arXiv subjects

Akio K. Inoue

Publications and source records attributed to Akio K. Inoue.

At least 127 records · Page 7Linked to original sources

arXiv2019

Balmer Break Galaxy Candidates at $z \sim 6$: a Potential View on the Star-Formation Activity at $z \gtrsim 14$

We search for galaxies with a strong Balmer break (Balmer Break Galaxies; BBGs) at $z \sim 6$ over a 0.41 deg$^2$ effective area in the COSMOS field. Based on rich imaging data, including data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), three candidates are identified by their extremely red $K - [3.6]$ colors as well as by non-detection in X-ray, optical, far-infrared (FIR), and radio bands. The non-detection in the deep ALMA observations suggests that they are not dusty galaxies but BBGs at $z \sim 6$, although contamination from Active Galactic Nuclei (AGNs) at $z \sim 0$ cannot be completely ruled out for the moment. Our spectral energy distribution (SED) analyses reveal that the BBG candidates at $z \sim 6$ have stellar masses of $\approx 5 \times 10^{10} M_{\odot}$ dominated by old stellar populations with ages of $\gtrsim 700$ Myr. Assuming that all the three candidates are real BBGs at $z \sim 6$, we estimate the stellar mass density (SMD) to be $2.4^{+2.3}_{-1.3} \times 10^{4} M_{\odot}$ Mpc$^{-3}$. This is consistent with an extrapolation from the lower redshift measurements. The onset of star formation in the three BBG candidates is expected to be several hundred million years before the observed epoch of $z \sim 6$. We estimate the star-formation rate density (SFRD) contributed by progenitors of the BBGs to be 2.4 -- 12 $\times 10^{-5} M_{\odot}$ yr$^{-1} $Mpc$^{-3}$ at $z > 14$ (99.7\% confidence range). Our result suggests a smooth evolution of the SFRD beyond $z = 8$.

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arXiv2019

Detections of [OIII] 88 $μ$m in Two Quasars in the Reionization Epoch

With the Atacama Large Millimeter/sub-millimeter Array (ALMA), we report detections of the far-infrared (FIR) [OIII] 88 $μ$m line and the underlying dust continuum in the two quasars in the reionization epoch, J205406.48-000514.8 (hereafter J2054-0005) at $z=6.0391\pm0.0002$ and J231038.88+185519.7 (hereafter J2310+1855) at $z=6.0035\pm0.0007$. The [OIII] luminosity of J2054-0005 and J2310+1855 are $L_{\rm [OIII]} = (6.8\pm0.6) \times 10^{9}$ and $(2.4\pm0.6) \times 10^{9}$ $L_{\rm \odot}$, corresponding to $\approx$ 0.05\%\ and 0.01\%\ of the total infrared luminosity, $L_{\rm TIR}$, respectively. Combining these [OIII] luminosities with [CII] 158 $μ$m luminosities in the literature, we find that J2054-0005 and J2310+1855 have the [OIII]-to-[CII] luminosity ratio of $2.1\pm0.4$ and $0.3\pm0.1$, respectively, the latter of which is the lowest among objects so far reported at $z>6$. Combining [OIII] observations at $z\approx6-9$ from our study and the literature, we identify the [OIII] line deficit: objects with larger $L_{\rm TIR}$ have lower $L_{\rm [OIII]}$-to-$L_{\rm TIR}$ ratios. Furthermore, we also find that the anti-correlation is shifted toward higher $L_{\rm TIR}$ value when compared to the local [OIII] line deficit.

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arXiv2019

Subaru narrow-band imaging search for Lyman continuum from galaxies at z>3 in the GOODS-N field

We report results of a search for galaxies at z>3 with Lyman continuum (LyC) emission using a narrow-band filter NB359 with Subaru / Suprime-Cam in a ~800 arcmin$^2$ blank field around the GOODS-N. We use 103 star-forming galaxies (SFGs) and 8 AGNs with spectroscopic redshifts in a range between 3.06 and 3.5, and 157 photometrically selected z=3.1 Lyman $α$ emitter (LAE) candidates as the targets. After removing galaxies spectroscopically confirmed to be contaminated by foreground sources, we found two SFGs and one AGN as candidate LyC emitting sources among the targets with spectroscopic redshifts. Among LAE candidates, five sources are detected in the NB359 image, and three among them may be contaminated by foreground sources. We compare the sample galaxies in the GOODS-N with those in the SSA22, where a prominent protocluster at z=3.1 is known and the LyC search using the same NB359 filter has been made. Frequency of galaxies with LyC leakage in the SSA22 field may be about two times higher than the galaxies in the GOODS-N with the sample UV magnitude range, although the numbers of LyC detections in these fields are too small to make a statistically significant conclusion. By combining the sample galaxies in these fields, we place the 3$σ$ upper limits of the observed LyC-to-UV flux density ratio and LyC escape fraction for galaxies at z=3.1 with absolute UV magnitude $M_{UV} < -18.8$ as ($f_{LyC}/f_{UV})_{obs} < 0.036$ and $f_{esc}^{abs}<8$\%, respectively.

