arXiv subjects

Nobunari Kashikawa

Publications and source records attributed to Nobunari Kashikawa.

At least 127 records · Page 7Linked to original sources

arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope and it started in March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years of observations (61.5 nights) and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i~26.4, ~26.5, and ~27.0 mag, respectively (5sigma for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) both internally and externally (against Pan-STARRS1), and ~10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp/.

astro-ph.IM↗

arXiv2017

A New Constraint on Reionization from Evolution of the Ly$α$ Luminosity Function at $z\sim6-7$ Probed by a Deep Census of $z=7.0$ Ly$α$ Emitter Candidates to 0.3 $L^*$

We detect 20 $z=7.0$ Ly$α$ emitter (LAE) candidates to $L({\rm Ly}α) \geq 2 \times 10^{42}$ erg s$^{-1}$ or $0.3$ $L^*_{z=7}$ and in $6.1\times 10^5$ Mpc$^3$ volume in the Subaru Deep Field and the Subaru/XMM-Newton Deep Survey field by 82 and 37 hours of Subaru Suprime-Cam narrowband NB973 and reddest optical $y$-band imaging. We compare their Ly$α$ and UV luminosity functions (LFs) and densities and Ly$α$ equivalent widths (EWs) to those of $z=5.7$, 6.6 and 7.3 LAEs from previous Suprime-Cam surveys. The Ly$α$ LF (density) rapidly declines by a factor of $\times$1.5 (1.9) in $L({\rm Ly}α)$ at $z=5.7-6.6$ (160 Myr), $\times$1.5 (1.6) at $z=6.6-7.0$ (60 Myr) at the faint end and $\times$2.0 (3.8) at $z=7.0-7.3$ (40 Myr). Also, in addition to the systematic decrease in EW at $z=5.7-6.6$ previously found, 2/3 of the $z=7.0$ LAEs detected in the UV continuum exhibit lower EWs than the $z=6.6$ ones. Moreover, while the UV LF and density do not evolve at $z=5.7-6.6$, they modestly decline at $z=6.6-7.0$, implying galaxy evolution contributing to the decline of the Ly$α$ LF. Comparison of the $z=7.0$ Ly$α$ LF to the one predicted by an LAE evolution model further reveals that galaxy evolution alone cannot explain all the decline of Ly$α$ LF. If we attribute the discrepancy to Ly$α$ attenuation by neutral hydrogen, the intergalactic medium transmission of Ly$α$ photons at $z=7.0$ would be $T_{{\rm Ly}α}^{\rm IGM} \leq 0.6-0.7$. It is lower (higher) than $T_{{\rm Ly}α}^{\rm IGM}$ at $z=6.6$ (7.3) derived by previous studies, suggesting rapid increase in neutral fraction at $z > 6$.

astro-ph.GA↗

arXiv2017

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8

We present spectroscopic identification of 32 new quasars and luminous galaxies discovered at 5.7 < z < 6.8. This is the second in a series of papers presenting the results of the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The photometric candidates were selected by a Bayesian probabilistic algorithm, and then observed with spectrographs on the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous paper, we have now identified 64 HSC sources over about 430 deg2, which include 33 high-z quasars, 14 high-z luminous galaxies, 2 [O III] emitters at z ~ 0.8, and 15 Galactic brown dwarfs. The new quasars have considerably lower luminosity (M1450 ~ -25 to -22 mag) than most of the previously known high-z quasars. Several of these quasars have luminous (> 10^(43) erg/s) and narrow (< 500 km/s) Ly alpha lines, and also a possible mini broad absorption line system of N V 1240 in the composite spectrum, which clearly separate them from typical quasars. On the other hand, the high-z galaxies have extremely high luminosity (M1450 ~ -24 to -22 mag) compared to other galaxies found at similar redshift. With the discovery of these new classes of objects, we are opening up new parameter spaces in the high-z Universe. Further survey observations and follow-up studies of the identified objects, including the construction of the quasar luminosity function at z ~ 6, are ongoing.

