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Recently scientists discovered a large stellar-mass black hole, which (previously) they believed that it shouldn't be possible in our galaxy!

Source

An international team of scientists say they have discovered a stellar-mass black hole with a mass 70 times greater than the sun — so large it defies current theories on how black holes of its kind form.

Why shouldn't it be present in our galaxy system? Are there any physical or mathematical calculations that defy its existence?

References:

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    $\begingroup$ Stellar black holes are created in supernova explosions, so not all of the original star mass ends up in the black hole. The answers here give some relevant details. $\endgroup$ – PM 2Ring Dec 2 at 7:15
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    $\begingroup$ Different sites report widely different claims from the authors; relying on those is an excercise in frustration, since they usually don't care about science or truth, just having a catchy title to bring in advertising revenues :) Unfortunately, the paper sumbitted to Nature (nature.com/articles/s41586-019-1766-2) disappeared, so... $\endgroup$ – Luaan Dec 3 at 9:02
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    $\begingroup$ @Luaan I'm able to access the paper (?) $\endgroup$ – Allure Dec 3 at 9:35
  • $\begingroup$ @Allure Yay, it's back online :) $\endgroup$ – Luaan Dec 3 at 10:08
  • $\begingroup$ @Luaan I found one article that put 100 million of these things in the Milky way. That's about 1 per 100 square light years. The reseachers seem to be using a radical new technique, so it's a good time for confirmation/or not confirmation. $\endgroup$ – Wayfaring Stranger Dec 3 at 16:07
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There're several pieces of information one needs to understand this.

Although stars more massive than 70 solar masses exist, when they become black holes, they usually lose mass in the process. The exact amount of mass lost depends on the metallicity (which is a technical term that describes how much "metals" - the astronomer's definition of metals is anything heavier than hydrogen & helium - is in the star). The companion star that we see today is at solar metallicity, so it's probable that the original star (the one that became the black hole) was also at solar metallicity. Unfortunately, that means it shouldn't leave a 70-mass black hole remnant. From the paper:

This [70 solar mass black hole] would strongly challenge current stellar evolution models, which only allow for the formation of black holes up to $25 M_{sun}$ at solar metallicity.

Where, then, did this 70-solar mass black hole come from? The paper discusses a few alternatives. The obvious one is that two smaller black holes simply merged to form this one. Problem with that is, you still need two 35-solar mass black holes, and 35 is clearly > 25. (In principle you could also have a 25-solar mass black hole merge with a 45-solar mass black hole, but that still leaves the question of where the 45-solar mass black hole came from in the first place.) Note also that this black hole probably didn't arise from neutron stars merging, since neutron stars have a mass limit of about 2 solar masses. Finally there is three black holes merging into one, but this is unlikely: mergers are already rare events, and having two mergers must be even rarer.

Here're a few more unlikely explanations I can think of:

  • Perhaps it's a primordial black hole. Problem with this is that primordial black holes are something of a unicorn - a last resort explanation - because they work whenever you need a missing mass. If you can explain an observation without invoking primordial black holes, that's much preferable. See also the next bullet point.
  • Perhaps the black hole formed elsewhere, in an environment that isn't at solar metallicity, then travelled to this place and captured its companion star. Problem with this is that stellar capture is not a likely process. In the same way we can say that the Earth probably formed around the Sun; it didn't form elsewhere and was captured by the Sun later. There's another problem, which is that the star is observed to have an eccentricity of almost zero (this means its orbit is roughly circular). Newton's laws predict an elliptical orbit. There are processes that will drive the orbits towards circular, but the time taken is long. A (rare) stellar capture event that leads to almost exactly a circular orbit is even more unlikely.

The paper discusses a couple of more credible alternatives:

  • Perhaps this was a triple system where one star became a black hole, and then this black hole "fell into" one of the other stars and ate it from the inside.
  • Perhaps there was a "fallback supernova". This is when a star goes supernova, but the ejected material somehow falls back onto the stellar remnant. This has never been directly observed, and this might be the first.
  • Perhaps something is wrong with the measurement, e.g. an unaccounted for systematic effect. This is the most mundane explanation.

In any case, the system is now an attractive target for telescopes.

Update: there are now several articles claiming an error in the analysis, and that there is no 70-solar mass black hole in this system.

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    $\begingroup$ Just to add: capture scenarios are unlikely because of the low-eccentricity orbit (e=0.03±0.01) and long timescale for tides to circularise the orbit, as noted in the discovery paper. This also likely rules out merging black holes (see this paper by Shen et al.) and other processes that involve sudden mass loss. $\endgroup$ – antispinwards Dec 2 at 21:08
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    $\begingroup$ The rarity of high mass star mergers comes first from the observation that high mass stars are rare. [Nice graphic from Uni Colorado][1] [1]: lasp.colorado.edu/outerplanets/images_solsys/big/… $\endgroup$ – TazAstroSpacial Dec 3 at 2:31
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    $\begingroup$ Secondly binary black hole mergers are rare because it takes a long time for 2 black holes in a stellar binary to merge. They just don't happen often and are over in a "flash". The initial orbits shrink by the emission of gravitational waves. This energy loss is very slow with orbits of days (millions years) but increases as the orbit shrinks. This is the in-spiral phase. The final chirp as the black holes merge is just the very of a very long process. [LIGO Sources and Types of Gravitational Radiation][2] $\endgroup$ – TazAstroSpacial Dec 3 at 2:35
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    $\begingroup$ The attempts to explain how you can get a $70\ M_{Sun}$ black hole seem to be glossing over the simplest answer -- one that the paper itself mentions. That the inferred distance to this system is wrong and thus the measured mass is wrong. If one uses other estimates, the mass drops to a very reasonable $10\ M_{Sun}$. $\endgroup$ – zephyr Dec 3 at 15:16
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    $\begingroup$ Note that forming a 70 solar mass black hole from stellar collapse is thought to be impossible at any metallicity due to the occurrence of pair instability supernovae. This rules out your "it was formed elsewhere" option. $\endgroup$ – mmeent Dec 4 at 8:58
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Perhaps this was a galactic black hole and is now a remnant from a previously accreted galaxy. Our Milky Way is rather large and could have swallowed up a smaller galaxy.

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    $\begingroup$ The question is not how it got there but why it "shouldn't" be there. $\endgroup$ – Mike G Dec 7 at 18:08
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    $\begingroup$ $70 M_\odot$ is far too small for a galactic central black hole. $\endgroup$ – PM 2Ring Dec 8 at 8:58

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