2.4. XRBs in Actively Starforming Galaxies
Observations show flatter XLF slopes (i.e., an increased presence of very luminous sources) in galaxies with more intense star formation. The best example is given by the merger system NGC 4038 / 39 (The Antennae), where nine ultra-luminous X-ray sources (ULXs; LX > 1039 ergs s-1, for a distance of 19 Mpc) were discovered with Chandra (Fabbiano, Zezas & Murray 2001). Other examples of exceptionally luminous sources are found in M82 (Kaaret et al. 2001; Matsumoto et al. 2001), the Circinus galaxy (Smith & Wilson 2001; Bauer et al. 2001) and NGC 1365 X-1 (Komossa & Schultz 1998). Consequently, flatter XLFs occur in galaxies with more intense star formation: the cumulative XLF slope is - 0.45 in The Antennae (Zezas & Fabbiano 2002; Kilgard et al. 2002; Fig. 6).
Figure 6. Left: Chandra ACIS image of The Antennae (Fabbiano et al. 2001); Right: the XLF of The Antennae (points with error bars) compared with other galaxies, as labelled. Note the steep XLFs of the Galactic HMLXBs (bulge) and of the early-type galaxy NGC 4697 (Zezas & Fabbiano 2002).
Grimm, Gilfanov & Sunyaev (2003) suggest that the XLFs of star forming galaxies scale with the star formation rate (SFR), thus advocating that HMXBs may be used as a star formation indicator in galaxies. They find that at high SFRs the total X-ray luminosity of a galaxy is linearly correlated to the SFR, and suggest a `universal' XLF of starforming galaxies described by a power law with cumulative slope of ~ - 0.6 and a cut-off at LX ~ few × 1040 ergs s-1. This result of course depends on how well is the SFR of a given galaxy known. This is a subject of considerable interest at this point, since various indicators are differently affected by extinction. The conclusion of a universal slope of the XLF of starforming galaxies may be at odd with the reported correlation between the XLF slope and the 60 µm luminosity from a minisurvey of spiral and starburst galaxies observed with Chandra (Kilgard et al. 2002). Also, theoretical models (Kalogera et al. 2003) suggest that XLF slopes depend on the age of the starburst, so it is possible that the `universal' XLF slope is not truly universal, but reflects a selection bias, in that the sample used by Grimm, Gilfanov & Sunyaev (2003) may be dominated by starburtsts of similar ages.
Comparison of the XLFs for different galaxies, and modeling of the same, provide powerful tools for understanding the nature of the X-ray sources and for relating them to the evolution of the parent galaxy and its stellar population. Early theoretical work has attempted to interpret the XLFs, using ad hoc power-law models, and accounting for aging and impulsive birth of XRB populations (Wu 2001, Kaaret 2002, Kilgard et al. 2002). Spurred by the recent observational developments, Kalogera and collaborators have developed the first models of synthetic XLFs, based on XRB evolutionary calculations (Belczynski et al. 2003). Such models provide us with a potentially powerful tool for studying the origin and evolution of XRB populations in stellar systems and their connection to galactic environments. A preliminary examination of such models for starburst galaxies (Kalogera et al. 2003; see Fig. 7) successfully shows that predictions and consistency checks for the shapes and normalizations of XLFs are possible with theoretical XRB modeling. These new developments demonstrate that the predictions of 1995 are coming true (see Section 1).
Figure 7. Comparison of XRB population models (from Kalogera et al. 2003) with the observed XLF of NGC 1569 (bottom points, in black; data taken from Martin et al. 2002, ApJ, 574, 663). Models were constructed to match the star-formation history of NGC 1569 (recent star-burst duration and metallicity) and model XLFs are shown at different times since the beginning of the starburst. Top to botom: 10Myr (blue), 50Myr (yellow), 110Myr (red), 150Myr (cyan), 200Myr (green). Note that based on observations in other wavelengths, the age of the starburst is estimated to be 105-110Myr.