Hinting a dark matter nature of Sgr A* via the S-stars

Abstract: The motion data of the S-stars around the Galactic center gathered in the last 28 yr imply that Sgr A* hosts a supermassive compact object of about $4\times 10^6$ $M\odot$, a result awarded with the Nobel Prize in Physics 2020. A non-rotating black hole (BH) nature of Sgr A* has been uncritically adopted since the S-star orbits agree with Schwarzschild geometry geodesics. The orbit of S2 has served as a test of General Relativity predictions such as the gravitational redshift and the relativistic precession. The central BH model is, however, challenged by the G2 post-peripassage motion and by the lack of observations on event-horizon-scale distances robustly pointing to its univocal presence. We have recently shown that the S2 and G2 astrometry data are better fitted by geodesics in the spacetime of a self-gravitating dark matter (DM) core - halo distribution of 56 keV-fermions, "darkinos", which also explains the outer halo Galactic rotation curves. This Letter confirms and extends this conclusion using the astrometry data of the $17$ best-resolved S-stars, thereby strengthening the alternative nature of Sgr A* as a dense core of darkinos.

• The paper demonstrates that the dark matter core model fits the observed S-star orbits slightly better than the standard black hole model, with reduced chi-squared values.
• It employs astrometric data from 17 S-stars, analyzing orbital parameters like semi-major axis, eccentricity, and inclination to compare the RAR dark matter model against General Relativity predictions.
• The findings challenge conventional views of Sgr A*, prompting reconsideration of galactic core dynamics and offering new insights into dark matter’s role in astrophysical processes.

Exploring the Dark Matter Nature of Sagittarius A* through S-Stars

This paper investigates the longstanding assumption that Sagittarius A* (Sgr A*), the supermassive compact object at the Galactic center, is a non-rotating black hole (BH). Traditionally, this assumption has been corroborated by the Keplerian orbits of the S-stars, particularly S2, which align with predictions made by the Schwarzschild geometry within the framework of General Relativity (GR). The authors here challenge this assumption by proposing an alternative explanation: that Sgr A* may instead be a dense core of dark matter (DM) particles, referred to as "darkinos," in a specific configuration known as the Ruffini-Argüelles-Rueda (RAR) model.

Methodology and Findings

The authors utilize astrometric data from 17 well-resolved S-stars to test two theoretical models: the traditional BH model and the RAR DM core model. The latter predicts a core-halo DM distribution, where the core is made up of quantum degenerate \SI{56}{\keV} fermions. The analysis reveals that both models can fit the observational data of the S-stars with similar effectiveness. However, in several instances, the DM core model demonstrates a slightly better fit, as indicated by reduced chi-squared ($\chi^2$) statistics.

• Key Numerical Findings:
  • The analysis examines numerous orbital parameters such as semi-major axis (a), eccentricity (e), and inclination (i).
  • The reduced chi-squared values for the RAR model averaged to 1.5741, compared to 1.6273 for the BH model, suggesting a marginal preference for the DM hypothesis.

Implications and Discussion

The implications of this research are significant, as they propose a non-standard interpretation of Sgr A*. If Sgr A* is a DM core rather than a BH, it challenges the GR predictions of S-star orbital dynamics. This alternative view of Sgr A* as a DM core not only aligns with the observed S-star dynamics but also provides an explanation for the Milky Way's rotation curves via the same darkino distribution.

The realization of a DM nature for Sgr A* carries broader astrophysical consequences. For instance, it raises new questions about the conditions under which such DM cores could collapse into supermassive black holes, potentially providing new insights into black hole formation, galactic evolution, and dark matter physics. Furthermore, this scenario fits within a framework where galactic DM profiles are derived from relativistic thermodynamic principles, as established by the RAR model.

Additionally, the authors propose future investigations into newly observed S-stars with highly compact orbits, which could further test the validity of the DM core hypothesis. As the quality and quantity of observational data improve, these tests will be crucial for constraining the size and nature of darkino cores.

Conclusion

The study presents a compelling argument for reevaluating the accepted black hole model of Sgr A*. By fitting the motion of S-stars with a RAR model of DM cores, the authors strengthen the case for considering alternative models of gravitational centers in the universe. This research prompts further inquiry into the intricate relationship between fermionic dark matter and galactic core dynamics, building a foundation for future explorations in dark matter astrophysics.