The Lyα Reference Sample. VIII. Characterizing Lyα Scattering in Nearby Galaxies

Abstract

We examine the dust geometry and Lyα scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby ( < < z 0.02 0.2) Lyα-emitting and starbursting systems with Hubble Space Telescope Lyα, Hα, and Hβ imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media, while others of them show significantly lower Lyα emission compared to their Balmer decrement. With the LARS imaging, we present Lyα/Hα and Hα/Hβ maps with spatial resolutions as low as ∼40pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Lyα photons only scatter. We show that the dust that affects the escape of Lyα is more restricted to the galaxies' central regions, while the larger Lyα halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Lyα scatters in the halo, and find that this "characteristic" scattering distance correlates with the measured size of the Lyα halo. We note that there exists a slight anti-correlation between the scattering distance of Lyα and global dust properties.

The dominant origin of diffuse Lyα halos around Lyα emitters explored by spectral energy distribution fitting and clustering analysis

“…Lyα photons are not attenuated by dust if dust is confined in HI clumps (the clumpy ISMs ;Neufeld 1991;Hansen & Peng Oh 2006) because Lyα photons are scattered on the surface of clumps before being absorbed by dust. Scarlata et al (2009) find that the clumpy dust screen (ISMs) can reproduce observed line ratios of Lyα to Hα (or fesc(Lyα)), and Hα to Hβ (orE(B − V )) of local LAEs (see also Bridge et al 2017). It is, however, not clear what causes such a clumpy ISM geometry especially for LAEs.…”

Deciphering the Lyman α blob 1 with deep MUSE observations

“…The difficulty in interpreting line-of-sight velocities and linewidths from Lyα emission is that resonant scattering diffuses the intrinsic Lyα radiation field in real and in the frequency space (see review by Dijkstra 2019). The spatial diffusion can be envisioned as a projected smoothing processes (Bridge et al 2018) that can enhance the apparent size of the Lyα blobs by reducing the steepness of their surface brightness profiles (Zheng et al 2011;Gronke & Bird 2017). Moreover, it can also "wash-out" cold-accretion features of small angular size (Smith et al 2019).…”

Emission from the Ionized Gaseous Halos of Low-redshift Galaxies and Their Neighbors

“…Although these are plausible explanations, physical reality is likely to be complicated by the detailed internal geometry of the central galaxy. Studies of the Lyα emission from starbursting systems have highlighted the complex role of dust content and geometry on the escaping radiation (Hayes et al 2013(Hayes et al , 2014Bridge et al 2017). At intermediate r p , 50 kpc, the emission flux is becoming more similar in the two subsamples and therefore less dependent on the SFR of the central galaxy.…”