Supernova Feedback Efficiency and Mass Loading in the Starburst and Galactic Superwind Exemplar M82

Abstract

We measure the net energy efficiency of supernova and stellar wind feedback in the starburst galaxy M82 and the degree of mass-loading of the hot gas piston driving its superwind by comparing a large suite of 1 and 2dimensional hydrodynamical models to a set of observational constraints derived from hard X-ray observations of the starburst region (the fluxes of the Heα and Lyα-like lines of S, Ar, Ca and Fe, along with the total diffuse E = 2 -8 keV X-ray luminosity). These are the first direct measurements of the feedback efficiency and mass-loading of supernova heated and enriched plasma in a starburst galaxy. We consider a broad range of plausible parameters for the M82 starburst, varying the age and mode of star formation, the starburst region size and geometry, and supernova metal yields. Over all these varied input parameters all the models that satisfy the existing observational constraints have medium to high thermalization efficiencies (30% ≤ ǫ ≤ 100%) and the volume-filling wind fluid that flows out of the starburst region is only mildly centrally mass loaded (1.0 ≤ β ≤ 2.8). These results imply a temperature of the plasma within the starburst region in the range 30 -80 million Kelvin, a mass flow rate of the wind fluid out of the starburst region ofṀ tot ∼ 1.4 -3.6 M ⊙ yr −1 and a terminal velocity of the wind in the range v ∞ = 1410 -2240 km s −1 . This velocity is considerably larger than the escape velocity from M82 (v esc 460 km s −1 ) and the velocity of the Hα emitting clumps and filaments within M82's wind (v Hα ∼ 600 km s −1 ). Drawing on these results we provide a prescription for implementing starburst-driven superwinds in cosmological models of galaxy formation and evolution that more accurately represents the energetics of the hot metal-enriched phases than the existing recipes do.

Gravitational and mass distribution effects on stationary superwinds

“…It has been also validated that the velocity, mass, momentum and energy of these outflows scale with the galaxy SFR, luminosity and mass (Rupke, Veilleux & Sanders 2005b); fact that seems consistent with the picture of a pressure driven superwind (Chevalier & Clegg 1985, CC85 hereafter). On the other hand, the bulk properties of the hot X-ray emitting haloes (T ∼ 10 6 -10 7 K) detected around some galaxies (Dahlen, Weaver & Heckman 1998;Heckman et al 2000;Strickland & Heckman 2009) also agree with the predictions of simple superwind models (see Stevens & Hartwell 2003).…”

“…Anyway, for our purposes, the behaviour at large radii is not that important, as the thermalization driven superwind solution is valid only close to the galaxy 8 (see e.g. Strickland & Heckman 2009). So, we will proceed to give the expression corresponding to the external gravitational potential.…”

Evolution of multiple supernova remnants

“…Recently have pointed out that this assumption is not valid in the conditions prevailing in the central regions of starbursts, because of prohibitive radiative loss in large ambient density. This criticism is, by the way, not in contradiction with the assumption (Strickland & Heckman 2009) that the heating efficiency can be large, of order 0.1. The question is whether or not the filling factor of X-ray emitting gas in the central (200-300 pc) of starbursts can be regarded as close to unity, as is required in the galactic wind models of Chevalier & Clegg (1985); .…”

“…Strickland & Heckman (2009) have inferred a value of 0.1-0.3 for the heating efficiency of SNe for the X-ray emitting gas. Our results put these inference and assumptions on a firm footing, and also provide scalings with the SNe rate density and gas density so that heating efficiency can be estimated for a general case.…”

“…This process assumes that although SNe lose most energy in radiation in isolated cases, the efficiency of heating the ISM can be large in the central region filled with hot and low density gas and that the gas in this region is thermalized (e.g., Chevalier & Clegg 1985;. Numerical simulations (e.g., Suchkov et al 1994Suchkov et al , 1996Strickland & Stevens 2000;Fujita et al 2009;Strickland & Heckman 2009) also implement the initial conditions leading to galactic winds making similar assumptions. It is believed that in a multiphase medium and in the case of multiple SNe events, the efficiency of SNe heating -the fraction of the total explosion energy transferred into thermal energy -can be larger than ∼ 0.1.…”