Evidence for an Additional Heat Source in the Warm Ionized Medium of Galaxies

Abstract

Spatial variations of the [S ii]/Halpha and [N ii]/Halpha line intensity ratios observed in the gaseous halo of the Milky Way and other galaxies are inconsistent with pure photoionization models. They appear to require a supplemental heating mechanism that increases the electron temperature at low densities, ne. This would imply that in addition to photoionization, which has a heating rate per unit volume proportional to n2e, there is another source of heat with a rate per unit volume proportional to a lower power of ne. One possible mechanism is the dissipation of interstellar plasma turbulence, which, according to Minter & Spangler, heats the ionized interstellar medium in the Milky Way at a rate of approximately 1x10-25ne ergs cm-3 s-1. If such a source were present, it would dominate over photoionization heating in regions where ne less, similar0.1 cm-3, producing the observed increases in the [S ii]/Halpha and [N ii]/Halpha intensity ratios at large distances from the galactic midplane as well as accounting for the constancy of [S ii]/[N ii], which is not explained by pure photoionization. Other supplemental heating sources, such as magnetic reconnection, cosmic rays, or photoelectric emission from small grains, could also account for these observations, provided they supply approximately 10-5 ergs s-1 per square centimeter of the Galactic disk to the warm ionized medium.

VLT-VIMOS integral field spectroscopy of luminous and ultraluminous infrared galaxies

“…2) display an increase on the line ratio in those areas with low Hα surface brightness. Similar results have been observed in other environments, like our Galaxy (Madsen et al 2006;Reynolds et al 1999), or other spiral galaxies (Collins & Rand 2001;Miller & Veilleux 2003;Blanc et al 2009) in the so-called diffuse ionized gas (DIG) or warm interstellar medium (WIM). In this section we further explore this result by looking at the relation between the ionization degree and surface brightness for the different interaction groups.…”

“…Miller & Veilleux 2003). If they are interpreted only as photoionization areas, there is a need for extra heating (Reynolds et al 1999;Mathis 2000) and even with this extra heating, it is difficult to reach [N ii]λ6584/Hα > ∼ 1 line ratios. Another possibility would be that these line ratios are the composite effect of photoionization and shocks or turbulent mixing layers (TML, Collins & Rand 2001;Miller & Veilleux 2003).…”

The origin of the ionization of the diffuse ionized gas in spirals

“…, as discussed by a number of authors, including Reynolds (1984), Reynolds et al (1999), Greenawalt et al (1997), Hoopes & Walterbos (2000), Collins & Rand (2001), and also from the presence of nonthermal velocity widths of emission lines (e.g. by Minter & Balser 1999;Tufte et al 1999).…”

Cosmic rays in astrospheres

“…The stellar wind parameters are taken from Henrichs & Sudnik (2013). As interstellar temperature we use the diffuse ionized medium temperature derived by Reynolds et al (1999). The density value is estimated from integrating the HI slice at the position and the velocities expected for the distance of λ Cephei based on data of the Canadian Galactic Plane Survey (Taylor et al 2003).…”

“…For the heating function by photo-ionization and some supplemental heat source we use the approach by Reynolds et al (1999), see also Kosiński & Hanasz (2006). The heating rate for photo-ionization depending on electron collisions is limited by recombination and is thus proportional to n 2 e , while additional heating terms that can include photoelectric heating by dust, dissipation of turbulence, interactions with cosmic rays are proportional to n e :…”