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Register a new userCHORUS. III. Photometric and Spectroscopic Properties of Ly$alpha$ Blobs at $z=4.9-7.0$
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We report the Subaru Hyper Suprime-Cam (HSC) discovery of two Ly$alpha$ blobs (LABs), dubbed z70-1 and z49-1 at $z=6.965$ and $z=4.888$ respectively, that are Ly$alpha$ emitters with a bright ($log L_{rm Lyalpha}/{rm [erg s^{-1}]}>43.4$) and spatially-extended Ly$alpha$ 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$alpha$ emission with a scale length of $1.4pm 0.2$ 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}$lambda$1240 emission, or Ly$alpha$ line broadening, while z49-1 has a strong {sc Civ}$lambda$1548 emission line indicating an AGN on the basis of the UV-line ratio diagnostics. We carefully model the point-spread functions of the HSC images, and conduct two-component exponential profile fitting to the extended Ly$alpha$ emission of the LABs. The Ly$alpha$ 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$alpha$ luminosity relation of the Ly$alpha$ halos around VLT/MUSE star-forming galaxies at the similar redshifts, suggesting that typical LABs at $zgtrsim5$ are not special objects, but star-forming galaxies at the bright end.
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The distribution of Ly$alpha$ emission is an presently accessible method for studying the state of the intergalactic medium (IGM) into the reionization era. We carried out deep spectroscopic observations in order to search for Ly$alpha$ emission from galaxies with photometric redshifts $z$ = 5.5 - 8.3 selected from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). Utilizing data from the Keck/DEIMOS spectrograph, we explore a wavelength coverage of Ly$alpha$ emission at $z$ ~ 5 - 7 with four nights of spectroscopic observations for 118 galaxies, detecting five emission lines with ~ 5$sigma$ significance: three in the GOODS-N and two in the GOODS-S field. We constrain the equivalent width (EW) distribution of Ly$alpha$ emission by comparing the number of detected objects with the expected number constructed from detailed simulations of mock emission lines that account for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The Ly$alpha$ EW distribution is well described by an exponential form, $text{dN/dEW}propto text{exp(-EW/}W_0)$, characterized by the $e$-folding scale ($W_0$) of ~ 60 - 100$AA$ at 0.3 < $z$ < 6. By contrast, our measure of the Ly$alpha$ EW distribution at 6.0 < $z$ < 7.0 rejects a Ly$alpha$ EW distribution with $W_0$ > 36.4$AA$ (125.3$AA$) at 1$sigma$ (2$sigma$) significance. This provides additional evidence that the EW distribution of Ly$alpha$ declines at $z$ > 6, suggesting an increasing fraction of neutral hydrogen in the IGM at that epoch.
In this work we model the observed evolution in comoving number density of Lyman-alpha blobs (LABs) as a function of redshift, and try to find which mechanism of emission is dominant in LAB. Our model calculates LAB emission both from cooling radiation from the intergalactic gas accreting onto galaxies and from star formation (SF). We have used dark matter (DM) cosmological simulation to which we applied empirical recipes for Ly$alpha$ emission produced by cooling radiation and SF in every halo. In difference to the previous work, the simulated volume in the DM simulation is large enough to produce an average LABs number density. At a range of redshifts $zsim 1-7$ we compare our results with the observed luminosity functions of LABs and LAEs. Our cooling radiation luminosities appeared to be too small to explain LAB luminosities at all redshifts. In contrast, for SF we obtained a good agreement with observed LFs at all redshifts studied. We also discuss uncertainties which could influence the obtained results, and how LAB LFs could be related to each other in fields with different density.
