Dynamical analysis of the dark matter and central black hole mass in the dwarf spheroidal Leo I (2111.04770v1)

Abstract: We measure the central kinematics for the dwarf spheroidal galaxy Leo I using integrated-light measurements and previously published data. We find a steady rise in the velocity dispersion from $300^{\prime\prime}$ into the center. The integrated-light kinematics provide a velocity dispersion of $11.76\pm0.66$ km/s inside $75^{\prime\prime}$. After applying appropriate corrections to crowding in the central regions, we achieve consistent velocity dispersion values using velocities from individual stars. Crowding corrections need to be applied when targeting individual stars in high density stellar environments. From integrated light, we measure the surface brightness profile and find a shallow cusp towards the center. Axisymmetric, orbit-based models measure the stellar mass-to-light ratio, black hole mass and parameters for a dark matter halo. At large radii it is important to consider possible tidal effects from the Milky Way so we include a variety of assumptions regarding the tidal radius. For every set of assumptions, models require a central black hole consistent with a mass $(3.3 \pm 2) \times 10^6\, M_\odot$. The no-black-hole case for any of our assumptions is excluded at over 95% significance, with $6.4<\Delta\chi^2<14$. A black hole of this mass would have significant effect on dwarf galaxy formation and evolution. The dark halo parameters are heavily affected by the assumptions for the tidal radii, with the circular velocity only constrained to be above 30 km/s. Reasonable assumptions for the tidal radius result in stellar orbits consistent with an isotropic distribution in the velocities. These more realistic models only show strong constraints for the mass of the central black hole.

• The paper finds a central black hole of about 3.4 million solar masses, ruling out a no-black-hole scenario with high confidence.
• The analysis employs axisymmetric, orbit-based models of integrated-light and stellar velocities to constrain dark matter halo properties under tidal influences.
• The study reveals low stellar mass-to-light ratios, suggesting a minor luminous mass contribution compared to the dominant dark matter component.

Dynamical Analysis of Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I

This paper presents a comprehensive dynamical analysis of the dwarf spheroidal galaxy Leo I, focusing on the galaxy's dark matter (DM) content and the mass of its central black hole (BH). Utilizing integrated-light measurements complemented by existing datasets, the authors investigate the central kinematics and propose a non-negligible black hole mass that has significant implications for the understanding of dwarf galaxy dynamics and evolution.

Methodology

The paper employs integrated-light kinematic measurements combined with individual stellar velocities, duly corrected for effects such as crowding, which can bias measurements toward lower velocity dispersion in dense environments. A notable methodological strength of the paper is the application of an axisymmetric, orbit-based model that includes parameters for a dark matter halo alongside a stellar mass-to-light ratio (M/L) and a potential central black hole.

Findings

1. Central BH Mass: The models consistently require a central black hole, with its mass estimated around 3.4 million solar masses. This excludes a no-black-hole scenario with a highly significant confidence level, indicated by a chi-square difference of 6.4 to 13.8 compared to black hole-inclusive models.
2. Dark Matter Halo: The parameters for Leo I’s dark halo vary significantly depending on the assumptions made regarding tidal effects from the Milky Way. The analyses support a circular velocity above 30 km/s, but precise values differ widely due to the tidal radius variations considered in the models. The halo scale length remains poorly constrained unless stricter truncations or priors are applied based on typical dwarf spheroidal profiles.
3. Stellar Mass-to-Light Ratio: Estimated stellar M/L ratios are typically low, with upper limits around 2 to 5, suggesting minimal luminous mass contribution compared to ordinary expectations for such dwarf systems.

Implications and Future Research

The presence of a notable central black hole in a dwarf spheroidal like Leo I challenges typical BH-host galaxy scaling relations. This paper fosters implications regarding black hole formation and merger history in early universe dwarf systems. Moreover, the results regarding dark matter profiles suggest that Leo I, and potentially similar galaxies, exhibit more complexity in DM distribution than simplified models predict, opening discussions about dark halo shapes in the context of tidal interactions.

Speculation on AI and Broader Impacts

The methodology employed in deriving kinematics and constraints on mass parameters involves sophisticated modeling that could benefit from advances in AI, particularly machine learning algorithms capable of managing vast parameter spaces and simulating more realistic scenarios. The development of AI-driven analytical models could enhance precision and reduce computation time, yielding more robust results even in data-limited environments.

In conclusion, the paper makes a significant contribution to dwarf galaxy studies, particularly in advancing our understanding of central black holes' roles and the nature of dark halos under tidal influences. Its approach sets a precedent for future studies aiming to unravel the complexities of low-mass galaxies, providing a pivotal link between observed dynamics and theoretical models.