Modelling optically dark clouds in clusters of galaxies
Advisor: Richard Wünsch (AI CAS)
Co-advisor: Rhys Taylor (AI CAS)
Funding: base scholarship supplemented from a national grant until the end of 2021. The net income will be at least 20 000 CZK/month until the end of 2021. Extension of the supplement beyond 2021 depends on availability of funds.
Website: http://galaxy.asu.cas.cz/page/projects

Surveys of the Virgo cluster of galaxies in the neutral hydrogen 21cm line
carried out with the 300 meter single-dish Arecibo radiotelescope have
discovered a new class of objects. They are spatially unresolved clouds with
masses of order 10^7 Solar masses and velocity dispersions more than hundred
km/s (see the attached spectrum). Such high velocity dispersions suggest that
dynamical masses of these objects are much higher (by factor 100-1000), in the
range typical for galaxies. However, deep optical observations show no signs of
galaxies (i.e stars) at those positions at all. One hypothesis of the origin of
these objects suggests that they are in fact so called dark galaxies, i.e. dark
matter halos that did not accrete enough baryons to form stars, predicted to
exist by cosmological simulations. This hypothesis is supported by a recent
discovery of the ultra diffuse galaxies consisting of a stellar population of
the very low surface density embedded in a dark matter halo ~1000 times more
massive that the stellar component. The optically dark clouds of the neutral
hydrogen would be their natural extension - more extreme objects with even lower
fraction of baryons leading to star formation below the detectable limit.

The aim of this project is to perform hydrodynamic simulations of the neutral
hydrogen clouds embedded in dark matter halos. They will interact with the hot
rarefied gas filling the galaxy clusters leading to an important question: under
which conditions the neutral hydrogen clouds can survive. Another interesting
question is what are the conditions necessary to prevent star formation inside
those clouds. The numerical model will be based on the hydrodynamic code Flash
which is widely used within our research group. Most of the physical modules
needed for the given problem are ready as they were implemented to test another
hypothesis of the origin of these clouds - a turbulent sphere in equilibrium
with the hot ambient gas (see the attached figure). The comparison with the
Arecibo data will be done with the help of R. Taylor who has an extensive
experience with analysing the Arecibo data. A new neutral hydrogen survey has
just been finished (WAVES) and discoveries of more such objects are expected.