The Nature of the X-Ray Emission and the Mass Distributions in Two Early-Type Galaxies

Abstract

We present spectral analysis of $ASCA$ observations of the early-type galaxies NGC 720 (E4) and NGC 1332 (E7/S0) with emphasis on constraining the relative contribution to the X-ray emission from hot gas and the integrated emission from X-ray binaries. Single-temperature spectral models yield poor fits to the spectrum ($χ^2_{red}\sim 3$) over the $\sim 0.5 - 5$ keV energy range. Two-temperature models significantly improve the spectral fits ($χ^2_{red}\sim 1.5$) and have soft-component temperatures and sub-solar abundances consistent with previous $ROSAT$ single-temperature models ($T_{soft}\sim 0.6$ keV, abundances $\sim 0.1$) and hard-component temperatures ($T_{hard}\gtrsim 3$ keV) consistent with those expected from a discrete component. The soft component dominates the emission in both galaxies, especially in the 0.4 - 2.4 keV band used in previous $ROSAT$ studies. Combining these spectral results with $ROSAT$ data we updated constraints on the mass distributions for NGC 720 and NGC 1332. For NGC 720, which yields the more precise constraints, the ellipticity of the intrinsic shape of the mass is slightly reduced $(Δε_{mass}\approx 0.05)$ when the discrete component is added, $ε_{mass}\sim 0.4-0.6$ $(90%)$. The estimates for the total mass increase with increasing discrete flux, and we find that models with $F_{hard}/F_{soft}=0.45$, the $2σ$ upper limit, have masses that exceed by $\sim 30% - 50%$ those where $F_{hard}/F_{soft}=0$.