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arXiv2019

ADF22: Blind detections of [CII] line emitters shown to be spurious

We report Atacama Large Millimetre/submillimeter Array (ALMA) Cycle-5 follow-up observations of two candidate [CII] emitters at z ~ 6 in the ALMA deep field in SSA22 (ADF22). The candidates were detected blindly in a Cycle-2 ALMA survey covering ~ 5 square arcmins, with a single tuning, along with two CO lines associated with galaxies at lower redshifts. Various tests suggested at least one of the two > 6-sigma [CII] candidates should be robust (Hayatsu et al. 2017). Nevertheless, our new, deeper observations recover neither candidate, demonstrating a higher contamination rate than expected. The cause of the spurious detections is under investigation but at present it remains unclarified.

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arXiv2019

SILVERRUSH. VIII. Spectroscopic Identifications of Early Large Scale Structures with Protoclusters Over 200 Mpc at z~6-7: Strong Associations of Dusty Star-Forming Galaxies

We have obtained three-dimensional maps of the universe in $\sim200\times200\times80$ comoving Mpc$^3$ (cMpc$^3$) volumes each at $z=5.7$ and $6.6$ based on a spectroscopic sample of 179 galaxies that achieves $\gtrsim80$\% completeness down to the Ly$α$ luminosity of $\log(L_{\rm Lyα}/[\mathrm{erg\ s^{-1}}])=43.0$, based on our Keck and Gemini observations and the literature. The maps reveal filamentary large-scale structures and two remarkable overdensities made out of at least 44 and 12 galaxies at $z=5.692$ (z57OD) and $z=6.585$ (z66OD), respectively, making z66OD the most distant overdensity spectroscopically confirmed to date with $>10$ spectroscopically confirmed galaxies. We compare spatial distributions of submillimeter galaxies at $z\simeq 4-6$ with our $z=5.7$ galaxies forming the large-scale structures, and detect a $99.97\%$ signal of cross correlation, indicative of a clear coincidence of dusty star-forming galaxy and dust unobscured galaxy formation at this early epoch. The galaxies in z57OD and z66OD are actively forming stars with star formation rates (SFRs) $\gtrsim5$ times higher than the main sequence, and particularly the SFR density in z57OD is 10 times higher than the cosmic average at the redshift (a.k.a. the Madau-Lilly plot). Comparisons with numerical simulations suggest that z57OD and z66OD are protoclusters that are progenitors of the present-day clusters with halo masses of $\sim10^{14}\ \mathrm{M_\odot}$.

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arXiv2019

Lyman continuum observations across cosmic time: recent developments, future requirements

Quantifying the physical conditions that allow radiation emitted shortward of the hydrogen ionization edge at 911.7 Å to escape the first collapsed objects and ultimately reionize the universe is a compelling problem for astrophysics. The escape of LyC emission from star-forming galaxies and AGN is intimately tied to the emergence and sustenance of the metagalactic ionizing background that pervades the universe to the present day and in turn is tied to the emergence of structure at all epochs. JWST was built in part to search for the source(s) responsible for reionization, but it cannot observe LyC escape directly, because of the progressive increase in the mean transmission of the intergalactic medium towards the epoch of reionization. Remarkable progress has been made to date in directly detecting LyC leaking from star-forming galaxies using space-based and the ground-based observatories, but there remain significant gaps in our redshift coverage of the phenomenon. Ongoing projects to measure LyC escape at low- and intermediate-z will provide guidance to JWST investigations by analyzing the robustness of a set of proposed LyC escape proxies, and also provide a closeup examination of the physical conditions that favor LyC escape. However, currently available facilities are inadequate for deeply probing LyC escape at the faint end of the galaxy luminosity function. Doing so will require facilities that can detect LyC emission in the restframe to limiting magnitudes approaching 28 $< m^*_{(1+z)900} <$ 32 for $M^*_{(1+z)1500}$ galaxies. The goal of acquiring statistically robust samples for determining LyC luminosity functions across cosmic time will require multi-object spectroscopy from spacebased flagship class and groundbased ELT class telescopes along with ancillary panchromatic imaging and spectroscopy spanning the far-UV to the mid-IR.

