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 ~200 × 200 × 80 comoving Mpc^3 (cMpc^3) volumes each at z = 5.7 and 6.6 based on a spectroscopic sample of 179 galaxies that achieves ≳80% completeness down to the Lyα luminosity of log(L_(Lyα)/[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 ≃ 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) ≳5 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 ~10^(14) M_⊙

Stellar Initial Mass Function (IMF) Probed with Supernova Rates and Neutrino Background: Cosmic Average IMF Slope is ≃2−3\simeq 2-3 Similar to the Salpeter IMF

The stellar initial mass function (IMF) is expressed by $\phi(m) \propto m^{-\alpha}$ with the slope $\alpha$, and known as the poorly-constrained but very important function in studies of star and galaxy formation. There are no sensible observational constraints on the IMF slopes beyond Milky Way and nearby galaxies. Here we combine two sets of observational results, 1) cosmic densities of core-collapse supernova explosion (CCSNe) rates and 2) cosmic far ultraviolet radiation (and infrared re-radiation) densities, which are sensitive to massive ($\simeq 8-50 \,{\rm M}_\odot$) and moderately massive ($\simeq 2.5-7 \,{\rm M}_\odot$) stars, respectively, and constrain the IMF slope at $m>1\,{\rm M}_\odot$ with a freedom of redshift evolution. Although no redshift evolution is identified beyond the uncertainties, we find that the cosmic average IMF slope at $z=0$ is $\alpha=1.8-3.2$ at the 95 % confidence level that is comparable with the Salpeter IMF, $\alpha=2.35$, which marks the first constraint on the cosmic average IMF. We show a forecast for the Nancy Grace Roman Space Telescope supernova survey that will provide significantly strong constraints on the IMF slope with $\delta \alpha\simeq 0.5$ over $z=0-2$. Moreover, as for an independent IMF probe instead of 1), we suggest to use diffuse supernovae neutrino background (DSNB), relic neutrinos from CCSNe. We expect that the Hyper-Kamiokande neutrino observations over 20 years will improve the constraints on the IMF slope and the redshift evolution significantly better than those obtained today, if the systematic uncertainties of DSNB production physics are reduced in the future numerical simulations.Comment: 7 pages, 6 figures, ApJ in pres

Fluorescent C II* 1335A emission spectroscopically resolved in a galaxy at z = 5.754

We report the discovery of the first spectroscopically resolved C II /C II* 1334, 1335A doublet in the Lyman-break galaxy J0215-0555 at z = 5.754. The separation of the resonant and fluorescent emission channels was possible thanks to the large redshift of the source and long integration time, as well as the small velocity width of the feature, 0.6 +- 0.2A. We model this emission and find that at least two components are required to reproduce the combination of morphologies of C II* emission, C II absorption and emission, and Lyman-alpha emission from the object. We suggest that the close alignment between the fluorescence and Lyman-alpha emission could indicate an ionisation escape channel within the object. While the faintness of such a C II /C II* doublet makes it prohibitively difficult to pursue for similar systems with current facilities, we suggest it can become a valuable porosity diagnostic in the era of JWST and the upcoming generations of ELTs.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter

The Mean Absorption Line Spectra of a Selection of Luminous z~6 Lyman Break Galaxies

We examine the absorption line spectra of a sample of 31 luminous (M_UV=-23) Lyman break galaxies at redshift z~6 using data taken with the FOCAS and OSIRIS spectrographs on the Subaru and GTC telescopes. For two of these sources we present longer exposure data taken at higher spectral resolution from ESO's X-shooter spectrograph. Using these data, we demonstrate the practicality of stacking our lower resolution data to measure the depth of various interstellar and stellar absorption lines to probe the covering fraction of low ionization gas and the gas-phase and stellar metallicities near the end of the era of cosmic reionization. From maximum absorption line depths of SiII1260 and CII1334, we infer a mean covering fraction of >0.85+/-0.16 for our sample. This is larger than that determined using similar methods for lower luminosity galaxies at slightly lower redshifts, suggesting that the most luminous galaxies appear to have a lower escape fraction than fainter galaxies, and therefore may not play a prominent role in concluding reionization. Using various interstellar absorption lines we deduce gas-phase metallicities close to solar indicative of substantial early enrichment. Using selected stellar absorption lines, we model our spectra with a range of metallicities using techniques successfully employed at lower redshift and deduce a stellar metallicity of 0.4 +0.3/-0.1 solar, consistent with the stellar mass - stellar metallicity relation recently found at z~3-5. We discuss the implications of these metallicity estimates for the typical ages of our luminous galaxies and conclude our results imply initial star formation at redshifts z~10, consistent with independent analyses of earlier objects.Comment: 13 pages, 10 figures, 3 tables, Accepted for Publication in Ap