A hidden graveyard of dead stars would envelop our galaxy

Simulations indicate that a significant number of neutron stars and stellar black holes born in the disk, or bulge, of our Milky Way were ejected into orbits outside of these structures. So there would be a hidden world of dead stars waiting to be discovered surrounding our galaxy.

The theory of the structure and evolution of stars, the foundations of which were laid in the 1930s and which has undergone strong development in the following decades thanks to the development of nuclear astrophysics, says that stars with more than eight solar masses will end their lives by explode in the form of supernovae.

The heart of these stars must at least yield neutron stars by gravitationally collapsing, but if the explosion does not eject enough matter, the mass of the remnant star, whose diameter is at most tens of kilometers, will not allow the neutron star to exist in a stable way and it will collapse to a stellar black hole – it is often estimated that the initial mass of the star must exceed 30 solar masses. This scenario must have repeated itself many, many times in our galaxy since its birth more than 12 billion years ago.

The explosion of a supernova is not symmetrical, neither is the resulting ejection of matter, and can sometimes cause the final compact star to propel itself like a rocket. If the speed detected is high enough, the neutron star or black hole will eventually leave the Milky Way’s disk or its central bulge and finally its halo to exit the intergalactic medium.

The direction in which this motion begins is random — and if we stay at velocities low enough for the compact stars resulting from SN II (gravitationally collapsed) supernovae to easily orbit around the central bulb of our galaxy – can we therefore expect the existence of a population of dead stars enveloping the Milky Way.

A presentation of the discovery of J0002. For a fairly accurate French translation, click the white rectangle at the bottom right. Then the English subtitles should appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © NASA Goddard

Ghostly star corpses

Isolated holes that take up almost no matter should not be surrounded by an accretion disk radiating in the X domain, and so they should not be easy to observe. In fact, one might think that only the evidence of a gravitational microlensing effect allows it to be highlighted. In fact, we know of an example, MOA-2011-BLG-191/OGLE-2011-BLG-0462, which is a black hole about seven times the mass of the Sun, whose existence has been proven by observations made with the Hubble and Subaru telescopes were carried out.

The case of neutron stars is more favorable because, even without accretion of X-ray and gamma-ray emitting matter, a neutron star behaves like a kind of radio beacon, in that its collimated emission of electromagnetic waves periodically intersects the Earth, which can be viewed with a radio telescope as a pulsar detected and this is how the first one was discovered in 1967 by Jocelyn Bell. As an example of such a pulsar, we can cite the case of PSR J0002+6216, located about 6,500 light-years from the solar system in the Milky Way when looking towards the constellation Cassiopeia.

In fact, the speed of J0002 is so extraordinary, about 1,100 kilometers per second, five times faster than the average speed of known pulsars, that this neutron star will eventually leave our galaxy at a speed greater than the escape velocity of our Milky Way.

40% of neutron stars ejected from the Milky Way?

A team of astronomers, mostly from the University of Sydney in Australia, wanted to know what the graveyard of dead stars that were thrown into new orbits and must have formed over the billions of years of our Milky Way’s existence might have looked like. Researchers therefore ran simulations to try to generate, theoretically, the equivalent of credible and expected distributions of neutron stars and black holes that could be constructed through observations. The result was published in an article by Monthly Bulletins of the Royal Astronomical Society

They then received what they called a in English galactic underworld, which translates to “an underground world” in French. This follows the shape of our Milky Way well, but envelops it by being more diffuse, to the point where it has a disk thickness more than tripled compared to that of the thin disk, and therefore a height of ±1,260 30 pc (remember that a parsec, denoted pc, is about 3.26 light-years across and that the thin disk itself is enveloped by the thick disk, which is notable for being composed almost entirely of old stars).

We also note that the spiral structure of our galaxy is not present in the galactic underworld.

The simulations show that about 30% of the dead stars in the galaxy actually left the galaxy at speeds of at least a million kilometers per hour and are therefore migrating in the intergalactic medium. This would even represent about 40% of all produced neutron stars but only 2% of the stellar black holes in the entire history of the Milky Way. In total, it would be about 0.4% of the stellar mass of our Milky Way that would have left it since its birth.

the galactic underworld itself would represent about 1% of the mass of our galaxy in star form. It therefore cannot explain dark matter if it really exists, and we don’t have to bring the MOON theory in its place.

Finally, the simulations show probable distances of 19 and 21 percent from the sun to the nearest neutron star and black hole, respectively.

Help with ghost hunting

The results obtained should help researchers know where to look and how to highlight their existence galactic underworld. As co-author, Peter Tuthill of the Institute of Astronomy Sydney, co-author of the paper, explained in a University of Sydney statement: One of the problems with finding these ancient objects is that until now we didn’t know where to look. The oldest neutron stars and black holes formed when the galaxy was younger and shaped differently, then underwent complex changes spanning billions of years. It was a big job modeling all of this to find her “.

And to add: To me, one of the coolest things we’ve found in this work is that even the local stellar neighborhood around our Sun is vulnerable to these spooky visitors. Statistically, our nearest remnant should be only 65 light-years away: galactically, more or less in our backyard. “.

In this context, it is interesting to recall that the AMS cosmic ray detector revealed an unusually high positron flux in the Solar System, a flux that we do not yet know whether reflects the existence of dark matter particle decay or on the contrary, a neutron star near the solar system that has not yet been discovered because it is not in the form of a pulsar or an X or gamma source.

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