The Compact UV Size of Green Pea Galaxies As Local Analogs of High Redshift Ly$α$-Emitters

Keunho J. Kim, Sangeeta Malhotra, James E. Rhoads, Huan Yang

Published 2021-04-16Version 1

We study the dependence of Ly$\alpha$ escape from galaxies on UV continuum size and luminosity using a sample of 40 Green Pea (GP) galaxies, which are the best local analogs of high-redshift Ly$\alpha$-emitters (LAEs). We use the Cosmic Origins Spectrograph near-ultraviolet images from the \textit{Hubble Space Telescope} to measure the UV size and luminosity with $0.047''$ spatial resolution. Like most galaxies the GPs show a log-normal size distribution. They also show a positive correlation between size and UV-continuum luminosity. The slope of the size-continuum luminosity relation for GPs is consistent with those of continuum-selected star-forming galaxies at low and high redshifts. A distinctive feature of GPs is a very compact typical radius of 0.33 kpc with a population spread (1$\sigma$) of 0.19 kpc. The peak of the size distribution and the intercept of the size-luminosity relation of GPs are noticeably smaller than those of continuum-selected star-forming galaxies at similar redshifts. There are statistically significant anti-correlations found between the circularized half-light radius ($r_{\rm cir,50}$), the Ly$\alpha$ equivalent width (EW(Ly$\alpha$)), and the Ly$\alpha$ escape faction ($f^{Ly\alpha}_{esc}$), suggesting that small UV-continuum radii are crucial for Ly$\alpha$ emission. GPs and high-redshift LAEs have similar sizes, once spatial resolution effects are properly considered. Our results show that a compact/small size is crucial for escape of Ly$\alpha$ photons, and that Ly$\alpha$-emitters show constant characteristic size independent of their redshift.