THE ORIGIN AND OPTICAL DEPTH OF IONIZING RADIATION IN THE “GREEN PEA” GALAXIES

Abstract

Although Lyman continuum (LyC) radiation from star-forming galaxies likely\ndrove the reionization of the Universe, observations of star-forming galaxies\nat low redshift generally indicate low LyC escape fractions. However, the\nextreme [O III]/[O II] ratios of the z=0.1-0.3 Green Pea galaxies may be due to\nhigh escape fractions of ionizing radiation. To analyze the LyC optical depths\nand ionizing sources of these rare, compact starbursts, we compare nebular\nphotoionization and stellar population models with observed emission lines in\nthe Peas' SDSS spectra. We focus on the six most extreme Green Peas, the\ngalaxies with the highest [O III]/[O II] ratios and the best candidates for\nescaping ionizing radiation. The Balmer line equivalent widths and He I\n{\\lambda}3819 emission in the extreme Peas support young ages of 3-5 Myr, and\nHe II {\\lambda}4686 emission in five extreme Peas signals the presence of hard\nionizing sources. Ionization by active galactic nuclei or high-mass X-ray\nbinaries is inconsistent with the Peas' line ratios and ages. Although stacked\nspectra reveal no Wolf-Rayet (WR) features, we tentatively detect WR features\nin the SDSS spectra of three extreme Peas. Based on the Peas' ages and line\nratios, we find that WR stars, chemically homogeneous O stars, or shocks could\nproduce the observed He II emission. If hot stars are responsible, the Peas'\noptical depths are ambiguous. However, accounting for emission from shocks\nlowers the inferred optical depth and suggests that the Peas may be optically\nthin. The Peas' ages likely optimize the escape of Lyman-continuum radiation;\nthey are old enough for supernovae and stellar winds to reshape the\ninterstellar medium, but young enough to possess large numbers of UV-luminous O\nor WR stars.\n