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arXiv2017

Clustering of quasars in a wide luminosity range at redshift 4 with Subaru Hyper Suprime-Cam wide field imaging

We examine the clustering of quasars over a wide luminosity range, by utilizing 901 quasars at $\overline{z}_{\rm phot}\sim3.8$ with $-24.73<M_{\rm 1450}<-22.23$ photometrically selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S16A Wide2 date release and 342 more luminous quasars at $3.4<z_{\rm spec}<4.6$ having $-28.0<M_{\rm 1450}<-23.95$ from the Sloan Digital Sky Survey (SDSS) that fall in the HSC survey fields. We measure the bias factors of two quasar samples by evaluating the cross-correlation functions (CCFs) between the quasar samples and 25790 bright $z\sim4$ Lyman Break Galaxies (LBGs) in $M_{\rm 1450}<-21.25$ photometrically selected from the HSC dataset. Over an angular scale of \timeform{10.0"} to \timeform{1000.0"}, the bias factors are $5.93^{+1.34}_{-1.43}$ and $2.73^{+2.44}_{-2.55}$ for the low and high luminosity quasars, respectively, indicating no luminosity dependence of quasar clustering at $z\sim4$. It is noted that the bias factor of the luminous quasars estimated by the CCF is smaller than that estimated by the auto-correlation function (ACF) over a similar redshift range, especially on scales below \timeform{40.0"}. Moreover, the bias factor of the less-luminous quasars implies the minimal mass of their host dark matter halos (DMHs) is $0.3$-$2\times10^{12}h^{-1}M_{\odot}$, corresponding to a quasar duty cycle of $0.001$-$0.06$.

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arXiv2017

The Quasar Luminosity Function at Redshift 4 with Hyper Suprime-Cam Wide Survey

We present the luminosity function of z=4 quasars based on the Hyper Suprime-Cam Subaru Strategic Program Wide layer imaging data in the g, r, i, z, and y bands covering 339.8 deg^2. From stellar objects, 1666 z~4 quasar candidates are selected by the g-dropout selection down to i=24.0 mag. Their photometric redshifts cover the redshift range between 3.6 and 4.3 with an average of 3.9. In combination with the quasar sample from the Sloan Digital Sky Survey in the same redshift range, the quasar luminosity function covering the wide luminosity range of M1450=-22 to -29 mag is constructed. It is well described by a double power-law model with a knee at M1450=-25.36+-0.13 mag and a flat faint-end slope with a power-law index of -1.30+-0.05. The knee and faint-end slope show no clear evidence of redshift evolution from those at z~2. The flat slope implies that the UV luminosity density of the quasar population is dominated by the quasars around the knee, and does not support the steeper faint-end slope at higher redshifts reported at z>5. If we convert the M1450 luminosity function to the hard X-ray 2-10keV luminosity function using the relation between UV and X-ray luminosity of quasars and its scatter, the number density of UV-selected quasars matches well with that of the X-ray-selected AGNs above the knee of the luminosity function. Below the knee, the UV-selected quasars show a deficiency compared to the hard X-ray luminosity function. The deficiency can be explained by the lack of obscured AGNs among the UV-selected quasars.

astro-ph.GA↗

arXiv2017

The Galaxy--Halo Connection in High-Redshift Universe: Details and Evolution of Stellar-to-Halo Mass Ratios of Lyman Break Galaxies on CFHTLS Deep Fields

We present the results of clustering analyses of Lyman break galaxies (LBGs) at $z\sim3$, $4$, and $5$ using the final data release of the Canada--France--Hawaii Telescope Legacy Survey (CFHTLS). Deep- and wide-field images of the CFHTLS Deep Survey enable us to obtain sufficiently accurate two-point angular correlation functions to apply a halo occupation distribution analysis. Mean halo masses, calculated as $\langle M_{h} \rangle=10^{11.7}-10^{12.8}h^{-1}M_{\odot}$, increase with stellar-mass limit of LBGs. The threshold halo mass to have a central galaxy follows the same increasing trend with the low-$z$ results, whereas the threshold halo mass to have a satellite galaxy shows higher values at $z=3-5$ than $z=0.5-1.5$ over the entire stellar mass range. Satellite fractions of dropout galaxies, even at less massive haloes, are found to drop sharply from $z=2$ down to less than $0.04$ at $z=3-5$. These results suggest that satellite galaxies form inefficiently within dark haloes at $z=3-5$ even for less massive satellites with $M_{\star}<10^{10}M_{\odot}$. We compute stellar-to-halo mass ratios (SHMRs) assuming a main sequence of galaxies, which is found to provide consistent SHMRs with those derived from a spectral energy distribution fitting method. The observed SHMRs are in good agreement with the model predictions based on the abundance-matching method within $1σ$ confidence intervals. We derive observationally, for the first time, $M_{\rm h}^{\rm pivot}$, which is the halo mass at a peak in the star-formation efficiency, at $3 3$. In addition, $M_{\rm h}^{\rm pivot}$ and its normalization are found to be almost unchanged during $0<z<5$. Our study shows an observational evidence that galaxy formation is ubiquitously most efficient near a halo mass of $M_{\rm h}\sim10^{12}M_{\odot}$ over cosmic time.

