[Paper Review] The ultra-diffuse galaxy NGC 1052-DF2 with MUSE: II. The population of DF2: stars, clusters and planetary nebulae

Jérémy Fensch, R. F. J. van der Burg+14 more|arXiv (Cornell University)|Dec 18, 2018

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy95 references20 citations

TL;DR

This study presents the first simultaneous spectroscopic analysis of the stellar population and globular clusters (GCs) in the ultra-diffuse galaxy NGC 1052-DF2 using MUSE/VLT data. It finds an old, metal-poor stellar body ([M/H] = −1.07 ± 0.12) and similarly old, more metal-poor GCs ([M/H] = −1.63 ± 0.09), both consistent with dwarf galaxy scaling relations, and reports the discovery of three planetary nebulae, supporting a distance of 13–20 Mpc and challenging the original dark matter-deficient interpretation.

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 20Mpc, suggested a new formation channel for UDGs. We present the first systematic spectroscopic analysis of both the stellar body and the GCs (six of which were previously known, and one newly confirmed member) of this galaxy using MUSE@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 GCs. The UDG's stellar population is old, 8.9$\pm$1.5 Gyr, metal-poor, with [M/H] = $-$1.07$\pm$0.12 with little or no $α$-enrichment. The stacked spectrum of all GCs indicates a similar age of 8.9$\pm$1.8 Gyr, but lower metallicity, with [M/H] = $-$1.63$\pm$0.09, and similarly low $α$-enrichment. There is no evidence for a variation of 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 alpha 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.

Motivation & Objective

• To resolve the debate over NGC 1052-DF2's dark matter content by conducting a comprehensive spectroscopic analysis of its stellar and globular cluster populations.
• To determine the age, metallicity, and alpha-enhancement of the galaxy's stellar body and its globular clusters using high-resolution MUSE data.
• To assess the galaxy's distance and its consistency with empirical mass-metallicity relations for dwarf galaxies.
• To investigate the origin of DF2 by testing tidal dwarf galaxy and alternative formation hypotheses using stellar population and nebular data.
• To detect and characterize planetary nebulae in a UDG for the first time, using their luminosity and number as distance indicators.

Proposed method

• Spectral fitting of the integrated light of NGC 1052-DF2's stellar body using the pPXF fitting routine and the eMILES stellar library.
• Stacking of individual globular cluster spectra to improve signal-to-noise for population synthesis modeling.
• Measurement of Lick indices from the stacked GC spectrum to cross-validate age and metallicity estimates.
• Identification of planetary nebulae via emission-line detection in the MUSE data cube, using narrowband filters and spatial-spectral analysis.
• Distance estimation based on the number of detected planetary nebulae, calibrated against known luminosity-metallicity relations and PN luminosity function.
• Comparison of the galaxy's stellar mass and metallicity with the empirical mass-metallicity relation for dwarf galaxies to assess consistency with standard formation models.

Experimental results

Research questions

• RQ1 Is the stellar population of NGC 1052-DF2 consistent with that of typical dwarf galaxies, or does it show signs of extreme metal-poor or anomalous formation?
• RQ2 What is the age and metallicity of the globular cluster system in DF2, and how do they compare to the field star population?
• RQ3 Can the number of detected planetary nebulae constrain the distance to DF2, and is this consistent with the 13 Mpc and 20 Mpc estimates?
• RQ4 Does the observed metallicity and age distribution of the stellar populations support a tidal dwarf galaxy origin for DF2?
• RQ5 Is the lack of extended emission lines in the MUSE data consistent with the absence of significant ionized gas, and what does this imply for the galaxy's evolutionary state?

Key findings

• The stellar population of NGC 1052-DF2 is old, with a mean age of 8.9 ± 1.5 Gyr, and metal-poor, with [M/H] = −1.07 ± 0.12, showing little to no α-enhancement.
• The stacked globular cluster spectrum indicates a similar age of 8.9 ± 1.8 Gyr, but a significantly lower metallicity of [M/H] = −1.63 ± 0.09, with no evidence of age or metallicity spread in the GC system.
• The galaxy's stellar mass and metallicity place it firmly on the empirical mass-metallicity relation observed for dwarf galaxies, contradicting expectations for a dark matter-deficient system.
• The discovery of three planetary nebulae is consistent with the number expected for a galaxy of similar luminosity and metallicity, supporting a distance range of 13–20 Mpc.
• The observed population properties, including low α-abundance and metallicity, are consistent with a formation scenario involving pre-enriched gas, possibly from a tidal origin, though not ruled out by standard dwarf galaxy evolution.
• The data do not support significant stellar mass loss or stripping as a mechanism to reconcile DF2’s metallicity with the mass-metallicity relation under the dark matter-deficient hypothesis.