Models of diffuse Hα in the interstellar medium: the relative contributions from in situ ionization and dust scattering

Abstract

Using three dimensional Monte Carlo radiation transfer models of photoionisation and dust scattering, we explore different components of the widespread diffuse Hα emission observed in the interstellar medium of the Milky Way and other galaxies. We investigate the relative contributions of Hα from recombination emission in ionised gas and Hα that originates in Hii regions near the Galactic midplane and scatters off high altitude dust in the diffuse interstellar medium. For the radiation transfer simulations we consider two geometries for the interstellar medium: a three dimensional fractal geometry that reproduces the average density structure inferred for hydrogen in the Milky Way, and a density structure from a magneto hydrodynamic simulation of a supernovae driven turbulent interstellar medium. Although some sight lines that are close to Hii regions can be dominated by scattered light, overall we find that less than ∼ 20% of the total Hα intensity in our simulations can be attributed to dust scattering. Our findings on the relative contribution of scattered Hα are consistent with previous observational and theoretical analyses. We also investigate the relative contributions of dust scattering and in situ ionisation of high density dust clouds in the diffuse gas. Dust scattering in these partially ionised clouds contribute ∼ 40% to the total intensity of Hα.

Planck2015 results

“…This is an average value for the high latitude sky, so individual regions with different electron temperature might be present a level higher than this. It is consistent with previous estimates of around 20% of scattered light on average at high latitudes (Wood & Reynolds 1999;Witt et al 2010;Dong & Draine 2011;Brandt & Draine 2012;Barnes et al 2015). If we repeat the calculation using a dust-corrected Hα map, assuming that 1/3 of the dust lies in front of the Hα-emitting gas (see Dickinson et al 2003), we find f scatt = (36 ± 12)%.…”

“…Previous estimates of this fraction suggested relatively low values (around 10%), while more recent analyses indicate that as much as half of the high latitude Hα intensity could be scattered (Witt et al 2010). Other analyses give values around the 10-20% level (Wood & Reynolds 1999;Brandt & Draine 2012;Barnes et al 2015). We attempt to estimate the scattered fraction in Sect.…”

Attenuation Modified by DIG and Dust as Seen in M31

“…In the DIG regions, the [SII](λ6717 + λ6731)/Hα ratio is higher (> 0.4, while for the HII regions it is usually < 0.2; Minter & Balser 1998;Haffner et al 2009). The origin of ionization in the DIG is speculative, but is likely due to a combination of supernova shocks, turbulent dissipation, leaked radiation from nearby OB stars, additional photons provided along channels in the neutral gas, and heating by cosmic rays or dust grains (Reynolds 1990;Reynolds & Cox 1992;Minter & Balser 1998;Madsen et al 2006;Haffner et al 2009;Barnes et al 2014Barnes et al , 2015Ascasibar et al 2016).…”

The physics of Lyman α escape from high-redshift galaxies

“…Different methodologies have been developed in a handful of studies with various advantages and limitations. We highlight applications focusing on the scattered light contribution of the diffuse galactic background (Wood & Reynolds 1999;Barnes et al 2015), star formation relations (without dust) from simulations of isolated dwarfs and high-redshift galaxies (Kim et al 2013(Kim et al , 2019, sub-resolution population synthesis for a Milky-Way like galaxy (Pellegrini et al 2020a,b), and periodic tall box simulations capable of exploring subparsec scale feedback and emission (Peters et al 2017;Kado-Fong et al 2020). Our work further adds to this class of detailed emission line modelling by achieving resolved emission and dust extinction throughout entire galaxies to better understand the physics and assist with observational interpretations.…”