples of LAEs at z=6.6-7.3.

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arXiv2017

Modelling of Lyman-alpha Emitting Galaxies and Ionized Bubbles at the Era of Reionization

Understanding Ly$α$ emitting galaxies (LAEs) can be a key to reveal cosmic reionization and galaxy formation in the early Universe. Based on halo merger trees and Ly$α$ radiation transfer calculations, we model redshift evolution of LAEs and their observational properties at $z \ge 6$. We consider ionized bubbles associated with individual LAEs and IGM transmission of Ly$α$ photons. We find that Ly$α$ luminosity tightly correlates with halo mass and stellar mass, while the relation with star formation rate has a large dispersion. Comparing our models with the observed luminosity function by Konno et al., we suggest that LAEs at $z \sim 7$ have galactic wind of $V_{\rm out} \gtrsim 100~\rm km\, s^{-1}$ and HI column density of $N_{\rm HI} \gtrsim 10^{20}~\rm cm^{-2}$. Number density of bright LAEs rapidly decreases as redshift increases, due to both lower star formation rate and smaller HII bubbles. Our model predicts future wide deep surveys with next generation telescopes, such as JWST, E-ELT and TMT, can detect LAEs at $z \sim 10$ with a number density of $n_{\rm LAE} \sim {\rm a~few~} \times10^{-6} ~\rm Mpc^{-3}$ for the flux sensitivity of $10^{-18} ~\rm erg\, cm^{-2}\, s^{-1}$. By combining these surveys with future 21-cm observations, it could be possible to detect both LAEs with $L_{Lyα} \gtrsim 10^{42}~\rm erg~s^{-1}$ and their associated giant HII bubbles with the size $\gtrsim 250 ~\rm kpc$ at $z \sim 10$.

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arXiv2017

Diversity of the Lyman continuum escape fractions of high-$z$ galaxies and its origins

The Lyman continuum (LyC) escape fraction is a key quantity to determine the contribution of galaxies to cosmic reionization. It has been known that the escape fractions estimated by observations and numerical simulations show a large diversity. However, the origins of the diversity are still uncertain. In this work, to understand what quantities of galaxies are responsible for controlling the escape fraction, we numerically evaluate the escape fraction by performing ray-tracing calculation with simplified disc galaxy models. With a smooth disc model, we explore the dependence of the escape fraction on the disposition of ionizing sources, and find that the escape fraction varies up to $\sim 3$ orders of magnitude. It is also found that the halo mass dependence of disc scale height determines whether the escape fraction increases or decreases with halo mass. With a clumpy disc model, it turns out that the escape fraction increases as the clump mass fraction increases because the density in the inter-clump region decreases. In addition, we find that clumpiness regulates the escape fraction via two ways when the total clump mass dominates the total gas mass; the escape fraction is controlled by the covering factor of clumps if the clumps are dense sufficient to block LyC photons, otherwise the clumpiness works to reduce the escape fraction by increasing the total number of recombination events in a galaxy.

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arXiv2017

Constraining the Contribution of Galaxies and Active Galactic Nuclei to Cosmic Reionization

Understanding the detailed process of cosmic reionization is one of the remaining problems in astrophysics and cosmology. Here we construct a model of cosmic reionization that includes contribution from high-$z$ galaxies and active galactic nuclei (AGNs), and calculate the reionization and thermal histories with the model. To keep the model to be general and realistic, we vary the escape fraction of ionizing photons, $f_{\rm esc}$, and the faint-end slope of the AGN luminosity function at high redshifts, $α_{\rm hz}$, within the constraints from the observed cosmic star formation history and the observed bright-end UV luminosity functions at $z \leq 6$. Additionally, we model the spectral energy distribution (SED) of AGNs which depends on the Eddington ratio and the black-hole mass. By comparing the computed reionization histories with the observed \ion{H}{i} fractions and the optical depth for Thomson scattering from Planck, we find that $α_{\rm hz} > -1.5$ and $f_{\rm esc} < 0.15$ are favored when we employ the bright-end luminosity function obtained by Giallongo et al. Our result suggests that an AGN-dominated model with the abundance of faint-AGNs being as large as the estimate by Giallongo et al. is allowed only if the contribution from high-z galaxies is almost negligible, while a galaxy-dominant model is also allowed. We also find that the shape of SED has a significant impact on the thermal history. Therefore it is expected that measurements of the thermal state of the IGM provide useful information on properties of ionizing sources.

