3. THE GLOBULAR CLUSTER SOURCES OUTSIDE THE GALAXY

In this section, we discuss the very luminous globular cluster X-ray sources observed in galaxies other than our own. The observations we discuss were all done with Chandra, except for the ROSAT observations of M31. Some of the sources were already detected with ROSAT, but the positional accurracy of Chandra allows more secure identifications with globular clusters. Table 3 gives an overview of the observations reported so far. The lowest detectable luminosities vary strongly between galaxies. With the exception of M31 and NGC5128, however, we are always talking about very luminous sources (the tip of the iceberg). In addition to the sources discussed in this chapter, sources in many other globular clusters associated with other galaxies have been observed but not (yet) recognized as such, e.g. because the required optical cluster studies are not available (see Chapter 12 and Table 12.1).

The number of globular clusters varies widely between galaxies. Precise numbers are difficult to determine: clusters are difficult to detect against a bright background of the central regions of a galaxy, and the cluster distribution may extend beyond the observed area. For example, globular clusters in NGC4697 have only been identified in an annulus from 1.5 to 2.5 arcmin from the center. And even for nearby M31 "the size of the globular cluster system is embarrassingly uncertain" (Barmby 2003). Estimates of the total number are often based on an uncertain extrapolation of the measured bright part of the globular cluster luminosity function and depend on the availability of multi-color images that go deep enough to probe a significant portion of the luminosity function (Kundu, private communication). In many galaxies the area in which positions of globular clusters are known with sufficient accuracy for comparison with X-ray positions is limited by the field-of-view of HST-WFPC2 observations: an example is seen in Figure 4.

The number N of globular clusters of a galaxy is sometimes scaled to the total luminosity of the galaxy (derived from absolute magnitude M_V), as a specific frequency S_N, defined as (Harris & van den Bergh 1981):

(1)

A `local' specific frequency is often defined for the field-of-view of the HST-WFPC2. The uncertainties in the total number of globular clusters are reflected in large uncertainties of the specific frequencies, and the uncertainty in the distance adds to this. For example, values for NGC1553 range from 1.22 ± 0.27 to 2.3 ± 0.5 (Bridges & Hanes 1990, Kissler-Patig 1997). Specific frequencies (most are meant to be global) are compiled by Harris (1991), Kissler-Patig (1997), and Ashman & Zepf (1998). Local specific frequencies of globular clusters have been measured in the inner region of 60 galaxies (Kundu & Whitmore, 2001a, b).

Many elliptical galaxies, and especially those in the center of clusters of galaxies, have large numbers of globular clusters (Harris 1991; Ashman & Zepf 1998). Per unit mass, most ellipticals have about twice as many globular clusters than spirals (Zepf & Ashman 1993, 1998). The globular cluster populations in most elliptical galaxies show a bimodal distribution in optical colors (Figure 3). Most of this is due to differences in metallicity, but differences in age may also play a role. Metal-poor clusters are bluer than metal-rich clusters of the same age; at the same metallicity, old clusters are redder than young ones. It has been suggested that the blue metal-poor globular clusters were formed at the proto-galactic epoch, and that the red metal-rich globular clusters resulted in later starbursts, e.g. as a consequence of the mergers that produce the galaxies that we observe today (Ashman & Zepf, 1992; Zepf & Ashman, 1993; for other possibilities see the review by West et al. 2004). However, to date there is no convincing evidence for difference in ages of red and blue subsystems (e.g., Puzia et al. 2002, Cohen et al. 2003).

Figure 3. Left: V-I colors of globular clusters vs. distance from the center of the elliptical galaxy NGC4472. LMXB-globular-cluster matches are indicated by filled circles. Most luminous X-ray sources are located in red globular clusters. The optical color distribution is shown on the right with a dashed line; notice the bimodal distribution. The distribution of the globular clusters that house the luminous X-ray sources is also shown. Courtesy of Kundu, Maccarone & Zepf (2002).