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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 Vsys ranges between 2317 km s−1 and 5198 km s−1 and is centred on the mean velocity of Hydra I (Vsys = 3683 ± 46 km s−1). Considering the mean velocity and the velocity dispersion of the cluster (σcluster ∼ 700 km s−1), 17 out of 20 targets are confirmed cluster members. The three objects with velocities of greater than 2σcluster 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/LV ∼ 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 NGC = 5.9−1.8+2.2, which corresponds to a GC-specific frequency of SN = 8.4−2.7+3.2.
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 Vsys ranges between 2317 km s−1 and 5198 km s−1 and is centred on the mean velocity of Hydra I (Vsys = 3683 ± 46 km s−1). Considering the mean velocity and the velocity dispersion of the cluster (σcluster ∼ 700 km s−1), 17 out of 20 targets are confirmed cluster members. The three objects with velocities of greater than 2σcluster 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/LV ∼ 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 NGC = 5.9−1.8+2.2, which corresponds to a GC-specific frequency of SN = 8.4−2.7+3.2.
Almost dark galaxies are objects that have eluded detection by traditional surveys such as the Sloan Digital Sky Survey (SDSS). The low surface brightness of these galaxies (μr(0) > 26 mag arcsec−2), and hence their low surface stellar mass density (a few solar masses per pc2 or less), suggest that the energy density released by baryonic feedback mechanisms is inefficient in modifying the distribution of the dark matter halos they inhabit. For this reason, almost dark galaxies are particularly promising for probing the microphysical nature of dark matter. In this paper, we present the serendipitous discovery of Nube, an almost dark galaxy with ⟨μV⟩e ∼ 26.7 mag arcsec−2. The galaxy was identified using deep optical imaging from the IAC Stripe82 Legacy Project. Follow-up observations with the 100 m Green Bank Telescope strongly suggest that the galaxy is at a distance of 107 Mpc. Ultra-deep multi-band observations with the 10.4 m Gran Telescopio Canarias favour an age of ∼10 Gyr and a metallicity of [Fe/H] ∼ −1.1. With a stellar mass of ∼4 × 108 M⊙ and a half-mass radius of Re = 6.9 kpc (corresponding to an effective surface density of ⟨Σ⟩e ∼ 0.9 M⊙ pc−2), Nube is the most massive and extended object of its kind discovered so far. The galaxy is ten times fainter and has an effective radius three times larger than typical ultradiffuse galaxies with similar stellar masses. Galaxies with comparable effective surface brightness within the Local Group have very low mass (tens of 105 M⊙) and compact structures (effective radius Re < 1 kpc). Current cosmological simulations within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. However, its highly extended and flattened structure is consistent with a scenario where the dark matter particles are ultralight axions with a mass of mB = (0.8−0.2+0.4) × 10−23 eV.
Dwarf galaxy abundances can serve as discernment tests for models of structure formation. Previous small-scale tensions between observations and dark matter-only cosmological simulations may have been resolved with the inclusion of baryonic processes; however, these successes have been largely concentrated on the Local Group dwarfs the feedback models were initially calibrated on. We investigate whether the Lambda CDM model can reliably reproduce dwarf abundances in the MATLAS low-to-moderate density fields that are centred upon early-type host galaxies beyond the Local Volume. We carried out mock observations of MATLAS-like fields with the high-resolution hydrodynamic simulation IllustrisTNG-50. We used matching selection criteria and compared the properties of dwarfs contained within them with their MATLAS analogues. Although simulated MATLAS-like dwarfs demonstrate photometric properties that are consistent with the observed galaxy population and follow the same scaling relations, TNG50 underestimates the number of dwarf galaxies in isolated MATLAS fields at the $6 level. This significance maintained within crowded fields containing more than a single bright host. Our $55-62<!PCT!>$ estimate of the fraction of background galaxies is in agreement with estimates by MATLAS, but is wholly insufficient to alleviate this discrepancy in dwarf abundances. Any incompleteness in the observed fields further exacerbates this tension. We identified a "too-many-satellites" problem in Lambda CDM, emphasising the need for the continued testing and refining of current models of galaxy formation in environments beyond the Local Group.
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