Dynamical Friction in dE Globular Cluster Systems

The dynamical friction timescale for globular clusters to sink to the centerof a dwarf elliptical galaxy (dE) is significantly less than a Hubble time ifthe halos have King-model or isothermal profiles and the globular clustersformed with the same radial density profile as the underlying stellarpopulation. We examine the summed radial distribution of the entire globularcluster systems and the bright globular cluster candidates in 51 Virgo andFornax Cluster dEs for evidence of dynamical friction processes. We find thatthe summed distribution of the entire globular cluster population closelyfollows the exponential profile of the underlying stellar population. However,there is a deficit of bright clusters within the central regions of dEs(excluding the nuclei), perhaps due to the orbital decay of these massiveclusters into the dE cores. We also predict the magnitude of each dE's nucleusassuming the nuclei form via dynamical friction. The observed trend ofdecreasing nuclear luminosity with decreasing dE luminosity is much strongerthan predicted if the nuclei formed via simple dynamical friction processes. Wefind that the bright dE nuclei could have been formed from the merger oforbitally decayed massive clusters, but the faint nuclei are several magnitudesfainter than expected. These faint nuclei are found primarily in M_V > -14 dEswhich have high globular cluster specific frequencies and extended globularcluster systems. In these galaxies, supernovae-driven winds, high central darkmatter densities, extended dark matter halos, the formation of new starclusters, or tidal interactions may act to prevent dynamical friction fromcollapsing the entire globular cluster population into a single bright nucleus.Comment: 15 pages, 2 tables, 7 figures; to appear in the Astrophysical Journal, April 20, 200