3.1. Elliptical galaxies
The most luminous X-ray sources in a galaxy are high-mass X-ray binaries, supernova-remnants, and low-mass X-ray binaries. Since elliptical galaxies do not house young stellar populations, virtually all luminous X-ray sources in them will be low-mass X-ray binaries. Table 3 provides an overview of the references and results; some additional remarks for individual galaxies follow.
NGC1399 is a giant elliptical galaxy in the center of the Fornax Cluster at 20.5 Mpc. A large fraction of the 2-10 keV X-ray emission in an 8' × 8' region is resolved into 214 discrete sources, including many background sources. 45 are in globular clusters (see Figure 4). Many of the globular cluster sources have super-Eddington luminosities (for an accreting neutron star), and their average luminosity is higher than that of the sources not associated with globular clusters. The most luminous source in a globular cluster, has an ultra-soft spectrum seen in the high state of black-hole binaries. This may indicate that some of the most luminous systems are massive black-hole binaries, rather than conglomerates of less luminous neutron-star binaries (Angelini et al. 2001).
Figure 4. The 0.3-10 keV Chandra image of NGC1399 centered on an HST pointing, smoothed with a Gaussian of about 0.8". The white line marks the HST/WFPC2 FOV. The circles show the X-ray source positions that are associated with globular clusters. The squares are the remaining sources. All 45 sources are marked; 38 have a significance in excess of 3. The top left image is an example of the Chandra contours overlaid on the HST field. Courtesy Angelini et al. (2001).
Dirsch et al. (2003) find that "within 7' the specific frequency of the blue clusters alone is a factor ~ 3 larger than for the red ones. Outside this radius, both populations have the same high local specific frequency", listed in Table 3.
Figure 5. DSS optical image of NGC4697. The circles show the positions of the X-ray sources detected with Chandra. The squares indicate X-ray sources in known globular clusters. One should note that globular clusters have only been identified in this galaxy in an annulus from 1.5 - 2.5 arcmin from the center. This figure was kindly provided by Craig Sarazin. It is adapted from Figure 3 of Sarazin, Irwin & Bregman (2001).
NGC4697. In this galaxy, most of the X-ray emission is also from point sources. The central source, with LX = 8 × 1038 erg s-1, may be an active nucleus and/or multiple LMXBs. The luminosity function of the LMXBs has a knee at 3.2 × 1038 erg s-1, which, Sarazin et al. (2000, 2001) suggest, separates the black-hole binaries from the neutron-star binaries.
NGC4472 is a giant elliptical galaxy. In the inner regions of the galaxy it has been shown that metal-rich red globular clusters are about 3 times more likely to host a very luminous LMXB than the blue metal-poor ones (Figure 3). The X-ray luminosity does not depend significantly on the properties of the host globular cluster. The X-ray luminosity function of both globular cluster-LMXBs and non-globular cluster-LMXBs show a break at ~ 3×1038 erg s-1, suggesting that the most luminous LMXBs may be black hole accretors (Kundu et al. 2002).
NGC4365, in the Virgo cluster, is one of a few early-type galaxies whose globular clusters do not have a bi-modal color distribution in V-I (but it does in infrared colours, Puzia et al. 2002). Kundu et al. (2003) find that the presence of very luminous LMXBs is correlated with metallicity, but not with cluster age. The LMXB fraction per unit mass of the globular clusters is ~ 10-7 M-1. In contrast, Sarazin et al. (2004) and Sivakoff et al. (2003) find that within the sample of IR-bright globular clusters studied by Puzia et al. (2002), the metal-rich, intermediate-age globular clusters, are four times as likely to contain LMXBs than the old globular clusters (with an uncertainty of a factor of two). The luminosity function is a power-law with a cutoff at ~ (0.9-2.0) × 1039 erg s-1, much higher than the cutoff measured for other ellipticals. No evidence is found here for a break in the luminosity function at or near the Eddington limit of a 1.4 M neutron star.
NGC3115 has a distinct bimodal color distribution of the globular clusters. The metal-poor blue and the metal-rich red globular clusters are both ~ 12 Gyr old (Puzia et al. 2002). There are roughly equal numbers of red and blue globular clusters in the WFPC2 image. Kundu et al. (2003) find that the red globular clusters are the preferred sites for LMXB formation, largely as a consequence of their higher metallicity.
NGC1407. White (2002) reported that about 90% of the 160 detected LMXBs have X-ray luminosities which exceed the Eddington limit for neutron stars. He suggests that many may be black hole binaries (rather than multiple neutron-star binaries within individual globular clusters), since 45% do not reside in globular clusters. To date (March 2004), these results have not yet been published in a refereed journal.
NGC1553 is an S0 galaxy. 30% of the emission in the 0.3-10 keV band and 60% of the emission in the 2.0-10 keV band is resolved into discrete sources. The luminosity function of all sources is well fitted by a power-law with a break at the Eddington luminosity for a 1.4 M neutron star. Kissler-Patig (1997) lists a global specific frequency of 2.3 ± 0.5, higher than the value listed in Table 3.
NGC4649 (M60) is a bright elliptical galaxy. Its X-ray luminosity function is well fit by a power-law with a break near the Eddington luminosity of a 1.4 M neutron star.
NGC1316 (Fornax A) is a disturbed elliptical radio galaxy with many tidal tails. Several mergers must have occurred over the past 2 Gyr (see Kim & Fabbiano 2003a, and references therein). One of the 5 globular cluster sources is super-soft (see Chapter 11). For an adopted distance of 18.6 Mpc, 35% of the sources are above the Eddington limit of a 1.4M neutron star (Kim & Fabbiano, 2003a). The luminosity function is well represented by an unbroken power law with a slope of -1.3.
NGC720. 3 of the 12 globular cluster sources have X-ray luminosities in excess of 1039 erg s-1 (at 35 Mpc). It is possible that this galaxy is much closer, and that none of the sources are ultra-luminous (Jeltema et al. 2003).
NGC4486 (M87) is a giant elliptical galaxy near the center of the Virgo cluster, and has the most populous GC system in the Local Supercluster. More luminous, redder and denser clusters are more likely to harbor a luminous X-ray source. Metal-rich red globular clusters are about three times more likely to host a luminous LMXB than the blue metal-poor ones; the trend with central density gives strong evidence that encounter rates are important in forming LMXBs in GCs. The trend with luminosity can arise as a consequence of the fact that more luminous clusters have higher encounter rates. The X-ray luminosity functions of both globular cluster-LMXBs and non-globular cluster LMXBs are well described by single power laws with an upper cutoff at ~ 1039 erg s-1 (Jordán et al. 2004).