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Register a new userA Lyman-{alpha} protocluster at redshift 6.9
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Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6. Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-$alpha$ galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of $3.7 times 10^{15}$ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.
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We report on the serendipitous discovery of a z=4.0, M1500=-22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60% escaping), a remarkable multiple peaked Lya emission, and significant Lya radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionising and Lya radiation possibly share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NV1240 profile, and has blue ultraviolet continuum (beta = -2.5 +/- 0.25, F_lambda~ lambda^beta) with weak low-ionisation interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric signature of the Halpha line with equivalent width of 1000A rest-frame emerging over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar mass of 1.5x10^9 Msun, SFR of 140 Msun/yr and age of ~10 Myr, with a low dust extinction, E(B-V)< 0.1, placing the source in the starburst region of the SFR-M^* plane. Ion3 shows similar properties of another LyC emitter previously discovered (z=3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents ideal high-redshift reference cases to guide the search for reionising sources at z>6.5 with JWST.
We report on a search for ultraluminous Lyman alpha emitting galaxies (LAEs) at z=6.6 using the NB921 filter on Hyper Suprime-Cam on the Subaru telescope. We searched a 30 degree squared area around the North Ecliptic Pole, which we observed in broadband g, r, i, z, and y and narrowband NB816 and NB921, for sources with NB921 < 23.5 and z - NB921 > 1.3. This corresponds to a selection of log L(Ly-alpha) > 43.5 erg/s. We followed up seven candidate LAEs (out of thirteen) with the Keck DEIMOS spectrograph and confirmed five z=6.6 LAEs, one z=6.6 AGN with a broad Ly-alpha line and a strong red continuum, and one low-redshift ([OIII]5007) galaxy. The five ultraluminous LAEs have wider line profiles than lower luminosity LAEs, and one source, NEPLA4, has a complex line profile similar to that of COLA1. In combination with previous results, we show that the line profiles of the z=6.6 ultraluminous LAEs are systematically different than those of lower luminosity LAEs at this redshift. This result suggests that ultraluminous LAEs generate highly ionized regions of the intergalactic medium in their vicinity that allow the full Lyman alpha profile of the galaxy---including any blue wings---to be visible. If this interpretation is correct, then ultraluminous LAEs offer a unique opportunity to determine the properties of the ionized zones around them, which will help in understanding the ionization of the z ~ 7 intergalactic medium. A simple calculation gives a very rough estimate of 0.015 for the escape fraction of ionizing photons, but more sophisticated calculations are needed to fully characterize the uncertainties.
I provide an overview about star-forming galaxies at high redshift and their physical properties. Starting from the populations of Ly-$alpha$ emitters and Lyman break galaxies, I summarize their common features and distinction. Then I summarize recent insight onto their physical properties gained from SED models including nebular emission, and various implications from these studies on the properties of star-formation at high redshift. Finally, I present new results and an overview on the dust content and UV attenuation of $z>6$ galaxies obtained from IRAM and ALMA observations.
We built a grid of photoionization models and compiled already available observational emission line intensities ($rm 1000 : < : lambda(AA) : < : 2000$) of confirmed star formation regions and Active Galactic Nucleus (AGNs) in order to classify five Ly$alpha$ emitter (LAE) objects at high redshift $(5.7 : < : z : < :7.2)$. We selected objects for which at least one metal emission-line was measured. The resulting sample is composed by the objects RXCJ2248.7-4431-ID3, HSCJ233408+004403, COSY, A1703-zd6, and CR7 (clump C). The photoionization models were built assuming a Power Law (associated with the presence of an AGN), a Direct Collapse Black Hole (DCBH), and Population II stars for the ionizing source. The resulting models were then compared with observational emission-line ratios in six diagnostic diagrams to produce a spectral classification of the sample. We found that CR7 (clump C), HSCJ233408+004403 and COSY probably have a non thermal ionizing source (AGN or DCBH) while the RXC J2248.7-4431-ID3 and A1703-zd6 seem to host a stellar cluster. Detailed photoionization models were constructed to reproduce observational emission line ratios of the sample of LAEs, and to derive chemical abundances and number of ionizing photons $Q(rm H)$ of these objects. From these models, we found metallicities in the range $(Z/Z_{odot})=0.1-0.5$ and $log Q(rm H) : > : 53$. Values for C/O abundance ratio derived for the LAEs seem to be consistent with those derived for local star forming objects with similar metallicities, while an overabundance of N/O was found for most of the LAEs.
We present the results from a submm survey of a sample of 23 giant Lya emitting nebulae in the overdensity at z=3.09 in the SA22 field. These objects, which have become known as Lya Blobs (LABs) have a diverse range of morphology and surface brightness, but the nature of their power source is unclear - with cooling flows and/or AGN/starburst ionised winds being possibilities. Using the SCUBA submm camera we measure the 850um flux of a sample of LABs, detecting four LABs at >3.5sigma individually, and a modest statistical detection of the full sample at about 3mJy. These fluxes correspond to bolometric luminosities in the ultraluminous regime, with star-formation rates of about 1e3 Msun/yr. We show there is a trend between Lya luminosity and bolometric output, which suggests that a galactic scale superwind generated from starbursts of age 10-100Myr may be responsible for the Lya emission. We estimate the star-formation rate density in SA22 to be >3 Msun/yr/Mpc^3 - greater than the field at this epoch, and note that there are now 7 submm galaxies in the SA22 structure, making this region the richest association of these intensely active galaxies. Finally we suggest that Lya haloes may be a common feature of the submm population in general, and have an important role in the heating and enrichment of the intergalactic medium.
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