AChandraX-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results

Abstract

(abridged) We present an analysis of Chandra X-ray observations of a sample of eight dwarf starburst galaxies (I Zw 18, VII Zw 403, NGC 1569, NGC 3077, NGC 4214, NGC 4449, NGC 5253, and He 2-10). Extended, diffuse X-ray emission is detected in all but two of the objects. Unresolved sources were found within all dwarf galaxies (total: 55 sources). These point sources are well fit by power law, thermal plasma, or black body models. Ten of the point sources exceed an X-ray luminosity of 10^39 erg s^-1 (ultraluminous X-ray sources). In those galaxies where diffuse X-ray emission is detected, this emission (with X-ray luminosities ranging from 4x10^38 erg s^-1 to 2x10^40 erg s^-1) contains most (60-80 per cent) of the X-ray photons. This diffuse emission can be well fit by MeKaL one-temperature thermal plasma models once the contribution from the unresolved point sources is subtracted properly. The diffuse X-ray component is significantly extended, reaching as far as 0.5-5 kpc into the outskirts of their hosts. Temperatures of various regions within the galaxies range from 1.6-7.6x10^6 K. With few exceptions, temperatures of the hot gas are remarkably uniform, hovering around 2-3x10^6 K. Temperatures of the coronal gas in the outer regions are in general ~2-3 times lower than those found in the central regions. Fits to the diffuse emission do not allow strong constraints to be put on the metallicities of the emitting plasmas. However, the derived metallicities are compatible with those determined from their HII regions. An alpha/Fe ratio of ~2 is indicated for the hot gas within at least three objects (NGC 1569, NGC 4449, and He 2-10).