The properties of dwarf spheroidal galaxies in the Cen A group

Abstract

Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at ∼50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315-45 is an ultradiffuse galaxy with an effective radius of ∼2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics (MOND). Surprisingly, in the dwarf KK 203 we find an extended Hα ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30-300 Myr. The Hα emission is most likely linked to a ∼40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out.

The coherent motion of Cen A dwarf satellite galaxies remains a challenge for ΛCDM cosmology

“…For the Cen A system, it has been suggested based on 16 lineof-sight velocities (Müller et al 2018b) that most dwarf galaxies are aligned in a co-rotating structure similar to what has been found for the MW and the Andromeda galaxy. We revis-ited this issue by adding 12 more satellites of Cen A, for which we acquired line-of-sight velocities using MUSE spectroscopic observations (Müller et al 2021). This increases the sample size from 16 to 28 satellites and enhances the representation of gaspoor dwarf spheroidal galaxies.…”

“…To date, 27 dwarf satellites of Cen A have both accurate distances and velocity measurements. The line-of-sight velocities were measured using the following two different techniques: emission lines from atomic and/or ionized gas for star forming galaxies (e.g., Koribalski et al 2004) or absorption line spectroscopy of the stars (e.g., Puzia & Sharina 2008;Müller et al 2021). The distances come from tip of the red giant branch (TRGB) measurements based on HST programs (Karachentsev et al 2007) and our VLT program (Müller et al 2019).…”

“…The simulated host halos are required to have a On-sky distribution of the Cen A satellite system within 800 kpc. The circles correspond to the dwarf galaxies studied in Müller et al (2018b), the stars to the newly observed dwarfs (Müller et al 2021). The colors indicate whether the galaxies are redshifted or blueshifted with respect to the systemic velocity of Cen A (shown with a large open circle).…”

“…In cosmological simulations, satellite systems with a similar degree of kinematic coherence and flattening were found in only ≤0.5% of Cen A analogs, indicating a similar degree of conflict as in earlier studies of the MW and Andromeda. In Müller et al (2021), we present spectroscopy taken with the Multi Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope (VLT) of 12 additional dwarf galaxies around Cen A. Here we use them to test our previous assessment of a co-rotating plane-ofsatellites around Cen A.…”

Dwarfs from the Dark (Energy Survey): a machine learning approach to classify dwarf galaxies from multi-band images

“…E-mail: oliver.muller@astro.unistra.fr by confusion with background galaxies, foreground galactic cirrus, or instrumental noise (Chiboucas et al 2013;Merritt et al 2016;Müller et al 2018Müller et al , 2019Bennet et al 2019). A prime example is the galaxy Cen 8/KK 198, which has been regarded as dwarf galaxy associated with the Centaurus group for more than two decades (Cote 1996;Jerjen et al 2000;Karachentsev et al 2013;Müller et al 2017), until VLT observations have uncovered it as a low-surface brightness spiral galaxy (Müller et al 2019(Müller et al , 2021. Once high-resolution imaging was available to study the morphology of the galaxy in detail, the spiral pattern in the older imaging becomes quite apparent, in other words, this spiral galaxy could have been spotted as an interloper in the dwarf galaxy catalogs before the costly follow-up observations.…”