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arXiv2017

Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH). I. Program Strategy and Clustering Properties of ~2,000 Lya Emitters at z=6-7 over the 0.3-0.5 Gpc$^2$ Survey Area

We present the SILVERRUSH program strategy and clustering properties investigated with $\sim 2,000$ Ly$α$ emitters at $z=5.7$ and $6.6$ found in the early data of the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey exploiting the carefully designed narrowband filters. We derive angular correlation functions with the unprecedentedly large samples of LAEs at $z=6-7$ over the large total area of $14-21$ deg$^2$ corresponding to $0.3-0.5$ comoving Gpc$^2$. We obtain the average large-scale bias values of $b_{\rm avg}=4.1\pm 0.2$ ($4.5\pm 0.6$) at $z=5.7$ ($z=6.6$) for $\gtrsim L^*$ LAEs, indicating the weak evolution of LAE clustering from $z=5.7$ to $6.6$. We compare the LAE clustering results with two independent theoretical models that suggest an increase of an LAE clustering signal by the patchy ionized bubbles at the epoch of reionization (EoR), and estimate the neutral hydrogen fraction to be $x_{\rm HI}=0.15^{+0.15}_{-0.15}$ at $z=6.6$. Based on the halo occupation distribution models, we find that the $\gtrsim L^*$ LAEs are hosted by the dark-matter halos with the average mass of $\log (\left < M_{\rm h} \right >/M_\odot) =11.1^{+0.2}_{-0.4}$ ($10.8^{+0.3}_{-0.5}$) at $z=5.7$ ($6.6$) with a Ly$α$ duty cycle of 1 % or less, where the results of $z=6.6$ LAEs may be slightly biased, due to the increase of the clustering signal at the EoR. Our clustering analysis reveals the low-mass nature of $\gtrsim L^*$ LAEs at $z=6-7$, and that these LAEs probably evolve into massive super-$L^*$ galaxies in the present-day universe.

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arXiv2017

Can HI 21 cm line trace the Missing Baryons in the Filamentary Structures?

A large fraction of baryons predicted from the standard cosmology has been missing observationally. Although previous numerical simulations have indicated that most of the missing baryons reside in large-scale filaments in the form of Warm Hot Intergalactic Medium (WHIM), it is generally very difficult to detect signatures from such a diffuse gas. In this work, we focus on the hyperfine transition of neutral hydrogen (HI) called 21-cm line as a tool to trace the WHIM. For the purpose, we first construct the map of the 21-cm signals by using the data provided by the state-of-the-art cosmological hydrodynamics simulation project, Illustris, in which detailed processes affecting the dynamical and thermal evolution of the WHIM are implemented. From the comparison with the constructed 21-cm signal map with the expected noise level of the Square Kilometre Array phase 1 mid-frequency instrument (SKA1-mid), we find that the 21-cm signals from the filamentary structures at redshifts z=0.5-3 are detectable with the SKA1-mid if we assume the angular resolution of Δθ> 10 arcmin and the observing time of t_obs > 100 hours. However, it also turns out that the signals mainly come from galaxies residing in the filamentary structures and the contribution from the WHIM is too small to detect with the SKA1-mid. Our results suggest that about 10 times higher sensitivity than the SKA1-mid is possibly enough to detect the WHIM at z=0.5-3.

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arXiv2017

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

Recent radio observations show that the giant molecular cloud (GMC) mass functions noticeably vary across galactic disks. High-resolution magnetohydrodynamics simulations show that multiple episodes of compression are required for creating a molecular cloud in the magnetized interstellar medium. In this article, we formulate the evolution equation for the GMC mass function to reproduce the observed profiles, for which multiple compression are driven by the network of expanding shells due to HII regions and supernova remnants. We introduce the cloud-cloud collision (CCC) terms in the evolution equation in contrast to the previous work (Inutsuka et al. 2015). The computed time evolution suggests that the GMC mass function slope is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC effect is limited only in the massive-end of the mass function. In addition, we identify a gas resurrection channel that allows the gas dispersed by massive stars to regenerate GMC populations or to accrete onto the pre-existing GMCs. Our results show that almost all of the dispersed gas contribute to the mass growth of pre-existing GMCs in arm regions whereas less than 60 per cent in inter-arm regions. Our results also predict that GMC mass functions have a single power-law exponent in the mass range < 10^5.5 Msun (where Msun represents the solar mass), which is well characterized by GMC self-growth and dispersal timescales. Measurement of the GMC mass function slope provides a powerful method to constrain those GMC timescales and the gas resurrecting factor in various environment across galactic disks.

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arXiv2017

Where are the Low-Mass Population III Stars ?

We study the number and the distribution of low mass Pop III stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and Milky Way sized halos are followed by a high resolution cosmological simulation. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa IMF from 0.15 to 1.0 Msun for low mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high and middle latitude fields by photometrically selecting low metallicity stars with optimized narrow band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than ten at <100 kpc for the tip of redgiant stars. Provided that available observations have not detected any survivors, the formation models of low mass Pop III stars with more than ten stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III star at a high mass range of > 10 Msun.

