Hans-Walter Rix receives ERC Advanced Grant worth 2.85 million euros

April 26, 2022

Hans-Walter Rix, Director at the Max Planck Institute for Astronomy in Heidelberg and head of the Galaxies and Cosmology Department, has been awarded one of the prestigious and highly endowed funding awards of the European Research Council – an ERC Advanced Grant. His project, entitled "Hunting Dormant Black Holes in the Galaxy", will be funded with 2.85 million euros.

Due to their exotic properties, black holes are among the most mysterious and exciting objects in the universe. Essentially, two types of these extremely dense entities exist: First, the supermassive black holes at the centers of galaxies like our Milky Way, which have millions of solar masses. The other is stellar black holes - objects that, according to stellar evolution models, are at the end of life of very massive stars. Simplified, such stars collapse to a black hole when the radiation pressure at the end of the nuclear energy production fails and there is nothing to prevent the collapse of the remaining mass of the star by gravity. Within a certain distance around such ultracompact objects, gravity is so strong that even light cannot escape. Therefore, black holes are invisible from the outside, but may reveal themselves by the impact on their environment.

Following common stellar evolution models, there should be many millions of such stellar black holes in our home galaxy, the Milky Way system. But only about 20 objects are verified with some certainty. In close binary systems they can attract attention if they collect matter from the neighboring star (the astronomer then uses the term accretion) and if short-wave X-rays are released during this process. Thus - interestingly enough, in contrast to the much more distant "big" black holes in the centers of galaxies - we know only a few objects from the relatively close stellar black holes in our Milky Way.

Hans-Walter Rix: „The key question is: are the predictions actually correct - have we really understood the evolution of massive stars and the formation of such stellar black holes? If so, there must be numerous "dormant" objects that have so far remained hidden from us due to a lack of accretion effects. And such we now want to and can find - if they really exist!"

Artist's impression of a stellar black hole in a binary system. This example shows the active case, when matter of the luminous star falls over a disk-like structure (blue) - the so-called accretion disk - onto the invisible companion - the black hole - and X-rays are emitted. But there should be numerous other objects, which are " dormant" due to the lack of accretion and therefore inconspicuous so far. Exactly such objects are now to be discovered.

© ESA, NASA, and Felix Mirabel (French Atomic Energy Commission and Institute for Astronomy and Space Physics/Conicet of Argentina)

Artist's impression of a stellar black hole in a binary system. This example shows the active case, when matter of the luminous star falls over a disk-like structure (blue) - the so-called accretion disk - onto the invisible companion - the black hole - and X-rays are emitted. But there should be numerous other objects, which are " dormant" due to the lack of accretion and therefore inconspicuous so far. Exactly such objects are now to be discovered.

© ESA, NASA, and Felix Mirabel (French Atomic Energy Commission and Institute for Astronomy and Space Physics/Conicet of Argentina)

The key to the success of the project is the targeted analysis of a huge, statistically significant quantity of observational data. The ERC project will analyze more than half a million spectra of massive stars from the SDSS-V sky survey in a novel way. SDSS stands for the Sloan Digital Sky Survey - the most comprehensive three-dimensional mapping of the sky ever made. Especially in the first phases of the SDSS since the beginning of the millennium, the survey provided famous data on the large-scale distribution of galaxies in the universe. In the more recent phases, stellar spectroscopy is now increasingly coming into focus to study the structure of our own Milky Way.

In combination with modeling and measurements from the space-based observatories TESS and Gaia, the SDSS-V will now be used to identify "hot" candidates for "dormant" stellar black holes orbiting still-living stars. By combining spectroscopy, astrometry (position measurements) and photometry (brightness measurements), it may be possible to identify typical time variations in the relative motions and brightnesses of such pairs of a luminous star and a dark companion - the stellar black hole.

„If the theory is correct, we should discover around 100 new objects“ said Hans-Walter Rix.

The ERC project thus represents a fundamental review of our current astrophysical models for the evolution of massive stars and the formation of stellar black holes. As SDSS-V project scientist, Hans-Walter Rix himself has been largely responsible for the realization of this novel spectroscopic survey of the entire sky, focusing on the physics of stars and the Milky Way.

The European Research Council has now allocated 2.85 million euros for the new ERC project. Over the next five years, this substantial funding will allow, among other things, the establishment of a working group consisting of several PhD students and postdocs, which will work closely with Hans-Walter Rix's department "Galaxies and Cosmology".

The ERC offers various funding schemes within its program and all applications for funding must demonstrate the excellence of the scientists involved and, above all, the outstanding importance of the projects applied for.

Hans-Walter Rix has been Director at the Max Planck Institute for Astronomy in Heidelberg since 1999 and heads the Department of Galaxies and Cosmology (GC).

Link to the press release of the European Research Council

KJ/HWR