The Interaction of Hot and Cold Gas in Early-Type Galaxies

Sa galaxies have approximately equal masses of HI and X-ray emitting gas and are ideal sites for studying the interaction between hot and cold gas. An X-ray observation of the Sa galaxy NGC 1291 with the ROSAT PSPC shows a striking spatial anticorrelation between hot and cold gas where X-ray emitting material fills the large central hole in the HI disk. This supports a previous suggestion that hot gas is a bulge phenomenon and neutral hydrogen is a disk phenomenon. The X-ray luminosity (1.5 times 10(40) erg s(-1) ) and radial surface brightness distribution (beta = 0.51) are the same as for elliptical galaxies with optical luminosities and velocity dispersions like that of the bulge of NGC 1291. Modeling of the X-ray spectrum requires a component with a temperature of 0.15 keV, similar to that expected from the velocity dispersion of the stars, and with a hotter component with kT = 1.07 keV. This hotter component is not due to emission from stars and its origin remains unclear. PSPC observations were obtained for the Sa galaxy NGC 3623, which also shows X-ray emission from the bulge.