Forbes: Milky Way Houses Up To 100 Million Black Holes

[bystander]

Milky Way Houses Up To 100 Million Black Holes
Forbes | Starts With A Bang | 2017 Aug 09

[img3="This illustration of a black hole, surrounded by X-ray emitting gas, showcases one of the major ways black holes are identified and found. Based on recent research, there may be as many as 100 million black holes in the Milky Way galaxy alone. Credit: ESA"]https://blogs-images.forbes.com/startsw ... KN7I_A.jpg[/img3][hr][/hr]

How many black holes are there in the Milky Way? This straightforward question has proven extremely difficult to answer, since black holes are so difficult to directly detect. However, scientists not only have developed indirect methods for locating and even weighing them, we also understand how the Universe forms them: from stars and stellar remnants. If we can understand the different stars that existed at all different times in our galaxy's history, we should be able to infer exactly how many black holes — and of what mass — exist in our galaxy today. Thanks to a comprehensive study by a trio of researchers from UC Irvine, the first accurate estimates of the number of black holes found in Milky Way-like galaxy have now been made. Not only is our galaxy filled with hundreds of billions of stars, but we also are home to up to 100 million black holes.

This is all the more remarkable when you consider that it wasn't so long ago — back in the 1980s — that scientists weren't yet certain that black holes existed. The best evidence we had came from X-ray and radio emitting sources that exerted a gravitational influence that exceeded that of neutron stars, and yet had no optical or infrared counterpart. Subsequently, we began measuring the motions of stars at the galactic center using multiwavelength astronomy, revealing that they appeared to orbit a large mass that needed to contain about four million Suns' worth of matter. Consistent with other observations of more active galaxies, we now believe that every massive galaxy, including our own, contains a supermassive black hole. ...

UCI Celestial Census Indicates that Black Holes Pervade the Universe
University of California, Irvine | 2017 Aug 07

Researchers expect more gravitational-wave evidence of collisions in the near future

After conducting a cosmic inventory of sorts to calculate and categorize stellar-remnant black holes, astronomers from the University of California, Irvine have concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way – far more than expected. ...

Counting Black Holes: The Cosmic Stellar Remnant Population
and Implications for LIGO - Oliver D. Elbert, James S. Bullock, Manoj Kaplinghat

• Monthly Notices of the RAS (online 02 Aug 2017) DOI: 10.1093/mnras/stx1959
  arXiv.org > astro-ph > arXiv:1703.02551 > 07 Mar 2017

[The What not the Who]

Does this mean 'the missing mass' has been found? Does this mean there is no Dark Matter? If there is no Dark Matter .. would that be mean to Dark Matter diehards .. or just advance in cosmology?

[Doum]

Does this mean 'the missing mass' has been found? Does this mean there is no Dark Matter? If there is no Dark Matter .. would that be mean to Dark Matter diehards .. or just advance in cosmology?

Does this mean there is no Dark Matter?

No. there is a 100 billion stars in the galaxy. So a 100 millions black hole represent 0.1 % of that. And black hole are start that collapse into blach hole. The mass it represent is not that important in the galaxy.

[The What Not The Who]

Does this mean 'the missing mass' has been found? Does this mean there is no Dark Matter? If there is no Dark Matter .. would that be mean to Dark Matter diehards .. or just advance in cosmology?

Does this mean there is no Dark Matter?

No. there is a 100 billion stars in the galaxy. So a 100 millions black hole represent 0.1 % of that. And black hole are start that collapse into blach hole. The mass it represent is not that important in the galaxy.

Star mass can be measured .. but what about the Black Hole mass. If the Black Hole mass is 1,000 times the mass they might be thought to be, then the decimal point moves quite a bit. Things aren't quite so black and white and red and yellow and orange and blue as a starry night.

[Ann]

Does this mean 'the missing mass' has been found? Does this mean there is no Dark Matter? If there is no Dark Matter .. would that be mean to Dark Matter diehards .. or just advance in cosmology?

Does this mean there is no Dark Matter?

No. there is a 100 billion stars in the galaxy. So a 100 millions black hole represent 0.1 % of that. And black hole are start that collapse into blach hole. The mass it represent is not that important in the galaxy.

Star mass can be measured .. but what about the Black Hole mass. If the Black Hole mass is 1,000 times the mass they might be thought to be, then the decimal point moves quite a bit. Things aren't quite so black and white and red and yellow and orange and blue as a starry night.

Doum is right. The mass that black holes represent is far too small for them to make up the unseen dark matter of the Milky Way.

Space.com wrote:
By calculating the speed of stars throughout the galaxy and conducting a detailed study of the Milky Way's outer edges, a team of astronomers in Australia determined that the amount of the unseen dark matter in the galaxy is just 80 billion times the mass of the sun — half the mass of recent estimates.

So the dark matter in the Milky Way is 80 billion times the mass of the Sun. Let's say that the dark matter in our galaxy would actually consist of black holes. If the Milky Way hosts 100 million black holes, and their combined mass is 80 billion solar masses, then the average mass of each black hole would be 800 solar masses. That's a lot, and it is much more than the mass of any star in the Milky Way.

Click to view full size image

Artist's impression of Cygnus X-1, a black hole accreting matter
from a normal companion. ESA/Hubble.

Such a high mass for the individual black holes in the Milky Way would mean that many if not most of them would have companions. In the same way that massive stars are typically members of binary or multiple systems, most 800 M_☉ black holes would have companions. Some of these black holes in multiple systems would be accreting mass from their companions and have outbursts. Because of this, we would see them. Not only that, but we would be able to calculate their mass from the orbit of their normal companions. But the best-known black hole in the Milky Way, apart from Sagittarius A*, is Cygnus X-1, which has given itself away because it is accreting mass from a normal companion. The orbit of the companion shows that Cygnus X-1 contains only 14.8 M_☉, which is a far cry from 800 M_☉.

We have no reason to believe that there are 100 million 800 M_☉ black holes in the Milky Way. Instead, Cygnus X-1 may be rather typical, and in fact Cygnus X-1 may be slightly massive as Milky Way black holes go. The average mass of such black holes may be close to 10 solar masses. So if there are 100 million 10 M_☉ black holes in the Milky Way, their combined mass would be 1 billion solar masses. That leaves 79 billion solar masses unaccounted for, out of the 80 billion M_☉ we were looking for. All that mass likely consists of dark matter.

Ann