[Paper Review] The ultra-diffuse galaxy NGC 1052-DF2 with MUSE: I. Kinematics of the stellar body

Éric Emsellem, R. F. J. van der Burg+15 more|arXiv (Cornell University)|Dec 18, 2018

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy83 references41 citations

TL;DR

This paper presents the first spectroscopic analysis of the stellar body of DF2 using MUSE, deriving its systemic velocity, a weak prolate-like rotation, and a low stellar velocity dispersion with implications for its mass/dark matter content.

ABSTRACT

The so-called ultra-diffuse galaxy NGC~1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2's stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9$^{-1.8}_{+1.4}$~\kms, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major-axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of $σ_{\mathrm{int}}=10.6^{+3.9}_{-2.3}$~\kms. The velocity dispersion $σ_{ m{DF2}\star}$( e) for the stellar body within one effective radius is $10.8^{-4.0}_{+3.2}$~\kms. Considering various sources of systemic uncertainties this central value varies between 5 and 13~\kms, and we conservatively report a 95\% confidence upper limit to the dispersion within one e\ of 21~\kms. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC~1052-DF2 that have been reported in the recent literature.

Motivation & Objective

• Motivate the study by the DF2 dark matter debate and the need to characterize the stellar body kinematics.
• Provide a robust measurement of the DF2 systemic velocity and velocity field using MUSE data.
• Characterize the velocity dispersion of the stellar body and discrete tracers (globular clusters and planetary nebulae).
• Assess rotation/kinematic structure and compare to GC/PN kinematics to infer dynamical mass implications.
• Offer updated mass estimates for DF2 under different distance assumptions discussed in the literature.

Proposed method

• Obtain integral-field spectroscopy of DF2 with VLT/MUSE across ~5.1 hours of on-target time.
• Reduce data with the MUSE esorex pipeline and calibrate wavelength using sky lines, with 2 km/s uniformity; perform sky subtraction with Zurich Atmosphere Purge (ZAP).
• Extract spectra for DF2, GCs, and PNe using PSF-matched apertures and Gaussian weighting; build a global spectrum and Voronoi-binned maps.
• Measure kinematics with pPXF using two stellar libraries (eMiles and Pegase-HR) to fit templates and derive velocities and dispersions, including bootstrap realisations for uncertainties.
• Account for wavelength-dependent spectral resolution (LSF) and test robustness against template choice, spectral range, and background handling.

Experimental results

Research questions

• RQ1 What is the systemic velocity of the DF2 stellar body and is it consistent with its GC system?
• RQ2 What is the velocity field and any rotation signal of DF2's stellar body, and does it resemble oblate or prolate kinematics?
• RQ3 What is the velocity dispersion of DF2 within one effective radius, and how does it compare when using different templates and spectral regions?
• RQ4 How do the velocities of associated GCs and discovered PNe compare to the galaxy's systemic velocity and kinematic trend?
• RQ5 What are the implications of the measured kinematics for the dynamical mass and dark matter content of DF2 under different distance assumptions?

Key findings

• The stellar body systemic velocity is 1792.9 km/s with uncertainties -1.8/+1.4 (stat) and -1.3/+0.2 (systematic).
• A weak but significant velocity gradient exists with a kinematic axis near the photometric major axis, indicating prolate-like rotation.
• The inferred velocity dispersion from clusters and PNe is σ_int = 10.6^{+3.9}_{-2.3} km/s.
• Within one effective radius, the stellar body dispersion is σ_DF2★(R_e) = 10.8^{+3.2}_{-4.0} km/s, with a 95% upper limit of 21 km/s considering systematics.
• Dispersion remains consistent across 0.5–1.5 R_e, with nominal values around 9–11 km/s depending on templates and spectral region.
• Updated mass estimates for DF2 are provided for different assumed distances (13 vs 20 Mpc) in light of the measured dispersions.