A 3.5-million Solar Masses Black Hole in the Centre of the Ultracompact Dwarf Galaxy Fornax UCD3 (1804.02938v1)

Abstract: The origin of ultracompact dwarfs (UCDs), a class of compact stellar systems discovered two decades ago, still remains a matter of debate. Recent discoveries of central supermassive black holes in UCDs likely inherited from their massive progenitor galaxies provide support for the tidal stripping hypothesis. At the same time, on statistical grounds, some massive UCDs might be representatives of the high luminosity tail of the globular cluster luminosity function. Here we present a detection of a $3.3^{+1.4}{-1.2}\times10^6\,M{\odot}$ black hole ($1\sigma$ uncertainty) in the centre of the UCD3 galaxy in the Fornax cluster, that corresponds to 4 per cent of its stellar mass. We performed isotropic Jeans dynamical modelling of UCD3 using internal kinematics derived from adaptive optics assisted observations with the SINFONI spectrograph and seeing limited data collected with the FLAMES spectrograph at the ESO VLT. We rule out the zero black hole mass at the $3\sigma$ confidence level when adopting a mass-to-light ratio inferred from stellar populations. This is the fourth supermassive black hole found in a UCD and the first one in the Fornax cluster. Similarly to other known UCDs that harbour black holes, UCD3 hosts metal rich stars enhanced in $\alpha$-elements that supports the tidal stripping of a massive progenitor as its likely formation scenario. We estimate that up to 80 per cent of luminous UCDs in galaxy clusters host central black holes. This fraction should be lower for UCDs in groups, because their progenitors are more likely to be dwarf galaxies, which do not tend to host central black holes.

• The paper confirms the detection of a ~3.3-million solar mass black hole at UCD3’s core, representing about 4% of its stellar mass and ruling out a zero mass scenario at the 3σ level.
• It employs isotropic Jeans dynamical modeling with adaptive optics and FLAMES spectrograph data to derive UCD3’s internal kinematics and velocity dispersion profiles.
• The findings support the tidal stripping theory by indicating UCD3 as the remnant nucleus of a formerly larger, metal-rich galaxy.

A Supermassive Black Hole in UCD3: Insights and Implications

The paper authored by Afanasiev et al. presents a comprehensive analysis aimed at understanding the enigmatic nature of ultracompact dwarf galaxies (UCDs), specifically targeting the detection of a supermassive black hole (SMBH) within the ultracompact dwarf galaxy Fornax UCD3. The detection of a $3.3^{+1.4}_{-1.2}\times10^6\,M_{\odot}$ black hole at the core of UCD3, constituting approximately 4% of its stellar mass, offers significant support for the hypothesis of tidal stripping in UCD formation.

Methodology and Observations

A core aspect of the paper involves utilizing isotropic Jeans dynamical modeling to determine the internal kinematics of UCD3. Leveraging adaptive optics with the SINFONI spectrograph and data from the FLAMES spectrograph at the VLT, the research team was able to ascertain velocity dispersion profiles indicative of the black hole’s presence. The data compellingly ruled out a zero black hole mass scenario at the $3\sigma$ confidence level, given mass-to-light (M/L) ratio estimations inferred from stellar populations.

Implications for UCD Formation

This detection marks the fourth confirmed instance of an SMBH within a UCD and represents the first such discovery in the Fornax cluster. The metal-rich stars with enhanced $\alpha$-elements in UCD3 further substantiate the theory that such UCDs are remnants of more massive progenitor galaxies stripped down to their nuclear components. This is concurrent with findings in other UCDs that host central black holes and suggests a common formation mechanism through the tidal stripping process.

Broader Consequences and Future Research

Afanasiev et al. estimate that as many as 80% of luminous UCDs in galaxy clusters might host central black holes, a proportion expected to diminish in UCDs within galaxy groups due to their tendency to originate from dwarf galaxies that typically do not possess central black holes. These insights have profound implications for our understanding of galaxy evolution and the demographics of black holes in the local universe.

Moving forward, this research underscores the need for further observational campaigns targeting other UCDs with similar methodologies to refine estimates of the frequency and properties of SMBHs in UCDs. Understanding the environments and formation conditions that favor the formation of such compact dense stellar systems with central black holes could shed light on galactic evolution processes in varying gravitational landscapes.

Additionally, the juxtaposition of metal-rich, $\alpha$-enhanced stellar populations with SMBHs in UCDs could serve as an observational marker to identify other tidally stripped systems. Deepening our understanding of the stripping mechanisms and constraints could significantly diverge our grasp of UCD origins and their initial progenitor mass distribution.

Conclusion

The research presented by Afanasiev et al. makes significant strides in elucidating the relationship between supermassive black holes and ultracompact dwarfs. Through a meticulously crafted kinematic model and careful analysis, the paper not only confirms the presence of an SMBH in UCD3 but also enriches the larger narrative of UCD formation theories. The implications of this paper are expected to ripple through future galactic studies, emphasizing the need to integrate dynamical and chemical perspectives in understanding the diverse evolutionary paths of compact stellar systems.