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arXiv2019

CHORUS. III. Photometric and Spectroscopic Properties of Ly$α$ Blobs at $z=4.9-7.0$

We report the Subaru Hyper Suprime-Cam (HSC) discovery of two Ly$α$ blobs (LABs), dubbed z70-1 and z49-1 at $z=6.965$ and $z=4.888$ respectively, that are Ly$α$ emitters with a bright ($\log L_{\rm Lyα}/{\rm [erg\ s^{-1}]}>43.4$) and spatially-extended Ly$α$ emission, and present the photometric and spectroscopic properties of a total of seven LABs; the two new LABs and five previously-known LABs at $z=5.7-6.6$. The z70-1 LAB shows the extended Ly$α$ emission with a scale length of $1.4\pm 0.2$ kpc, about three times larger than the UV continuum emission, making z70-1 the most distant LAB identified to date. All of the 7 LABs, except z49-1, exhibit no AGN signatures such as X-ray emission, {\sc Nv}$λ$1240 emission, or Ly$α$ line broadening, while z49-1 has a strong {\sc Civ}$λ$1548 emission line indicating an AGN on the basis of the UV-line ratio diagnostics. We carefully model the point-spread functions of the HSC images, and conduct two-component exponential profile fitting to the extended Ly$α$ emission of the LABs. The Ly$α$ scale lengths of the core (star-forming region) and the halo components are $r_{\rm c}=0.6-1.2$ kpc and $r_{\rm h}=2.0-13.8$ kpc, respectively. The average $r_{\rm h}$ of the LABs falls on the extrapolation of the $r_{\rm h}$-Ly$α$ luminosity relation of the Ly$α$ halos around VLT/MUSE star-forming galaxies at the similar redshifts, suggesting that typical LABs at $z\gtrsim5$ are not special objects, but star-forming galaxies at the bright end.

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arXiv2019

Radiation-pressure-driven dust transport to galaxy halos at $z\sim 10$

The origin of dust in galaxy halos or in the circum-galactic medium (CGM) is still a mystery. We investigate if the radiation pressure in high-redshift ($z\sim 10$) galaxies can efficiently transport dust to halos. To clarify the first dust enrichment of galaxy halos in the early Universe, we solve the motion of a dust grain considering radiation pressure, gas drag, and gravity in the vertical direction of the galactic disc. Radiation pressure is estimated in a consistent manner with the stellar spectra and dust extinction. As a consequence, we find that dust grains with radii $a\sim 0.1~μ$m successfully escape from the galactic disc if the ongoing star formation episode converts more than 15 per cent of the baryon content into stars and lasts $\gtrsim 30$ Myr, while larger and smaller grains are trapped in the disc because of gravity and gas drag, respectively. We also show that grain charge significantly enhances gas drag at a few--10 scale heights of the galactic disc, where the grain velocities are suppressed to $\sim 1$ km s$^{-1}$. There is an optimum dust-to-gas ratio ($\sim 10^{-3}$) in the galactic disc and an optimum virial mass $\sim 10^{10}$--$10^{11}$ M$_{\odot}$ for the transport of $a\sim 0.1~μ$m grains to the halo. We conclude that early dust enrichment of galaxy halos at $z\gtrsim 10$ is important for the origin of dust in the CGM.

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arXiv2019

"Big Three Dragons": a z = 7.15 Lyman BreakGalaxy Detected in [OIII] 88 $μ$m, [CII] 158 $μ$m, and Dust Continuum with ALMA

We present new ALMA observations and physical properties of a Lyman Break Galaxy at z=7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, $M_{\rm UV}\approx-22.4$, and has been spectroscopically identified in Ly$α$ with a small rest-frame equivalent width of $\approx4$ Å. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [OIII] 88 $μ$m, [CII] 158 $μ$m, and their underlying dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII] lines have consistent redshifts of $7.1520\pm0.0003$, and the [OIII] luminosity, $(34.4\pm4.1)\times10^{8}L_{\rm \odot}$, is about three times higher than the [CII] luminosity, $(11.0\pm1.4)\times10^{8}L_{\rm \odot}$. With our two continuum flux densities, the dust temperature is constrained to be $T_{\rm d}\approx50-60$ K under the assumption of the dust emissivity index of $β_{\rm d}=2.0-1.5$, leading to a large total infrared luminosity of $L_{\rm TIR}\approx1\times10^{12}L_{\rm \odot}$. Owing to our high spatial resolution data, we show that the [OIII] and [CII] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by $\approx200$ km s$^{-1}$. We also find these two clumps have comparable UV, infrared, [OIII], and [CII] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR$=260^{+119}_{-57}$ Gyr$^{-1}$, inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [OIII]-to-[CII] luminosity ratio.

