Hierarchical Mergers Of Black Holes (Cosmology)

While most of the fusion events between extreme objects detected by Ligo and Virgo are produced by so-called “first generation” black holes formed by the collapse of stars, others could instead be second (or third) generation, in which one of the that melts already comes from one (or more) previous fusion of black holes. An article published today in Nature Astronomy reviews all the theoretical results, models and events of gravitational waves detected and coming from hierarchical mergers of stellar-mass black holes

Generations in comparison. Generations of black holes, which merge into a single entity producing space-time “vibrations”, detected on Earth thanks to the Ligo and Virgo interferometers . And if the first generation comes from two black holes formed by the collapse of massive stars at the end of their lives, it is possible that history will repeat itself, and the resulting black hole fuses again with a similar object, giving rise to a black hole of second generation. To recognize the exact position of these extreme stellar objects in the family tree of black holes, it is necessary to observe the imprint that mergers leave on gravitational waves. And that’s not all, because in reality such black holes may have already been observed. This is what we read in an articlereview published today in Nature Astronomy by David Gerosa , young astrophysicist at the University of Birmingham close to return to Italy with a one-way ticket signed ERC , which will see him in the role of associate professor at the University Milano-Bicocca. Gerosa deals with the astronomy of gravitational waves, both studying the dynamics of the sources (the binary pairs of black holes, in fact) from a theoretical point of view, statistically analyzing the data.

Your review talks about second or third generation black holes. Are there statistical estimates on how many such objects may exist, on what are the probabilities of forming them and, among these, how many are actually observable?

«To date, Ligo and Virgo have observed about 50 events. One of these ( Gw 190521 ) has characteristics (in particular the mass) typical of a second generation event. Based on the observed sample alone, a rough estimate of the incidence rate is therefore 1/50. However, we must take into account the observational bias – events involving high masses are easier to observe – and how different astrophysical environments can assemble successive generations of mergers . It is a very open problem, and I hope this transpires from our review ».

Are these newly formed objects, to be looked for in the nearby universe?

“Not necessarily, but with current equipment we are sensitive only to events that occur at redshifts less than 1″

What differentiates them from black holes that come from a first generation merger?

«The mass involved, first of all, which is higher for second or third generation events. This is particularly interesting because stellar evolution models predict an upper limit on the mass of black holes that form from the collapse of massive stars, which is about 50 times the mass of the Sun. If Ligo or Virgo measure a higher mass event, a different training scenario must be involved. We talked about this idea in a 2017 article  (and also presented in another independent article released the same day) and the 2019 event seems to have confirmed it. The mass, alone, however, can also have other origins ».

What else, then?

“The production of black holes of a generation other than the first also has a very characteristic effect on spin. It is a relativistic phenomenon called orbital hang up which regulates the number of orbits performed by the binary as the spin varies: hierarchical black holes typically have spin around the characteristic value 0.7 (in dimensionless units in which the spin varies between 0 and 1) ” .

Being a signal generated by massive objects, is it easier to detect with current detectors?

“And how. In fact, we tend to have already seen one, Gw 190521. The signal is easier to identify for more massive objects. Care must be taken that they are not too massive, otherwise the signal goes out of the band sampled by the detectors, even if these objects are intrinsically few ».

Are there any observed cases that have dubious characteristics and could belong to this category?

«In the case of Gw 190521, as we have said, the main interpretation is that it is a second generation merger . But it is not the only one . Another second generation event, albeit a little more uncertain, could be Gw 190412 ».

How come?

“Because the mass involved is lower, and in this case the ratio between the masses and the spin suggest that it is a mixed merger in which one black hole is second generation and the other is the first. These two data – relationship between masses and spin – are however “weaker” indicators: forming such an event using “normal” theoretical models is rare but not impossible ».

How to definitively confirm that the event derives from a second or third generation merging ?

“Certainty in science builds up slowly. With more events (we expect thousands within a few years) it will become clear whether a subpopulation of next-generation black holes is needed to explain the data. ‘

Featured image: Davide Gerosa, astrophysicist at the University of Birmingham and first author of the review article on generations of black holes © INAF


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