Two formation channels of ultra-compact dwarf galaxies in Hickson compact groups

Abstract

Context. The formation of ultra-compact dwarf galaxies (UCDs) is believed to be driven by interaction, and UCDs are abundant in the cores of galaxy clusters, environments that mark the end-point of galaxy evolution. Nothing is known about the properties of UCDs in compact groups of galaxies, environments where most of galaxy evolution and interaction is believed to occur and where UCDs in an intermediate stage in their evolution may be expected. Aims. The main goal of this study is to detect and characterize, for the first time, the UCD population of compact groups of galaxies. For that, two nearby groups in different evolutionary stages, HCG 22 and HCG 90, were targeted. Methods. We selected about 40 UCD candidates from pre-existing photometry of both groups, and obtained spectra of these candidates using the VLT FORS2 instrument in MXU mode. Archival HST/ACS imaging was used to measure their structural parameters. Results. We detect 16 and 5 objects belonging to HCG 22 and HCG 90, respectively, covering the magnitude range −10.0 > M R > −11.5 mag. Their integrated colours are consistent with old ages covering a broad range in metallicities (metallicities confirmed by the spectroscopic measurements). Photometric mass estimates put 4 objects in HCG 90 and 9 in HCG 22 in the mass range of UCDs (>2 × 10 6 M ) for an assumed age of 12 Gyr. These UCDs are on average 2−3 times larger than the typical size of Galactic GCs, covering a range of 2 < ∼ r h < ∼ 21 pc. The UCDs in HCG 22 are more concentrated around the central galaxy than in HCG 90, at the 99% confidence level. They cover a broad range in [α/Fe] abundances from sub-to super-solar. The spectra of 3 UCDs (2 in HCG 22, 1 in HCG 90) show tentative evidence of intermediate age stellar populations. The clearest example is the largest and most massive UCD (∼10 7 M ) in our sample, which is detected in HCG 22. Its properties are most consistent with a stripped dwarf galaxy nucleus. We calculate the specific frequency (S N ) of UCDs for both groups, finding that HCG 22 has about three times higher S N than HCG 90. Conclusions. The ensemble properties of the detected UCDs supports two co-existing formation channels: a star cluster origin (lowluminosity, compact sizes, old ages, super-solar α/Fe), and an origin as tidally stripped dwarf nuclei (more extended and younger stellar populations). Our results imply that the UCDs detected in both groups do not, in their majority, originate from relatively recent galaxy interactions. Most of the detected UCDs have likely been brought into the group along with their host galaxies.

A large population of ultra-compact dwarf galaxies in the Hydra I cluster

“…8 and 10), we find objects extending the red GC population towards higher luminosities, as well as objects with fairly blue colours, which coincide with nuclei of dwarf galaxies in the colour−magnitude diagram. Norris & Kannappan (2011) suggest a general scheme of GC/UCD formation and the connection to galaxy nuclei (see also Da Rocha et al 2011;Chilingarian et al 2011). In this picture, objects below M V ∼ −10 mag are "normal" GCs (red and blue) with a common mean size, plus a fraction of low-mass nuclei, indistinguishable from GCs.…”

Observational dynamics of low‐mass stellar systems

“…More recently, objects of this type have also been discovered in less rich environments, around the dominant giant galaxies in nearby loose groups, namely NGC 5128 (Rejkuba et al 2007;Taylor et al 2010), M 104 (Hau et al 2009), NGC 4546 and NGC 3923 (Norris & Kannappan 2011), NGC 4494 (Foster et al 2011), as well as in the compact groups HCG 22 and HCG 90 (Da Rocha et al 2011) and in the fossil group NGC 1132 (Madrid 2011).…”

“…Consequently, a consensus seems to emerge in the literature, that the morphological class of UCDs comprises objects from different evolutionary tracks (e.g Brodie et al 2011;Da Rocha et al 2011;Hilker 2009;Norris & Kannappan 2011), even though the fraction of threshed nuclei among UCDs may be very low ).…”

A catalog of extended clusters and ultra-compact dwarf galaxies

“…EOs were detected in a large number of elliptical galaxies: NGC 5128 (Gomez et al 2006;McLaughlin et al 2008;Chattopadhyay et al 2009;Taylor et al 2010;Mouhcine et al 2010 (Haşegan et al 2005;Evstigneeva et al 2008;Brodie et al 2011;Chies-Santos et al 2011), M 84 (Chies-Santos et al 2011, the central galaxy of the Fornax Cluster NGC 1399 (Richtler et al 2005;Evstigneeva et al 2007;Hilker et al 2007;Evstigneeva et al 2008;Chilingarian et al 2011), NGC 3923 (Norris & Kannappan 2011), NGC 4476 (Haşegan et al 2005), NGC 5846 (Chies-Santos et al 2006), NGC 4696 (Mieske et al 2007), NGC 3311 ), IC 4041, NGC 4889, IC 3998, IC 4030, IC 4041, and NGC 4908 (Chiboucas et al 2011), NGC 4874 (Madrid et al 2010Chiboucas et al 2011), NGC 1132(Madrid 2011), NGC 1275(Penny et al 2012, NGC 4365 (Blom et al 2012), NGC 1316 (Goudfrooij 2012), NGC 1199(Da Rocha et al 2011, and ESO325-G004 (Blakeslee & Barber 2008).…”