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      Lyman Continuum Emission Escaping from Green Pea Galaxies at z=0.5



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          Compact starburst galaxies are thought to include many or most of the galaxies from which substantial Lyman continuum emission can escape into the intergalactic medium. Li and Malkan (2018) used SDSS photometry to find a population of such starburst galaxies at z~0.5. They were discovered by their extremely strong [OIII]4959+5007 emission lines, which produce a clearly detectable excess brightness in the i bandpass, compared with surrounding filters. We therefore used the HST/COS spectrograph to observe two of the newly discovered i-band excess galaxies around their Lyman limits. One has very strongly detected continuum below its Lyman limit, corresponding to an escape fraction of ionizing photons of 20+/-2%. The other, which is less compact in UV imaging, has a 3-sigma upper limit to its Lyman escape fraction of <5%. Before the UV spectroscopy, the existing data could hardly distinguish these two galaxies. This suggests that roughly ~half of the strong [OIII] emitters as a class have significant ionizing photons escaping, the differences being determined by the luck of our particular viewing geometry. Obtaining the HST spectroscopy, revealed that the Lyman-continuum emitting galaxy differs in having no central absorption in its prominent Ly{\alpha} emission line profile. The other target, with no escaping Lyman continuum, shows the more typical double-peaked Ly{\alpha} emission. This profile signals the presence of a significant column of HI along our line-of-sight, which absorbs both Ly{\alpha} photons at the systemic redshift, and also Lyman limit photons which were emitted in our direction.

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          23 December 2019

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          19 pages, submitted for publication to The Astrophysical Journal December 9, 2019, under review

          Galaxy astrophysics


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