Ly$\alpha$ and UV Sizes of Green Pea Galaxies

Abstract

Green Peas are nearby analogs of high-redshift Ly$\alpha$-emitting galaxies. To probe their Ly$\alpha$ escape, we study the spatial profiles of Ly$\alpha$ and UV continuum emission of 24 Green Pea galaxies using the Cosmic Origins Spectrograph (COS) on Hubble Space Telescope (HST). We extract the spatial profiles of Ly$\alpha$ emission from their 2D COS spectra, and of UV continuum from both the 2D spectra and NUV images. The Ly$\alpha$ emission shows more extended spatial profiles than the UV continuum in most Green Peas. The deconvolved Full Width Half Maximum (FWHM) of the Ly$\alpha$ spatial profile is about 2 to 4 times that of the UV continuum in most cases. Since Green Peas are analogs of high-z LAEs, it suggests that most high-z LAEs likely have larger Ly$\alpha$ sizes than UV sizes. We also compare the spatial profiles of Ly$\alpha$ photons at blueshifted and redshifted velocities in eight Green Peas with sufficient data quality, and find the blue wing of the Ly$\alpha$ line has a larger spatial extent than the red wing in four Green Peas with comparatively weak blue Ly$\alpha$ line wings. We show that Green Peas and MUSE $z=3-6$ LAEs have similar Ly$\alpha$ and UV continuum sizes, which probably suggests starbursts in both low-z and high-z LAEs drive similar gas outflows illuminated by Ly$\alpha$ light. Five Lyman continuum (LyC) leakers in this sample have similar Ly$\alpha$ to UV continuum size ratios (~1.4-4.3) to the other Green Peas, indicating their LyC emission escape through ionized holes in the interstellar medium.