The hot gas content of low-luminosity early-type galaxies and the implications regarding supernova heating and active galactic nucleus feedback

We have analyzed Chandra observations of 18 low-luminosity early-type galaxies with L-B less than or similar to 3 x 10(10) L circle dot(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 keVemission 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 lifetime of the galaxies. The average ratio of gas mass to stellar mass in our sample is M-gas/M-* = 0.001, compared to more luminous early-type galaxies that typically have M-gas/M-* = 0.01. The time required to accumulate the observed gas mass from stellar mass loss in these galaxies is typically 3 x 10(8) 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 NGC 4552, which is the most optically luminous galaxy in our sample and has the highest gas content. Using recent estimates of the Type la 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 L-B less than or similar to 3 x 10(10) L circle dot(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 2 or the present stellar mass-loss rate is underestimated by a factor of 2.