Smooth Boundaries to Cosmological HII Regions from Galaxy Clustering

The HII regions around quasars and galaxies at redshifts beyond the epoch of reionisation will provide prime targets for upcoming 21cm campaigns using a new generation of low-frequency radio observatories. Here we show that the boundaries of these HII regions will not be sharp. Rather, the clustering of sources near massive galaxies results in a neutral fraction that rises gradually towards large radii from an interior value near zero. A neutral fraction corresponding to the global background value is typically reached at a distance of 2-5 times the radius of the HII region around the central massive galaxy.Comment: 5 Pages, 3 figures. Submitted to MNRA

Very Massive Stars in High-Redshift Galaxies

A significant fraction of Lyman Alpha (Lya) emitting galaxies (LAEs) at z> 5.7 have rest-frame equivalent widths (EW) greater than ~100 Angstrom. However only a small fraction of the Lya flux produced by a galaxy is transmitted through the IGM, which implies intrinsic Lya EWs that are in excess of the maximum allowed for a population-II stellar population having a Salpeter mass function. In this paper we study characteristics of the sources powering Lya emission in high redshift galaxies. We propose a simple model for Lya emitters in which galaxies undergo a burst of very massive star formation that results in a large intrinsic EW, followed by a phase of population-II star formation with a lower EW. We confront this model with a range of high redshift observations and find that the model is able to simultaneously describe the following eight properties of the high redshift galaxy population with plausible values for parameters like the efficiency and duration of star formation: i-iv) the UV and Lya luminosity functions of LAEs at z=5.7 and 6.5, v-vi) the mean and variance of the EW distribution of Lya selected galaxies at z=5.7, vii) the EW distribution of i-drop galaxies at z~6, and viii) the observed correlation of stellar age with EW. Our modeling suggests that the observed anomalously large intrinsic equivalent widths require a burst of very massive star formation lasting no more than a few to ten percent of the galaxies star forming lifetime. This very massive star formation may indicate the presence of population-III star formation in a few per cent of i-drop galaxies, and in about half of the Lya selected galaxies.Comment: 11 pages, 7 figures, MNRAS in press, comments by referee included, references added+update

The stellar mass function and star formation rate-stellar mass relation of galaxies at z ~ 4 - 7

We investigate the evolution of the star formation rate-stellar mass relation (SFR-M*) and Galaxy Stellar Mass Function (GSMF) of z ~ 4-7 galaxies, using cosmological simulations run with the smoothed particle hydrodynamics code P-GADGET3(XXL). We explore the effects of different feedback prescriptions (supernova driven galactic winds and AGN feedback), initial stellar mass functions and metal cooling. We show that our fiducial model, with strong energy-driven winds and early AGN feedback, is able to reproduce the observed stellar mass function obtained from Lyman-break selected samples of star forming galaxies at redshift 6 < z < 7. At z ~ 4, observed estimates of the GSMF vary according to how the sample was selected. Our simulations are more consistent with recent results from K-selected samples, which provide a better proxy of stellar masses and are more complete at the high mass end of the distribution. We find that in some cases simulated and observed SFR-M* relations are in tension, and this can lead to numerical predictions for the GSMF in excess of the GSMF observed. By combining the simulated SFR(M*) relationship with the observed star formation rate function at a given redshift, we argue that this disagreement may be the result of the uncertainty in the SFR-M* (Luv-M*) conversion. Our simulations predict a population of faint galaxies not seen by current observations.Comment: 23 Pages, 13 figures, modified to match accepted version to MNRA

Luminosity Functions of Lyman Alpha Emitting Galaxies and Cosmic Reionization of Hydrogen

Recent observations imply that the observed number counts of Lya Emitters (LAEs) evolved significantly between z=5.7 and z=6.5. It has been suggested that this was due to a rapid evolution in the ionisation state, and hence transmission of the IGM which caused Lya flux from z=6.5 galaxies to be more strongly suppressed. In this paper we consider the joint evolution of the Lya and UV luminosity functions (LFs) and show that the IGM transmission evolved between z=6.5 and z=5.7 by a factor 1.1 <R < 1.8 (95% CL). This result is insensitive to the underlying model of the Lya LF (as well as cosmic variance). Using a model for IGM transmission, we find that the evolution of the mean IGM density through cosmic expansion alone may result in a value for the ratio of transmissions as high as R=1.3. Thus, the existing LFs do not provide evidence for overlap. Furthermore, the constraint R<1.8 suggests that the Universe at z=6.5 was more than half ionised by volume, i.e. x_i,V>0.5.Comment: MNRAS in press. Constraints from rest-frame UV LF added. Discussion added on cosmic variance. Lower limit on x_i,V lowered to 0.5 (from 0.8

Improved Constraints on The Neutral Intergalactic Hydrogen Surrounding Quasars at Redshifts z>6

We analyze the evolution of HII regions around the seven known SDSS quasars at z>6. The comparison between observed and model radii of the HII regions generated by these quasars individually, suggests that the surrounding intergalactic hydrogen is significantly neutral. When all constraints are combined, the existing quasar sample implies a volume averaged neutral fraction that is larger than 10% at z>6. This limited sample permits a preliminary analysis of the correlations between the quasar parameters, the sizes of their HII regions, and the associated constraints on the neutral hydrogen fraction. We find no evidence in these correlations to contradict the interpretation that the red side of the Gunn-Peterson trough corresponds to the boundary between an HII region and a partially neutral IGM.Comment: 9 pages, 4 figures. Submitted to Ap