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arXiv2016

Formation of globular clusters induced by external ultraviolet radiation II: Three-dimensional radiation hydrodynamics simulations

We explore the possibility of the formation of globular clusters under ultraviolet (UV) background radiation. One-dimensional spherical symmetric radiation hydrodynamics (RHD) simulations by Hasegawa et al. have demonstrated that the collapse of low-mass (10^6-10^7 solar masses) gas clouds exposed to intense UV radiation can lead to the formation of compact star clusters like globular clusters (GCs) if gas clouds contract with supersonic infall velocities. However, three-dimensional effects, such as the anisotropy of background radiation and the inhomogeneity in gas clouds, have not been studied so far. In this paper, we perform three-dimensional RHD simulations in a semi-cosmological context, and reconsider the formation of compact star clusters in strong UV radiation fields. As a result, we find that although anisotropic radiation fields bring an elongated shadow of neutral gas, almost spherical compact star clusters can be procreated from a "supersonic infall" cloud, since photo-dissociating radiation suppresses the formation of hydrogen molecules in the shadowed regions and the regions are compressed by UV heated ambient gas. The properties of resultant star clusters match those of GCs. On the other hand, in weak UV radiation fields, dark matter-dominated star clusters with low stellar density form due to the self-shielding effect as well as the positive feedback by ionizing photons. Thus, we conclude that the "supersonic infall" under a strong UV background is a potential mechanism to form GCs.

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arXiv2016

Japanese Cosmic Dawn/Epoch of Reionization Science with the Square Kilometre Array

Cosmic reionization is known to be a major phase transition of the gas in the Universe. Since astronomical objects formed in the early Universe, such as the first stars, galaxies and black holes, are expected to have caused cosmic reionization, the formation history and properties of such objects are closely related to the reionization process. In spite of the importance of exploring reionization, our understandings regarding reionization is not sufficient yet. Square Kilometre Array (SKA) is a next-generation large telescope that will be operated in the next decade. Although several programs of next-generation telescopes are currently scheduled, the SKA will be the unique telescope with a potential to directly observe neutral hydrogen up to z~30, and provide us with valuable information on the Cosmic Dawn (CD) and the Epoch of Reionization (EoR). The early science with the SKA will start in a few years; it is thus the time for us to elaborate a strategy for CD/EoR Science with the SKA. The purpose of this document is to introduce Japanese scientific interests in the SKA project and to report results of our investigation.

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arXiv2015

A new ray-tracing scheme for 3D diffuse radiation transfer on highly parallel architectures

We present a new numerical scheme to solve the transfer of diffuse radiation on three-dimensional mesh grids which is efficient on processors with highly parallel architecture such as recently popular GPUs and CPUs with multi- and many-core architectures. The scheme is based on the ray-tracing method and the computational cost is proportional to $N_{\rm m}^{5/3}$ where $N_{\rm m}$ is the number of mesh grids, and is devised to compute the radiation transfer along each light-ray completely in parallel with appropriate grouping of the light-rays. We find that the performance of our scheme scales well with the number of adopted CPU cores and GPUs, and also that our scheme is nicely parallelized on a multi-node system by adopting the multiple wave front scheme, and the performance scales well with the amount of the computational resources. As numerical tests to validate our scheme and to give a physical criterion for the angular resolution of our ray-tracing scheme, we perform several numerical simulations of the photo-ionization of neutral hydrogen gas by ionizing radiation sources without the "on-the-spot" approximation, in which the transfer of diffuse radiation by radiative recombination is incorporated in a self-consistent manner.

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arXiv2014

The Mass Spectrum of the First Stars

We perform cosmological hydrodynamics simulations with non-equilibrium primordial chemistry to obtain 59 minihalos that host first stars. The obtained minihalos are used as initial conditions of local three dimensional radiation hydrodynamics simulations to investigate the formation of the first stars. We find two-thirds of the minihalos host multiple stars, while the rest of them have single stars. The mass of the stars found in our simulations are in the range of 1 Msun \la M \la 300 Msun, peaking at several x 10 Msun. Most of the very massive stars of \ga 140 Msun are born as single stars, although not all of the single stars are very massive. We also find a few stars of \la 1 Msun that are kicked by the gravitational three body interactions to the position distant from the center of mass. The frequency that a star forming minihalo contains a binary system is \sim 50%. We also investigate the abundance pattern of the stellar remnants by summing up the contributions from the first stars in the simulations. Consequently, the pattern is compatible with that of the low metallicity Damped Lyman-alpha systems or the Extremely Metal Poor (EMP) stars, if the mass spectrum obtained in our experiment is shifted to the low mass side by 0.2 dex. If we consider the case that an EMP star is born in the remnant of the individual minihalo without mixing with others, the chemical signature of the pair instability supernova is more prominent, because most of them are born as single stars.

