Stellar Population Properties of Ultracompact Dwarfs in M87: A Mass–Metallicity Correlation Connecting Low-metallicity Globular Clusters and Compact Ellipticals

Abstract

We derive stellar population parameters for a representative sample of ultracompact dwarfs (UCDs) and a large sample of massive globular clusters (GCs) with stellar masses 10 6 M ⊙ in the central galaxy M87 of the Virgo galaxy cluster, based on model fitting to the Lick-index measurements from both the literature and new observations. After necessary spectral stacking of the relatively faint objects in our initial sample of 40 UCDs and 118 GCs, we obtain 30 sets of Lick-index measurements for UCDs and 80 for GCs. The M87 UCDs have ages 8 Gyr and [α/Fe] ≃ 0.4 dex, in agreement with previous studies based on smaller samples. The literature UCDs, located in lower-density environments than M87, extend to younger ages and smaller [α/Fe] (at given metallicities) than M87 UCDs, resembling the environmental dependence of the Virgo dE nuclei. The UCDs exhibit a positive mass-metallicity relation (MZR), which flattens and connects compact ellipticals at stellar masses 10 8 M ⊙ . The Virgo dE nuclei largely follow the average MZR of UCDs, whereas most of the M87 GCs are offset towards higher metallicities for given stellar masses. The difference between the mass-metallicity distributions of UCDs and GCs may be qualitatively understood as a result of their different physical sizes at birth in a self-enrichment scenario or of galactic nuclear cluster star formation efficiency being relatively low in a tidal stripping scenario for UCD formation. The existing observations provide the necessary but not sufficient evidence for tidally stripped dE nuclei being the dominant contributors to the M87 UCDs.

Keck Cosmic Web Imager (KCWI) spectra of globular clusters and ultracompact dwarfs in the halo of M87

“…8 we show [α/Fe] versus total metallicity. The M87 UCDs and GCs, from our work and from Zhang et al (2018), tend to have super-solar alpha ratios indicative of rapid star formation. These ratios also cover a similar range to those of Virgo dE nuclei (although the dE nuclei have a lower mean value).…”

“…The alpha-element metallicity relation for a range of objects. The plot shows Virgo dE nuclei, UCDs and GCs (Zhang et al 2018) and Coma cluster ultra diffuse galaxies (Ferré-Mateu et al 2018), along with the 3 UCDs and 3 GCs from this study. The well studied M87 UCD S999 from Janz et al (2015) is also shown.…”

“…In Fig. 7 we summarise the stellar mass-metallicity relation for our M87 UCDs and GCs compared to UCDs and GCs from the combined study of Zhang et al (2018). We also highlight the detailed study by Janz et al (2015) of the M87 inner halo UCD S999 for which they measured an age of 7.6 Gyr, [Z/H] = -0.95, [α/Fe] = +0.34.…”

“…From their dE sample, Paudel et al (2010) noticed significant age differences in the stellar populations of dEs located in high and low density regions. Zhang et al (2018) combined a Lick line index analysis of their new data with that from the literature including Cohen et al (1998) and Paudel et al (2010) mentioned above, along with Evstigneeva et al (2007), Firth et al (2007), Francis et al (2012) and Janz et al (2016). This gave them a total sample of 40 UCDs (defined to have R e > 10 pc) and 118 GCs around M87 with spectra.…”

“…Literature samples of dE nuclei, UCDs and GCs are shown, along with the 3 UCDs and 3 GCs from this study. Individual M87 UCDs and GCs fromZhang et al (2018) are shown along with mean values (larger symbols with error bars). The well studied M87 UCD S999 from Janz et al (2015) is also shown.…”