ChandraObservations of NGC 253. II. On the Origin of Diffuse X‐Ray Emission in the Halos of Starburst Galaxies

Abstract

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.

XMM-Newtonunveils the complex iron Kαregion of Mrk 279

“…Masai et al 2002) and the contamination by point-like, hard sources in the diffuse emission spectrum (e.g. in NGC 253, Strickland et al 2002). Another way to model the high ionization spectrum in terms of a galactic wind, is to invoke quasi-thermal acceleration of electrons which then would produce X-rays with thermal photons.…”

Stellar and Gaseous Abundances in M82

“…On the other hand, high angular resolution alone is not enough: recent Chandra studies (Strickland et al 2002;Martin, Kobulnicky, & Heckman 2002) demonstrated that the relatively low spectral resolution of the ACIS detector makes individual element abundance analysis still problematic. The high spectral resolution of the Chandra gratings rapidly degrades for sources that are much more extended than the instrumental point-spread function (PSF), and thus these gratings are almost useless for the study of nearby SB galaxies.…”

Studying the Nearby Universe with Chandra

“…Naturally, most of the work on this topic has been concentrated on edge-on disk galaxies, which allow for detection of extraplanar gas unambiguously. Vertical X-ray plumes are detected along the minor axes of many nuclear starburst galaxies (e.g., NGC 253, Strickland et al 2002;NGC 3079, Cecil et al 2002;NGC 4945, Weaver 2001;M82, Lehnert, Heckman, & Weaver 1999 , Fig. 4).…”

“…Deep X-ray observations of nuclear starburst galaxies have further revealed large-scale, low surface brightness diffuse X-ray halos, which seem to be confined within faint limb-brightened Hα-emitting boundaries (e.g., NGC 253, Strickland et al 2002;NGC 1569, Martin et al 2002M82, Lehnert et al 1999, Fig. 4).…”

“…The most logical explanation for the characteristics of the hot gas outflows and halos is a confined galactic wind model (e.g., Strickland et al 2002). A free-streaming wind is not efficient in radiating X-rays, because it is cooled and rarefied rapidly during the adiabatic expansion phase within or just outside the galactic disks or nuclear regions.…”