The chemical composition of ultracompact dwarf galaxies in the Virgo and Fornax clusters

We present spectroscopic observations of ultracompact dwarf (UCD) galaxies in the Fornax and Virgo clusters made to measure and compare their stellar populations. The spectra were obtained on the Gemini-North (Virgo) and Gemini-South (Fornax) telescopes using the respective Gemini Multi-Object Spectrographs.

We estimated the ages, metallicities and abundances of the objects from measurements of Lick line-strength indices in the spectra; we also estimated the ages and metallicities independently using a direct spectral fitting technique. Both methods revealed that the UCDs are old (mean age 10.8 ± 0.7 Gyr) and (generally) metal rich (mean [Fe/H] = -0.8 ± 0.1). The α-element abundances of the objects measured from the Lick indices are super-solar.

We used these measurements to test the hypothesis that UCDs are formed by the tidal disruption of present-day nucleated dwarf elliptical galaxies. The data are not consistent with this hypothesis because both the ages and abundances are significantly higher than those of observed dwarf galaxy nuclei (this does not exclude disruption of an earlier generation of dwarf galaxies). They are more consistent with the properties of globular star clusters, although at higher mean metallicity. The UCDs display a very wide range of metallicity (-1.7 < [Fe/H] < 0.0), spanning the full range of both globular clusters and dwarf galaxy nuclei.

We confirm previous reports that most UCDs have high metalliticities for their luminosities, lying significantly above the canonical metallicitiy-luminosity relation followed by early-type galaxies. In contrast to previous work, we find that there is no significant difference in either the mean ages or the mean metallicities of the Virgo and Fornax UCD populations.