X-Ray Morphology

A large x-ray halo has been observed in M82 ([Watson, Stanger, & Griffiths 1984]; [Schaaf et al. 1989]; [Tsuru et al. 1990]; [Bregman, Schulman, & Tomisaka 1995]), extending at least 5-6 kiloparsecs in radius along the minor axis of the galaxy. Originally believed to be thermal emission from the hot ($T\sim10^7$-10⁸ K) gas within the outflow, current models suggest an origin in wind-shocked clouds. J.N. Bregman has kindly provided us with a recently published ([Bregman, Schulman, & Tomisaka 1995]) image of M82 taken with the Röntgensatellit (ROSAT; [Trümper 1984]) High-Resolution Imager (HRI; [Pfeffermann et al. 1987]). The pair of 13' square images with total exposure time 33.8 kiloseconds were taken in early 1991 and late 1992. The resolution is about 5''.

Contours representing the ROSAT soft x-ray flux have been overlayed on our deep H$\alpha$ image (Fig. 8). The morphologies are markedly similar, with specific filaments and knots clearly enhanced in both emission bands. (Note, for example, the two extensive filaments in the north, and the knots approximately 500 pc SW of the nucleus.) But in light of the more extensive distribution of the X-rays versus the optical emission, it seems unlikely that all of the X-ray flux is associated with specific filaments of optical emission. Since the regions of high optical/X-ray correlation are also some of the brightest in H$\alpha$, these knots presumably represent density enhancements in the outflow bubbles. The large extent of the X-ray halo and close spatial correlation with the H$\alpha$ emission suggest an interpretation in terms of shocks driven by a fast, rarefied wind plowing into denser halo gas (e.g. [Sutherland, Bicknell, & Dopita 1993]; [Dopita & Sutherland 1996]). Our observed H$\alpha$/X-ray luminosity ratio of $\approx$30 in the brightest filaments, identical to that derived by [Pérez-Olea & Colina 1996], suggests a similar scenario. We defer detailed modeling of the H$\alpha$ and soft x-ray emission to a subsequent paper, in which the shocks will be constrained by deep spectroscopy and imaging from the Keck and Hubble Space Telescopes.