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arXiv2017

Extended ionized gas clouds in the Abell 1367 cluster

We surveyed a central 0.6 deg^2 region of Abell 1367 cluster for extended ionized gas clouds (EIGs) using the Subaru prime-focus camera (Suprime-Cam) with a narrow-band filter that covers Halpha. We discovered six new EIGs in addition to five known EIGs. We also found that the Halpha tail from the blue infalling group (BIG) is extended to about 330 kpc in projected distance, which is about twice longer than previously reported. Candidates of star-forming blobs in the tail are detected. The properties of the EIG parent galaxies in Abell 1367 basically resemble those in the Coma cluster. A noticeable difference is that the number of detached EIGs is significantly fewer in Abell 1367, while the fraction of blue member galaxies is higher. The results suggest a difference in the evolutionary stage of the clusters; Abell 1367 is at an earlier stage than the Coma cluster.

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arXiv2017

A search for Ly-alpha emitters around a concentrated-region of strong Ly-alpha absorbers at z=2.3

In order to investigate the physical relationship between strong Ly-alpha absorbers (logN_HI>20.0 cm^-2) such as damped Ly-alpha absorption systems (DLAs) and young star-forming galaxies at high redshift, we have conducted narrow-band observations of Ly-alpha emitters (LAEs) in a concentrated region of strong Ly-alpha absorbers at z= 2.3, the J1230+34 field. Using a catalog of Ly-alpha absorbers with logN_HI>20.0 cm^-2 based on the baryon oscillation spectroscopic survey (BOSS), we found 6 fields where 3 or more absorbers are concentrated within a (50 Mpc)^3 cubic box in the comoving scale. Among them, we focus on the J1230+34 field, where 2 DLAs and 2 sub-DLAs present. Our narrow-band imaging observations with Subaru/Suprime-Cam using a custom-made filter, NB400 (lambda_c=4003 A and FWHM=92 A) yield a sample of 149 LAEs in this field. In the large scale (~50 Mpc), we have found no differences between the obtained Ly-alpha luminosity function and those in the blank fields at similar redshifts. We also compare the frequency distribution of the Ly-alpha rest-frame equivalent width (EW_0) in the target field and other fields including both overdensity region and blank field, but find no differences. On the other hand, in the small scale (~10 Mpc), we have found a possible overdensity of LAEs around a DLA with the highest HI column density (N_HI=21.08 cm^-2) in the target field while there are no density excess around the other absorbers with a lower N_HI.

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arXiv2017

Direct evidence for Ly$α$ depletion in the protocluster core

We have carried out a panoramic Ly$α$ narrowband imaging with Suprime-Cam on Subaru towards the known protocluster USS1558--003 at $z=2.53$. Our previous narrowband imaging at near-infrared has identified multiple dense groups of H$α$ emitters (HAEs) within the protocluster. We have now identified the large-scale structures across a $\sim$50 comoving Mpc scale traced by Ly$α$ emitters (LAEs) in which the protocluster traced by the HAEs is embedded. On a smaller scale, however, there are remarkably few LAEs in the regions of HAE overdensities. Moreover, the stacking analyses of the images show that HAEs in higher-density regions show systematically lower escape fractions of Ly$α$ photons than those of HAEs in lower-density regions. These phenomena may be driven by the extra depletion of Ly$α$ emission lines along our line of sight by more intervening cold circumgalactic/intergalactic medium and/or dust existing in the dense core. We also caution that all the past high-$z$ protocluster surveys using LAEs as the tracers would have largely missed galaxies in the very dense cores of the protoclusters where we would expect to see any early environmental effects.

