REVISITING WITH CHANDRA THE SCALING RELATIONS OF THE X-RAY EMISSION COMPONENTS (BINARIES, NUCLEI, AND HOT GAS) OF EARLY-TYPE GALAXIES

Abstract

We have selected a sample of 30 normal (non-cD) early-type galaxies, for all of which optical spectroscopy is available and which have been observed with Chandra to a depth such as to ensure the detection of bright low-mass X-ray binaries (LMXBs) with L X > 10 38 erg s −1 . This sample includes a larger fraction of gas-poor galaxies than previously studied samples and covers a wide range of stellar luminosity ( L K ), velocity dispersion (σ * ), globular cluster specific frequency ( S N ), and stellar age. We derive X-ray luminosities (or upper limits) from the different significant X-ray components of these galaxies: nuclei, detected and undetected LMXBs, coronally active binaries (ABs), cataclysmic variables (CVs), and hot gas. The ABs and CVs contribution is estimated from the L X – L K scaling relation of M31 and M32. The contribution of undetected LMXBs is estimated both by fitting the spectra of the unresolved X-ray emission and by extrapolating the LMXB X-ray luminosity function. On average, the X-ray luminosity of LMXBs is a factor of ∼10 higher than that of ABs+CVs. By spectral fitting the emission (also considering gas emission in the regions of point sources), we estimate the contribution of the hot gas. We find our sample equally divided among galaxies with L X (gas) > L X (LMXB), L X (ABCV) ≤ L X (gas) ≤ L X (LMXB), and L X (gas) < L X (ABCV). The results for the nuclei are consistent with those discussed by Pellegrini. We derive a revised scaling relation between the integrated X-ray luminosity of LMXBs in a galaxy and the L K luminosity of the host galaxy: L X (LMXB)/ L K ∼ 10 29 erg s −1 L K −1 with 50% 1σ rms; moreover, we also obtain a tighter L X (LMXB)/ L K – S N relation than previously published. We revisit the relations between hot gas content and other galaxy parameters ( L K , σ * ), which in most previous work was based on the integrated total X-ray luminosity of the galaxy, finding a steeper L X (gas)– L K relation with larger scatter than reported in the literature. We find a positive correlation between the luminosity and temperature of the hot interstellar medium, significantly tighter than reported by earlier studies. This relation is particularly well defined in the subsample with σ * >240 km s −1 , where it may be related to the analogous correlation found in cD galaxies and groups/clusters. However, the gas-poor galaxies with the shallowest potentials (σ * < 200 km s −1 ) also follow this relation, contrary to the expected anti-correlation in a simple outflow/wind scenario. Galaxies with intermediate values of σ * instead tend to have the same kT , while L X (gas) spans a factor of ∼20; among these galaxies, we find a moderate, positive correlation between L X (gas) and the average stellar age, possibly suggesting a transition from halo retention to outflow caused by rejuvenated star formation associated with recent mergers.