Galaxies with strong Lyman-alpha (Lyα) emission line (also called Lyα galaxies or emitters) offer an unique probe of the epoch of reionization—one of the important phases when most of the neutral hydrogen in the universe was ionized. In addition, Lyα galaxies at high redshifts are a powerful tool to study low-mass galaxy formation.
Since current observations suggest that the reionization is complete by redshift z ∼ 6, it is therefore necessary to discover galaxies at z > 6, to use their luminosity function (LF) as a probe of reionization. I found five z = 7.7 candidate Lyα galaxies with line fluxes > 7×10−18 erg/s/cm/ 2, from three different deep near-infrared (IR) narrowband (NB) imaging surveys in a volume > 4×104Mpc3.
From the spectroscopic followup of four candidate galaxies, and with the current spectroscopic sensitivity, the detection of only the brightest candidate galaxy can be ruled out at 5 sigma level. Moreover, these observations successfully demonstrate that the sensitivity necessary for both, the NB imaging as well as the spectroscopic followup of z ∼ 8 Lyα galaxies can be reached with the current instrumentation. While future, more sensitive spectroscopic observations are necessary, the observed Lyα LF at z = 7.7 is consistent with z = 6.6 LF, suggesting that the intergalactic medium (IGM) is relatively ionized even at z = 7.7, with neutral fraction x HI≤ 30%.
On the theoretical front, while several models of Lyα emitters have been developed, the physical nature of Lyα emitters is not yet completely known. Moreover, multi-parameter models and their complexities necessitates a simpler model. I have developed a simple, single-parameter model to populate dark mater halos with Lyα emitters. The central tenet of this model, different from many of the earlier models, is that the star-formation rate (SFR), and hence the Lyα luminosity, is proportional to the mass accretion rate rather than the total halo mass. This simple model is successful in reproducing many observable including LFs, stellar masses, SFRs, and clustering of Lyα emitters from z ∼ 3 to z ∼ 7. Finally, using this model, I find that the mass accretion, and hence the star-formation in > 30% of Lyα emitters at z ∼ 3 occur through major mergers, and this fraction increases to ∼ 50% at z ∼ 7.
