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Register a new userCANDELSz7: a large spectroscopic survey of CANDELS galaxies in the reionization epoch
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We present the results of CANDELSz7, an ESO large program aimed at confirming spectroscopically a homogeneous sample of z~6 and z~7 star forming galaxies. The candidates were selected in the GOODS-South, UDS and COSMOS fields using the official CANDELS catalogs based on H160-band detections. Standard color criteria, which were tailored depending on the ancillary multi-wavelength data available for each field, were applied to select more than 160 candidate galaxies at z~6 and z~7. Deep medium resolution FORS2 spectroscopic observations were then conducted with integration times ranging from 12 to 20 hours, to reach a Lyalpha flux limit of approximately 1-3x 10-18 erg/s/cm^2 at 3sigma. For about 40% of the galaxies we could determine a spectroscopic redshift, mainly through the detection of a single emission line that we interpret as Lyalpha emission, or for some of the brightest objects (H160< 25.5) from the presence of faint continuum and sharp drop that we interpret as a Lyman break. In this paper we present the redshifts and main properties of 65 newly confirmed high redshift galaxies. Adding previous proprietary and archival data we assemble a sample of ~260 galaxies that we use to explore the evolution of the Lyalpha fraction in Lyman break galaxies and the change in the shape of the emission line between z~6 and z~7. We also discuss the accuracy of the CANDELS photometric redshifts in this redshift range.
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Within one billion years of the Big Bang, intergalactic hydrogen was ionized by sources emitting ultraviolet and higher energy photons. This was the final phenomenon to globally affect all the baryons (visible matter) in the Universe. It is referred to as cosmic reionization and is an integral component of cosmology. It is broadly expected that intrinsically faint galaxies were the primary ionizing sources due to their abundance in this epoch. However, at the highest redshifts ($z>7.5$; lookback time 13.1 Gyr), all galaxies with spectroscopic confirmations to date are intrinsically bright and, therefore, not necessarily representative of the general population. Here, we report the unequivocal spectroscopic detection of a low luminosity galaxy at $z>7.5$. We detected the Lyman-$alpha$ emission line at $sim 10504$ {AA} in two separate observations with MOSFIRE on the Keck I Telescope and independently with the Hubble Space Telescopes slit-less grism spectrograph, implying a source redshift of $z = 7.640 pm 0.001$. The galaxy is gravitationally magnified by the massive galaxy cluster MACS J1423.8+2404 ($z = 0.545$), with an estimated intrinsic luminosity of $M_{AB} = -19.6 pm 0.2$ mag and a stellar mass of $M_{star} = 3.0^{+1.5}_{-0.8} times 10^8$ solar masses. Both are an order of magnitude lower than the four other Lyman-$alpha$ emitters currently known at $z > 7.5$, making it probably the most distant representative source of reionization found to date.
Candidates for the modest galaxies that formed most of the stars in the early universe, at redshifts $z > 7$, have been found in large numbers with extremely deep restframe-UV imaging. But it has proved difficult for existing spectrographs to characterise them in the UV. The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant UV-selected galaxy detected in dust emission is only at $z = 3.25$, and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at this early epoch. Here we report thermal dust emission from an archetypal early universe star-forming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be $z = 7.5pm0.2$ from a spectroscopic detection of the Ly{alpha} break. A1689-zD1 is representative of the star-forming population during reionisation, with a total star-formation rate of about 12M$_odot$ yr$^{-1}$. The galaxy is highly evolved: it has a large stellar mass, and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at $z > 7$, in spite of the very short time since they first appeared.
We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 z--band dropout candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we report on two newly discovered galaxies at redshift ~6.7 in the NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for the fraction of Ly-alpha emission in LBGs that is constantly increasing from z~3 to z~6 is most probably reversed from z~6 to z~7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
We present new results on [CII]158$mu$ m emission from four galaxies in the reionization epoch. These galaxies were previously confirmed to be at redshifts between 6.6 and 7.15 from the presence of the Ly$alpha$ emission line in their spectra. The Ly$alpha$ emission line is redshifted by 100-200 km/s compared to the systemic redshift given by the [CII] line. These velocity offsets are smaller than what is observed in z~3 Lyman break galaxies with similar UV luminosities and emission line properties. Smaller velocity shifts reduce the visibility of Ly$alpha$ and hence somewhat alleviate the need for a very neutral IGM at z~7 to explain the drop in the fraction of Ly$alpha$ emitters observed at this epoch. The galaxies show [CII] emission with L[CII]=0.6-1.6 x10$^8 L_odot$: these luminosities place them consistently below the SFR-L[CII] relation observed for low redshift star forming and metal poor galaxies and also below z =5.5 Lyman break galaxies with similar star formation rates. We argue that previous undetections of [CII] in z~7 galaxies with similar or smaller star formation rates are due to selection effects: previous targets were mostly strong Ly$alpha$ emitters and therefore probably metal poor systems, while our galaxies are more representative of the general high redshift star forming population .
The James Webb Space Telescope will provide deep imaging and spectroscopy for sources at redshifts above 6, covering the Epoch of Reionization (EoR, 6 < z < 10). The Mid-IR instrument (MIRI) integral field spectrograph (MRS) will be the only instrument on board JWST able to observe the brightest optical emission lines H$alpha$ and [OIII]0.5007$mu$m at redshifts above 7 and 9, respectively. This paper presents a study of the H$alpha$ fluxes predicted by FIRSTLIGHT cosmological simulations for galaxies at redshifts of 6.5 to 10.5, and its detectability with MIRI. Deep (40 ks) spectroscopic integrations with MRS will be able to detect (S/N > 5) EoR sources at redshifts above 7 with intrinsic star formation rates of more than 2 M$_{odot}$ yr$^{-1}$, and stellar masses above 4-9 $times$ 10$^7$ M$_{odot}$. In addition, the paper presents realistic MRS simulated observations of the expected (rest-frame) optical and near-infrared spectra for some spectroscopically confirmed EoR sources detected by ALMA as [OIII]88$mu$m emitters. The MRS simulated spectra cover a wide range of low metallicities from about 0.2 to 0.02Z$_{odot}$, and different [OIII]88$mu$m/[OIII]0.5007$mu$m line ratios. The simulated 10ks MRS spectra show S/N in the range of 5 to 90 for H$beta$, [OIII]0.4959,0.5007$mu$m, H$alpha$ and HeI1.083$mu$m emission lines of MACS1149-JD1 at z = 9.11, independent of metallicity. In addition, deep 40 ks simulated spectra of the luminous merger candidate B14-65666 at z=7.15 shows the MRS capabilities of detecting, or putting strong upper limits, on the [NII]0.6584$mu$m, [SII]0.6717,0.6731$mu$m, and [SIII]0.9069,0.9532$mu$m emission lines. In summary, MRS will enable the detailed study of key physical properties like internal extinction, instantaneous star formation, hardness of the ionising continuum, and metallicity, in bright (intrinsic or lensed) EoR sources.
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