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Intermediate mass black holes and the case of GW190521

Intermediate-mass black holes (IMBHs) constitute a class of black holes with masses in the range 100-100,000 solar masses (MSUN) that could bridge the gap between stellar BHs -- formed from the death of massive stars -- and supermassive BHs -- which inhabit galactic nuclei. IMBHs evaded all the observational efforts put in place to detect them until the recent detection of GW190521, a gravitational wave (GW) source originated by the merger of two stellar mass black holes (BHs) with a total mass of around 150 MSUN, operated by the LIGO-Virgo-Kagra collaboration.


The discovery of GW190521 opened a new window on the physics of BHs, as this can be considered the first, undoubted detection of a BH falling in the IMBH mass-range. Beside this, GW190521 has several peculiarities that makes it an even more interesting and exciting source. First, the two BHs that participated in the merger are massive: the least massive component has a mass of 66 MSUN while the primary component mass is around 85 MSUN. Both masses fall in a special region of the BH mass spectrum, called the “upper-mass gap”, where no BHs are expected to form according to our current knowledge of stellar evolution. Second, the effective spin parameter, a quantity that serves to estimate the level of alignment of the two BH spin vectors, is close to zero, thus implying that the spins of the two BHs are likely strongly misaligned or they are very slowly spinning. Third, it might be possible that the two BHs moved on a highly eccentric orbit, a feature suggesting that the GW190521 BHs could have paired in a dense star cluster.


In our recent work (fruit of a big international collaboration involving also Mirek Giersz from the Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences), we analyzed a suite of over 80 simulations of young massive clusters containing 110,000 stars to look for BH mergers capable to build-up GW190521-like mergers and, more in general, mergers involving an IMBH. In 80 simulations, around 17 IMBHs formed via a sequence of collisions among stars that build-up a very massive star with masses ~ 200-400 MSUN first, that is later accreted onto a stellar BH. However, only in three cases an IMBH captures a stellar BH and eventually merges. In one further serendipitous case, though, we found a series of BH mergers leading to a final BH binary merger with properties resembling those of GW190521.


About the GW190521-like merger, we found that it is a byproduct of a triple merger chain. At first, two stellar BHs merge together and the remnant later capture another BH and merge again, leaving behind a BH with a mass of 70 MSUN. Meanwhile, another BH captured a red giant star and accreted some of its material, reaching a mass of 68 MSUN. These two BHs meet together after around 84 Myr and merge, leaving behind an IMBH with a mass of 138 MSUN.


Comparing the final spin of our triple merger-driven IMBH with the value obtained assuming that the same BH formed out of a single BH or a double BH and with the value measured for GW190521, we found that the triple- and single-merger scenarios are the most likely to explain the origin of GW190521. In fact, we were able to recover GW190521 properties with a probability of around 70% in the case of single- and triple-scenario, while this percentage decreases to around 50% in the case of the double-merger scenario. This has important implications for interpreting the physics that potentially regulated GW190521 formation. If this source formed through a single merger, it must mean that BHs with masses in the so-called “upper mass-gap” can form without the intervention of previous BH mergers, while if the source originated through a triple merger scenario it would mean that the cluster where this occurred was extremely dense. In fact, when two BHs merge the remnant receive a kick that can be as large as 1,000 km/s. These GW recoil kicks can eject the BH remnant from the cluster, thus quenching the merger chain and preventing the possible BH growth.


Using numerical relativity fitting formulae, we show that for the triple merger scenario to occur the host cluster should have had a mass of at least 100,000 MSUN and a half-mass radius smaller than 1-0.5 pc, a condition met in several nuclear clusters and dense, massive and young star clusters observed in the local Universe.


We apply the same type of analysis to the merger involving heavier IMBHs in our model. However, to proceed with the analysis we had to make assumptions on the value of the IMBH spin. We demonstrated that a single merger with a stellar BH will leave a clear imprint on the IMBH final spin but it will permit us to distinguish very clearly whether the IMBH has a low or high spin at formation. Therefore, we predict that from the future detection of mergers between massive IMBHs (with masses > 300 MSUN) and stellar BH, which will be made possible by LISA, we will be able to place strong constraints on IMBH “natal” spins and, from them, on IMBH formation pathways and nursing environments.



Sketch of the formation of the GW190521-like merger in one of our simulations. Along the top row, two BHs with masses (17+28)MSUN undergo a first generation (Gen-1) merger, whose remnant capture a new companion with mass 25 MSUN and undergoes a further merger event (Gen-2). The remnant, with a mass of ~70MSUN, is the primary of the third generation merger (Gen-3) that resembles GW190521. Similarly, along the bottom row a main sequence and a red giant stars merge and their product is accreted onto a stellar BHs, leading to a final BH with mass of 68 MSUN, which is the secondary companion in the in the GW190521-like merger (Gen-3). The Gen-3 merger leads to the formation of a BH with a mass ~ 140MSUN, thus in the IMBH mass range.


Further reading:

Arca Sedda M et al (2021), Breaching the limit: formation of GW190521-like and IMBH mergers in young massive clusters