A Supermassive Black Hole in an Ultracompact Dwarf Galaxy (1409.4769v2)

Abstract: Ultracompact dwarf galaxies (UCDs) are among the densest stellar systems in the universe. These systems have masses up to 200 million solar masses, but half light radii of just 3-50 parsecs. Dynamical mass estimates show that many UCDs are more massive than expected from their luminosity. It remains unclear whether these high dynamical mass estimates are due to the presence of supermassive black holes or result from a non-standard stellar initial mass function that causes the average stellar mass to be higher than expected. Here we present the detection of a supermassive black hole in a massive UCD. Adaptive optics kinematic data of M60-UCD1 show a central velocity dispersion peak above 100 km/s and modest rotation. Dynamical modeling of these data reveals the presence of a supermassive black hole with mass of 21 million solar masses. This is 15% of the object's total mass. The high black hole mass and mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our analysis also shows that M60-UCD1's stellar mass is consistent with its luminosity, implying many other UCDs may also host supermassive black holes. This suggests a substantial population of previously unnoticed supermassive black holes.

Supermassive Black Hole in an Ultracompact Dwarf Galaxy: Insights and Implications

The paper presents an intriguing investigation into the presence of a supermassive black hole (BH) within the ultracompact dwarf galaxy (UCD) M60-UCD1. The findings leverage high-resolution adaptive optics integral field spectroscopic data to unravel the complexities of this astronomical phenomenon. This paper enriches the understanding of UCDs and the dynamics of black holes within these dense stellar systems.

Context and Findings

Ultracompact dwarf galaxies are among the universe's densest stellar systems, with significant implications for galactic evolution and black hole formation theories. With masses reaching up to 200 million solar masses but exceptionally small half-light radii of just 3-50 parsecs, these systems pose intriguing questions about their formation and composition. Dynamical mass estimates of UCDs often exceed those predicted by their luminosity, suggesting the influence of factors like supermassive black holes or non-standard stellar populations.

The core finding of this research is the detection of a supermassive black hole in M60-UCD1, the brightest known UCD. Through adaptive optics kinematic data, the researchers identified a central velocity dispersion exceeding 100 km/s, indicative of the gravitational influence of a supermassive black hole. Dynamical modeling estimates the black hole's mass at approximately 21 million solar masses, constituting 15% of the total mass of M60-UCD1. This discovery is significant, as it confirms that M60-UCD1 is likely the stripped nucleus of a once larger galaxy.

Methodology

The paper utilized integral field spectroscopic observations with Gemini/NIFS, facilitating a high-resolution examination of stellar kinematics through the CO absorption bandheads. The researchers constructed self-consistent dynamical models using the Schwarzschild method, integrating kinematic data with Hubble Space Telescope imaging to tightly constrain the mass-to-light ratio and black hole mass. The meticulous modeling ruled out a constant mass-to-light ratio model without a central black hole with high confidence (>99.99%).

Implications

The findings surface several implications for the paper of UCDs and black holes:

1. Stripped Nuclei Hypothesis: The significant black hole mass fraction supports the hypothesis that UCDs, exemplified by M60-UCD1, are remnants of tidally stripped galactic nuclei rather than merely large globular clusters.
2. Prevalence of Supermassive Black Holes: The research suggests that a substantial number of UCDs may host supermassive black holes, potentially doubling the known population of such black holes in the local universe. This alters the understanding of black hole distribution and frequency, invoking a revision of models regarding galaxy and black hole formation history.
3. Mass-to-Light Ratio Insights: The paper offers insights into the interpretation of mass-to-light ratios in UCDs, discouraging the possibility of alternative explanations such as variations in the stellar initial mass function without invoking central black holes.
4. Dark Matter Content: The high density of baryonic matter in UCDs like M60-UCD1 means that dark matter is unlikely to be a significant factor in their central dynamics, as dissipationless dark matter cannot reach the central densities achieved by baryonic processes.

Future Prospectives

The research provides a foundation for future observational campaigns and theoretical models. Ongoing efforts aim to detect similar black holes in other UCDs and massive local star clusters. Although the dynamical detection of black holes may be challenging in fainter and distant objects, alternative approaches like identifying accretion signatures or tidal disruption events might offer new avenues for exploration.

In summary, this paper advances the comprehension of UCDs and their supermassive black holes, contributing significantly to the understanding of these complex systems within a broader cosmic context. It urges a renewed investigation into the processes of galaxy evolution and the dynamics of black holes in dense stellar environments.