2.3. M83 and M101

M83 (NGC 5236) and M101 (NGC 5457) are both face-on Sc galaxies. M83 is likely to be a member of the Centaurus group, with a distance of ~ 4 Mpc (de Vaucouleurs et al. 1991); M101 is more distant (~ 7 Mpc; Stetson et al. 1998), but still in the nearby universe.

M83 is a grand design, barred spiral, with a starburst nucleus. Is has been observed extensively in the pre-Chandra era, but here we discuss only the Chandra observations, that are the most relevant for the study of the X-ray source population. M83 was observed with Chandra ACIS-S3 for ~ 50 ks (Fig. 5). Soria & Wu (2002) detect 81 sources in these data, of which 18 had been detected previously with ROSAT; 15 sources are resolved in the previously confused nuclear region, which has the highest source density. The XLF of the sources in the nuclear-bar region, where a young stellar population is likely to prevale, follows a fairly flat unbroken power-law (cumulative slope -0.8). The XLF of the disk sources is instead steeper (slope -1.3), with a break at ~ 6 × 10³⁷ ergs s^-1, becoming flatter at the lower luminosities. This behaviour is reminiscent of the XLFs of the bulges of M31 and M81, and suggests an older XRB population.

Figure 5. M83 as seen with Chandra. Note the population of point-like sources and the softs diffuse emission (possibly from hot ISM), associated with the spiral arms (from http://chandra.cfa.harvard.edu/photo/2003/1154/index.html).

In M101, 110 sources (27 of which are expected to be backround AGN) were detected in a 98 ks Chandra ACIS-S3 observation, with a limiting luminosity of 10³⁶ ergs s^-1 (Pence, Snowden & Mukai 2001). The sources cluster along the spiral arms, and, interestingly, sources in the interarm regions tend to have X-ray colors compatible with AGNs. Twelve sources are spatially coincident with supernova remnants, but, based on their variability, two of them are identified with XRBs. Eight other luminous sources exhibit variability in the Chandra data, and two more are found variable by comparison with previous ROSAT observations. Ten sources are supersoft, and a correlation between black-body temperature and total source luminosity is suggested by the data. The XLF of the M101 sources can be modelled with a power-law (cumulative slope -0.8) in the 10³⁶ -10³⁸ ergs s^-1 range.

In summary, with Chandra, X-ray source population studies are finally coming of age. The sub-arcsecond resolution of the Chandra mirrors (Van Speybroeck et al. 1997) allows both the separation of discrete sources from surrounding diffuse emission and the detection of much fainter sources than previously possible.

The XLFs of sources in a given system reflect the formation, evolution, and physical properties of the X-ray source population. These differences are evident in different regions of M31, M81 and M83. Comparison of the XLFs of nearby galaxies (and components thereof) with the XLFs of more distant systems provides a general coherent picture, pointing to steeper XLFs in older stellar populations (relative lack of very luminous sources). The XLFs of E and S0 galaxies have cumulative slopes in the range -1.0 to -2.0 (see Section 4.3), generally consistent with those of the bulges of M31 and M81. These slopes are significantly steeper than those of sources associated with younger stellar fields in M31, M81, and M83. A recent study of 32 nearby galaxies extracted from the Chandra archive (Colbert et al. 2003) confirms this basic difference between XLFs of old and younger stellar populations, finding cumulative slopes of ~ 1.4 and ~ 0.6 - 0.8 for elliptical and spiral galaxies respectively.