astro-ph.GA↗

arXiv2017

Similarities and uniqueness of Ly$α$ emitters among star-forming galaxies at z=2.5

We conducted a deep narrow-band imaging survey with the Subaru Prime Focus Camera on the Subaru Telescope and constructed a sample of Ly$α$ emitters (LAEs) at z=2.53 in the UDS-CANDELS field where a sample of H$α$ emitters (HAEs) at the same redshift is already obtained from our previous narrow-band observation at NIR. The deep narrow-band and multi broadband data allow us to find LAEs of stellar masses and star-formation rates (SFRs) down to $\gtrsim$$10^8$ M$_\odot$ and $\gtrsim$0.2 M$_\odot$/yr, respectively. We show that the LAEs are located along the same mass-SFR sequence traced by normal star-forming galaxies such as HAEs, but towards a significantly lower mass regime. Likewise, LAEs seem to share the same mass--size relation with typical star-forming galaxies, except for the massive LAEs, which tend to show significantly compact sizes. We identify a vigorous mass growth in the central part of LAEs: the stellar mass density in the central region of LAEs increases as their total galaxy mass grows. On the other hand, we see no Ly$α$ line in emission for most of the HAEs. Rather, we find that the Ly$α$ feature is either absent or in absorption (Ly$α$ absorbers; LAAs), and its absorption strength may increase with reddening of the UV continuum slope. We demonstrate that a deep Ly$α$ narrow-band imaging like this study is able to search for not only LAEs but also LAAs in a certain redshift slice. This work suggests that LAEs trace normal star-forming galaxies in the low-mass regime, while they remain as a unique population because the majority of HAEs are not LAEs.

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arXiv2017

The redshift selected sample of long gamma-ray burst host galaxies: the overall metallicity distribution at $z < 0.4$

We discuss the host galaxy metallicity distribution of all long gamma-ray bursts (GRBs) whose redshifts are known to be $< 0.4$, including newly obtained spectroscopic datasets of the host galaxies of GRB 060614, 090417B, and 130427A. We compare the metallicity distribution of the low-redshift sample to the model predictions, and constrain the relation between metallicity and GRB occurrence. We take account of spatial variation of metallicities among star forming regions within a galaxy. We found that the models, in which only low-metallicity stars produce GRBs with a sharp cutoff of GRB production efficiency around 12+log(O/H) $\sim$ 8.3, can well reproduce the observed distribution, while the models with no metallicity dependence are not consistent with the observations. We also discuss possible sampling biases we may suffer by collecting long GRBs whose redshifts are known, presenting the photometric observations of the host galaxy of GRB 111225A at $z = 0.297$ whose redshift has been undetermined until $\sim$ 2.3 years after the burst.

astro-ph.HE↗

arXiv2016

The Metal Abundances across Cosmic Time ($\mathcal{MACT}$) Survey. I. Optical Spectroscopy in the Subaru Deep Field

Deep rest-frame optical spectroscopy is critical for characterizing and understanding the physical conditions and properties of the ionized gas in galaxies. Here, we present a new spectroscopic survey called "Metal Abundances across Cosmic Time" or $\mathcal{MACT}$, which will obtain rest-frame optical spectra for $\sim$3000 emission-line galaxies. This paper describes the optical spectroscopy that has been conducted with MMT/Hectospec and Keck/DEIMOS for $\approx$1900 $z=0.1-1$ emission-line galaxies selected from our narrowband and intermediate-band imaging in the Subaru Deep Field. In addition, we present a sample of 164 galaxies for which we have measured the weak [OIII]$λ$4363 line (66 with at least 3$σ$ detections and 98 with significant upper limits). This nebular emission line determines the gas-phase metallicity by measuring the electron temperature of the ionized gas. This paper presents the optical spectra, emission-line measurements, interstellar properties (e.g., metallicity, gas density), and stellar properties (e.g., star formation rates, stellar mass). Paper II of the $\mathcal{MACT}$ survey (Ly et al.) presents the first results on the stellar mass--gas metallicity relation at $z\lesssim1$ using the sample with [OIII]$λ$4363 measurements.

astro-ph.GA↗

arXiv2016

MApping the Most Massive Overdensities (MAMMOTH) II -- Discovery of an Extremely Massive Overdensity BOSS1441 at $z=2.32$