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arXiv2019

Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the Reionization Era

We present the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [O III] 88 $μ$m line and rest-frame 90 $μ$m dust continuum emission in a Y-dropout Lyman break galaxy (LBG), MACS0416_Y1, lying behind the Frontier Field cluster MACS J0416.1-2403. This [O III] detection confirms the LBG with a spectroscopic redshift of $z = 8.3118 \pm 0.0003$, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 $μ$m flux density of $137 \pm 26$ $μ$Jy corresponds to a de-lensed total infrared (IR) luminosity of $L_{\rm IR} = (1.7 \pm 0.3) \times 10^{11} L_{\odot}$ if assuming a dust temperature of $T_{\rm dust} = 50$ K and an emissivity index of $β= 1.5$, yielding a large dust mass of $4 \times 10^6 M_{\odot}$. The ultraviolet-to-far IR spectral energy distribution modeling where the [O III] emissivity model is incorporated suggests the presence of a young ($τ_{\rm age} \approx 4$ Myr), star-forming (SFR $\approx 60 M_{\odot}$ yr$^{-1}$), moderately metal-polluted ($Z \approx 0.2 Z_{\odot}$) stellar component with a mass of $M_{\rm star} = 3 \times 10^8 M_{\odot}$. An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in $τ_{\rm age} \approx 4$ Myr, suggesting a pre-existing evolved stellar component with $M_{\rm star} \sim 3 \times 10^9 M_{\odot}$ and $τ_{\rm age} \sim 0.3$ Gyr as the origin of the dust mass.

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arXiv2018

Disentangling the physical parameters of gaseous nebulae and galaxies

We present an analysis to disentangle the connection between physical quantities that characterize the conditions of ionized HII regions -- metallicity ($Z$), ionization parameter ($U$), and electron density ($n_\mathrm{e}$) -- and the global stellar mass ($M_\ast$) and specific star formation rate ($\mathrm{sSFR}=\mathrm{SFR}/M_\ast$) of the host galaxies. We construct composite spectra of galaxies at $0.027 \le z \le 0.25$ from Sloan Digital Sky Survey, separating the sample into bins of $M_\ast$ and sSFR, and estimate the nebular conditions from the emission line flux ratios. Specially, metallicity is estimated from the direct method based on the faint auroral lines [OIII]$λ$4363 and [OII]$λλ$7320,7330. The metallicity estimates cover a wide range from $12+\log\mathrm{O/H}\sim7.6\textrm{--}8.9$. It is found that these three nebular parameters all are tightly correlated with the location in the $M_\ast$--sSFR plane. With simple physically-motivated ansätze, we derive scaling relations between these physical quantities by performing multi regression analysis. In particular, we find that $U$ is primarily controlled by sSFR, as $U \propto \mathrm{sSFR}^{0.43}$, but also depends significantly on both $Z$ and $n_\mathrm{e}$. The derived partial dependence of $U \propto Z^{-0.36}$ is weaker than the apparent correlation ($U\propto Z^{-1.52}$). The remaining negative dependence of $U$ on $n_\mathrm{e}$ is found to be $U \propto n_\mathrm{e}^{-0.29}$. The scaling relations we derived are in agreement with predictions from theoretical models and observations of each aspect of the link between these quantities. Our results provide a useful set of equations to predict the nebular conditions and emission-line fluxes of galaxies in semi-analytic models.

