A new way to measure the distance to NGC1052-DF2^⋆

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea, E-38206 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Spain
³ Centre for Astrophysics and Supercomputing, Swinburne University, John Street, Hawthorn, VIC 3122, Australia
⁴ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria
⁵ European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁶ Department of Physics, Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193, 08193 Cerdanyola del Valles, Spain

^⋆⋆ Corresponding author: mikey.beasley@gmail.com

Received: 1 October 2024
Accepted: 18 February 2025

Abstract

We have employed a novel way to measure the distance to NGC1052-DF2, using the internal stellar velocity dispersions (σ) of its globular clusters (GCs). We obtained deep (15.1 h), R = 18 200, calcium triplet integrated-light spectra for 10 GCs in NGC1052-DF2 using FLAMES GIRAFFE on VLT. For five GCs, we measured the σ values, along with precision velocities for the whole sample. We also present a new photometric analysis based on 40 orbits of archival Hubble Space Telescope imaging for 16 spectroscopically confirmed GCs. Assuming that the NGC1052-DF2 GCs obey the M_V – log(σ) relation followed by the Milky Way and M31 GCs, the NGC1052-DF2 GCs indicate a distance, d = 16.2 ± 1.3 (stat.) ±1.7 (sys.) Mpc. In contrast, adopting a literature distance of d = 21.7 Mpc from forward modelling of the tip of the red giant branch (TRGB), the GCs lie above the Milky Way + M31 relation by ∼0.6 magnitudes. For a shorter literature distance of 13 Mpc, the GCs fall below the relation by ∼0.4 mag. At d = 16.2 Mpc, we obtain mean dynamical M/L_V = 1.61 ± 0.44 M_⊙/L_⊙ and median half-light radii of r_h = 3.0 ± 0.5 pc. This is entirely consistent with Milky Way GCs, with mean M/L_V = 1.77 ± 0.10 M_⊙/L_⊙ and median r_h = 3.2 ± 0.6 pc. For the further distance of 21.7 Mpc, we obtain systematically lower M/L_V ratios (M/L_V = 1.19 ± 0.33 M_⊙/L_⊙), which could suggest ages of ∼6 Gyr assuming canonical mass functions. However, such young ages are inconsistent with a MUSE stellar population analysis of the NGC1052-DF2 GCs, which indicates they are ∼10 Gyr old. For d = 16.2 Mpc, coupled with our new photometry, we find that the properties of the GCs in NGC1052-DF2 appear to be entirely consistent with those in the Milky Way and other Local Group galaxies. To reconcile the further distance with our results, a mass function that is more dwarf-depleted than the Milky Way GCs must be invoked for the GCs of NGC1052-DF2.