Ly$alpha$ emission from galaxies can be utilized to characterize the ionization state in the intergalactic medium (IGM). We report our search for Ly$alpha$ emission at $z>7$ using a comprehensive Keck/MOSFIRE near-infrared spectroscopic dataset, as part of the Texas Spectroscopic Search for Ly$alpha$ Emission at the End of Reionization Survey. We analyze data from 10 nights of MOSFIRE observations which together target 72 high-$z$ candidate galaxies in the GOODS-N field, all with deep exposure times of 4.5-19 hr. Utilizing an improved automated emission-line search, we report 10 Ly$alpha$ emission lines detected ($>$4$sigma$) at $z>7$, significantly increasing the spectroscopically confirmed sample. Our sample includes large equivalent-width (EW) Ly$alpha$ emitters ($>$50r{A}), and additional tentative Ly$alpha$ emission lines detected at 3 - 4$sigma$ from five additional galaxies. We constrain the Ly$alpha$ EW distribution at $zsim7.6$, finding a significant drop from $zlesssim6$, suggesting an increasing fraction of neutral hydrogen (HI) in the IGM in this epoch. We estimate the Ly$alpha$ transmission through the IGM ($=$EW$_{zsimtext{7.6}}$/EW$_{zsimtext{2-6}}$), and infer an IGM HI fraction ($X_{text{HI}}$) of $49^{+19}_{-19}%$ at $zsim7.6$, which is lower in modest tension ($>$1$sigma$) with recent measurements at $z sim$ 7.6. The spatial distribution of the detected Ly$alpha$ emitters implies the presence of a potential highly ionized region at $zsim7.55$ which hosts four Ly$alpha$ emitters within a $sim$ 40 cMpc spatial separation. The prominence of this ionized region in our dataset could explain our lower inferred value of $X_{text{HI}}$, though our analysis is also sensitive to the chosen reference Ly$alpha$ EW distribution values and reionization models.
We present a new catalog of $9318$ Ly$alpha$ emitter (LAE) candidates at $z = 2.2$, $3.3$, $4.9$, $5.7$, $6.6$, and $7.0$ that are photometrically selected by the SILVERRUSH program with a machine learning technique from large area (up to $25.0$ deg$^2$) imaging data with six narrowband filters taken by the Subaru Strategic Program with Hyper Suprime-Cam (HSC SSP) and a Subaru intensive program, Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS). We construct a convolutional neural network that distinguishes between real LAEs and contaminants with a completeness of $94$% and a contamination rate of $1$%, enabling us to efficiently remove contaminants from the photometrically selected LAE candidates. We confirm that our LAE catalogs include $177$ LAEs that have been spectroscopically identified in our SILVERRUSH programs and previous studies, ensuring the validity of our machine learning selection. In addition, we find that the object-matching rates between our LAE catalogs and our previous results are $simeq 80$-$100$% at bright NB magnitudes of $lesssim 24$ mag. We also confirm that the surface number densities of our LAE candidates are consistent with previous results. Our LAE catalogs will be made public on our project webpage.
Ly-alpha blobs (LABs) offer insight into the complex interface between galaxies and their circumgalactic medium. Whilst some LABs have been found to contain luminous star-forming galaxies and active galactic nuclei that could potentially power the Ly-alpha emission, others appear not to be associated with obvious luminous galaxy counterparts. It has been speculated that LABs may be powered by cold gas streaming on to a central galaxy, providing an opportunity to directly observe the `cold accretion mode of galaxy growth. Star-forming galaxies in LABs could be dust obscured and therefore detectable only at longer wavelengths. We stack deep SCUBA-2 observations of the SSA22 field to determine the average 850um flux density of 34 LABs. We measure S_850 = 0.6 +/- 0.2mJy for all LABs, but stacking the LABs by size indicates that only the largest third (area > 1794 kpc^2) have a mean detection, at 4.5 sigma, with S_850 = 1.4 +/- 0.3mJy. Only two LABs (1 and 18) have individual SCUBA-2 > 3.5 sigma detections at a depth of 1.1mJy/beam. We consider two possible mechanisms for powering the LABs and find that central star formation is likely to dominate the emission of Ly-alpha, with cold accretion playing a secondary role.
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