Feedback processes in early-type galaxies

We present a simple phenomenological model of feedback in early-type galaxies that tracks the evolution of the interstellar medium gas mass, metallicity, and temperature. Modeling the star formation rate as a Schmidt law with a temperature-dependent efficiency, we find that intermittent episodes of star formation are common in moderate-size ellipticals. Our model is applicable in the case in which the thermalization time from supernovae is sufficiently long that spatial variations are relatively unimportant, an appropriate assumption for the empirical parameters adopted here, but one that can only be demonstrated conclusively through more detailed numerical studies. The departure from a standard scenario of passive evolution implies significantly younger luminosity-weighted ages for the stellar populations of low-mass galaxies at moderate redshifts, even though the more physically meaningful mass-weighted ages are changed only slightly. Secondary bursts of star formation also lead to a natural explanation of the large scatter in the near-UV optical relation observed in clusters at moderate redshift and account for the population of E+A galaxies that display a spheroidal morphology. As the late-time formation of stars in our model is due to the gradual cooling of the interstellar medium, which is heated to temperatures similar to1 keV by the initial burst of supernovae, our conclusions do not rely on any environmental effects or external mechanisms. Furthermore, a simple estimate of the X-ray emission from this supernova-heated gas leads to an L-X-L-B correlation that is in good agreement with observed values. Thus, feedback processes may be essential to understanding the observed properties of early-type galaxies from the optical to the X-ray.