Uncovering black holes hiding in a globular cluster

Title: A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5

Authors: Mark Gieles, Denis Erkal, Fabio Antonini, Eduardo Balbinot, and Jorge Peñarrubia

First Author’s Institution: 1. ICREA, Barcelona, Spain. 2. Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Barcelona, Spain.

Status: Published in Nature Astronomy [closed access]

If you have ever looked through a telescope at Messier Objects, you’ll agree with me that globular clusters are absolutely stunning. Compared to the faint fuzzy patches of external galaxies and Milky Way nebulae, globular clusters glow like millions of jewels crowded together. That is the reason they are a favorite target at star parties. To astronomers, though, globular clusters are fascinating because they are relics of the early universe, carrying signatures from their creation through billions of years (see this astrobite). In today’s paper, we will embark on a journey through the life of globular clusters.

Act I. The life story of globular clusters

Globular clusters (GCs) consist of hundreds of thousands of tightly bound stars, and we have around one hundred in our Milky Way galaxy. In standard galaxy formation theory, galaxies like our Milky Way formed through the hierarchical merging of many smaller galaxies. According to this picture, some GCs in our Galaxy were born in external galaxies and merged into the Milky Way later. GCs entering the strong gravitational field of the Milky Way may experience tidal stripping and lose some of their stars. When the stars get spread into a stellar stream, what happens to the original GC? Astronomers know of a dozen stellar streams from tidally stripped GCs, but very few progenitors have been observed. You may think that that’s because the original GCs completely dissolved. But wait! The authors of today’s paper investigate the possible role of stellar mass black holes in this story.

Act II. Enter black holes

How can star clusters have black holes? Massive stars (much higher than 10 solar mass) eventually become black holes at the end of their lives. These black holes are named stellar mass black holes, to be distinguished from the supermassive black holes sitting in the center of galaxies. GCs are expected to have some high mass stars and some low mass stars according to the initial mass function (IMF). The most massive stars quickly evolve to become black holes. Thus it is completely reasonable to ask the question, what is the role they play in the life of a GC?

The authors of today’s paper focus on a particular Milky Way GC called Palomar 5 (Pal 5). Although there is no direct observation of black holes in Pal 5, it has long stellar streams and is the least dense of all the Milky Way GCs. This makes it especially interesting: a low-density, fluffy GC like Pal 5 is more vulnerable to tidal disruption.

The authors investigate whether invisible black holes can explain why Pal 5 is so fluffy. Black holes in GCs quickly sink to the center due to something called dynamical friction. The authors of today’s paper ran N-body simulations with black holes in the GC model, and they found that the stars are less concentrated than the overall mass distribution in these GCs. Due to dynamical friction, the black holes sit at the center while the stars “float” in the outskirts. In Figure 1, the best-fit GC model with black holes agrees well with observational data.

Fig 1. Comparison between the best-fit model (blue) and observation (orange) of Pal 5. The model matches observations in panel a) density profile of stars, b) spatial distribution of the stellar stream track, c) stream density, and d) stream width. Black dots in panel b) represent black holes, and the zoom-in panel shows that most of the bound black holes are in the center of the cluster. Reproduced from Fig 1 of the paper.

The authors also ran models without black holes and found that those models are less likely. For a stars-only model to fit the present-day Pal 5, it needs the GC to have a velocity dispersion much lower than the observed value. In addition, the no black hole models require very specific initial conditions to match present-day Pal 5. That means it’s less likely to occur than the models with black holes.

Act III. The fate of GCs with black holes

Now let’s think about what would happen to these GCs. The main reason they lose stars is due to tidal stripping from the Milky Way, which is more efficient now that the stars are “fluffed up.” The main mechanism for losing black holes is two-body gravitational interaction. If there are many black holes packed into the central region, they can form binaries and hurl other black holes out of the GC. Which of these mechanisms (loss of stars from the GC due to tidal stripping or loss of BH due to two-body interactions) dominate depends on the initial black hole fraction in the GC. If the black holes make up around 10% of the total GC mass, then the mass loss rate for stars and black holes are equal. But, if the initial black hole fraction is higher, then the GC will lose stars faster and eventually will only contain black holes. If the initial black hole fraction is lower, the opposite happens and the GC will contain no black holes in the end. 

Fig 2. Density of the Pal 5 stellar stream with different initial black hole fractions (f_BH). Models above black hole fraction of 10% have prominent tidal tails, resulting from the GC losing more stars. Reproduced from Fig 4 of the paper.

The stars that are lost by the GC may still be visible as stellar streams. As shown in Figure 2, higher black hole fraction leads to more stars being lost into the stellar streams. For Pal 5, the density of its stellar stream increases with the black hole fraction. To achieve a stellar stream density like we observe, a black hole fraction of 22% is necessary. That is higher than 10% and it means Pal 5 will eventually be 100% black holes. Although black holes are mysterious and almost impossible to directly observe, the authors have, through the power of theoretical modelling, revealed a possible life story for Pal 5 and other GCs affected by their own black holes.

Astrobite edited by Pratik Gandhi

Featured image credit: NASA