Abstract
NGC 1052-DF2, an ultra-diffuse galaxy (UDG), has been the subject of intense debate. Its alleged absence of dark matter, and the brightness and number excess of its globular clusters (GCs) at an initially assumed distance of 20 Mpc suggest a new formation channel for UDGs. We present the first systematic spectroscopic analysis of the stellar body and the GCs in this galaxy (six previously known and one newly confirmed member) using MUSE at the VLT. Even though NGC 1052-DF2 does not show any spatially extended emission lines, we report the discovery of three planetary nebulae (PNe). We conduct full spectral fitting on the UDG and the stacked spectra of all the GCs. The UDG’s stellar population is old, 8.9 ± 1.5 Gyr; metal poor, [M/H] = −1.07 ± 0.12; and with little or no α-enrichment. The stacked spectrum of all GCs indicates a similar age of 8.9 ± 1.8 Gyr, but a lower metallicity of [M/H] = −1.63 ± 0.09 and a similarly low α-enrichment. There is no evidence for a variation in age and metallicity in the GC population with the available spectra. The significantly more metal-rich stellar body with respect to its associated GCs, the age of the population, its metallicity, and its α-enrichment are all in line with other dwarf galaxies. NGC 1052-DF2 thus falls on the same empirical mass–metallicity relation as other dwarfs for the full distance range assumed in the literature. We find that both debated distance estimates (13 and 20 Mpc) are similarly likely, given the three discovered PNe.
Highlights
Ultra-diffuse galaxies (UDGs) are a particular type of low surface brightness galaxies, defined as having central surface brightnesses of μg,0 > 24 mag arcsec−2, and sizes of Reff > 1.5 kpc
Paper I focuses on the kinematics of the UDG; this paper presents a stellar population analysis of this galaxy and its associated globular clusters (GCs)
We present the first simultaneous analysis of the stellar population of a UDG and its surrounding globular clusters
Summary
Ultra-diffuse galaxies (UDGs) are a particular type of low surface brightness galaxies, defined as having central surface brightnesses of μg,0 > 24 mag arcsec−2, and sizes of Reff > 1.5 kpc (van Dokkum et al 2015). Based on observations collected at the European Southern Observatory under ESO programs 2101.B-5008(A) and 2101.B-5053(A) To explain their high abundance in overdense regions such as the Coma cluster, van Dokkum et al (2015) proposed that UDGs may be hosted by massive, Milky Way-like, dark matter (DM) halos that could protect them from environmental effects. The high number of GCs around DF44 (∼100) would confirm the hypothesis of it being hosted by a very massive DM halo, along with a few other UDGs with a GC excess, but most UDGs have GC systems typical of dwarf galaxy DM halos (Beasley & Trujillo 2016; Amorisco et al 2018; Lim et al 2018) This is in line with a stacked weak-lensing study performed by Sifón et al (2018), showing that not all UDGs can have halo masses similar to those.
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