A critical analysis of the UV luminosity function at redshift  ~7 from deep WFC3 data

Abstract

Context. The study of the luminosity function (LF) of Lyman break galaxies (LBGs) at z = 7 is very important for ascertaining their role in the reionization of the Universe. These galaxies can be used also to investigate in detail the processes of formation and evolution of galactic structures in the infancy of our Universe. Aims. In this work we plan to perform a detailed and critical analysis of the statistical and systematic errors in the z ∼ 7 LF determination. Methods. To this aim, we have assembled a large sample of candidate LBGs at z ∼ 7 from different surveys, spanning a large variety of areas and depths. In particular, we have combined data from the deep (J < 27.4) and ultradeep (J < 29.2) surveys recently acquired with the new WFC3 NIR camera on HST, over the GOODS-ERS (∼40 sq. arcmin) and the HUDF (∼4 sq. arcmin) fields, with ground based surveys in wide and shallow areas from Hawk-I@VLT and HyperSuprimecam@Subaru. We have used public ACS images in the z band to select z-dropout galaxies, and other public data both in the blue (BVI) and in the red bands to reject possible low-redshift interlopers. We have compared our results with extensive Monte Carlo simulations to quantify the observational effects of our selection criteria as well as the effects of photometric scatter, color selections or the morphology of the candidates. Results. We have found that the number density of faint LBGs at z ∼ 7 is only marginally sensitive to the color selection adopted, but it is strongly dependent from the assumption made on the half light distributions of the simulated galaxies, used to correct the observed sample for incompleteness. The slope of the faint end of the LBGs LF has thus a rather large uncertainty, due to the unknown distribution of physical sizes of the z ∼ 7 LBGs. The implications of these uncertainties have been neglected by previous works. Conclusions. We conclude that galaxies at z ∼ 7 are unable to reionize the Universe unless there is a significant evolution in the clumpiness of the IGM or in the escape fraction of ionising photons or, alternatively, there is a large population of z ∼ 7 LBGs with large physical dimensions but still not detected by the present observations.

The size-luminosity relation atz = 7 in CANDELS and its implication on reionization

“…Their characteristics are described in Tables 1 and 2 of Grazian et al (2011). For the GDS and P12HUDF we recover 21 and 23 galaxies down to J < 27.8 and J < 29.2, respectively.…”

“…Thus a detailed analysis on the typical sizes of high-z galaxies and the relation between galaxy size and luminosity is necessary to understand whether galaxies alone are the responsibles for reionization. In Grazian et al (2011) we did not provide a definitive answer to these questions, which therefore motivated the investigation here with a larger sample of galaxies at z ∼ 7. Of course, other hypotheses on the sources responsible for reionization at such high-z are possible, like a top-heavy IMF, a large contribution from PopIII stars, or other sources of ionizing photons i.e., high-z active galactic nuclei (AGNs).…”

“…As for the ERS, a full description of this dataset and of the reduction procedures adopted are described in Grazian et al (2011).…”

“…This ensures both precise colors for extended objects and unbiased photometry for faint sources. Further details on the photometric measurement can be found in Grazian et al (2011).…”

“…One of the main motivations of this work is to answer to the questions raised in Grazian et al (2011). In our previous work we have explored different distributions for the half light radius of z = 7 galaxies.…”