We present the results of a new search for galaxies at redshift z ~ 9 in the first two Hubble Frontier Fields with completed HST WFC3/IR and ACS imaging. To ensure robust photometric redshift solutions, and to minimize incompleteness, we confine our
search to objects with H_{160} < 28.6 (AB mag), consider only image regions with an rms noise sigma_{160} > 30 mag (within a 0.5-arcsec diameter aperture), and insist on detections in both H_{160} and J_{140}. The result is a survey covering an effective area (after accounting for magnification) of 10.9 sq. arcmin, which yields 12 galaxies at 8.4 < z < 9.5. Within the Abell-2744 cluster and parallel fields we confirm the three brightest objects reported by Ishigaki et al. (2014), but recover only one of the four z > 8.4 sources reported by Zheng et al. (2014). In the MACSJ0416.1-240 cluster field we report five objects, and explain why each of these eluded detection or classification as z ~ 9 galaxies in the published searches of the shallower CLASH data. Finally, we uncover four z ~ 9 galaxies from the previously unsearched MACSJ0416.1-240 parallel field. Based on the published magnification maps we find that only one of these 12 galaxies is likely boosted by more than a factor of two by gravitational lensing. Consequently we are able to perform a fairly straightforward reanalysis of the normalization of the z ~ 9 UV galaxy luminosity function as explored previously in the HUDF12 programme. We conclude that the new data strengthen the evidence for a continued smooth decline in UV luminosity density (and hence star-formation rate density) from z ~ 8 to z ~ 9, contrary to recent reports of a marked drop-off at these redshifts. This provides further support for the scenario in which early galaxy evolution is sufficiently extended to explain cosmic reionization.
We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 HST WFC3/IR campaign to explore the rest-frame UV properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of t
he average UV power-law index, beta, for faint galaxies at z ~ 7, the first meaningful measurements of beta at z ~ 8, and tentative estimates for a new sample of galaxies at z ~ 9. Utilising galaxy selection in the new F140W imaging to minimize colour bias, and applying both colour and power-law estimators of beta, we find beta = -2.1 (+/-0.2) at z ~ 7 for galaxies with M_UV ~ -18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z ~ 8 we find a similar value, beta = -1.9 (+/-0.3). At z ~ 9, we find beta = -1.8 (+/-0.6), essentially unchanged from z ~ 6 - 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in beta within our new, robust z ~ 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either ~ solar metallicity and zero dust, or moderately sub-solar (~ 10-20%) metallicity with modest dust obscuration (A_V ~ 0.1-0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with beta ~ -2.5 may not emerge until M_UV > -16, a regime likely to remain inaccessible until the James Webb Space Telescope.
We have exploited the new, deep, near-infrared UltraVISTA imaging of the COSMOS field, in tandem with deep optical and mid-infrared imaging, to conduct a new search for luminous galaxies at redshifts z ~ 7. The unique multi-wavelength dataset provide
d by VISTA, CFHT, Subaru, HST and Spitzer over a common area of 1 deg^2 has allowed us to select galaxy candidates at z > 6.5 by searching first for Y+J-detected (< 25 AB mag) objects which are undetected in the CFHT+HST optical data. This sample was then refined using a photometric redshift fitting code, enabling the rejection of lower-redshift galaxy contaminants and cool galactic M,L,T dwarf stars.The final result of this process is a small sample of (at most) ten credible galaxy candidates at z > 6.5 which we present in this paper. The first four of these appear to be robust galaxies at z > 6.5, and fitting to their stacked SED yields z = 6.98+-0.05 with a stellar mass M* = 5x10^9 Msun, and rest-frame UV spectral slope beta = -2.0+-0.2. The next three are also good candidates for z > 6.5 galaxies, but the possibility that they are low-redshift galaxies or dwarf stars cannot be excluded. Our final subset of three additional candidates is afflicted not only by potential dwarf-star contamination, but also contains objects likely to lie at redshifts just below z = 6.5. We show that the three even-brighter z > 7 galaxy candidates reported in the COSMOS field by Capak et al. (2011) in fact all lie at z ~ 1.5-3.5. Consequently the new z ~ 7 galaxies reported here are the first credible z ~ 7 Lyman-break galaxies discovered in the COSMOS field and, as the most UV-luminous discovered to date at these redshifts, are prime targets for deep follow-up spectroscopy. We explore their physical properties, and briefly consider the implications of their inferred number density for the form of the galaxy luminosity function at z = 7.
We identify and investigate the nature of the 20 brightest 250-micron sources detected by the BLAST within the central 150 sq. arcmin of the GOODS-South field. Aided by the available deep VLA radio imaging, reaching S_1.4 = 30 micro-Jy, we have ident
ified radio counterparts for 17/20 of the 250-micron sources. The resulting enhanced positional accuracy of ~1 arcsec has then allowed us to exploit the deep multi-frequency imaging of GOODS-South to establish secure galaxy counterparts for the 17 radio-identified sources, and plausible galaxy candidates for the 3 radio-unidentified sources. Confusion is a serious issue for this deep BLAST 250-micron survey, due to the large size of the beam. Nevertheless, we argue that our chosen counterparts are significant, and often dominant contributors to the measured BLAST flux densities. For all of these 20 galaxies we have been able to determine spectroscopic (8) or photometric (12) redshifts. The result is the first `complete redshift distribution for a deep 250-micron selected galaxy sample. This reveals that 250-micron surveys reaching detection limits of ~30 mJy contain not only low-redshift spirals/LIRGs, but also the extreme z~2 dust-enshrouded starburst galaxies previously discovered at sub-millimetre wavelengths. Based on their IRAC colours, we find that virtually all of the BLAST galaxy identifications appear better described as analogues of the M82 starburst galaxy, or Sc star-forming discs rather than highly obscured ULIRGs. Inspection of the LABOCA 870-micron imaging of the GOODS-South field yields detections of 7/11 of the z>1 BLAST sources, and reveals 250/870 flux-density ratios consistent with a standard 40K modified black-body fit with a dust emissivity index beta=1.5.
