2. Diffuse X-ray emission

The total X-ray luminosity emitted from M83 in the 0.3-8.0 keV band is 2.5 × 10⁴⁰ erg s^{-1 (1)}. Of this, 6 × 10³⁹ erg s^-1 comes from the resolved sources and the rest is unresolved. The nuclear starburst contributes for 5 × 10³⁹ erg s^-1 (of which 1 × 10³⁹ erg s^-1 from resolved sources). The unresolved X-ray emission is dominated by a multitemperature, optically thin plasma component, at kT ~ 0.2-0.7 keV, slightly hotter in the nuclear region than in the arms (Figures 1, 2). It is likely to originate from gas shock-heated by core-collapse SN explosions ⁽²⁾. The unresolved emission from the disk region has a power-law-like tail that dominates above 3 keV. Its origin is still unclear: a population of faint, unresolved XRBs can produce a power-law component. The emission can also be due to a second thermal plasma component at kT > 2 keV, or to Compton upscattering of far-IR photons by relativistic electrons (Valinia & Marshall 1998).

From the observed temperature and luminosity, we estimate an average density n_e 5 × 10^-2 cm^-3 and a total mass ~ 10⁷ M for the X-ray emitting gas in M83; the cooling timescale is 10⁸ yr. For the hot gas in the starburst nuclear region, n_e 0.2 cm^-3, M 5 × 10⁵ M, t_c 4 × 10⁷ yr. Hence, the hot gas is an indicator of recent star formation.

Figure 2. Left: the Chandra/ACIS spectrum of the unresolved emission in the starburst nucleus of M83 shows a thermal plasma component (kT 0.6 keV, determined from the Fe L line complex) with strong metal lines; in particular, the 1.33 keV MgXI line suggests that the gas has been enriched by core-collapse SNe. Right: the unresolved emission in the spiral arms has a larger contribution from cooler gas (kT 0.2 keV) and a high-energy power-law component whose origin is yet to be determined.

¹ In other bands: L_B 2.5 × 10⁴³ erg s^-1, L_FIR 2.5 × 10⁴³ erg s^-1, L_H 1.5 × 10⁴⁰ erg s^-1. In general, L_X ~ L_H ~ 10^-3L_FIR for starburst spirals (Fabbiano & Shapley 2002; Calzetti et al. 1995; Condon et al. 1998; de Vaucouleurs et al. 1991); all three bands can be used as indicators of star formation. Back.

² SN ejecta can easily provide v_sh ~ T^1/2 > 650 km s^-1, required to heat the gas to kT 0.5 keV. Assuming a SN rate ~ 0.05 yr^-1 for M83, and a total mechanical energy 10⁵¹ erg injected into the ISM by each SN, the total mechanical luminosity is 1.5 × 10⁴² erg s^-1. Back.