X-ray observations of the starburst galaxy NGC 253 II. Extended emission from hot gas in the nuclear area, disk, and halo

Spatial and spectral analysis of deep ROSAT HRI and PSPC observations of th e near edge-on starburst galaxy NGC 253 reveal diffuse soft X-ray emission, which contributes 80% to its total X-ray luminosity (L-X = 5 10(39) ergs(- 1), corrected for foreground absorption). The nuclear area, disk, and halo contribution to the luminosity is about equal. The starburst nucleus itself is highly absorbed and not visible in the ROSAT band. The emission from the nuclear area stems from a heavily absorbed source wit h an extent of 250 pc (FWHM) about 100 pc above the nucleus along the SE mi nor axis ("nuclear source", X34), and the "X-ray plume". The nuclear source is best described as having a thermal bremsstrahlung spectrum with a tempe rature of T = 1.2 keV (N-H = 3 10(21) cm(-2)) and L-X(exgal) = 3 10(38) erg s(-1) (corrected for Galactic foreground absorption). The spectrum of the h ollow-cone shaped plume (opening angle of 32 degrees and extent of similar to 700 pc along the SE minor axis) is best modeled by a composite of a ther mal bremsstrahlung (N-H = 310(20) cm(-2), T = 1.2keV, L-X(exgal) = 4.610(38 ) ergs(-1)) and a thin thermal plasma (Galactic foreground absorption, T = 0.33 keV, L-X(exgal) = 4 10(38) erg s(-1)). The diffuse nuclear emission co mponents trace interactions between the galactic superwind emitted by the s tarburst nucleus, and the dense interstellar medium of the disk. Diffuse emission from the disk is heavily absorbed and follows the spiral s tructure. It can be described by a thin thermal plasma spectrum (T = 0.7 ke V, intrinsic luminosity L-X(intr) = 1.2 10(39) erg s(-1)), and most likely reflects a mixture of sources (X-ray binaries, supernova remnants, and emis sion from H II regions) and the hot interstellar medium. The surface bright ness profile reveals a bright inner and a fainter outer component along the major axis with extents of +/-3.4 kpc and 17.5 kpc. We analysed the total halo emission separated into two geometrical areas; t he "corona" (scale height similar to 1 kpc) and the "outer halo". The coron al emission (T = 0.2keV, L-X(intr) = 7.8 1038 erg s(-1)) is only detected f rom the near side of the disk (in the SE), emission from the back (in the N W) is shadowed by the intervening interstellar medium unambiguously deter-m ining the orientation of NGC 253 in space. Tn the NW we see the near edge o f the disk is seen, but the far component of the halo, and vice versa in th e SE. The emission in the outer halo can be traced to projected distances f rom the disk of 9 kpc, and shows a horn-like structure. Luminosities are hi gher (10 and 5 10(38) ergs(-1), respectively) and spectra harder in the NW halo than in the SE. The emission in the corona and outer halo is most like ly caused by a strong galactic wind emanating from the starburst nucleus. A s an additional contribution to the coronal emission floating on the disk l ike a spectacle-glass, we propose hot gas fueled from galactic fountains or iginating within the boiling star-forming disk. A two temperature thermal p lasma model with temperatures of 0.13 and 0.62 keV or a thin thermal plasma model with temperature of 0.15 keV and Gaussian components above 0.7 keV a nd Galactic foreground absorption are needed to arrive at acceptable fits f or the NW halo. This may be explained by starburst-driven super-winds or by effects of a non-equilibrium cooling function in a plasma expanding in fou ntains or winds. We compare our results to observations at other wavelengths and from other galaxies.