Optical signatures of cooling flows in isolated early-type galaxies.

Abstract

This thesis examines the expected consequences of cooling flows in individual early-type galaxies by studying whether the structure of the cool interstellar medium is related to the hot gas. In a cooling flow the hot gas is expected to cool and produce cold gas at the astrophysically interesting rate of 0.1 to 10 M$\sb\odot$ per year. In order to test these expectations, an observing program to search for optical emission was developed. The observing program consists of long-slit spectroscopy and imaging of a sample of 40 early-type galaxies with a range of X-ray luminosities and hot gas masses. The observations cover the wavelength range of $\lambda\lambda$4500-5200 and $\lambda\lambda$5900-7100 where the strongest lines are H$\beta$, (OIII) $\lambda\lambda$4959,5007, (OI) $\lambda$6300, (NII) $\lambda\lambda$6548,6584, H$\alpha$ and (SII) $\lambda\lambda$6715,6731. The data were processed and analysed using standard techniques. In order to remove the stellar component, the galaxy spectra were subtracted by a template. We fail to find the expected relationship between the cooling rate of the hot gas and the luminosity of the 10$\sp4$ K gas, or between the hot gas cooling time and emission line luminosity. We find evidence that supports an intrinsic origin for the 10$\sp4$ K gas. The 10$\sp4$ K gas has masses of 10$\sp3$ to 10$\sp7$ M$\sb\odot$, electron densities of n$\sb{e} < 10$ cm$\sp{-3}$to $n\sb{e}$ = 1.5 $\times$ 10$\sp3$ cm$\sp{-3}$, and electron pressures of P/k = 10$\sp4$ cm$\sp{-3}$K to 10$\sp7$ cm$\sp{-3}$K. The pressure of this 10$\sp4$ K gas is comparable and often greater than the pressure in the surrounding hot gas. The line emission detection rate of 60 percent is consistent with the result of other surveys of early-type galaxies. We find a relationship between dust emission of 60$\mu$ and 100$\mu$ and the 10$\sp4$ K gas, suggesting a common ionization and heating source for the dust and optical gas. Photoionization from post asymptotic giant branch stars is the most likely source of ionization for the gas, vitiating collisional ionization or an active galactic nucleus. We conclude that if optical signatures of cooling flows are present, they are masked by other processes.