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2. Diffuse X-ray emission

The total X-ray luminosity emitted from M83 in the 0.3-8.0 keV band is approx 2.5 × 1040 erg s-1 (1). Of this, approx 6 × 1039 erg s-1 comes from the resolved sources and the rest is unresolved. The nuclear starburst contributes for approx 5 × 1039 erg s-1 (of which approx 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 ne approx 5 × 10-2 cm-3 and a total mass ~ 107 Modot for the X-ray emitting gas in M83; the cooling timescale is approx 108 yr. For the hot gas in the starburst nuclear region, ne approx 0.2 cm-3, M approx 5 × 105 Modot, tc approx 4 × 107 yr. Hence, the hot gas is an indicator of recent star formation.

Figure 2a Figure 2b

Figure 2. Left: the Chandra/ACIS spectrum of the unresolved emission in the starburst nucleus of M83 shows a thermal plasma component (kT approx 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 approx 0.2 keV) and a high-energy power-law component whose origin is yet to be determined.

1 In other bands: LB approx 2.5 × 1043 erg s-1, LFIR approx 2.5 × 1043 erg s-1, LHalpha approx 1.5 × 1040 erg s-1. In general, LX ~ LHalpha ~ 10-3LFIR 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 vsh ~ T1/2 > 650 km s-1, required to heat the gas to kT gtapprox 0.5 keV. Assuming a SN rate ~ 0.05 yr-1 for M83, and a total mechanical energy approx 1051 erg injected into the ISM by each SN, the total mechanical luminosity is approx 1.5 × 1042 erg s-1. Back.

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