Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra-I cluster

I. Project description and preliminary results

¹ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
e-mail: enrichetta.iodice@inaf.it
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ University of Naples “Federico II”, C.U. Monte Sant’Angelo, Via Cinthia, 80126 Naples, Italy
⁴ INAF – Astronomical Observatory of Abruzzo, Via Maggini, 64100 Teramo, Italy
⁵ Gran Sasso Science Institute, viale Francesco Crispi 7, 67100 L’Aquila, Italy
⁶ Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, 20014 University of Turku, Finland
⁷ Tuorla Observatory, Department of Physics and Astronomy, Quantum, Vesilinnantie 5, 20014 University of Turku, Finland
⁸ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁹ Cambridge Astronomy Survey Unit (CASU), Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, John st., Hawthorn, VIC 3122, Australia
¹¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹³ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁴ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38200 La Laguna, Spain
¹⁵ European Space Agency (ESA), European Space Exploration and Research Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁶ Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, A.P. 3-72, México C.P. 58089, Mexico
¹⁷ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
¹⁸ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁹ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK

Received: 8 June 2023
Accepted: 10 August 2023

Abstract

Looking into the faintEst WIth MUSE (LEWIS) is an ESO large observing programme that aims 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). Data acquisition started in December 2021 and is expected to be concluded by March 2024. Up to June 2023, 29 targets were observed and the redshift has been derived for 20 of them. The distribution of systemic velocities V_sys ranges between 2317 km s⁻¹ and 5198 km s⁻¹ and is centred on the mean velocity of Hydra I (V_sys = 3683 ± 46 km s⁻¹). Considering the mean velocity and the velocity dispersion of the cluster (σ_cluster ∼ 700 km s⁻¹), 17 out of 20 targets are confirmed cluster members. The three objects with velocities of greater than 2σ_cluster away from the cluster mean velocity could be two background galaxies and one foreground galaxy. 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 (i) derived the stellar kinematics, including the two-dimensional maps of line-of-sight velocity and velocity dispersion, (ii) constrained age and metallicity, and (iii) studied the globular cluster (GC) population hosted by the UDG. Results are compared with the available measurements for UDGs and dwarf galaxies in the literature. By fitting the stacked spectrum inside one effective radius, we find that UDG11 has a velocity dispersion of σ = 20 ± 8 km s⁻¹ and is old (10 ± 1 Gyr), metal-poor ([M/H] = −1.17 ± 0.11 dex), and has a total dynamical mass-to-light ratio of M/L_V ∼ 14, which is 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 its major or minor photometric axis, and the average value of the velocity dispersion is ⟨σ⟩_e = 27 ± 8 km s⁻¹. 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_−1.8^+2.2, which corresponds to a GC-specific frequency of S_N = 8.4_−2.7^+3.2.