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- ItemThe dwarf galaxy satellite system of Centaurus A(Les Ulis : EDP Sciences, 2019) Müller, Oliver; Rejkuba, Marina; Pawlowski, Marcel S.; Ibata, Rodrigo; Lelli, Federico; Hilker, Michael; Jerjen, HelmutDwarf galaxy satellite systems are essential probes to test models of structure formation, making it necessary to establish a census of dwarf galaxies outside of our own Local Group. We present deep FORS2 VI band images from the ESO Very Large Telescope (VLT) for 15 dwarf galaxy candidates in the Centaurus group of galaxies. We confirm nine dwarfs to be members of Cen A by measuring their distances using a Bayesian approach to determine the tip of the red giant branch luminosity. We have also fit theoretical isochrones to measure their mean metallicities. The properties of the new dwarfs are similar to those in the Local Group in terms of their sizes, luminosities, and mean metallicities. Within our photometric precision, there is no evidence of a metallicity spread, but we do observe possible extended star formation in several galaxies, as evidenced by a population of asymptotic giant branch stars brighter than the red giant branch tip. The new dwarfs do not show any signs of tidal disruption. Together with the recently reported dwarf galaxies by the complementary PISCeS survey, we study the luminosity function and 3D structure of the group. By comparing the observed luminosity function to the high-resolution cosmological simulation IllustrisTNG, we find agreement within a 90% confidence interval. However, Cen A seems to be missing its brightest satellites and has an overabundance of the faintest dwarfs in comparison to its simulated analogs. In terms of the overall 3D distribution of the observed satellites, we find that the whole structure is flattened along the line-of-sight, with a root-mean-square (rms) height of 130 kpc and an rms semi-major axis length of 330 kpc. Future distance measurements of the remaining dwarf galaxy candidates are needed to complete the census of dwarf galaxies in the Centaurus group.
- ItemVLT/FLAMES high-resolution chemical abundances in Sculptor: A textbook dwarf spheroidal galaxy(Les Ulis : EDP Sciences, 2019) Hill, Vanessa; Skúladóttir, Ása; Tolstoy, Eline; Venn, Kim A.; Shetrone, Matthew D.; Jablonka, Pascale; Primas, Francesca; Battaglia, Giuseppina; de Boer, Thomas J. L.; François, Patrick; Helmi, Amina; Kaufer, Andreas; Letarte, Bruno; Starkenburg, Else; Spite, MoniqueWe present detailed chemical abundances for 99 red-giant branch stars in the centre of the Sculptor dwarf spheroidal galaxy, which have been obtained from high-resolution VLT/FLAMES spectroscopy. The abundances of Li, Na, -elements (O, Mg, Si, Ca Ti), iron-peak elements (Sc, Cr, Fe, Co, Ni, Zn), and r- and s-process elements (Ba, La, Nd, Eu) were all derived using stellar atmosphere models and semi-automated analysis techniques. The iron abundances populate the whole metallicity distribution of the galaxy with the exception of the very low metallicity tail, 2:3 [Fe/H] 0:9. There is a marked decrease in [ /Fe] over our sample, from the Galactic halo plateau value at low [Fe/H] and then, after a “knee”, a decrease to sub-solar [ /Fe] at high [Fe/H]. This is consistent with products of core-collapse supernovae dominating at early times, followed by the onset of supernovae type Ia as early as 12 Gyr ago. The s-process products from low-mass AGB stars also participate in the chemical evolution of Sculptor on a timescale comparable to that of supernovae type Ia. However, the r-process is consistent with having no time delay relative to core-collapse supernovae, at least at the later stages of the chemical evolution in Sculptor. Using the simple and well-behaved chemical evolution of Sculptor, we further derive empirical constraints on the relative importance of massive stars and supernovae type Ia to the nucleosynthesis of individual iron-peak and -elements. The most important contribution of supernovae type Ia is to the iron-peak elements: Fe, Cr, and Mn. There is, however, also a modest but non-negligible contribution to both the heavier -elements: S, Ca and Ti, and some of the iron-peak elements: Sc and Co. We see only a very small or no contribution to O, Mg, Ni, and Zn from supernovae type Ia in Sculptor. The observed chemical abundances in Sculptor show no evidence of a significantly di erent initial mass function, compared to that of the Milky Way. With the exception of neutron-capture elements at low [Fe/H], the scatter around mean trends in Sculptor for [Fe=H] 2:3 is extremely low, and compatible with observational errors. Combined with the small scatter in the age-elemental abundances relation, this calls for an effcient mixing of metals in the gas in the centre of Sculptor since 12 Gyr ago.