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arXiv2018

CHORUS II. Subaru/HSC Determination of the Ly$α$ Luminosity Function at $z=7.0$: Constraints on Cosmic Reionization Model Parameter

We present the Ly$α$ luminosity function (LF) derived from 34 Ly$α$ emitters (LAEs) at $z=7.0$ on the sky of $3.1$ deg$^2$, the largest sample compared to those in the literature obtained at a redshift $z\gtrsim7$. The LAE sample is made by deep large-area Subaru narrowband observations conducted by the Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) project. The $z=7.0$ Ly$α$ LF of our project is consistent with those of the previous DECam and Subaru studies at the bright and faint ends, respectively, while our $z=7.0$ Ly$α$ LF has uncertainties significantly smaller than those of the previous study results. Exploiting the small errors of our measurements, we investigate the shape of the faint to bright-end Ly$α$ LF. We find that the $z=7.0$ Ly$α$ LF shape can be explained by the steep slope of $α\simeq -2.5$ suggested at $z=6.6$, and that there is no clear signature of a bright-end excess at $z\simeq 7$ claimed by the previous work, which was thought to be made by the ionized bubbles around bright LAEs whose Ly$α$ photons could easily escape from the partly neutral IGM at $z \simeq 7$. We estimate the Ly$α$ luminosity densities (LDs) with Ly$α$ LFs at $z\simeq 6-8$ given by our and the previous studies, and compare the evolution of the UV-continuum LD estimated with dropouts. The Ly$α$ LD monotonically decreases from $z\sim 6$ to $8$, and evolves stronger than the UV-continuum LD, indicative of the Ly$α$ damping wing absorption of the IGM towards the heart of the reionization epoch.

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arXiv2018

The distribution and physical properties of high-redshift [OIII] emitters in a cosmological hydrodynamics simulation

Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) detected far-infrared emission lines such as the [OIII] 88 μm line from galaxies at $z \sim 7 - 9$. Far-infrared lines can be used to probe the structure and kinematics of such high-redshift galaxies as well as to accurately determine their spectroscopic redshifts. We use a cosmological simulation of galaxy formation to study the physical properties of [OIII] 88 μm emitters. In a comoving volume of 50 $h^{-1}$ Mpc on a side, we locate 34 galaxies with stellar masses greater than $10^8\ {\rm M_\odot}$ at $z = 9$, and more than 270 such galaxies at $z = 7$. We calculate the [OIII] 88 μm luminosities ($L_{\rm OIII}$) by combining a physical model of HII regions with emission line calculations using the photoionization code CLOUDY. We show that the resulting $L_{\rm OIII}$, for a given star formation rate, is slightly higher than predicted from the empirical relation for local galaxies, and is consistent with recent observations of galaxies at redshifts 7 - 9. Bright [OIII] emitters with $L_{\rm OIII} > 10^8 {\rm L_\odot}$ have stellar masses greater than $10^9\ {\rm M_\odot}$, star formation rates higher than $3\ {\rm M_\odot\ yr}^{-1}$, and the typical metallicity is $\sim 0.1\ {\rm Z_\odot}$. The galaxies are hosted by dark matter halos with masses greater than $10^{10.5}\ {\rm M_\odot}$. Massive galaxies show characteristic structure where the [OIII] emitting gas largely overlaps with young stars, but the emission peak is separated from the main stellar population, suggesting the stochastic and localized nature of star formation in the first galaxies. We propose to use the [OIII] 5007 Å line, to be detected by James Webb Space Telescope (JWST), to study the properties of galaxies whose [OIII] 88 μm line emission has been already detected with ALMA.

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arXiv2018

SILVERRUSH. V. Census of Lya, [OIII]5007, Ha, and [CII]158um Line Emission with ~1000 LAEs at z=4.9-7.0 Revealed with Subaru/HSC

We investigate Lya, [OIII]5007, Ha, and [CII]158um emission from 1124 galaxies at z=4.9-7.0. Our sample is composed of 1092 Lya emitters (LAEs) at z=4.9, 5.7, 6.6, and 7.0 identified by Subaru/Hyper Suprime-Cam (HSC) narrowband surveys covered by Spitzer large area survey with Subaru/HSC (SPLASH) and 34 galaxies at z=5.148-7.508 with deep ALMA [CII]158um data in the literature. Fluxes of strong rest-frame optical lines of [OIII] and Ha (Hb) are constrained by significant excesses found in the SPLASH 3.6 and 4.5um photometry. At z=4.9, we find that the rest-frame Ha equivalent width and the Lya escape fraction f_Lya positively correlate with the rest-frame Lya equivalent width EW^0_Lya. The f_Lya-EW^0_Lya correlation is similarly found at z~0-2, suggesting no evolution of the correlation over z~0-5. The typical ionizing photon production efficiency of LAEs is logxi_ion/[Hz erg^-1]~25.5 significantly (60-100%) higher than those of LBGs at a given UV magnitude. At z=5.7-7.0, there exists an interesting turn-over trend that the [OIII]/Ha flux ratio increases in EW^0_Lya~0-30 A, and then decreases out to EW^0_Lya~130 A. We also identify an anti-correlation between a [CII] luminosity to star-formation rate ratio (L_[CII]/SFR) and EW^0_Lya at the >99% confidence level. We carefully investigate physical origins of the correlations with stellar-synthesis and photoionization models, and find that a simple anti-correlation between EW_Lya^0 and metallicity explains self-consistently all of the correlations of Lya, Ha, [OIII]/Ha, and [CII] identified in our study, indicating detections of metal-poor (~0.03 Zo) galaxies with EW^0_Lya~200 A.

