Lyα Escape from Low-mass, Compact, High-redshift Galaxies

Abstract

Abstract We investigate the effects of stellar populations and sizes on Ly α escape in 27 spectroscopically confirmed and 35 photometric Ly α emitters (LAEs) at z ≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive ( M ⋆ ≈ 10 7 –10 9 M ⊙ ), younger (ages ≲1 Gyr), smaller ( r e &lt; 1 kpc), and less dust-attenuated ( E ( B − V ) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1–100 M ⊙ yr −1 ). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities ( Z neb ≲ 0.2 Z ⊙ ) and/or high ionization parameters ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">U</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.1em"/> <mml:mo>≳</mml:mo> <mml:mspace width="0.1em"/> <mml:mo>−</mml:mo> <mml:mn>2.4</mml:mn> </mml:math> ). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Ly α escape by computing SFR surface density, Σ SFR , and specific SFR surface density, Σ sSFR . For a given Σ SFR , the Ly α escape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher Σ sSFR , on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Ly α photons. These results have important implications for the physics of Ly α radiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization.