Leo I: The classical dwarf spheroidal galaxy with the highest dark matter density

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia ‘Augusto Righi’, Alma Mater Studiorum – Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
³ Department of Astronomy, Indiana University, Swain West, 727 E. 3rd Street, IN 47405 Bloomington, USA

^★ Corresponding author: raffaele.pascale@inaf.it

Received: 2 April 2025
Accepted: 16 June 2025

Abstract

Dwarf spheroidal galaxies (dSphs) are known for being strongly dark matter (DM) dominated, which makes them convenient targets for investigating the DM nature and distribution. Recently, renewed interest in the dSph Leo I has resulted from claims suggesting the presence of a central supermassive black hole, with mass estimates that challenge the typical expectations for dSphs, which are generally thought to host intermediate-mass black holes (IMBHs). However, a recent study presented new upper limits on the black hole mass that are consistent with the expected range for IMBHs, solving the concerns raised in the literature. Building on the analysis of this study, we examine in this follow-up work the DM properties of Leo I that we inferred from the most recent dynamical models presented by the authors. Our results indicate that the galaxy Leo I has the highest DM density among the classical dSphs, with a central DM density (measured at a distance of 150 pc from the galaxy center) ρ150 = 35.5_−4.7^+3.8 × 10⁷M_⊙kpc⁻³. According to our model, the DM density profile has a logarithmic slope γ150 = −0.89_−0.17^+0.21 at 150 pc, which agrees with the literature values. At smaller distances, the DM distribution flattens into a core of constant density, with a core radius of r_c = 72₋₃₂⁺⁴⁰ pc. Combined with the small pericentric distance of the Leo I orbit in the Milky Way, the new estimate of ρ₁₅₀ makes Leo I decisive in the study of the anticorrelation between pericenter and central DM density. It also suggests that the anticorrelation might be significantly steeper and more pronounced than previously estimated. Finally, despite its DM dominance, Leo I does not emerge as the most favorable target for an indirect DM detection: The inferred DM decay D and annihilation J factors, log D(0.5°)[ GeV cm ²] = 17.94_−0.25^+0.17 and log J(0.5°)[GeV²cm⁻⁵] = 18.13_−0.18^+0.17, are consistent with previous estimates and lower than the highest values measured in dSphs.