Title: Fast-moving stars around an intermediate-mass black hole in \(\omega\) Centauri
Authors: Maximilian Häberle , Nadine Neumayer , Anil Seth , Andrea Bellini, Mattia Libralato, Holger Baumgardt, Matthew Whitaker, Antoine Dumont, Mayte Alfaro Cuello, Jay Anderson, Callie Clontz, Nikolay Kacharov, Sebastian Kamann, Anja Feldmeier-Krause, Antonino Milone, Maria Selina Nitschai, Renuka Pechetti, Glenn van de Ven
First Author’s Institution: Max Planck Institute for Astronomy, Heidelberg, Germany.
Status: Published in Nature [open access]
The Missing Link Between Stellar and Supermassive Black Holes
Astronomers have evidence that most galaxies contain millions of black holes (BHs) with masses in the range 5-100 \(M_{\odot}\), called stellar-mass BHs. They also believe that nearly every galaxy contains a central supermassive black hole (SMBH), with mass ranging from \(10^6\) to \(10^9\) \(M_{\odot}\). However, another more elusive class of BHs is thought to exist, intermediate-mass black holes (IMBHs). IMBHs have masses greater than 100 \(M_{\odot}\), but are not massive enough to be considered supermassive. While astronomers have detected plenty of stellar-mass and supermassive BHs, the link between the two remains elusive. By understanding the population of IMBHs and the galaxies they inhabit, we can better grasp the formation and evolution of SMBHs and galaxies across cosmic time. We know that SMBHs must have been IMBHs at some point, but are any still around today?
\(\omega\)Centauri: The Corpse of a Dwarf Galaxy
\(\omega\)Centauri is a hyper-dense group of stars, known as a globular cluster (GC). It is the most massive GC in the Milky Way, and the chemical makeup and motions of its stars are different from most GCs native to our Galaxy, implying that \(\omega\)Centauri was once a dwarf galaxy that was eaten by the Milky Way! If \(\omega\)Centauri is indeed the remnant of a long-dead dwarf galaxy, it could harbor a central BH that didn’t quite reach supermassive status, matching its host galaxy’s smaller size. In fact, the existence of a central IMBH in \(\omega\)Centauri has been debated for nearly twenty years, but today’s authors hope to resolve the mystery surrounding \(\omega\)Centauri’s central IMBH for good.
A Supermassive Analogy
The closest SMBH to us is Sagittarius A* (SgA*), located at the center of our own Milky Way (see Figure 1). SgA* was detected by carefully monitoring the orbits of stars very close to the center of our Galaxy over long periods. Massive objects will influence the orbits of stars in their vicinity (think of satellites around Earth, or the motion of planets around the Sun). Otherwise invisible BHs exhibit the same behavior, allowing us to infer their presence. Our relative proximity to the center of our Galaxy (~8 kiloparsec) makes it possible to monitor individual stars, something impossible for large, more distant BHs. However, today’s authors used this same principle to show the likely presence of an IMBH in the (relatively) nearby \(\omega\)Centauri. By monitoring stars in the central region of \(\omega\)Centauri, they showcase that the presence of a central IMBH is the only explanation for the stars’ anomalous motion.
The Speedy Star Problem
Using 20 years of Hubble data targeting \(\omega\)Centauri, the authors noticed seven unusually speedy stars in the cluster’s central region. Figure 2 showcases the seven stars in pink, along with their abnormally \(\omega\)Centauri is unknown, so the authors also show three possible locations of the center as crosses. In \(\omega\)Centauri’s central region, the escape velocity is about 62 km/s when only considering the stars visible in data. With some of the velocities measured at 90+ km/s, these stars should have fled their host long ago, but instead seem to be content in the central regions of \(\omega\)Centauri. This suggests the presence of an unseen, yet influential, member of the cluster retaining these stars.
Is it really an IMBH?
Using models of the distribution of stars in \(\omega\)Centauri, the authors compare predictions of the central escape velocity in \(\omega\)Centauri with and without a central IMBH. Figure 3 showcases this, with our speedy stars plotted in pink. Since these stars have velocities well above the escape velocity without a central IMBH (the flat line), it’s clear they should have abandoned \(\omega\)Centauri if no IMBH was present. This argument provides the minimum IMBH mass needed to explain the presence of these stars in \(\omega\)Centauri’s nucleus, about 8,200 \(M_{\odot}\). Without further observations, the IMBH’s mass can’t be pinned down for sure, but the authors plan to carry out followup research to get a more accurate estimate.
Other scenarios, such as a smaller stellar-mass BH or interactions between two or more stars, also fail to explain the existence of the unusual stars. If they were orbiting a smaller, stellar-mass BH, their orbital period would be shorter than 10 years, implying that complete orbits would be seen in the data. Likewise, acceleration due to star-star interactions would produce substantially more abnormally fast stars, and deplete \(\omega\)Centauri of its stellar reservoir too quickly. This evidence leads the authors to claim the only solution to the speedy star problem is a central IMBH in \(\omega\)Centauri.
The likely presence of an IMBH in a dwarf galaxy remnant like \(\omega\)Centauri provides a crucial piece of the galaxy-BH evolution puzzle, although further studies are needed to confirm its exact position and mass. Detecting more IMBHs will allow astronomers to better understand the evolution of their supermassive cousins, and inform us of the impact that central BHs have on their host galaxies. This detection hints at the possible presence of a previously undetected population of BHs scattered throughout the Universe, one of which is our intermediate-mass cosmic neighbor!
Astrobite edited by Chloe Klare and Cole Meldorf
Featured image credit: NASA
