The Andromeda Galaxy, which is visible to the naked eye, is one of
the nearest galaxies to the Milky Way. In its centre it houses a very
massive black hole, which has a mass more than 100 million times that of
the Sun. Even so, this black hole, as well as the one at the centre of
our own galaxy called Sagittarius A* are among the least active known of
the supermassive black holes at galaxy centres: they emit very little
radiation.
The activity of a black hole depends on how it is being “fed”, that
is to say on how the material which falls into it gets closer as it
falls. It is hard to track this in the Milky Way because of our position
very close to the plane of the Galaxy, where dust obscuration is very
high, and the field is crowded with stars, but the situation with
Andromeda is different, and we can observe the black hole with much less
impediment.
Now, by combining observations from the Hubble Space Telescope and the Spitzer Space Telescoope, a team of scientists, led by the group of Computational Astrophysics from
the Munich University Observatory (USM) and the Instituto de
Astrofísica de Canarias (IAC) has been able to study in detail how the
black hole of the Andromeda galaxy is carefully fed.
“Black holes are greedy feeders, but nevertheless sensitive” explains Christian Alig,
a researcher at the USM who is the first author of the article. “When
they are fed slowly and bit by bit they don’t show signs of feeding, but
if the feeding is forced and excessive they react violently and
aggressively.”
Using powerful computers it has been possible to simulate the transport and whereabouts of this material through time and space. The image shows snapshots of the path versus time (indicated in the upper panels in units of millions of years) increasing from left to right. The network of paths, or filaments (streaks of colour in each image) appears to get more complicated as they approach the centre of the galaxy. However the motion is ordered and progressive, with the filaments rotating slowly in a spiral towards the black hole. The journey lasts for over 100 million years.
While the Hubble can see the darkening produced by the dust of the filaments in visible light, the Spitzer telescope can pick out the same filaments in the infrared. Using joint observations with the both telescopes has revealed a complete view of the accretion process of the material around the black hole. Because the Andromeda galaxy is so near, the Spitzer observations of its centre are the most detailed observations of the centre of a galaxy made with this telescope until now, and have a level of precision comparable to that achieved by the Hubble Space Telescope.
This study is part of the PARSEC project, whose aim is to investigate over a wide range of wavelengths, the nuclei of the nearest galaxies, and the accretion processes of their central black holes. Led by the IAC the project has almost 50 members in institutions in a large number of countries.
Article: C. Alig, A. Prieto et al. “The Accretion Mode in Sub-Eddington Supermassive Black Holes: Getting into the Central Parsecs of Andromeda”. 2023 ApJ 953 109. DOI: 10.3847/1538-4357/ace2c3
Contact at the IAC:
Almudena Prieto, almudena.prieto@iac.es
