High Lyman Continuum Escape Fraction in a Lensed Young Compact Dwarf Galaxy at z = 2.5

Abstract

We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift z≈ 2.5. The stellar mass of A2218-Flanking is {log}({M}_* /{M}_⊙ )={9.14}_-0.04^+0.07 and SFR is {12.5}_-7.4^+3.8 {M}_⊙ yr^-1 after correcting the magnification. This galaxy has a young galaxy age of 127 Myr and a compact galaxy size of {r}_1/2=2.4 {kpc}. The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (∼800 Å) from A2218-Flanking. We firmly detect (14σ ) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is {f}_{abs,{esc}}> 28 % {--}57 % based on the flux density ratio between 1700 and 800 Å ({f}₁₇₀₀/{f}₈₀₀). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f ₈₀₀ is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower {f}₁₇₀₀/{f}₈₀₀ (higher {f}₈₀₀/{f}₁₇₀₀) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.