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arXiv2014

On the evolution of gas clouds exposed to AGN radiation. I. Three-dimensional radiation hydrodynamic simulations

We perform three-dimensional radiation hydrodynamic simulations of uniform dusty gas clouds irradiated by an active galactic nucleus (AGN) to investigate the dependence of evolution of clouds on the ionization parameter $\mathcal{U}$ and the Str{ö}mgren number $\mathcal{N}_{S}$. We find that the evolution can be classified into two cases depending on $\mathcal{U}$. In low $\mathcal{U}$ cases ($\mathcal{U}\approx 10^{-2}$), the evolution is mainly driven by photo-evaporation. A approximately spherically-symmetric evaporation flow with velocity of $100\operatorname{-}150\;\mathrm{km\;s^{-1}}$ is launched from the irradiated face. The cloud is compressed by a D-type shock with losing its mass due to photo-evaporation and is finally turned into a dense filament by $t\lesssim 1.5t_{\mathrm{sc}}$. In high $\mathcal{U}$ cases ($\mathcal{U}\approx 5\times 10^{-2}$), radiation pressure suppresses photo-evaporation from the central part of the irradiated face, reducing photo-evaporation rate. A evaporation flow from the outskirts of the irradiated face is turned into a high velocity ($\lesssim 500\;\mathrm{km\;s^{-1}}$) gas wind because of radiation pressure on dust. The cloud is swept by a radiation pressure-driven shock and becomes a dense gas disk by $t\approx t_{\mathrm{sweep}}$. Star formation is expected in these dense regions for both cases of $\mathcal{U}$. We discuss the influences of the AGN radiation on the clumpy torus. A simple estimate suggests that the clumps are destroyed in timescales shorter than their orbital periods. For the clumpy structure to be maintained over long period, the incident radiation field needs to be sufficiently weaken for most of the clumps, or, some mechanism that creates the clumps continuously is needed.

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arXiv2013

The impacts of ultraviolet radiation feedback on galaxies during the epoch of reionization

We explore the impacts of ultraviolet (UV) radiation feedback on galaxies during the epoch of reionisation by cosmological simulations in which hydrodynamics and the transfer of the H and He ionising photons are consistently coupled. Moreover we take into account H_2 non-equilibrium chemistry, including photo-dissociation. The most striking feature of the simulations is a high spatial resolution for the radiative transfer (RT) calculation which enables us to start considering not only external UV feedback processes but also internal UV feedback processes in each galaxy. We find that the star formation is significantly suppressed due to the internal UV and supernova (SN) feedback. In low mass galaxies with M<10^9Msun, a large amount of gas is evacuated by photo-evaporation as previous studies have shown, which results in the suppression of star formation. Surprisingly, star formation in massive halos is also strongly suppressed despite the fact that these halos hardly lose any gas by photo-evaporation. The suppression of star formation in massive halos is mainly caused by following two factors; (i) small scale clumpy structures in the galaxies are smoothened by the internal feedback, (ii) although the dense gas in the galaxies is mostly neutral, the H_2 formation and cooling processes are disturbed by mild photo-heating. Photo-dissociating radiations actually suppress star formation, but the magnitude of the effect is not so large in massive galaxies. Even though our simulation volume is too small to be a representative patch of the Universe during reionisation, we find that our simulated star formation rate densities and HI fractions at z=6-7 are consistent with those found in observations.

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arXiv2012

Formation and Radiative Feedback of First Objects and First Galaxies

First, the formation of first objects driven by dark matter is revisited by high-resolution hydrodynamic simulations. It is revealed that dark matter haloes of ~10^4M_sun can produce first luminous objects with the aid of dark matter cusps. Therefore, the mass of first objects is smaller by roughly two orders of magnitude than in the previous prediction. This implies that the number of Pop III stars formed in the early universe could be significantly larger than hitherto thought. Secondly, the feedback by photo-ionization and photo-dissociation photons in the first objects is explored with radiation hydrodynamic simulations, and it is demonstrated that multiple stars can form in a 10^5M_sun halo. Thirdly, the fragmentation of an accretion disk around a primordial protostar is explored with photo-dissociation feedback. As a result, it is found that the photo-dissociation can reduce the mass accretion rate onto protostars. Also, protostars as small as 0.8M_sun may be ejected and evolve with keeping their mass, which might be detected as "real first stars" in the Galactic halo. Finally, state-of-the-art radiation hydrodynamic simulations are performed to investigate the internal ionization of first galaxies and the escape of ionizing photons. We find that UV feedback by forming massive stars enhances the escape fraction even in a halo as massive as > 6* 10^9M_sun, while it reduces the star formation rate significantly. This may have a momentous impact on the cosmic reionization.

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