Using Lyman-αto detect galaxies that leak Lyman continuum

Abstract

Aims. We propose to infer the output of the ionising continuum-leaking properties of galaxies based upon their Lyα line profiles. Methods. We carried out Lyα radiation transfer calculations in two models of H ii regions. These models are porous to ionising continuum escape: 1) we define Lyman-continuum (LyC) optically thin star clusters, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, in other words, almost totally ionised; and 2) we define riddled ionisation-bounded media that are surrounded by neutral interstellar medium, but have holes, which results in a covering fraction lower than unity. Results. The Lyα spectra that emerge from these configurations have distinctive features: 1) a classical asymmetric redshifted profile in the first case, but with a small shift of the profile maximum compared to the systemic redshift (v peak ≤ 150 km s −1 ); 2) a main peak at the systemic redshift in the second case (v peak = 0), with a non-zero Lyα flux bluewards of the systemic redshift as a consequence. If in a galaxy that leaks ionising photons the Lyα component that emerges from the leaking star cluster(s) is assumed to dominate the total Lyα spectrum, the Lyα shape may be used as a pre-selection tool for detecting LyC-leaking galaxies in objects with high spectral resolution Lyα spectra (R ≥ 4000). Our predictions are corroborated by examination of a sample of ten local starbursts with high-resolution HST/COS Lyα spectra that are known in the literature as LyC leakers or leaking candidates. Conclusions. Observations of Lyα profiles at high resolution are expected to show definite signatures revealing the escape of Lyman-continuum photons from star-forming galaxies.

The Lyman alpha reference sample

“…When assuming n IC /n C = 0 in each component, we always obtain a strong emission at the Lyα line center. Verhamme et al 2015) enables us to use the Lyα line of Mrk 1486 as a probe of the clumpiness of the HI distribution inside the disk and outflowing halos. As shown in Duval et al (2014), a clumpy distribution of HI and dust can affect the Lyα line profile in three different ways:…”

Inclination dependence of Lyman-αproperties in a turbulent disk galaxy

“…Galaxies detected by their strong Lyman-α (Lyα) emission, called Lyα emitters (LAEs), have become an important tool in understanding cosmology at intermediate-to-high redshifts (z > 1.5), such as for the large-scale matter distribution (Hill et al 2008;Adams et al 2011) or the epoch of reionization via the connection of LAEs and Lyman-continuum leakers (Behrens et al 2014;Verhamme et al 2014;Dijkstra 2014). The physical type(s) of objects classified as LAEs and the connection of the observed line profiles to the physical properties of the emitters are still a matter of debate, though progress has been made in recent years (e.g., Ahn et al 2003;Verhamme et al 2006;Dijkstra & White 2010).…”

“…In some recent publications, work has been focused on understanding how anisotropies of LAEs in terms of column densities and velocity fields affect observed Lyα properties with respect to the observer's position, mainly for simplified models of LAEs (Zheng & Wallace 2014;Laursen et al 2013;Behrens et al 2014;Verhamme et al 2014;Gronke & Dijkstra 2014;Duval et al 2014). For quantifying the inclination dependence of Lyα transmission, more realistic models of LAEs are needed, for example, for estimating how large-scale surveys are affected by a possible alignment of galaxies with the large-scale structure in combination with inclination effects (Hirata 2009) or other correlations with the large-scale structure (Zheng et al 2010(Zheng et al , 2011Behrens & Niemeyer 2013).…”

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

“…On top of the bulk velocity of the scattering medium, the width and the shift of the maximum of the Lyα peak correlate with the column density of the scattering medium (e.g. Verhamme et al 2015). From the velocity map, the shift is between ∼150 and 200 km s −1 everywhere, which is typical of high-redshift LAEs (Hashimoto et al 2015) tracing typical column densities of the order of log(NHI)∼20.…”

“…17 in Rivera- Thorsen et al 2015, for an illustration). Given the shape of the Lyα profile emergent from this galaxy, the covering fraction of the scattering medium is most certainly 1, since the profile does not peak at the systemic redshift, but is redshifted by ∼170 km s −1 , which means no intrinsic Lyα emission is seen (Behrens, Dijkstra & Niemeyer 2014;Verhamme et al 2015).…”