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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

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

Hubble Space Telescope Wide Field Camera 3 Observations of Escaping Lyman Continuum Radiation from Galaxies and Weak AGN at Redshifts $z$$\,\simeq\,$2.3-4.1

We present observations of escaping Lyman Continuum (LyC) radiation from 34 massive star-forming galaxies and 12 weak AGN with reliably measured spectroscopic redshifts at $z$$\simeq$2.3-4.1. We analyzed Hubble Space Telescope Wide Field Camera 3 (WFC3) mosaics of the Early Release Science field in three UVIS filters to sample the rest-frame LyC over this redshift range. With our best current assessment of the WFC3 systematics, we provide 1$σ$ upper limits for the average LyC emission of galaxies at $z$=2.35, 2.75, and 3.60 to $\sim$28.5, 28.1, and 30.7 mag in image stacks of 11-15 galaxies in the WFC3/UVIS F225W, F275W, and F336W, respectively. The LyC flux of weak AGN at $z$=2.62 and 3.32 are detected at 28.3 and 27.4 mag with SNRs of $\sim$2.7 and 2.5 in F275W and F336W for stacks of 7 and 3 AGN, respectively, while AGN at $z$=2.37 are constrained to $\gtrsim$27.9 mag at 1$σ$ in a stack of 2 AGN. The stacked AGN LyC light profiles are flatter than their corresponding non-ionizing UV continuum profiles out to radii of r$\lesssim$0."9, which may indicate a radial dependence of porosity in the ISM. With synthetic stellar SEDs fit to UV continuum measurements longwards of Ly$α$ and IGM transmission models, we constrain the absolute LyC escape fractions to $f_{\rm esc}^{\rm abs}$$\simeq$$22^{+44}_{-22}$% at $z$=2.35 and $\lesssim$55% at $z$=2.75 and 3.60, respectively. All available data for galaxies, including published work, suggests a more sudden increase of $f_{\rm esc}$ with redshift at $z$$\simeq$2. Dust accumulating in (massive) galaxies over cosmic time correlates with increased HI column density, which may lead to reducing $f_{\rm esc}$ more suddenly at $z$$\lesssim$2. This may suggest that star-forming galaxies collectively contributed to maintaining cosmic reionization at redshifts $z$$\gtrsim$2-4, while AGN likely dominated reionization at $z$$\lesssim$2.

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arXiv2017

The effect of radiation pressure on spatial distribution of dust inside HII regions

We investigate the impact of radiation pressure on spatial dust distribution inside H$_\mathrm{II}$ regions using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. In order to investigate grain size effects as well, we introduce two additional fluid components describing large and small dust grains in the simulations. Relative velocity between dust and gas strongly depends on the drag force. We include collisional drag force and coulomb drag force. We find that, in a compact H$_\mathrm{II}$ region, a dust cavity region is formed by radiation pressure. Resulting dust cavity sizes (~0.2 pc) agree with observational estimates reasonably well. Since dust inside an H$_\mathrm{II}$ region is strongly charged, relative velocity between dust and gas is mainly determined by the coulomb drag force. Strength of the coulomb drag force is about 2-order of magnitude larger than that of the collisional drag force. In addition, in a cloud of mass $10^5$ $M_{\odot}$, we find that the radiation pressure changes the grain size distribution inside H$_\mathrm{II}$ regions. Since large (0.1 $\mathrm{μm}$) dust grains are accelerated more efficiently than small (0.01 $\mathrm{μm}$) grains, the large to small grain mass ratio becomes smaller by an order of magnitude compared with the initial one. Resulting dust size distributions depend on the luminosity of the radiation source. The large and small grain segregation becomes weaker when we assume stronger radiation source, since dust grain charges become larger under stronger radiation and hence coulomb drag force becomes stronger.

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