Chandra Observation of NGC4449. Analysis of the X-ray Emission from a Dwarf Starburst Galaxy

Abstract

We present CHANDRA X-ray data of the nearby Magellanic Irregular dwarf starburst galaxy NGC4449. Contributions to the X-ray emission come from discrete point sources and extended diffuse emission. The extended emission has a complex morphology with an extent of 2.4x1.6kpc down to a flux density of 1.3E-13 erg/s/cm2/arcmin2. The best spectral fit to this emission is obtained with an absorbed, two temperature model giving temperatures for the two gas components of 0.28keV and 0.86keV, a total mass content of ~10^7 Msun compared with a galactic mass of several 10^{10} Msun and a total thermal energy content of ~2.5E55erg, with an average energy injection rate for the galaxy of a few 1E41 erg/s. Comparison of the morphology of the diffuse X-ray emission with that of the observed Halpha emission shows similarities in the two emissions. An expanding super-bubble is suggested by the presence of diffuse X-ray emission within what appears to be a cavity in the Halpha emission. The kinematics of this bubble suggest an expansion velocity of ~220km/s and a mass injection rate of 0.14Msun/yr, but the presence of NGC4449's huge HI halo (r~40kpc) may prevent the ejection, into the IGM, of the metal-enriched material and energy it contains. The arcsecond-resolution of CHANDRA has detected 24 X-ray point sources down to a completeness level corresponding to a flux of ~2E-14 erg/s/cm2, within the optical extent of NGC4449 and analysis of their spectra has shown them to be from at least 3 different classes of object. As well as the known SNR in this galaxy, it also harbours several X-ray binaries and super-soft sources. The point source X-ray luminosity function, for the higher luminosity sources, has a slope of ~-0.51, comparable to those of other starburst galaxies.