A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy

Abstract: We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1. We have obtained velocity measurements for 98.2% of the stars within 67 pc (10 arcmin, or 2.3 half-light radii) of the center of Segue 1 that have colors and magnitudes consistent with membership, down to a magnitude limit of r=21.7. Based on photometric, kinematic, and metallicity information, we identify 71 stars as probable Segue 1 members, including some as far out as 87 pc. After correcting for the influence of binary stars using repeated velocity measurements, we determine a velocity dispersion of 3.7^{+1.4}_{-1.1} km/s, with a corresponding mass within the half-light radius of 5.8^{+8.2}_{-3.1} x 10^5 Msun. The stellar kinematics of Segue 1 require very high mass-to-light ratios unless the system is far from dynamical equilibrium, even if the period distribution of unresolved binary stars is skewed toward implausibly short periods. With a total luminosity less than that of a single bright red giant and a V-band mass-to-light ratio of 3400 Msun/Lsun, Segue 1 is the darkest galaxy currently known. We critically re-examine recent claims that Segue 1 is a tidally disrupting star cluster and that kinematic samples are contaminated by the Sagittarius stream. The extremely low metallicities ([Fe/H] < -3) of two Segue 1 stars and the large metallicity spread among the members demonstrate conclusively that Segue 1 is a dwarf galaxy, and we find no evidence in favor of tidal effects. We also show that contamination by the Sagittarius stream has been overestimated. Segue 1 has the highest measured dark matter density of any known galaxy and will therefore be a prime testing ground for dark matter physics and galaxy formation on small scales.

• The paper presents a nearly complete spectroscopic survey within 67 pc, identifying 71 probable members for a robust dynamic and chemical analysis of Segue 1.
• The paper determines an intrinsic velocity dispersion of 3.7 km/s, leading to a mass-to-light ratio of ~3400 M☉/L☉ that underscores its dark matter-dominated nature.
• The paper reveals a wide metallicity spread (mean [Fe/H] = -2.5 over nearly 2 dex) and finds no evidence of tidal disruption, confirming Segue 1 as an ultra-faint dwarf galaxy.

Spectroscopic Survey of the Milky Way Satellite Segue 1: Analysis of an Ultra-Faint Dwarf Galaxy

The study presented by Simon et al. focuses on a detailed Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1. The research aims to elucidate the nature of Segue 1, particularly testing its classification as the darkest known galaxy given its high mass-to-light ratio. The principal outcomes of this investigation are derived from measuring a comprehensive set of stellar velocities and metallicities within Segue 1, allowing the researchers to assess its dynamic and chemical profile.

Key Findings

1. Sample Completeness and Membership: Researchers achieved a 98.2% velocity measurement for stars within 67 pc of Segue 1, identifying 71 as probable members based on photometric, kinematic, and metallicity criteria. This nearly complete sample facilitates a robust analysis of the galaxy’s properties.
2. Stellar Kinematics: The intrinsic velocity dispersion was determined to be 3.7^{+1.4}_{-1.1}~\kms. This finding implies a high mass-to-light ratio of approximately 3400~M$_{\odot}$/L$_{\odot}$, reinforcing the assertion that Segue 1 is a dark matter-dominated galaxy. This high ratio remains consistent even after correcting for possible binary star effects, which are statistically accounted for using repeated velocity measurements.
3. Mass and Dark Matter Density: Within its half-light radius, Segue 1's mass is estimated at 5.8^{+8.2}_{-3.1} \times 10^5 M$_{\odot}$. This corresponds to the highest estimated dark matter density of any known galaxy, indicating that Segue 1 presents an important case for studying dark matter physics.
4. Metallicity Dispersion: The metallicity analysis identified a significant spread, with mean [Fe/H] = -2.5 and variations across nearly 2 dex, alongside the presence of extremely metal-poor stars ([Fe/H] < -3). Such a metallicity range is characteristic of dwarf galaxies rather than star clusters, supporting the reclassification of Segue 1.
5. Tidal Interaction Analysis: The researchers reevaluated claims that Segue 1 is undergoing tidal disruption from the Milky Way. They found no compelling evidence for significant tidal disturbances or contamination from the Sagittarius stream, thus reinforcing the conclusion that Segue 1 is dynamically stable and dark matter-dominated.

Implications and Future Directions

The detailed spectroscopic work on Segue 1 informs both practical and theoretical frameworks within astrophysics, notably:

• Galactic Evolution and Formation: The variations and extreme properties of Segue 1 provide valuable insights into the limits of galaxy formation and evolution processes, particularly in dark matter contexts.
• Constraints on Dark Matter Models: With its high density, Segue 1 remains a critical target for indirect detection methods probing dark matter particle properties and annihilation signatures.
• Astrophysical Anomalies and Binary Star Effects: The study underscores the significance of accounting for binary star populations when interpreting velocity dispersions in dwarf galaxies, promoting the development of more sophisticated, parameter-diverse analysis frameworks.

In conclusion, the extensive spectroscopic survey supports the hypothesis that Segue 1 is an ultra-faint dwarf galaxy with significant dark matter predominance. This conclusion positions Segue 1 as a pivotal subject for continued dark matter research and for refining models of galaxy formation in low-density environments. Future research could benefit from expanded spectroscopic studies and indirect detection efforts to further elucidate the properties and implications of such dark matter-rich systems.