A Chandra X‐ray survey of nearby dwarf starburst galaxies – II. Starburst properties and outflows

We present a comprehensive comparison of the X‐ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (I Zw 18, VII Zw 403, NGC 1569, NGC 3077, NGC 4214, NGC 4449, NGC 5253 and He 2–10). In Paper I, we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X‐ray sources, we find the following: point sources are in general located close to bright H  ii regions, rims of superbubbles or young stellar clusters. The number of X‐ray point sources appears to be a function of the current star formation (SF) rate and the blue luminosity of the hosts. Ultraluminous X‐ray sources (ULXs) are only found in those dwarf galaxies that are currently interacting. The power‐law (PL) index of the combined cumulative X‐ray point‐source luminosity function is α= 0.24 ± 0.06 , shallower than that of more massive starburst galaxies (α= 0.4 –0.8) and of non‐starburst galaxies (α∼ 1.2) . For those galaxies showing extended X‐ray emission (six out of the eight galaxies), we derive the following: superwinds develop along the steepest gradient of the H  i distribution with volume densities of 0.02–0.06 cm −3 , pressures of 1–3 × 10 5  K cm −3 , thermal energies of 2–30 × 10 54  erg and hot gas masses of 2–20 × 10 6  M ⊙ ( ∼1 per cent of the H  i masses). On global scales, the distribution of the X‐ray emission looks remarkably similar to that seen in Hα (comparing azimuthal averages); locally, however, their distribution is clearly distinct in many cases, which can be explained by the different emission mechanisms (forward versus reverse shocks). Mass loading of order 1 to 5 is required to explain the differences between the amount of hot gas and the modelled mass loss from massive stars. The metallicity of the dwarf galaxies correlates with the diffuse X‐ray luminosity and anticorrelates with the cooling time of the hot gas. The diffuse X‐ray luminosity is also a function of the current star formation rate (SFR). The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared with the ambient interstellar medium (ISM). Extended H  i envelopes such as tidal tails, however, may delay outflows on time‐scales exceeding those of the cooling time of the hot gas.