Diffuse X‐Ray Emission in Spiral Galaxies

We compare the soft diffuse X-ray emission from Chandra images of 12 nearbyintermediate inclination spiral galaxies to the morphology seen in Halpha,molecular gas, and mid-infrared emission. We find that diffuse X-ray emissionis often located along spiral arms in the outer parts of spiral galaxies buttends to be distributed in a rounder morphology in the center. The X-raymorphology in the spiral arms matches that seen in the mid-infrared or Halphaand so implies that the X-ray emission is associated with recent active starformation. We see no strong evidence for X-ray emission trailing the locationof high mass star formation in spiral arms. However, population synthesismodels predict a high mechanical energy output rate from supernovae for a timeperiod that is about 10 times longer than the lifetime of massive ionizingstars, conflicting with the narrow appearance of the arms in X-rays. Thefraction of supernova energy that goes into heating the ISM must depend onenvironment and is probably higher near sites of active star formation. TheX-ray estimated emission measures suggest that the volume filling factors andscale heights are high in the galaxy centers but low in the outer parts ofthese galaxies. The differences between the X-ray properties and morphology inthe centers and outer parts of these galaxies suggest that galactic fountainsoperate in outer galaxy disks but that winds are primarily driven from galaxycenters.Comment: 28 pages, 4 figures, to be submitted to Ap