The "Dark-Matter Dominated" Galaxy Segue 1 Modeled with a Black Hole and no Dark Halo

Nathaniel Lujan, Karl Gebhardt, Richard Anantua, Owen Chase, Maya H. Debski, Claire Finley, Loraine V. Gomez, Om Gupta, Alex J. Lawson, Izabella Marron, Zorayda Martinez, Connor A. Painter, Yonatan Sklansky, Hayley West

First listed 2025-05-09 | Last updated 2025-05-13

Abstract

The dwarf spheroidal galaxy, Segue 1, is thought to have one of the largest ratios of dark matter to stellar mass. Using orbit-based dynamical models, we model Segue 1, including a dark halo and a central black hole. The best-fit model requires a black hole mass of $4 \pm 1.5 \times 10^5\ M_\odot$. The value of the black hole mass is the same with or without a dark halo. The mass-to-light ratio of the stars is poorly constrained by the dynamical modeling, reflecting that Segue 1 is dominated by mass other than stars. Dynamical models that exclude a black hole provide a worse fit and require a dark halo with very small scale radii of around 100 parsecs. Additionally, the zero black hole models require a stellar orbital distribution that is highly radially biased. The model with a black hole provides an orbital structure that is close to isotropic, more similar to other well-studied systems. We argue that the two-parameter models of stars and black hole provide a better description of Segue 1 than the three-parameter models of stars and two dark halo components. Additional support for a central black hole comes from a significant increase in the central rotation. Using individual velocities, we measure a rotation amplitude of $9.0 \pm 2.4\ \mathrm{km\ s^{-1}}$. Segue 1 is likely being tidally stripped at large radii, and we might be witnessing the remnant nucleus of a more massive system. Alternatively, given the high black hole mass relative to the stellar mass, Segue 1 is analogous to Little Red Dots seen in the early Universe.

Short digest

Orbit-based dynamical models of Segue 1 favor a central intermediate-mass black hole of 4 ± 1.5 × 10^5 M⊙, reproducing the stellar kinematics with a near-isotropic orbital structure. Zero–black-hole models both fit worse and require an extremely compact dark halo (scale radius ~100 pc) plus highly radial stellar orbits, while the BH model is also supported by a central rotation spike of 9.0 ± 2.4 km s−1. The tracer profile is built from number counts with tidal-stream subtraction and deprojection, and the stellar mass-to-light ratio remains poorly constrained, underscoring non-stellar mass dominance. Taken together, Segue 1 looks like a tidally stripped nuclear remnant and a nearby analog of Little Red Dots with overmassive black holes.

Key figures to inspect

• Figure 1: Inspect the number-count profile before/after tidal subtraction to see how the tracer density used for modeling is constructed and extrapolated inward from ~1′–9′ via a smoothing spline.
• Figure 2: Examine the rotation amplitude versus radius; the pronounced central rise (>99% significance) and lack of rotation beyond ~2′ bolster the case for a central massive object.
• Figure 3: Read the χ² trends against stellar M/L, BH mass, dark-halo circular velocity, and scale radius; note the preferred BH mass (~4×10^5 M⊙) and that no-BH fits push toward very small halo scale radii (~100 pc).
• Figure 4: Compare internal dispersion ratios for BH-included versus halo-only models; BH models remain near isotropic while halo-only demands strong radial anisotropy, highlighting the dynamical plausibility of the BH solution.