High-resolution X-ray spectroscopy and imaging of the nuclear outflow of the starburst galaxy NGC 253^*

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany e-mail: mbauer@mpe.mpg.de
² INAF Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
³ Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
⁴ European Space Astronomy Centre, ESA, PO Box 50727, 28080 Madrid, Spain
⁵ Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Received: 4 September 2006
Accepted: 18 January 2007

Abstract

Aims.Using XMM-Newton data, we have studied the nuclear outflow of the nearby starburst galaxy NGC 253 in X-rays with respect to its morphology and to spectral variations along the outflow.

Methods.We analysed XMM-Newton RGS spectra, RGS brightness profiles in cross-dispersion direction, narrow-band RGS and EPIC images, and EPIC PN brightness profiles of the nuclear region and of the outflow from NGC 253.

Results.We detect a diversity of emission lines along the outflow of NGC 253. This includes the He-like ions of Si, Mg, Ne, and O and their corresponding ions in the next higher ionisation state. Additionally transitions from and are prominent. The derived temperatures from line ratios along the outflow range from 0.21±0.01 to 0.79±0.06 keV, and the ratio of lines indicates a predominantly collisionally ionised plasma. Additionally, we see indications of either a recombining or an underionised plasma in the line ratio. Derived electron densities are 0.106±0.018 cm^-3 for the nuclear region and 0.025±0.003 cm^-3 for the outflow region closest to the centre. The RGS image in the line energy clearly shows the morphology of an outflow extending out to ~750 pc along the southeast minor axis, while the northwest part of the outflow is not seen in due to the heavy absorption by the galactic disc. This is the first time that the hot wind fluid has been detected directly. The limb brightening in Chandra and XMM-Newton EPIC observations is only seen in the energy range that contains the lines (550-750 eV). In all other energy ranges between 400 and 2000 eV, no clear evidence of limb brightening could be detected.