Search

Following our previous spectroscopic observations of z > 7 galaxies with Gemini/Gemini Near Infra-Red Spectrograph (GNIRS) and Very Large Telescope (VLT)/XSHOOTER, which targeted a total of eight objects, we present here our results from a deeper and larger VLT/FOcal Reducer and Spectrograph (FORS2) spectroscopic sample of Wide Field Camera 3 selected z > 7 candidate galaxies. With our FORS2 setup we cover the 737–1070 nm wavelength range, enabling a search for Lyman α in the redshift range spanning 5.06–7.80. We target 22 z-band dropouts and find no evidence of Lyman α emission, with the exception of a tentative detection (<5σ, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3-6.5 is inconsistent with our non-detections at z ∼ 7.8 at the 96 per cent confidence level. This may indicate that a significant neutral H i fraction in the intergalactic medium suppresses Lyman α, with an estimated neutral fraction $\chi _{\rm H\,\small {I}}\sim 0.5$, in agreement with other estimates. [ABSTRACT FROM PUBLISHER]

ABSTRACT We present Gemini/Gemini Near Infrared Spectrograph (GNIRS) spectroscopic observations of four z-band ( z ≈ 7) dropout galaxies and Very Large Telescope (VLT)/XSHOOTER observations of one z-band dropout and three Y-band ( z ≈ 8-9) dropout galaxies in the Hubble Ultra Deep Field, which were selected with Wide Field Camera 3 imaging on the Hubble Space Telescope. We find no evidence of Lyman α emission with a typical 5σ sensitivity of 5 × 10−18 erg cm−2 s−1, and use the upper limits on Lyman α flux and the broad-band magnitudes to constrain the rest-frame equivalent widths for this line emission. Accounting for incomplete spectral coverage, we survey 3.0 z-band dropouts and 2.9 Y-band dropouts to a Lyman α rest-frame equivalent width limit >120 Å (for an unresolved emission line); for an equivalent width limit of 50 Å the effective numbers of drop-outs surveyed fall to 1.2 z-band drop-outs and 1.5 Y-band drop-outs. A simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3-6.5 is inconsistent with our non-detections at z = 7-9 at the ≈1σ level for spectrally unresolved lines, which may indicate that a significant neutral H i fraction in the intergalactic medium suppresses the Lyman α line in z-drop and Y-drop galaxies at z > 7. [ABSTRACT FROM AUTHOR]

ABSTRACT The acquisition of deep near-IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very high redshift Universe. For galaxies up to z≈ 7.7 sufficient wavelength coverage exists to probe the rest-frame ultraviolet (UV) continuum without contamination from either Lyman α emission or the Lyman α break. In this work we use near-infrared (near-IR) imaging to measure the rest-frame UV continuum colours of galaxies at 4.7 < z < 7.7. We have carefully defined a colour-colour selection to minimize any inherent bias in the measured UV continuum slope for the drop-out samples. For the highest redshift sample (6.7 < z < 7.7), selected as z f850 lp-band drop-outs, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust-free, solar metallicity, star-forming population (or a moderately dusty population of low metallicity). At lower redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower redshift and more luminous galaxies are dustier; however, this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours. [ABSTRACT FROM AUTHOR]

We present a search for galaxies at using the latest Hubble Space Telescope/Wide Field Camera 3 (WFC3) near-infrared data, based on the Lyman-break technique. We search for galaxies which have large colours (the '-drops') on account of the Lyman α forest absorption, and with colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift (15 are robust and a further nine more marginal but consistent with being high redshift) over an area of arcmin. Previous searches for -drops with WFC3 have focused only on the Hubble Ultra Deep Field, and our larger survey (involving two other nearby deep fields and a wider area survey) has trebled the number of robust -drop candidates. For the first time, we have sufficient galaxies to fit both and of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from to , in the sense that there are fewer UV-bright galaxies at , consistent with an evolution mainly in . The candidate galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxies below our detection limit provide a significant UV contribution. The faint-end slope, α, is not well constrained. However, adopting a similar faint-end slope to that determined at () and a Salpeter initial mass function (IMF), then the ionizing photon budget still falls short if , even integrating down to . A steeper faint-end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

We have searched for star-forming galaxies at by applying the Lyman-break technique to newly released Y-, J- and H-band images (m) from Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. By comparing these images of the Hubble Ultra Deep Field with the Advanced Camera for Surveys (ACS) z′-band (m) images, we identify objects with red colours, , consistent with the Lyman α forest absorption at . We identify 12 of these z′-drops down to a limiting magnitude (equivalent to a star formation rate of at ), all of which are undetected in the other ACS filters. We use the WFC-band image to eliminate contaminant low-mass Galactic stars, which typically have redder colours than galaxies. One of our z′-drops is probably a T-dwarf star. The -drops appear to have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z′-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining candidates we determine a lower limit on the star formation rate density of for a Salpeter initial mass function, which rises to after correction for luminosity bias. The star formation rate density is a factor of ≈10 less than that of Lyman-break galaxies at , and is about half the value at . We also present the discovery of seven Y-drop objects with and which are candidate star-forming galaxies at higher redshifts (). We find no robust J-drop candidates at . While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit (a steep faint-end slope). The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are of low metallicity or have a top-heavy initial mass function. [ABSTRACT FROM AUTHOR]

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the Great Observatories Origins Deep Survey (GOODS)-South field to search for star-forming galaxies at . By comparing Wide Field Camera 3 (WFC3) m Y-band images with Advanced Camera for Surveys (ACS) z-band ( m) images, we identify objects with colours consistent with Lyman-break galaxies at . This new data cover an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B, V and i bands from GOODS v2.0 and the WFC band, we attempt to remove any low-redshift interlopers. Our selection criteria yields six candidates brighter than , of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all six candidates are at this implies a surface density of objects brighter than of , a value significantly smaller than the prediction from luminosity function. This suggests continued evolution of the bright end of the luminosity function between and 7, with number densities lower at higher redshift. [ABSTRACT FROM AUTHOR]