4.3. Globular Clusters

Young star clusters are observed in star-forming galaxies like the LMC [79] and in intense starburst galaxies [80, 81]. These clusters have half-light radii of less than 5 pc, masses of 10⁴ to 10⁷ M, and metal abundances comparable to their parent starbursts. Their luminosity functions follow power laws with slopes of -1.6 to -2, intriguingly close to the mass function of giant molecular clouds [82]. However, it's not entirely clear that cluster luminosity is a good indicator of mass since some range of cluster ages is usually present.

Evidence is accumulating that the globular cluster systems of field ellipticals are partly due to cluster formation in merger-induced starbursts:

• Ongoing and recent mergers (eg., NGC 4038 / 9, NGC 7252, NGC 3921) have populations of blue luminous clusters with ages of less than 1 Gyr [81, 83, 84].

• Older remnants (eg., NGC 3610) have redder and fainter clusters with ages of a few Gyr [85].

• Predicted specific frequencies ⁽¹⁾ in merger remnants increase to S_N 2 or 3 over ~ 10 Gyr as the stellar populations fade [83, 84].

• Globulars in elliptical galaxies have bimodal color (metalicity) distributions.

These findings imply that metal-rich star clusters form during mergers and are gradually assimilated into existing globular cluster populations [86]. However, the large populations of metal-poor globulars found in cluster ellipticals are not consistent with mergers of field spirals [87]; predicted specific frequencies of metal-poor clusters are S_N^P 1, while in fact S_N^P 4. This problem is even worse for cluster systems in cD galaxies, which have S_N^P 10; obviously, no amount of merging between metal-rich systems will produce metal-poor clusters!

The question of high-S_N in cluster ellipticals boils down to this: fewer stars, or more globulars? One way to get fewer stars is to merge galaxies after their metal-poor globulars have formed but before they build up substantial disks. For example, the Milky Way as it was ~ 10 Gyr ago could serve as a building-block for cluster ellipticals; the halo of our galaxy, considered alone, has S_N^P 4. However, mergers of Milky Way halos (or dwarf elliptical galaxies [88]) still fall short of the high S_N^P values of cD galaxies. Another way to end up with fewer stars is to eject most of the gas after the initial epoch of cluster formation; the problem here is that the ejection efficiency must be higher in cD galaxies, which have the deeper potential wells and should be better at retaining gas [89].

Alternately, the production of globular clusters may have been more efficient in high-redshift starbursts. Even at low-z, about 20% of the UV emitted by starbursts comes from knots identified with young clusters [80]; if all these clusters survive, the specific frequency for a pure starburst population is S_N 60. Moreover, these clusters are concentrated where the surface densities are highest; it's likely that net yields of star clusters increase rapidly with increasing surface density.

If so, then globular cluster systems reflect the starburst histories of their parent galaxies: Large populations of metal-poor globulars are due to efficient cluster production in early starbursts, while predominantly metal-rich systems (eg., NGC 5846) formed in more recent starbursts. Metallicity distributions for cluster systems support this idea; giant elliptical galaxies have a range of distributions with multiple peaks between [Fe/H] -1.2 and 0.2 [90]. Such variety seems hard to explain in a picture where internal events determine the timing of cluster formation (eg., [87]); on the other hand, it's easy to imagine that different distributions result from the different merging histories of individual galaxies.