We present a detailed case study of the diffuse X-ray and Hα emission in the halo of NGC 253, a nearby edge-on starburst galaxy driving a galactic superwind. The arcsecond spatial resolution of the ACIS imaging spectroscope on the Chandra X-ray Observatory allows us to study the spatial and spectral properties of the diffuse X-ray emitting plasma, at a height of between 3 and 9 kpc above the disk in the northern halo of NGC 253, with greatly superior spatial and spectral resolution compared to previous X-ray instruments. We find statistically significant structure within the diffuse emission on angular scales down to ∼ 10 ′′ (∼ 130 pc), and place limits on the luminosity of any X-ray-emitting "clouds" on smaller scales. There is no statistically significant evidence for any spatial variation in the spectral properties of the diffuse emission over scales from several ∼ 400 pc to ∼ 3 kpc. The spectrum of the diffuse X-ray emission is clearly thermal, although with the higher spectral resolution and sensitivity of Chandra it is clear that current simple spectral models do not provide a physically meaningful description of the spectrum. In particular, the fitted metal abundances are unphysically low. There is no convincing evidence for diffuse X-ray emission at energies above 2 keV in the halo.We show that the X-shaped soft X-ray morphology of the superwind previously revealed by ROSAT is matched by very similar X-shaped Hα emission, extending at least 8 kpc above the plane of the galaxy. In the northern halo the X-ray emission appears to lie slightly interior to the boundary marked by the Hα emission. The total 0.3 -2.0 keV energy band X-ray luminosity of the northern halo L X ∼ 5 × 10 38 erg s −1 , is very similar to the halo Hα luminosity of L Hα ∼ 4 × 10 38 erg s −1 , both of which are a small fraction of the estimated wind energy injection rate of ∼ 10 42 erg s −1 from supernovae in the starburst. We show that there are a variety of models that can simultaneously explain spatially-correlated X-ray and Hα emission in the halos of starburst galaxies, although the physical origin of the various emission components can be very different in different models. These findings indicate that the physical origin of the X-ray-emitting milliondegree plasma in superwinds is closely linked to the presence of much cooler and denser T ∼ 10 4 gas, not only within the central kpc regions of starbursts, but also on ∼ 10 kpc-scales within the halos of these galaxies.

We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3−8.0 keV) luminosity L XP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies, and is well correlated with the B-band and K-band luminosities for elliptical galaxies. This suggests an intimate connection between L XP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star-formation rate SF R. We derive proportionality constants α = 1.3 × 10 29 erg s −1 M −1

We investigate how the empirical properties of hot X-ray-emitting gas in a sample of seven starburst and three normal edge-on spiral galaxies (a sample that covers the full range of star formation intensity found in disk galaxies) correlate with the size, mass, star formation rate, and star formation intensity in the host galaxies. From this analysis we investigate various aspects of mechanical energy ''feedback''-the return of energy to the interstellar medium from massive star supernovae and stellar winds-on galactic scales. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-Ray Observatory to remove X-ray emission from point sources more accurately than in any previous study and hence obtain the X-ray properties of the diffuse thermal emission alone. Intriguingly, the diffuse X-ray properties of the normal spirals (in both their disks and halos) fall where extrapolation of the trends from the starburst galaxies with superwinds would predict. We demonstrate, using a variety of multiwavelength star formation rate and intensity indicators, that the luminosity of diffuse X-ray emission in the disk (and, where detected, in the halo) is directly proportional to the rate of mechanical energy feedback from massive stars in the host galaxies. Accretion of gas from the intergalactic medium (IGM) does not appear to be a significant contributor to the diffuse X-ray emission in this sample. Nevertheless, with only three nonstarburst normal spiral galaxies it is hard to exclude an accretion-based origin for extraplanar diffuse X-ray emission around normal star-forming galaxies. Larger galaxies tend to have more extended X-ray-emitting halos, but galaxy mass appears to play no role in determining the properties of the disk or extraplanar X-ray-emitting plasma. The combination of these luminosity and size correlations leads to a correlation between the surface brightness of the diffuse X-ray emission and the mean star formation rate per unit area in the disk (calculated from the far-infrared luminosity and the optical size of the galaxy, L FIR =D 2 25 ). Further observational work of this form will allow empirical constraints to be made on the critical star formation rate per unit disk area necessary to blow hot gas out of the disk into the halo. We show that a minor generalization of standard superbubble theory directly predicts a critical star formation rate per unit area for superbubble blowout from the disk and by extension for superwinds to blow out of the gaseous halos of their host galaxy. At present there are a variety of poorly known parameters in this theory that complicate comparison between observation and theory, making it impossible to assess the quantitative accuracy of standard superbubble blowout theory. We argue that the crucial spatial region around a galaxy that controls whether gas in starburst-driven superwinds will escape into the IGM is not the outer halo $100 kpc from the host galaxy, but the inner few halo scale heights, within $20 kpc of the galaxy plan...

Results obtained from an X-ray spectral survey of nearby Seyfert galaxies using XMM-Newton are reported. The sample was optically selected, well defined, complete in B magnitude, and distance limited: it consists of the nearest (D < ∼ 22 Mpc) 27 Seyfert galaxies (9 of type 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315) sample. This is one of the largest atlases of hard X-ray spectra of low-luminosity active galaxies ever assembled. All nuclear sources except two Seyfert 2s are detected between 2 and 10 keV, half for the first time ever, and average spectra are obtained for all of them. Nuclear luminosities reach values down to 10 38 erg s −1 . The shape of the distribution of X-ray parameters is affected by the presence of Compton-thick objects ( > ∼ 30% among type 2s). The latter have been identified either directly from their intense FeK line and flat X-ray spectra, or indirectly with flux diagnostic diagrams which use isotropic indicators. After taking into account these highly absorbed sources, we find that (i) the intrinsic X-ray spectral properties (i.e., spectral shapes and luminosities above 2 keV) are consistent between type 1 and type 2 Seyferts, as expected from "unified models"; (ii) Seyfert galaxies as a whole are distributed fairly continuously over the entire range of N H , between 10 20 and 10 25 cm −2 ; and (iii) while Seyfert 1s tend to have lower N H and Seyfert 2s tend to have the highest, we find 30% and 10% exceptions, respectively. Overall the sample is of sufficient quality to well represent the average intrinsic X-ray spectral properties of nearby active galactic nuclei, including a proper estimate of the distribution of their absorbing columns. Finally, we conclude that, with the exception of a few cases, the present study agrees with predictions of unified models of Seyfert galaxies, and extends their validity down to very low luminosities.