- ItemThe coherent motion of Cen A dwarf satellite galaxies remains a challenge for ΛcDM cosmology(Les Ulis : EDP Sciences, 2021) Müller, Oliver; Pawlowski, Marcel S.; Lelli, Federico; Fahrion, Katja; Rejkuba, Marina; Hilker, Michael; Kanehisa, Jamie; Libeskind, Noam; Jerjen, HelmutThe plane-of-satellites problem is one of the most severe small-scale challenges for the standard Λ cold dark matter (ΛCDM) cosmological model: Several dwarf galaxies around the Milky Way and Andromeda co-orbit in thin, planar structures. A similar case has been identified around the nearby elliptical galaxy Centaurus A (Cen A). In this Letter, we study the satellite system of Cen A, adding twelve new galaxies with line-of-sight velocities from VLT/MUSE observations. We find that 21 out of 28 dwarf galaxies with measured velocities share a coherent motion. Similarly, flattened and coherently moving structures are found only in 0.2% of Cen A analogs in the Illustris-TNG100 cosmological simulation, independently of whether we use its dark-matter-only or hydrodynamical run. These analogs are not co-orbiting, and they arise only by chance projection, thus they are short-lived structures in such simulations. Our findings indicate that the observed co-rotating planes of satellites are a persistent challenge for ΛCDM, which is largely independent from baryon physics. © O. Müller et al. 2021.
- ItemIncorporating baryon-driven contraction of dark matter halos in rotation curve fits(Les Ulis : EDP Sciences, 2022) Li, Pengfei; McGaugh, Stacy S.; Lelli, Federico; Schombert, James M.; Pawlowski, Marcel S.The condensation of baryons within a dark matter (DM) halo during galaxy formation should result in some contraction of the halo as the combined system settles into equilibrium. We quantify this effect on the cuspy primordial halos predicted by DM-only simulations for the baryon distributions observed in the galaxies of the SPARC database. We find that the DM halos of high surface brightness galaxies (with Σeff 3; 100L pc-2 at 3.6 μm) experience strong contraction. Halos become more cuspy as a result of compression: the inner DM density slope increases with the baryonic surface mass density. We iteratively fit rotation curves to find the balance between initial halo parameters (constrained by abundance matching), compression, and stellar mass-to-light ratio. The resulting fits often require lower stellar masses than expected for stellar populations, particularly in galaxies with bulges: stellar mass must be reduced to make room for the DM it compresses. This trade off between dark and luminous mass is reminiscent of the cusp-core problem in dwarf galaxies, but occurs in more massive systems: the present-epoch DM halos cannot follow from cuspy primordial halos unless (1) the stellar mass-to-light ratios are systematically smaller than expected from standard stellar population synthesis models, and/or (2) there is a net outward mass redistribution from the initial cusp, even in massive galaxies widely considered to be immune from such effects.
- ItemStellar metallicity gradients of Local Group dwarf galaxies(Les Ulis : EDP Sciences, 2022) Taibi, S.; Battaglia, G.; Leaman, R.; Brooks, A.; Riggs, C.; Munshi, F.; Revaz, Y.; Jablonka, P.Aims. We explore correlations between the strength of metallicity gradients in Local Group dwarf galaxies and their stellar mass, star formation history timescales, and environment. Methods. We performed a homogeneous analysis of literature spectroscopic data of red giant stars and determined radial metallicity profiles for 30 Local Group dwarf galaxies. This is the largest compilation of this type to date. Results. The dwarf galaxies in our sample show a variety of metallicity profiles, most of them decreasing with radius and some with rather steep profiles. The derived metallicity gradients as a function of the half-light radius, [Fe/H](R/Re), show no statistical differences when compared with the morphological type of the galaxies, nor with their distance from the Milky Way or M31. No correlations are found with either stellar mass or star formation timescales. In particular, we do not find the linear relation between [Fe/H](R/Re) and the galaxy median age t50, which has been reported in the literature for a set of simulated systems. On the other hand, the high angular momentum in some of our galaxies does not seem to affect the gradient strengths. The strongest gradients in our sample are observed in systems that are likely to have experienced a past merger event. When these merger candidates are excluded, the analysed dwarf galaxies show mild gradients (ã-0.1 dex Re-1) with little scatter between them, regardless of their stellar mass, dynamical state, and their star formation history. These results agree well with different sets of simulations presented in the literature that were analysed using the same method as for the observed dwarf galaxies. Conclusions. The interplay between the multitude of factors that could drive the formation of metallicity gradients likely combine in complex ways to produce in general comparable mild [Fe/H](R/Re) values, regardless of stellar mass and star formation history. The strongest driver of steep gradients seems to be previous dwarf-dwarf merger events in a system.