Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster: hints of UCD formation in low-density environments

Steffen Mieske,Gijs Verdoes Kleijn,Edwin A Valentijn,Joachim Janz,Michele Cantiello,Teymoor Saifollahi,Reynier F Peletier,Aku Venhola,Michael Hilker

doi:10.1093/mnras/stab1118

Abstract

ABSTRACT Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster 20 yr ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the virial radius of the galaxy cluster. Our search is complete down to magnitude mg = 21 mag or M$_g\, \sim$ −10.5 mag at the distance of the Fornax cluster. The UCD candidates are identified and separated from foreground stars and background galaxies by their optical and near-infrared colours. This primarily utilizes the u−i/i−Ks diagram and a machine learning technique is employed to incorporate other colour combinations to reduce the number of contaminants. The newly identified candidates (44) in addition to the spectroscopically confirmed UCDs (61), increase the number of known Fornax UCD considerably (105). Almost all of the new UCD candidates are located outside the Fornax cluster core (360 kpc), where all of the known UCDs were found. The distribution of UCDs within the Fornax cluster shows that a population of UCDs may form in low-density environments. This most likely challenges the current models of UCD formation.

Highlights

In the late 90s, through the spectroscopic surveys of the Fornax galaxy cluster, Hilker et al (1999) and Drinkwater et al (2000a) independently reported the detection of very compact objects at the redshift of the cluster, brighter than globular clusters (GCs) and fainter than compact dwarf galaxies
The known ultracompact dwarf galaxies (UCDs) were found through spectroscopic surveys around massive galaxies (Norris et al 2014) or in the cores of galaxy clusters/groups
Using the optical data from the Fornax Deep Survey (FDS; Iodice et al 2016; Venhola et al 2018) combined with the near-infrared observations of the Vista Hemisphere Survey (VHS) and ESO/VISTA archival data, we aim to identify the possible population of UCDs in the outskirts of the Fornax galaxy cluster within its virial radius (700 kpc Drinkwater, Gregg & Colless 2001)

Summary

Introduction

In the late 90s, through the spectroscopic surveys of the Fornax galaxy cluster, Hilker et al (1999) and Drinkwater et al (2000a) independently reported the detection of very compact objects at the redshift of the cluster, brighter than globular clusters (GCs) and fainter than compact dwarf galaxies. UCDs are larger, brighter, and more massive than typical GCs with typical half-light radii of 10 ≤ rh ≤ 100 pc, luminosities between −13.0 ≤ Mg ≤ −10.0 mag, and masses in a range from 2 × 106 to < 108 M (Mieske et al 2008; Misgeld & Hilker 2011) with predominantly old stellar populations and a wide range of metallicities (Firth, Evstigneeva & Drinkwater 2009; Janz et al 2016; Zhang et al 2018; Forbes et al 2020; Fahrion et al 2020a). An SMBH with a mass of 15 per cent on the average can explain the elevated Mdyn/M∗ (Mieske et al 2013)

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Discussion

Conclusion