Lyα輝線銀河の性質 : 星形成率、星質量、ダークマターハロー質量

東京大学The University of Tokyo博士(理学)学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 柏川 伸成, （副査）東京大学教授 河野 孝太郎, 東京大学准教授 藤井 通子, 東北大学教授 兒玉 忠恭, 大阪産業大学准教授 井上 昭雄doctoral thesi

The Stellar Mass, Star Formation Rate and Dark Matter Halo Properties of LAEs at z∼2z\sim2

We present average stellar population properties and dark matter halo masses of $z \sim 2$ \lya emitters (LAEs) from SED fitting and clustering analysis, respectively, using $\simeq$ $1250$ objects ($NB387\le25.5$) in four separate fields of $\simeq 1$ deg$^2$ in total. With an average stellar mass of $10.2\, \pm\, 1.8\times 10^8\ {\mathrm M_\odot}$ and star formation rate of $3.4\, \pm\, 0.4\ {\mathrm M_\odot}\ {\rm yr^{-1}}$, the LAEs lie on an extrapolation of the star-formation main sequence (MS) to low stellar mass. Their effective dark matter halo mass is estimated to be $4.0_{-2.9}^{+5.1} \times 10^{10}\ {\mathrm M_\odot}$with an effective bias of$1.22^{+0.16}_{-0.18}$which is lower than that of$z \sim 2$LAEs ($1.8\, \pm\, 0.3$), obtained by a previous study based on a three times smaller survey area, with a probability of$96\%. However, the difference in the bias values can be explained if cosmic variance is taken into account. If such a low halo mass implies a low HI gas mass, this result appears to be consistent with the observations of a high \lya escape fraction. With the low halo masses and ongoing star formation, our LAEs have a relatively high stellar-to-halo mass ratio (SHMR) and a high efficiency of converting baryons into stars. The extended Press-Schechter formalism predicts that at z=0$our LAEs are typically embedded in halos with masses similar to that of the Large Magellanic Cloud (LMC); they will also have similar SHMRs to the LMC, if their SFRs are largely suppressed after$z \sim 2$ as some previous studies have reported for the LMC itself.Comment: 34 pages, 15 figures, 6 tables. Accepted for publication in PAS

A hard ionizing spectrum in z=3-4 Lyα emitters with intense [OIII] emission: Analogs of galaxies in the reionization era?

We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [O iii]λλ5007,4959, [O ii]λ3727, and Hβ for a sample of 15 redshift z≃3.1\mbox-3.7 Lyα emitters (LAEs) and Lyman break galaxies (LBGs). In conjunction with spectra from other surveys, we confirm earlier indications that LAEs have a much higher [O iii]/[O ii] line ratio than is seen in similar redshift LBGs. By comparing their distributions on a [O iii]/[O ii] versus R23 diagram, we demonstrate that this difference cannot arise solely because of their lower metallicities but most likely is due to a harder ionizing spectrum. Using measures of Hβ and recombination theory, we demonstrate, for a subset of our LAEs, that ξion—the number of Lyman continuum photons per UV luminosity—is indeed 0.2–0.5 dex larger than for typical LBGs at similar redshifts. Using photoionization models, we estimate the effect this would have on both [O iii]/[O ii] and R23 and conclude such a hard spectrum can only partially explain such intense line emission. The additional possibility is that such a large [O iii]/[O ii] ratio is in part due to density rather than ionization bound nebular regions, which would imply a high escape fraction of ionizing photons. We discuss how further observations could confirm this possibility. Clearly LAEs with intense [O iii] emission represent a promising analog of those z > 7 sources with similarly strong lines that are thought to be an important contributor to cosmic reionization

The dominant origin of diffuse Lyα halos around Lyα emitters explored by spectral energy distribution fitting and clustering analysis

The physical origin of diffuse Lyα halos (LAHs) around star-forming galaxies is still a matter of debate. We present the dependence of LAH luminosity [L(Lyα)_H] on the stellar mass (M⋆), star formation rate, color excess [E(B − V)⋆], and dark matter halo mass (M_h) of the parent galaxy for ∼900 Lyα emitters (LAEs) at z ∼ 2 divided into ten subsamples. We calculate L(Lyα)_H using the stacked observational relation between L(Lyα)H and central Lyα luminosity of Momose et al. (2016, MNRAS, 457, 2318), which we find agrees with the average trend of VLT/MUSE-detected individual LAEs. We find that our LAEs have relatively high L(Lyα)_H despite low M⋆ and M_h, and that L(Lyα)_H remains almost unchanged with M⋆ and perhaps with M_h. These results are incompatible with the cold stream (cooling radiation) scenario and the satellite-galaxy star-formation scenario, because the former predicts fainter L(Lyα)_H and both predict steeper L(Lyα)_H vs. M⋆ slopes. We argue that LAHs are mainly caused by Lyα photons escaping from the main body and then scattering in the circum-galactic medium. This argument is supported by LAH observations of Hα emitters (HAEs). When LAHs are taken into account, the Lyα escape fractions of our LAEs are about ten times higher than those of HAEs with similar M⋆ or E(B − V)⋆, which may partly arise from lower H I gas masses implied from lower M_h at fixed M⋆, or from another Lyα source in the central part

SILVERRUSH. III. Deep Optical and Near-Infrared Spectroscopy for Lya and UV-Nebular Lines of Bright Lya Emitters at z=6-7

We present Lya and UV-nebular emission line properties of bright Lya emitters (LAEs) at z=6-7 with a luminosity of log L_Lya/[erg s-1] = 43-44 identified in the 21-deg2 area of the SILVERRUSH early sample developed with the Subaru Hyper Suprime-Cam (HSC) survey data. Our optical spectroscopy newly confirm 21 bright LAEs with clear Lya emission, and contribute to make a spectroscopic sample of 96 LAEs at z=6-7 in SILVERRUSH. From the spectroscopic sample, we select 7 remarkable LAEs as bright as Himiko and CR7 objects, and perform deep Keck/MOSFIRE and Subaru/nuMOIRCS near-infrared spectroscopy reaching the 3sigma-flux limit of ~ 2x10^{-18} erg s-1 for the UV-nebular emission lines of He II1640, C IV1548,1550, and O III]1661,1666. Except for one tentative detection of C IV, we find no strong UV-nebular lines down to the flux limit, placing the upper limits of the rest-frame equivalent widths (EW_0) of ~2-4 A for He II, C IV, and O III] lines. Here we also investigate the VLT/X-SHOOTER spectrum of CR7 whose 6 sigma detection of He II is claimed by Sobral et al. Although two individuals and the ESO-archive service carefully re-analyze the X-SHOOTER data that are used in the study of Sobral et al., no He II signal of CR7 is detected, supportive of weak UV-nebular lines of the bright LAEs even for CR7. Spectral properties of these bright LAEs are thus clearly different from those of faint dropouts at z~7 that have strong UV-nebular lines shown in the various studies. Comparing these bright LAEs and the faint dropouts, we find anti-correlations between the UV-nebular line EW_0 and UV-continuum luminosity, which are similar to those found at z~2-3.Comment: 26 pages, 12 figures. Accepted for publication in PASJ special issu