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Friday, January 3, 2025

The treasure hunter



Justo A. González Villalba

Welcome, my dear visitor. My name is Justo Antonio González Villalba, and everything you will read in this blog is my very direct typewriting.  I am originally from Spain, Madrid, one of the most welcoming and culturally diverse cities I know in the country and abroad, where I studied applied physics at the Universidad Complutense de Madrid (UCM). I started my decades-long career as a software engineer working on Earth-observation projects for the European Space Agency (ESA), such as SMOS and SWARM, to study the evolution of the Earth's temperature and magnetic fields.



ALMA antennas observing an event horizon

Then I moved into astronomy, thanks to the European Southern Observatory (ESO), where I have been working for Atacama Large Millimetre Array (ALMA), one of the largest astronomy endeavours in the world. ALMA is famous for many things, such as unveiling protoplanetary discs and contributing to making the first image of a black hole event horizon. I started with ALMA a couple of years before the first scientific observations, at the time when we were at the edge of what astronomy can deliver. At first I joined the ALMA Archive team and developed the first working version of the data acquisition and distribution system, which is still running smoothly nowadays. Then I moved into the Common Astronomy Software Applications (CASA), where I lead developments in many areas, such as the data transformation and parallel processing frameworks. Currently I am the lead of the ALMA telescope calibration software, which runs continuously in the observatory (24/7) and performs the online calibration of the array, including many hardware components such as antennas, receivers, and electronics.

University Observatory of Munich

Some years ago I decided to dig further into the scientific aspects of the projects I work with. So I enrolled to study astrophysics at the Ludwig Maximilian University of Munich (LMU), in particular in the University Observatory of Munich (USM). This is the same university where giants of quantum mechanics, such as Max Planck and Wolfgang Pauli, studied too. However, my main research focus is quite far from the quantum realm, rather on the completely opposite side: galaxy clusters, which are some of the biggest structures known in the Universe. Also, nowadays we can use the immense computational power available to run and analyze cosmological simulations that put to the test our models for the physics of galaxy clusters. I can do this thanks to the Computational Astrophysics (CAST) group of the USM, in particular Prof. Dr. Klaus Dolag.


The cosmic web
The reason why I study galaxy clusters instead of quantum mechanics? This is simply because they represent the ultimate frontier of physics. We know in great detail what happens at the quantum level, and this knowledge is solid up to the stellar evolution models; also, classical gravity works really well at the scale of the solar system, but beyond is where things start to get very complicated. At the scale of galaxy clusters, unclear elements of physics, such as dark matter, become dominant, and the borders between the Universe as a whole and local structures are fuzzy. Galaxy clusters represent metaphorically and physically the Gordian Knot of astrophysics as they build up in the knots between filaments of the cosmic web.



Jet-driven black hole feedback
There is one particular class of galaxy clusters that has drawn my attention since it escapes all explanations in terms of classical physics, and it touches on one of the concepts of physics that is crucial for our society: energy. I am talking about the so-called cool-core galaxy clusters. These objects are cooler and denser in the central regions, so theoretically they should radiate away their energy reservoir quickly, producing significant amounts of even colder gas and triggering star formation. This situation is described as the cooling flow problem or cooling catastrophe in analogy to the ultraviolet catastrophe of the black body radiation back in the early XX century. However, the significant amounts of cold gas and star formation that this model predicted have never been observed. The most established theory is that cool core galaxy clusters host a central black hole that accretes material and ejects some of it, generally in the form of jets at relativistic speeds, heating up the intra-cluster medium (ICM) gas. I learnt all this directly from Prof. Dr. Hans Böhringer at the LMU, who is one of the founding fathers of this model.

But is this everything to it? Although the black hole energy injection theory is very well established, we have recurring questions as to why this energy perfectly balances the radiation losses (the so-called fine-tuning problem) and how it can be distributed across the intra-cluster medium. I think it is undeniable that black holes have an energetic imprint in galaxy clusters. However, we have to keep in mind that galaxy clusters are the biggest structures in the Universe, so it looks challenging to explain the whole problem with the mere contribution of the central black hole. One interesting avenue to explore is the contribution from mergers. Actually, mergers of galaxy clusters are the most energetic events since the Big Bang! The images below show different views of a galaxy cluster merger from the Magneticum Box2b simulation. One of the largest simulations for its resolution class.


Different views of a galaxy cluster merger

In this blog I will post my ideas, thoughts, and findings in the realm of galaxy clusters, the cool-core problem, dark matter, dark energy, and astrophysics in general. I hope you will find it informative and enjoyable!

And here is my list of publications. Don't hesitate to contact me if you have any questions or proposals for collaboration:

on
Location: Múnich, Alemania