Dissection of Hα emitters: Low-z analogs of z > 4 star-forming galaxies

Strong Hα emitters (HAEs) dominate the z ∼ 4 Lyman-break galaxy (LBG) population. We have identified local analogs of these HAEs using the Sloan Digital Sky Survey. At z < 0.4, only 0.04% of the galaxies are classified as HAEs with Hα equivalent widths (≳ 500) comparable to that of z ∼ 4 HAEs. Local HAEs have lower stellar mass and lower ultraviolet (UV) luminosity than z ∼ 4 HAEs, yet the Hα-to-UV luminosity ratio, as well as their specific star formation rate, is consistent with that of z ∼ 4 HAEs, indicating that they are scaled-down versions of high-z star-forming galaxies. Compared to the previously studied local analogs of LBGs selected using rest-frame UV properties, local HAEs show similar UV luminosity surface density, weaker D_n (4000) break, lower metallicity, and lower stellar mass. This implies that the local HAEs are less evolved galaxies than the traditional Lyman break analogs. In the stacked spectrum, local HAEs show a significant He II λ4686 emission line suggesting a population of hot, massive stars similar to that seen in some Wolf-Rayet galaxies. Low [N II]/[O III] line flux ratios imply that local HAEs are inconsistent with being systems that host bright active galactic nuclei. Instead, it is highly likely that local HAEs are galaxies with an elevated ionization parameter, either due to a high electron density or large escape fraction of hydrogen ionizing photons as in the case of Wolf-Rayet galaxies.