TheChandraview of NGC 1800 and the X-ray scaling properties of dwarf starbursts

Abstract

The superb spatial resolution of Chandra is utilized to study the X-ray morphology of the dwarf starburst galaxy NGC1800 embedded in a small group of galaxies. Diffuse galactic emission is detected, extending several kpc above the galactic plane, with an overall morphology similar to the galactic winds seen in nearby X-ray bright starburst galaxies. This makes NGC1800 the most distant dwarf starburst with a clear detection of diffuse X-ray emission. The diffuse X-ray luminosity of 1.3+/-0.3 *10^38 erg/s accounts for at least 60 per cent of the total soft X-ray output of the galaxy. A hot gas temperature of kT=0.25 keV and metallicity Z~0.05Z_Sun are derived, the latter in consistency with results from optical spectroscopy of the interstellar medium. Our failure to detect any hot gas associated with the embedding galaxy group translates into an upper limit to the group X-ray luminosity of L_X<10^41 erg/s. There is no convincing evidence that the outflowing wind of NGC1800 is currently interacting with any intragroup gas, and mechanical considerations indicate that the wind can escape the galaxy and its surrounding HI halo, eventually delivering energy and metals to the intragroup gas. Properties of NGC1800 are compared to those of other dwarf starburst galaxies, and a first detailed discussion of the X-ray scaling properties of this population of objects is given, set against the equivalent results obtained for normal starburst galaxies. Results indicate that dwarf starbursts to a large degree behave as down-scaled versions of normal starburst galaxies.