A Chandra X-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results

Abstract

We present an analysis of Chandra X-ray observations of a sample of eight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253 and He2-10). 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 fitted by power-law (PL), thermal plasma (TP) or blackbody (BB) models. 10 of the point sources exceed an X-ray luminosity of 10³⁹ erg s^-1 (ultraluminous X-ray sources, ULXs). In those galaxies where diffuse X-ray emission is detected, this emission (with X-ray luminosities ranging from 4 × 10³⁸ to 2 × 10⁴⁰ erg s^-1) contains most (60-80 per cent) of the X-ray photons. This diffuse emission can be well fitted by MEKAL one-temperature TP models once the contribution from the unresolved point sources is subtracted properly. The diffuse X-ray component is significantly extended, reaching as far as 0.5-5kpc into the outskirts of their hosts. Azimuthally averaged X-ray surface brightness profiles are well approximated by exponential functions. Temperatures of various regions within the galaxies range from 1.6 to 7.6 × 10⁶K. With few exceptions, temperatures of the hot gas are remarkably uniform, hovering around 2-3 × 10⁶K. 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 α/Fe ratio of ~2 is indicated for the hot gas within at least three objects (NGC1569, NGC4449 and He2-10). Shadowing of the diffuse X-ray emission by the cooler disc gas is used to constrain the orientation of the galaxies.