How galaxies leak radiation reveals clues about cosmic reionization
Astronomers found that galaxies leaking ionizing radiation have distinctly compact glowing halos, offering the first direct observational signatures to predict which star-forming galaxies lose their energy into space. The discovery could improve models used in cosmology and inform understanding of how the early universe transformed from opaque to transparent.
Originaltitel: The Ly <i>α</i> and Continuum Origins Survey. II. The Connection between the Escape of Ionizing Radiation and Ly <i>α</i> Halos in Star-forming Galaxies
Abstract One of the current challenges in galaxy evolution studies is to establish the mechanisms that govern the escape of ionizing radiation from galaxies. Here, we investigate the connection between Lyman continuum (LyC) escape and the conditions of the circumgalactic medium (CGM), as probed by Ly α halos (LAHs) in emission. We use Ly α and UV continuum imaging data from the Ly α and Continuum Origins Survey (LaCOS), targeting 42 nearby ( z ≃0.3), star-forming galaxies with LyC observations (escape fractions of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">esc</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">LyC</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≃</mml:mo> <mml:mn>0.01</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.49</mml:mn> </mml:math> ). LaCOS galaxies show extended Ly α emission ubiquitously, with LyC emitters (LCEs) having more compact Ly α morphologies than non-LCEs, and Ly α spatial offsets that do not exceed the extent of the UV continuum. We model the diffuse LAHs using a combined Sérsic plus exponential 2D profile, and find that the characteristic scale length of the Ly α halo is 10 times larger than the UV, on average. We unveil a significant anticorrelation between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">esc</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">LyC</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> and the Ly α halo fraction (HF; or contribution of the halo to the total Ly α luminosity), which we propose as a new LyC indicator. Our observations show that halo scale lengths and HFs both scale positively with the optical depth of the neutral gas in the interstellar medium, revealing a picture in which Ly α and LyC photons in LCEs either emerge directly from the central starbursts or escape isotropically, and in the case of Ly α , minimize the number of scattering interactions in a less-extended CGM.