Lyman-αspectral properties of five newly discovered Lyman continuum emitters

Abstract

Aims. We have recently reported the discovery of five low redshift Lyman continuum (LyC) emitters (LCEs, hereafter) with absolute escape fractions f LyC esc ranging from 6 to 13%, higher than previously found, and which more than doubles the number of low redshift LCEs. We use these observations to test theoretical predictions about a link between the characteristics of the Lyman-alpha (Lyα) line from galaxies and the escape of ionising photons. Methods. We analyse the Lyα spectra of eight LCEs of the local Universe observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five leakers and three galaxies from the litterature), and compare their strengths and shapes to the theoretical criteria and comparison samples of local galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas, and the high-redshift strong LyC leaker Ion2. Results. Our LCEs are found to be strong Lyα emitters, with high equivalent widths, EW(Lyα)> 70 Å, and large Lyα escape fractions, f Lyα esc > 20%. The Lyα profiles are all double-peaked with a small peak separation, in agreement with our theoretical expectations. They also have no underlying absorption at the Lyα position. All these characteristics are very different from the Lyα properties of typical star-forming galaxies of the local Universe. A subset of the comparison samples (2-3 Green Pea galaxies) share these extreme values, indicating that they could also be leaking. We also find a strong correlation between the star formation rate surface density and the escape fraction of ionising photons, indicating that the compactness of star-forming regions plays a role in shaping low column density paths in the interstellar medium of LCEs. Conclusions. The Lyα properties of LCEs are peculiar: Lyα can be used as a reliable tracer of LyC escape from galaxies, in complement to other indirect diagnostics proposed in the literature.

Kinematics and Optical Depth in the Green Peas: Suppressed Superwinds in Candidate LyC Emitters*

“…These same galaxies tend to have low outflow velocities. The GPs with outflow velocities < 100 km s −1 all have ∆v Lyα < 400 km s −1 , as narrow as known GP LyC emitters (Verhamme et al 2017). With burst ages of only a few Myr (e.g., Jaskot & Oey 2013;Izotov et al 2017b), some GPs may be too young for SN feedback, although stellar winds should still be present.…”

“…Galaxies with low N HI should also have high f Lyα esc (e.g., Yajima et al 2014;Dijkstra et al 2016;Verhamme et al 2017). However, aperture effects may lower f Lyα esc in the new z < 0.12 GPs compared to other GP samples (Jaskot & Oey 2014;Henry et al 2015;Izotov et al 2016b).…”

ALMA [C ii] 158 μm Detection of a Redshift 7 Lensed Galaxy behind RX J1347.1−1145*

“…In the case of a multi-phase model (Dijkstra et al 2016), a low-velocity offset translates to a low covering fraction of neutral gas, independent of its neutral column density and outflow velocity. In both models, however, the low-velocity offset is a consequence of the presence of low HI-column density escape routes for Lyα photons, which may also allow ionizing photons to escape (see Verhamme et al 2015Verhamme et al , 2017Dijkstra et al 2016 for more detailed discussions). For a more general conclusion, a larger sample is needed, but if future observations systematically show smaller velocity offsets, this would imply that the observed drop in LAE fraction between z∼6 and 7 (e.g., Schenker et al 2012Schenker et al , 2014Caruana et al 2014;Pentericci et al 2014;Tilvi et al 2014;Schmidt et al 2016) is more easily explained by changes in the IGM than in the circumgalactic medium or galactic intrinsic properties (Dijkstra 2014;Choudhury et al 2015;Mesinger et al 2015).…”

Small-scale Intensity Mapping: Extended Halos as a Probe of the Ionizing Escape Fraction and Faint Galaxy Populations during Reionization

“…Various indirect alternative approaches, including relations between the gas covering fraction and reddening (e.g., Jones et al 2012;Leethochawalit et al 2016;Reddy et al 2016), Hβ line equivalent width and UV spectral slope (Zackrisson et al 2013(Zackrisson et al , 2017, or analyses of the Lyα spectral line profile Verhamme et al 2017), have been proposed, but the values for the escape fraction remain within a fairly broad range: f 0.3 0.01 esc ion   . Additionally, measurements of the Thomson scattering optical depth to the CMB have placed the "galaxypopulation-averaged" ionizing escape fraction value within the range f 0.1 0.2 esc ion~-(e.g., Robertson et al 2015;Mitra et al 2016;Sun & Furlanetto 2016).…”