Chandraobservation of NGC 4449: analysis of the X-ray emission from a dwarf starburst galaxy

Abstract

We present Chandra X-ray data on the nearby Magellanic irregular dwarf starburst galaxy NGC 4449. 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 ∼2.4 × 1.6 kpc down to a flux density of 1.3 × 10−13 erg s−1 cm−2 arcmin−2. The best spectral fit to this emission is obtained with an absorbed, two-temperature model giving temperatures for the two gas components of 0.28 ± 0.01 and 0.86 ± 0.04 keV, a total mass content of ∼107 M⊙ compared with a galactic mass of several 1010 M⊙ and a total thermal energy content of ∼2.5 × 1055 erg, with an average energy injection rate for the galaxy of a few 1041 erg s−1. Comparison of the morphology of the diffuse X-ray emission with that of the observed Hα 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 Hα emission. The kinematics of this bubble suggest an expansion velocity of ∼220 km s−1 and a mass injection rate of ∼0.14 M⊙ yr−1, but the presence of a huge H i halo (r∼ 40 kpc) in NGC 4449 may prevent the ejection, into the intergalactic medium, 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 ∼2 × 10−14 erg s−1 cm−2, within the optical extent of NGC 4449 and analysis of their spectra has shown them to be from at least three different classes of object. As well as the known supernova remnant in this galaxy, it also harbours several X-ray binaries and supersoft 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.