Cosmological simulations suggest a strong correlation between high optical-depth Ly$α$ absorbers, which arise from the intergalactic medium (IGM), and 3-D mass overdensities on scales of $10-30$ $h^{-1}$ comoving Mpc. By examining the absorption spectra of $\sim$ 80,000 QSO sight-lines over a volume of 0.1 Gpc$^3$ in the Sloan Digital Sky Survey III (SDSS-III), we have identified an extreme overdensity, BOSS1441, which contains a rare group of strong Ly$α$ absorbers at $z=2.32\pm 0.02$. This absorber group is associated with six QSOs at the same redshift on a 30 comoving Mpc scale. Using Mayall/MOSAIC narrowband and broadband imaging, we detect Ly$α$ emitters (LAEs) down to $0.7\times L_{\rm{Lyα}}^*$, and reveal a large-scale structure of Ly$α$ emitters (LAEs) in this field. Our follow-up Large Binocular Telescope (LBT) observations have spectroscopically confirmed 19 galaxies in the density peak. We show that BOSS1441 has an LAE overdensity of $10.8\pm 2.6$ on a 15 comoving Mpc scale which could collapse to a massive cluster with $M\gtrsim10^{15}$ M$_\odot$ at $z\sim0$. This overdensity is among the most massive large-scale structures at $z\sim2$ discovered to date.

astro-ph.GA↗

arXiv2016

Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9

We report the discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9. This is the initial result from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the exquisite multiband imaging data produced by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. The candidate selection is performed by combining several photometric approaches including a Bayesian probabilistic algorithm to reject stars and dwarfs. The spectroscopic identification was carried out with the Gran Telescopio Canarias and the Subaru Telescope for the first 80 deg2 of the survey footprint. The success rate of our photometric selection is quite high, approaching 100 % at the brighter magnitudes (zAB < 23.5 mag). Our selection also recovered all the known high-z quasars on the HSC images. Among the 15 discovered objects, six are likely quasars, while the other six with interstellar absorption lines and in some cases narrow emission lines are likely bright Lyman-break galaxies. The remaining three objects have weak continua and very strong and narrow Ly alpha lines, which may be excited by ultraviolet light from both young stars and quasars. These results indicate that we are starting to see the steep rise of the luminosity function of z > 6 galaxies, compared with that of quasars, at magnitudes fainter than M1450 ~ -22 mag or zAB ~24 mag. Follow-up studies of the discovered objects as well as further survey observations are ongoing.

astro-ph.GA↗

arXiv2016

Detection of an oxygen emission line from a high redshift galaxy in the reionization epoch

The physical properties and elemental abundances of the interstellar medium in galaxies during cosmic reionization are important for understanding the role of galaxies in this process. We report the Atacama Large Millimeter/submillimeter Array detection of an oxygen emission line at a wavelength of 88 micrometers from a galaxy at an epoch about 700 million years after the Big Bang. The oxygen abundance of this galaxy is estimated at about one-tenth that of the Sun. The non-detection of far-infrared continuum emission indicates a deficiency of interstellar dust in the galaxy. A carbon emission line at a wavelength of 158 micrometers is also not detected, implying an unusually small amount of neutral gas. These properties might allow ionizing photons to escape into the intergalactic medium.

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arXiv2016

A Systematic Survey of Protoclusters at $z\sim3\mathrm{-}6$ in the CFHTLS Deep Fields

We present the discovery of three protoclusters at $z\sim3\mathrm{-}4$ with spectroscopic confirmation in the Canada-France-Hawaii Telescope (CFHT) Legacy Survey Deep Fields. In these fields, we investigate the large-scale projected sky distribution of $z\sim3\mathrm{-}6$ Lyman break galaxies and identify 21 protocluster candidates from regions that are overdense at more than $4σ$ overdensity significance. Based on cosmological simulations, it is expected that more than $76\%$ of these candidates will evolve into a galaxy cluster of at least a halo mass of $10^{14}\,\mathrm{M_\odot}$ at $z=0$. We perform follow-up spectroscopy for eight of the candidates using Subaru/FOCAS, KeckII/DEIMOS, and Gemini-N/GMOS. In total we target 462 dropout candidates and obtain 138 spectroscopic redshifts. We confirm three real protoclusters at $z=3\mathrm{-}4$ with more than five members spectroscopically identified, and find one to be an incidental overdense region by mere chance alignment. The other four candidate regions at $z\sim5\mathrm{-}6$ require more spectroscopic follow-up in order to be conclusive. A $z=3.67$ protocluster, which has eleven spectroscopically confirmed members, shows a remarkable core-like structure composed of a central small region ($<0.5\,\mathrm{physical\>Mpc}$) and an outskirts region ($\sim1.0\,\mathrm{physical\>Mpc}$). The Ly$α$ equivalent widths of members of the protocluster are significantly smaller than those of field galaxies at the same redshift while there is no difference in the UV luminosity distributions. These results imply that some environmental effects start operating as early as at $z\sim4$ along with the growth of the protocluster structure.

