The Next Generation Fornax Survey (NGFS). IV. Mass and Age Bimodality of Nuclear Clusters in the Fornax Core Region

Abstract

We present the analysis of 61 nucleated dwarf galaxies in the central regions ( R vir /4) of the Fornax galaxy cluster. The galaxies and their nuclei are studied as part of the Next Generation Fornax Survey (NGFS) using optical imaging obtained with the Dark Energy Camera (DECam) mounted at Blanco/CTIO and near-infrared data obtained with VIRCam at VISTA/ESO. We decompose the nucleated dwarfs in nucleus and spheroid, after subtracting the surface brightness profile of the spheroid component and studying the nucleus using PSF photometry. In general, nuclei are consistent with colors of confirmed metal-poor globular clusters, but with significantly smaller dispersion than other confirmed compact stellar systems in Fornax. We find a bimodal nucleus mass distribution with peaks located at log(M * /M ) 5.4 and ∼ 6.3. These two nucleus sub-populations have different stellar population properties, the more massive nuclei are older than ∼ 2 Gyr and have metal-poor stellar populations (Z ≤ 0.02 Z ), while the less massive nuclei are younger than ∼ 2 Gyr with metallicities in the range 0.02 < Z/Z ≤ 1. We find that the nucleus mass (M nuc ) vs. galaxy mass (M gal ) relation becomes shallower for less massive galaxies starting around 10 8 M and the mass ratio η n = M nuc /M gal shows a clear anti-correlation with M gal for the lowest masses, reaching 10%. We test current theoretical models of nuclear cluster formation and find that they cannot fully reproduce the observed trends. A likely mixture of in-situ star formation and star-cluster mergers seem to be acting during nucleus growth over cosmic time.

Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters

“…Both in KKs58 and KK197, we found metal-poor NSCs with metallicities comparable to metal-poor GCs. This is in agreement with a study of 61 NSCs of dwarf galaxies in the Fornax galaxy by Ordenes-Briceño et al (2018) that found that most most dwarf nuclei have colours consistent with metal-poor GCs. They also report a bimodal mass distribution of dwarf galaxy NSCs with peaks at log(M * /M ) ≈ 5.4 and 6.3.…”

“…These values use rough estimates of the host stellar mass, based on integrated magnitudes and a fixed mass-to-light ratio. These ratios are within the scatter for galaxies of similar mass (Ordenes-Briceño et al 2018;Sánchez-Janssen et al 2019a). While the NSC-to-host mass relation appears to be independent of environment (Sánchez-Janssen et al 2019a), the mass ratio itself shows a trend with host stellar mass.…”

“…Nucleated dwarf galaxies have been studied in various environments, from dense galaxy clusters such as Virgo, Coma or Fornax to less populated groups (e.g. Georgiev et al 2009b;Georgiev & Böker 2014;den Brok et al 2014;Ordenes-Briceño et al 2018). In this work, we report on two nucleated dwarf elliptical galaxies that are confirmed members of the Centaurus group (Müller et al 2019).…”

“…Dwarf ellipticals can be further subdivided into nucleated and non-nucleated depending on the presence of a nuclear star cluster (NSC) in or near the photometric centre of the dwarf galaxy (e.g. Ordenes-Briceño et al 2018). However, bright star clusters at central positions have also been found in dwarf irregulars (e.g.…”

“…In reality, most likely a mixture of both processes occurs (Da Rocha et al 2011;Spengler et al 2017;Ordenes-Briceño et al 2018;Sills et al 2019), but the relative contributions of both formation channels might depend on the host's mass. Studies of early-type galaxies in the Fornax and Virgo galaxy clusters have suggested that the in-situ formation might be the dominant channel of NSC formation in massive galaxies (> 10 9 M ), while the GC accretion is more efficient in low mass galaxies because of shorter dynamical friction timescales (e.g.…”