Astron. Astrophys. 360, 24-48 (2000)

X-ray observations of the starburst galaxy NGC 253

II. Extended emission from hot gas in the nuclear area, disk, and halo

W. Pietsch ¹, A. Vogler ^2,1, U. Klein ³ and H. Zinnecker <sup>4

1</sup> Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85740 Garching bei München, Germany
² CEA/Saclay, DAPNIA, Service d'Astrophysique, L'Ormes des Merisiers, Bât. 709, 91191 Gif-sur-Yvette, France
³ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received 22 October 1999 / Accepted 9 June 2000

Abstract

Spatial and spectral analysis of deep ROSAT HRI and PSPC observations of the near edge-on starburst galaxy NGC 253 reveal diffuse soft X-ray emission, which contributes 80% to its total X-ray luminosity ( erg s^-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 with an extent of 250 pc (FWHM) about 100 pc above the nucleus along the SE minor axis ("nuclear source", X34), and the "X-ray plume". The nuclear source is best described as having a thermal bremsstrahlung spectrum with a temperature of T = 1.2 keV ( cm^-2) and erg s^-1 (corrected for Galactic foreground absorption). The spectrum of the hollow-cone shaped plume (opening angle of 32^o and extent of 700 pc along the SE minor axis) is best modeled by a composite of a thermal bremsstrahlung ( cm^-2, T = 1.2 keV, erg s^-1) and a thin thermal plasma (Galactic foreground absorption, T = 0.33 keV, erg s^-1). The diffuse nuclear emission components trace interactions between the galactic super-wind emitted by the starburst nucleus, and the dense interstellar medium of the disk.

Diffuse emission from the disk is heavily absorbed and follows the spiral structure. It can be described by a thin thermal plasma spectrum (T = 0.7 keV, intrinsic luminosity erg s^-1), and most likely reflects a mixture of sources (X-ray binaries, supernova remnants, and emission from H II regions) and the hot interstellar medium. The surface brightness profile reveals a bright inner and a fainter outer component along the major axis with extents of 3.4 kpc and 7.5 kpc.

We analysed the total halo emission separated into two geometrical areas; the "corona" (scale height kpc) and the "outer halo". The coronal emission (T = 0.2 keV, erg s^-1) is only detected from the near side of the disk (in the SE), emission from the back (in the NW) is shadowed by the intervening interstellar medium unambiguously determining the orientation of NGC 253 in space. In the NW we see the near edge of the disk is seen, but the far component of the halo, and vice versa in the SE. The emission in the outer halo can be traced to projected distances from the disk of 9 kpc, and shows a horn-like structure. Luminosities are higher (10 and erg s^-1, respectively) and spectra harder in the NW halo than in the SE. The emission in the corona and outer halo is most likely caused by a strong galactic wind emanating from the starburst nucleus. As an additional contribution to the coronal emission floating on the disk like a spectacle-glass, we propose hot gas fueled from galactic fountains originating within the boiling star-forming disk. A two temperature thermal plasma 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 and Galactic foreground absorption are needed to arrive at acceptable fits for 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 fountains or winds.

We compare our results to observations at other wavelengths and from other galaxies.

Key words: ISM: jets and outflows galaxies: individual: NGC 253 galaxies: spiral galaxies: starburst X-rays: galaxies

Send offprint requests to: W. Pietsch (wnp@mpe.mpg.de)

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Contents

• 1. Introduction
• 2. Observations and point source analysis
• 3. Analysis of diffuse emission and results
  • 3.1. Iso-intensity contour and "true" colour maps
  • 3.2. Separating diffuse emission components
    • 3.2.1. Diffuse emission from nuclear area and X-ray plume
    • 3.2.2. Diffuse emission from disk and halo
• 4. Discussion
  • 4.1. X-ray luminosity of NGC 253 emission components
  • 4.2. Emission from the area of the starburst nucleus and X-ray plume of NGC 253
    • 4.2.1. Source X34: extended emission from the starburst nucleus of NGC 253?
    • 4.2.2. X-ray plume along SE minor axis
  • 4.3. Diffuse emission from the NGC 253 disk and the flat SE halo component immediately above (coronal emission)
  • 4.4. Shadowing of halo emission by the disk
  • 4.5. Extended soft X-ray halo of NGC 253
  • 4.6. Comparison with diffuse X-ray emission detected in other spiral galaxies
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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