Silvio Lorenzoni

The contribution of high-redshift galaxies to cosmic reionization: new results from deep WFC3 imaging of the Hubble Ultra Deep Field

We have searched for star-forming galaxies at z≈ 7–10 by applying the Lyman-break technique to newly released Y-, J- and H-band images (1.1, 1.25 and 1.6 μm) 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 (0.85 μm) images, we identify objects with red colours, (z′−Y)AB > 1.3, consistent with the Lyman α forest absorption at z≈ 6.7–8.8. We identify 12 of these z′-drops down to a limiting magnitude YAB 1.0 and JAB 0.5), 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.

The ultraviolet properties of star-forming galaxies - I. HST WFC3 observations of very high redshift galaxies

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 zf850lp-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.

Spectroscopy of z ∼ 7 candidate galaxies: using Lyman α to constrain the neutral fraction of hydrogen in the high-redshift universe

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 χHI∼0.5, in agreement with other estimates.

No evidence for Lyman emission in spectroscopy of z > 7 candidate galaxies

We present Gemini/Gemini Near Infrared Spectrograph (GNIRS) spectroscopic observations of four z-band (z approximate to 7) dropout galaxies and Very Large Telescope (VLT)/XSHOOTER observations of one z-band dropout and three Y-band (z approximate to 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 alpha emission with a typical 5 sigma sensitivity of 5 x 10(-18) erg cm(-2) s(-1), and use the upper limits on Lyman alpha 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 alpha rest-frame equivalent width limit > 120 angstrom (for an unresolved emission line); for an equivalent width limit of 50 angstrom 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 approximate to 1 sigma level for spectrally unresolved lines, which may indicate that a significant neutral H I fraction in the intergalactic medium suppresses the Lyman alpha line in z-drop and Y-drop galaxies at z > 7.

Probing ∼L* Lyman‐break galaxies at z≈ 7 in GOODS‐South with WFC3 on Hubble Space Telescope

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the GOODS-South field to search for star forming galaxies at z~7.0. By comparing WFC 3 0.98 micron Y-band images with ACS z-band (0.85 micron) images, we identify objects with colours consistent with Lyman break galaxies at z~6.4-7.4. This new data covers 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 WFC3 J-band we attempt to remove any low-redshift interlopers. Our selection criteria yields 6 candidates brighter than Y_AB = 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all 6 candidates are at z~7 this implies a surface density of objects brighter than Y_AB = 27.0 of 0.30\pm0.12 arcmin-2, a value significantly smaller than the prediction from the z ~ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function between z = 6 to 7, with number densities lower at higher redshift.

Probing

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the GOODS-South field to search for star forming galaxies at z~7.0. By comparing WFC 3 0.98 micron Y-band images with ACS z-band (0.85 micron) images, we identify objects with colours consistent with Lyman break galaxies at z~6.4-7.4. This new data covers 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 WFC3 J-band we attempt to remove any low-redshift interlopers. Our selection criteria yields 6 candidates brighter than Y_AB = 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all 6 candidates are at z~7 this implies a surface density of objects brighter than Y_AB = 27.0 of 0.30\pm0.12 arcmin-2, a value significantly smaller than the prediction from the z ~ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function between z = 6 to 7, with number densities lower at higher redshift.

\sim L_{*}

The recent Hubble Space Telescope near-infrared imaging with the Wide-Field Camera #3 (WFC 3) of the Great Observatories Origins Deep Survey South (GOODS-S) field in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) programme covering nearly 100 arcmin2, along with already existing Advanced Camera for Surveys optical data, makes possible the search for bright galaxy candidates at redshift z ≈ 7–9 using the Lyman break technique. We present the first analysis of z′-drop z ≈ 7 candidate galaxies in this area, finding 19 objects. We also analyse Y-drops at z ≈ 8, trebling the number of bright (HAB < 27 mag) Y-drops from our previous work, and compare our results with those of other groups based on the same data. The bright high-redshift galaxy candidates we find serve to better constrain the bright end of the luminosity function at those redshift, and may also be more amenable to spectroscopic confirmation than the fainter ones presented in various previous work on the smaller fields (the Hubble Ultra Deep Field and the WFC 3 Early Release Science observations). We also look at the agreement with previous luminosity functions derived from WFC 3 drop-out counts, finding a generally good agreement, except for the luminosity function of Yan et al. at z ≈ 8, which is strongly ruled out.

Lyman-break Galaxies at

The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright the Royal Astronomical Society

z\approx 7

The addition of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) has led to a dramatic increase in our ability to study the z>6 Universe. The increase in the near-infrared (NIR) sensitivity of WFC3 over previous instruments has enabled us to reach apparent magnitudes approaching 29 (AB). This allows us to probe the rest-frame ultraviolet (UV) continuum, redshifted into the NIR at

in GOODS-South with WFC3 on HST

z>6