The Next Generation Virgo Cluster Survey. XXXIV. Ultracompact Dwarf Galaxies in the Virgo Cluster

Chengze Liu,Joel Roediger,Sungsoon Lim,Puragra Guhathakurta,Jean-Charles Cuilandre,Youkyung Ko,John P Blakeslee,Laura Ferrarese,Eric W Peng,Patrick Côté,Chelsea Spengler,Xiaohu Yang,J Christopher Mihos,Mathieu Powalka,Elisa Toloba,Stephen Gwyn,Pierre–Alain Duc ,A Boselli ,Andrés Jordán ,Patrick R Durrell ,Y P Jing ,Ariane Lançon ,Thomas H Puzia ,Roberto P Muñoz ,Hong Xin Zhang ,A Longobardi ,R Sánchéz-Janssen ,S Mei ,Karla Álamo-Martínez

doi:10.3847/1538-4365/abad91

Abstract

Abstract We present a study of ultracompact dwarf (UCD) galaxies in the Virgo cluster based mainly on imaging from the Next Generation Virgo Cluster Survey (NGVS). Using ∼100 deg2 of u*giz imaging, we have identified more than 600 candidate UCDs, from the core of Virgo out to its virial radius. Candidates have been selected through a combination of magnitudes, ellipticities, colors, surface brightnesses, half-light radii, and, when available, radial velocities. Candidates were also visually validated from deep NGVS images. Subsamples of varying completeness and purity have been defined to explore the properties of UCDs and compare to those of globular clusters and the nuclei of dwarf galaxies with the aim of delineating the nature and origins of UCDs. From a surface density map, we find the UCDs to be mostly concentrated within Virgo’s main subclusters, around its brightest galaxies. We identify several subsamples of UCDs—i.e., the brightest, largest, and those with the most pronounced and/or asymmetric envelopes—that could hold clues to the origin of UCDs and possible evolutionary links with dwarf nuclei. We find some evidence for such a connection from the existence of diffuse envelopes around some UCDs and comparisons of radial distributions of UCDs and nucleated galaxies within the cluster.

Highlights

Two decades ago, investigators reported the discovery of a potentially new class of stellar system in the Fornax cluster (Hilker et al 1999; Drinkwater et al 2000; Phillipps et al 2001)
Note that only objects detected in our short exposures lack u∗- and iband data, and so are drawn from the bright end of the ultra compact dwarf (UCD) luminosity function
We find about half of the 22 UCD candidates with HST imaging are embedded in diffuse envelopes that are visible in the space-based images

Summary

Introduction

Investigators reported the discovery of a potentially new class of stellar system in the Fornax cluster (Hilker et al 1999; Drinkwater et al 2000; Phillipps et al 2001) These systems appeared to bridge the gap between normal globular clusters (GCs) and early-type galaxies (including the subset of compact elliptical galaxies), and so were named as ultra-compact dwarf galaxies (UCDs). Color-selected UCD candidates on the u∗gz diagram (Equation 4) for those objects having u∗-band data; otherwise, we consider the (g−z) color alone (Equation 6) These selection criteria dramatically reduce our original photometric sample from 346,948 objects to 779 candidate UCDs. With the assist of Gaia (see Figure 7), 11 of these candidates are re-classified as stars. To further reduce the number of contaminants, we place a cut on radial velocities (when available) and visually inspect the candidates (see Figure 8).

Objectives

Results

Discussion

Conclusion