astrobites | Daily Paper Summaries | 2019 Sep 20
Jessica May Hislop wrote:
The effect of dark matter on galaxies was first observed back in the 1920s by various astronomers. When studying the rotation of the outer parts of the galaxy, they could see it was going much faster than it should be given the mass of the galaxy (estimated from the light emitted by the stars). It was therefore inferred that there must be more mass in the galaxy that we can’t see. Moreover, it wasn’t just a little bit of mass that we couldn’t see, it was 5 times the mass of the visible stars within the galaxy.
Within the standard cosmological model, dark matter makes up approximately 27% of the Universe, whereas normal matter, known as baryons, only makes up 4.6%. It is thought that dark matter is spread all through our Universe. It is not smoothly distributed however. It is formed of filaments, very much like a very disorganised spiders web. These filaments join together at certain points, and it’s at these points where we have a lot of dark matter where the biggest galaxies form. A large group of dark matter particles causes a deep potential well, in which gas can fall in and form the first stars and build up a whole galaxy. The more dark matter there is, the deeper the potential well and the larger the galaxy that forms there.
In order to determine the dark matter content, we can define the ‘half optical light radius’ which is the radius at which half of the optical light of the galaxy is contained. For low mass galaxies known as ‘dwarf galaxies’ within the local group, whilst they have less dark matter than very massive galaxies, they are still dominated by dark matter even within this half optical light radius.
Today’s paper presents 19 galaxies that seem to be mostly baryons well beyond the half optical light radius, rather than being dominated by dark matter as expected. 14 of these galaxies are isolated galaxies, meaning they have no nearby larger galaxies which may be affecting them. ...
A population of dwarf galaxies deficient in dark matter ~ Qi Guo et al
- arXiv.org > astro-ph > arXiv:1908.00046 > 31 Jul 2019