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Galaxy evolution in dense environments

Periodic Reporting for period 3 - Fornax (Galaxy evolution in dense environments)

Reporting period: 2019-10-01 to 2021-03-31

On a "small" scale (up to hundreds of millions of light years) the structure of the Universe is far from homogeneous. Galaxies are distributed on long sheets and filaments, and avoid the large and empty cosmic voids. At the crossing between filaments we find the most massive bound structures in the Universe: galaxy clusters. To this day, clusters continue to pull more and more galaxies towards them thanks to their gravitational attraction. Galaxies falling into clusters undergo rapid evolution as they collide with one another and feel the pressure of the hot (up to hundreds of millions of degrees) ambient gas. Eventually, these interactions lead to the exhaustion of galaxies' cold gas reservoir and shut down of star formation within them. From that point on these galaxies evolves "passively", that is, they continue to age without forming new stars.

This ERC project, called FORNAX, ivestigatesn the exact physical mechanisms that determine the evolution of galaxies in clusters. In particular, as clusters come in different size and mass, we want to understand down to what cluster mass events such as galaxy collision and interaction with the cluster hot ambient gas are relevant. To do so we will study one of the most nearby small clusters, called Fornax. We will do so using a brand new radio telescope, MeerKAT, located in South Africa. This telescope will allow us to observe the cold gas of galaxies in Fornax with unprecedented sensitivity and resolution, and answer the questions of whether, through what processes and how quickly this gas is being removed from them.
The first part of this project consisted in preparing the tools we will need to process the data of the MeerKAT telescope. This is challenging because sensitive observations such as those we will perform require a detailed understanding of the instrument -- something that is never trivial especially with a new telescope. Furthermore, MeerKAT produces very large amounts of data, and efficient algorithms are necessary to process them in a reasonable amount of time. For this reason the FORNAX team has focused on the development of a new data processing pipeline and on its testing. We have also worked on analysing the first data coming from the new MeerKAT telescope to help verifying that it is working well. For the moment these are commissioning data, but they are already giving us exciting new insights in the nature of the Fornax cluster.
One of our main scientific results during this period has been the study of the most famous examples in Fornax of a galaxy undergoing removal of its cold gas because of the pressure from the hot ambient gas. Our data, taken in this case with an Australian telescope, have actually proven that hypothesis wrong. In a paper led by Karen Lee-Waddell we have shown that, in fact, that galaxy owes its distorted optical appearance to a collision with another galaxy. So the search for an evidence of the interaction between galaxies and the hot ambient gas of Fornax is still on.

Another important success, which we are currently in the process of finalizing for publication, is the new detection of long tails of cold gas in the Fornax region. This detection gives us new insights on the recent evolution of some galaxies in the cluster and, in particular, on their recent merging with other galaxies. This work, which we will disseminate once published, is but an appetizer for what MeerKAT is capable of delivering for our projects.