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

Abstract

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.

Formation of ultra-compact dwarf galaxies from supergiant molecular clouds

“…The properties of UCDs are similar to those of nuclear clusters: as well as those listed above, the metallicities (Brodie et al 2011;Francis et al 2012), internal velocity dispersions ) and colour-magnitude re- (Cote et al 2006;Evstigneeva et al 2008;Brüns et al 2011) are similar to those of dwarf galaxy nuclei. Simulations of the threshing scenario performed by Bekki et al (2001Bekki et al ( , 2003 produce objects with sizes and luminosities comparable with observed UCDs.…”

Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies

“…In both objects, we assumed solar metallicities. However, if the metallicity were significantly below solar, this could lead to an overestimate of the population mass estimates; this seems possible in VUCD3,where the existing measurements span a wide range from [Z/H]=−0.28 to 0.35 (Evstigneeva et al 2007;Firth et al 2009;Francis et al 2012). GC dynamical mass estimates also seem to be lower than expected Kimmig et al 2015), though this appears to be in part because of mass segregation within the clusters combined with the assumption of mass-traces-light models (Shanahan & Gieles 2015;Baumgardt 2017).…”

“…VUCD3 is located 14kpc in projection from the center of M87 and has M 12.75 V = - (Mieske et al 2013). The metallicity of VUCD3 has been estimated to be between −0.28 and 0.35 in several studies (Evstigneeva et al 2007;Firth et al 2009;Francis et al 2012), and it has an [α/Fe]∼0.5 (Francis et al 2012). M59cO is located 10kpc in projection from the center of M59 and has M 13.26 V = - (Mieske et al 2013).…”

The Slowest Spinning X-Ray Pulsar in an Extragalactic Globular Cluster

“…This makes them analogous to ultra-compact dwarf (UCDs) galaxies (Drinkwater et al 2003) observed in the nearby Virgo and Fornax clusters, which are known to have mass-tolight ratios consistent with the Kroupa IMF when the two-body relaxation timescales are long (Chilingarian et al 2011). UCDs are proven to be tidally stripped galactic nuclei by the recent discovery of central massive black holes (Seth et al 2014) and by their stellar population properties (Francis et al 2012) similar to much more massive tidally stripped M32-like compact elliptical galaxies (Chilingarian et al 2009;Chilingarian & Zolotukhin 2015). The only known stellar systems where central luminosity densities exceed 10 5 L pc −3 are nuclear star clusters in low and intermediate luminosity galaxies, cEs, some UCDs (Evstigneeva et al 2008) and some globular clusters.…”