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arXiv2016

Detection of emission lines from z ~ 3 DLAs towards the QSO J2358+0149

Using VLT/X-shooter we searched for emission line galaxies associated to four damped Lyman-$α$ systems (DLAs) and one sub-DLA at 2.73<=z<=3.25 towards QSO J2358+0149. We detect [O III] emission from a "low-cool" DLA at z_abs = 2.9791 (having log N(HI)=21.69+\-0.10, [Zn/H] = -1.83+\-0.18) at an impact parameter of, $ρ$ ~12 kpc. The associated galaxy is compact with a dynamical mass of (1-6)x10^9 M_solar, very high excitation ([O III]/[O II] and [O III]/[H$β$] both greater than 10), 12+[O/H]<=8.5 and moderate star formation rate (SFR <=2 M_solar yr^{-1}). Such properties are typically seen in the low-z extreme blue compact dwarf galaxies. The kinematics of the gas is inconsistent with that of an extended disk and the gas is part of either a large scale wind or cold accretion. We detect Ly$α$ emission from the z_abs = 3.2477 DLA (having log N(HI)=21.12+\-0.10 and [Zn/H]=-0.97+\-0.13).The Ly$α$ emission is redshifted with respect to the metal absorption lines by 320 km s^{-1}, consistent with the location of the red hump expected in radiative transport models. We derive SFR ~0.2-1.7 M_solar yr^{-1} and Ly$α$ escape fraction of >=10 per cent. No other emission line is detected from this system. Because the DLA has a small velocity separation from the quasar (~500 km s^{-1}) and the DLA emission is located within a small projected distance ($ρ<5$ kpc), we also explore the possibility that the Ly$α$ emission is being induced by the QSO itself. QSO induced Ly$α$ fluorescence is possible if the DLA is within a physical separation of 340 kpc to the QSO. Detection of stellar continuum light and/or the oxygen emission lines would disfavor this possibility. We do not detect any emission line from the remaining three systems.

astro-ph.GA↗

arXiv2016

A New Constraint on the Ly$α$ Fraction of UV Very Bright Galaxies at Redshift 7

We study the extent to which very bright (-23.0 < MUV < -21.75) Lyman-break selected galaxies at redshifts z~7 display detectable Lya emission. To explore this issue, we have obtained follow-up optical spectroscopy of 9 z~7 galaxies from a parent sample of 24 z~7 galaxy candidates selected from the 1.65 sq.deg COSMOS-UltraVISTA and SXDS-UDS survey fields using the latest near-infrared public survey data, and new ultra-deep Subaru z'-band imaging (which we also present and describe in this paper). Our spectroscopy has yielded only one possible detection of Lya at z=7.168 with a rest-frame equivalent width EW_0 = 3.7 (+1.7/-1.1) Angstrom. The relative weakness of this line, combined with our failure to detect Lya emission from the other spectroscopic targets allows us to place a new upper limit on the prevalence of strong Lya emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1-sigma upper limit on the fraction of UV bright galaxies at z~7 that display EW_0 > 50 Angstrom, which we estimate to be < 0.23. This result may indicate a weak trend where the fraction of strong Lya emitters ceases to rise, and possibly falls between z~6 and z~7. Our results also leave open the possibility that strong Lya may still be more prevalent in the brightest galaxies in the reionization era than their fainter counterparts. A larger spectroscopic sample of galaxies is required to derive a more reliable constraint on the neutral hydrogen fraction at z~7 based on the Lya fraction in the bright galaxies.

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