We analyze archival Lyα spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Lyα profiles with radiative transfer models, and explore the dependence of Lyα escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]λ5007 equivalent widths of hundreds ofÅ. All 12 Green Pea galaxies in our sample show Lyα lines in emission, with a Lyα equivalent width distribution similar to high redshift Lyα emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Lyα escape fractions and find correlations between Lyα escape fraction and kinematic features of Lyα profiles. The escape fraction of Lyα in these galaxies ranges from 1.4% to 67%. We also find that the Lyα escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Lyα profiles with single HI shell radiative transfer models and find that the Lyα escape fraction anticorrelates with the derived HI column densities. Single shell models fit most Lyα profiles well, but not the ones with highest escape fractions of Lyα. Our results suggest that low HI column density and low metallicity are essential for Lyα escape, and make a galaxy a Lyα emitter.

Cosmological simulations suggest a strong correlation between high optical-depth Lyα absorbers, which arise from the intergalactic medium (IGM), and 3-D mass overdensities on scales of 10 − 30 h −1 comoving Mpc. By examining the absorption spectra of ∼ 80,000 QSO sight-lines over a volume of 0.1 Gpc 3 in the Sloan Digital Sky Survey III (SDSS-III), we have identified an extreme overdensity, BOSS1441, which contains a rare group of strong Lyα absorbers at z = 2.32 ± 0.02. This absorber group is associated with six QSOs at the same redshift on a 30 comoving Mpc scale. Using Mayall/MOSAIC narrowband and broadband imaging, we detect Lyα emitters (LAEs) down to 0.7 × L * Lyα , and reveal a large-scale structure of Lyα emitters (LAEs) in this field. Our follow-up Large Binocular Telescope (LBT) observations have spectroscopically confirmed 19 galaxies in the density peak. We show that BOSS1441 has an LAE overdensity of 10.8 ± 2.6 on a 15 comoving Mpc scale which could collapse to a massive cluster with M 10 15 M at z ∼ 0. This overdensity is among the most massive large-scale structures at z ∼ 2 discovered to date.

We present the largest-ever sample of 79 Lyα emitters (LAEs) at z ∼ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in the literature. The bright end bump in the Lyα luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L Lyα > 10 43.3 erg s −1 ). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at z ∼ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of x HI ∼ 0.2 -0.4 is derived at z ∼ 7.0 based on the cosmic evolution of the Lyα luminosity function.