The Hot Gas Content of Low-luminosity Early-type Galaxies and the Implications regarding Supernova Heating and Agn Feedback

We have analyzed Chandra observations of 18 low-luminosity early-type galaxies with LB ∼ 3×10L⊙B. Thermal emission from hot gas with temperatures between 0.2 and 0.8 keV comprises 5-70% of the total 0.5-2.0 keV emission from these galaxies. We find that the total X-ray luminosity from LMXBs (resolved plus the power-law component of the unresolved emission) scales roughly linearly with the K-band luminosity of the galaxies with a normalization comparable to that found in more luminous early-type galaxies. All of the galaxies in our sample are gas poor with gas masses much less than that expected from the accumulation of stellar mass loss over the life time of the galaxies. The average ratio of gas mass to stellar mass in our sample is Mgas/M∗ = 0.001, compared to more luminous early-type galaxies which typically have Mgas/M∗ = 0.01. The time required to accumulate the observed gas mass from stellar mass loss in these galaxies is typically 3× 108 yr. Since the cooling time of the gas is longer than the replenishment time, the gas cannot be condensing out of the hot phase and forming stars, implying that the gas is most likely being expelled from these galaxies in a wind. The one exception to this is NGC4552, which is the most optically luminous galaxy in our sample and has the highest gas content. Using recent estimates of the Type Ia supernova rate and AGN heating rate in early-type galaxies, we find that, on average, heating by Type Ia supernovae should exceed AGN heating in galaxies with LB ∼ 3× 10L⊙B. We also find that heating by Type Ia supernovae is energetically sufficient to drive winds in these galaxies, even if the present Type Ia supernova rate is overestimated by a factor of two or the present stellar mass loss rate is underestimated by a factor of two. There is significant scatter in the gas properties of galaxies with comparable optical luminosities. Nearly continuous heating by Type Ia supernovae alone cannot account for the large scatter in observed gas mass. We suggest that the scatter in gas properties could arise from periodic AGN outbursts, during which time AGN heating dominates over Type Ia supernova heating, or environmental factors, such as a high pressure environment which would suppress the formation of strong galactic winds. Subject headings: galaxies:elliptical and lenticular – galaxies:ISM – X-ray:binaries – X-ray:galaxies – X-ray:ISM