[Paper Review] Looking into the faintEst WIth MUSE (LEWIS): on the nature of ultra-diffuse galaxies in the Hydra-I cluster.I. Project description and preliminary results

E. Iodice, M. Hilker+24 more|arXiv (Cornell University)|Aug 22, 2023

Astronomy and Astrophysical ResearchPhysics and Astronomy3 citations

TL;DR

This paper presents the LEWIS survey, an ESO Large Programme using MUSE to conduct the first homogeneous integral-field spectroscopic survey of 30 ultra-diffuse galaxies (UDGs) in the Hydra I cluster. For UDG11, the study reveals a low velocity dispersion (σ = 20 ± 8 km/s), old age (10 ± 1 Gyr), metal-poor stellar population ([M/H] = −1.17 ± 0.11 dex), and a high dynamical mass-to-light ratio (M/L_V ≈ 14), indicating a dark matter-dominated halo consistent with classical dwarf galaxies, suggesting external quenching during cluster accretion.

ABSTRACT

Looking into the faintEst WIth MUSE (LEWIS) is an ESO large observing programme aimed at obtaining the first homogeneous integral-field spectroscopic survey of 30 extremely low-surface brightness (LSB) galaxies in the Hydra I cluster of galaxies, with MUSE at ESO-VLT. The majority of LSB galaxies in the sample (22 in total) are ultra-diffuse galaxies (UDGs). The distribution of systemic velocities Vsys ranges between 2317 km/s and 5198 km/s and is centred on the mean velocity of Hydra I (Vsys = 3683 $\pm$ 46 km/s). Considering the mean velocity and the velocity dispersion of the cluster, 17 out of 20 targets are confirmed cluster members. To assess the quality of the data and demonstrate the feasibility of the science goals, we report the preliminary results obtained for one of the sample galaxies, UDG11. For this target, we derived the stellar kinematics, including the 2-dimensional maps of line-of-sight velocity and velocity dispersion, constrained age and metallicity, and studied the globular cluster (GC) population hosted by the UDG. Results are compared with the available measurements for UDGs and dwarf galaxies in literature. By fitting the stacked spectrum inside one effective radius, we find that UDG11 has a velocity dispersion $σ= 20 \pm 8$ km/s, it is old ($10\pm1$ Gyr), metal-poor ([M/H]=-1.17$\pm$0.11 dex) and has a total dynamical mass-to-light ratio M$/L_V\sim 14$, comparable to those observed for classical dwarf galaxies. The spatially resolved stellar kinematics maps suggest that UDG11 does not show a significant velocity gradient along either major or minor photometric axes. We find two GCs kinematically associated with UDG11. The estimated total number of GCs in UDG11, corrected for the spectroscopic completeness limit, is $N_{GC}= 5.9^{+2.2}_ {-1.8}$, which corresponds to a GC specific frequency of $S_N = 8.4^{+3.2}_{-2.7}$.

Motivation & Objective

• To conduct the first homogeneous integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in a galaxy cluster using MUSE.
• To determine the kinematic, stellar population, and globular cluster (GC) properties of UDGs in the Hydra I cluster.
• To assess whether UDGs form via internal processes or external environmental quenching by comparing their dynamical and structural properties to classical dwarfs.
• To establish a benchmark for UDG formation mechanisms by studying a nearly complete sample in a dynamically active cluster environment.

Proposed method

• Acquisition of deep, high-spectral-resolution integral-field spectroscopy with MUSE on the ESO Very Large Telescope (VLT) for 30 low-surface brightness galaxies in Hydra I.
• Reduction and calibration of MUSE data cubes using the MUSE Python Data Analysis Framework (MPDAF) and standard pipelines.
• Stellar kinematics derived via Voronoi binning with signal-to-noise ratio ≥10 to produce 2D maps of line-of-sight velocity and velocity dispersion.
• Spectral energy distribution (SED) fitting and stellar population synthesis modeling to estimate age, metallicity, and mass-to-light ratios from stacked spectra within one effective radius.
• Identification and kinematic association of globular clusters (GCs) via radial velocity measurements and completeness correction for spectroscopic detection limits.
• Comparison of derived dynamical masses and mass-to-light ratios with those of classical dwarf galaxies and other UDGs in the literature.

Experimental results

Research questions

• RQ1 What is the intrinsic kinematic structure of UDG11, as traced by its 2D stellar velocity and velocity dispersion maps?
• RQ2 What are the age, metallicity, and mass-to-light ratio of UDG11, and how do they compare to those of classical dwarf galaxies?
• RQ3 How many globular clusters does UDG11 host, and what is its specific frequency, indicating its formation history?
• RQ4 Is UDG11 a cluster member, and what does its radial velocity distribution suggest about its dynamical state and environmental history?
• RQ5 What do the dynamical and stellar population properties of UDG11 imply about the formation mechanism of ultra-diffuse galaxies?

Key findings

• The velocity dispersion of UDG11, measured from the stacked spectrum within one effective radius, is σ = 20 ± 8 km/s, consistent with a dynamically cold system.
• UDG11 is old, with an estimated age of 10 ± 1 Gyr, and metal-poor, with a metallicity of [M/H] = −1.17 ± 0.11 dex.
• The total dynamical mass-to-light ratio within one effective radius is M/L_V ≈ 14 M⊙/L⊙, comparable to that of classical dwarf galaxies.
• Spatially resolved kinematics show no significant velocity gradient along the major or minor photometric axes, with a mean velocity dispersion of ⟨σ⟩_e = 27 ± 8 km/s.
• Two globular clusters are kinematically associated with UDG11, and after completeness correction, the total number of GCs is estimated as N_GC = 5.9+2.2−1.8, yielding a specific frequency S_N = 8.4+3.2−2.7.
• The absence of emission lines in the MUSE cube indicates that UDG11 lacks ionized gas, supporting a quenched state, and its properties suggest it may have been quenched externally during cluster accretion.