A hidden graveyard of dead stars would envelop our galaxy

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

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

The heart of these stars must at least give neutron stars by collapsing gravitationally, but if the explosion does not emit enough matter, the mass of the remaining star, whose diameter is at most a few tens of kilometers, will not allow the neutron star to exist in a stable way , and it will collapse into a star-sized hole – it is often estimated that the initial mass of the star for this 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, so the resulting ejection of matter is not either and can sometimes lead to the propulsion of the final compact star like a rocket. If the speed achieved is high enough, the neutron star or black hole will eventually leave the disk of the Milky Way or its central bulge and eventually its halo to exit the intergalactic medium.

The direction in which this motion will begin is random – and if we stick to velocities low enough that the compact stars originating from SN II-type supernovae (with gravitational collapse) can simply find themselves in orbit around the central bulb of our galaxy – we can therefore expect the existence of a population of dead stars surrounding the Milky Way.

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

Ghostly starry

Isolated holes accreting almost indiscriminately should not be surrounded by an accretion disc radiating into the X domain and should therefore not be easy to observe. In fact, one might think that only the detection of a gravitational microlensing effect makes it possible to highlight it. Indeed, we know of one example with MOA-2011-BLG-191/OGLE-2011-BLG-0462, which concerns a black hole containing about seven solar masses, whose existence has been demonstrated by observations made with the Hubble- and the Subaru telescopes.

The case of neutron stars is more favorable because a neutron star, even in the absence of accretion of matter giving X- and gamma-rays, behaves as a kind of beacon in the radio domain, and to the extent that its collimated emission of electromagnetic waves periodically intersects the Earth, can it is detected with a radio telescope as a pulsar, and it is in this way that it was first discovered in 1967 by Jocelyn Bell. As an example of a pulsar of this type, we can mention the case of PSR J0002+6216, located about 6,500 light-years from the solar system in the Milky Way, when we look towards the constellation Cassiopeia.

In fact, the speed of J0002 is so exceptional, 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, and its speed is greater than the escape speed of our Milky Way.

40% of the neutron stars ejected from the Milky Way?

A team of astronomers, mainly from the University of Sydney in Australia, wanted to know what the graveyard of dead stars ejected into new orbits might have looked like, which must have formed over the billions of years of our Milky Way’s existence . The researchers therefore carried out simulations to try to theoretically produce the equivalent of credible and expected distributions of neutron stars and black holes that could be drawn from observations. The result was published in an article by Monthly Notices of the Royal Astronomical Society

They then got what they called in English a galactic underworld, which could be translated into French as “an underground world”. This follows the shape of our Milky Way well, but it envelopes it by being more diffuse to the point that it has a disc thickness more than three times that of the thin disc and therefore with a height of 1,260 ± 30 pc . (remember that one parsec, denoted PC, is approximately 3.26 light-years, and that the thin disk itself is surrounded by the thick disk, which is characterized by the fact that it consists almost exclusively of old stars).

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

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

that galactic underworld itself would represent about 1% of the mass of our galaxy in stellar form. It therefore cannot account for dark matter, if it really exists, and we do not need to insert the MOND theory instead.

Finally, the simulations indicate probable distances of 19 and 21 per cent respectively. from the Sun to the nearest neutron star and black holes.

Ghost hunting help

The results obtained should help the researchers to know where to look and how to highlight the existence of this 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 were created when the galaxy was younger and formed differently and then underwent complex changes spanning billions of years. It was a huge task to model all this to find them “.

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

It is interesting in this regard to remember that the AMS cosmic ray detector has revealed an abnormally high flux of positrons in the solar system, a flux that we do not yet know whether it reflects the existence of the solution of dark matter particles or on opposite a neutron star close to the Solar System not yet discovered because it is not in the form of a pulsar or an X or gamma source.

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