by Joe Cairns, who worked at ESO – Garching with Dr. Andra Stroe (ESO Fellow)
This summer, I spent three months working on a research project at the European Southern Observatory (ESO) in Garching bei München, Germany. The 15 member states of ESO currently operate three observing facilities in the Atacama Desert: La Silla, Paranal and Chajnantor, boasting a range of instruments including the 3.5-metre New Technology Telescope and ESO’s flagship Very Large Telescope (VLT). ESO also jointly operates the Atacama Large Millimetre Array (ALMA) and are in the process of building the much anticipated Extremely Large Telescope (ELT) which is expected to see first light in 2024.
I lived in Garching for the duration of my research project; a small, quiet town in the outskirts of Munich just a short tube ride away from the ESO Headquarters. While the ESO observatories are located in Chile, the scientific research and the development of instrumentation is done primarily at these ESO Headquarters. Also on site is the ESO Supernova facility, a free museum and planetarium aiming to promote interest in astronomy and celebrate around six decades of scientific research at ESO. For example, the planetarium show I saw explained what could be seen in the night sky in the coming weeks, both with and without a telescope, and discussed the telescopes currently operated by ESO.
What makes ESO such an interesting place to work is that you meet people from a variety of different backgrounds and at different stages in their careers (e.g. PhD students, fellows, researchers). The astronomy department at ESO feels very much like a community and the opportunity to attend talks, seminars and other activities means that you get to constantly meet new people and take part in interesting discussions. Many of these talks were on topics that I had not studied in much detail during my degree, and so gave me the opportunity to learn exciting new things. For example, I attended talks on the development of astronomical instrumentation, on the hunt for exoplanets and on X-Ray binaries, as well as many others. That being said, there are many more relaxed events to take advantage of at ESO, including Wine and Cheese Seminars and weekly Beer Fridays.
Outside of ESO, Garching was an extremely exciting place to live. I was just 30 minutes from the centre of Munich by U-bahn, allowing me to easily explore the city in my free time. On Sundays, many museums and art galleries charge just €1 for entry, meaning that I could visit many of Munich’s tourist attractions (including the Museum Fünf Kontinente, the Neue Pinakothek and the Staatliche Museum Ägyptischer Kunst). I also visited many famous beer gardens and beer halls which are an important part of Munich’s heritage. Munich’s location and excellent transport system meant that I could visit places slightly further afield, such as Neuschwanstein Castle and Salzburg. I was also lucky enough to catch a pre-season football match at the Allianz Arena between Bayern Munich and Manchester United!
During my time at ESO, I worked on studying the molecular gas content of galaxies in disturbed cluster environments with Dr. Andra Stroe. It is well understood that the properties of a galaxy are linked to the environment in which it resides. Galaxies residing in local, relaxed clusters tend to be passive and elliptical whereas, in the field, you’re more likely to find star-forming spiral galaxies. However, recent studies have shown that interactions between gas-rich galaxies residing in merging clusters and the shocks generated by the merging clusters can enhance star formation activity in these disturbed cluster environments, resulting in a reversal of the typical environmental trends. In order to understand how the enhanced star formation rates in disturbed cluster members are fuelled, we need to link the star-forming properties of the cluster members to their reservoirs of cold, molecular gas.
In this project, we aimed to study the molecular gas reservoirs of disturbed cluster members by looking at galaxies in the nearby Antlia galaxy cluster. This is our most nearby disturbed galaxy cluster, residing around 130 million light years away and containing around 375 galaxies. This project made use of the Atacama Pathfinder Experiment (APEX) telescope, an ALMA prototype antenna modified to operate as a 12-metre single dish radio telescope at ESO’s Llano de Chajnantor observatory in Chile. This telescope was used to probe the 2-1 rotational transition of carbon monoxide (otherwise known as the CO(2-1) transition) in 92 galaxies in the Antlia cluster. CO(2-1) is the best tracer of molecular gas in galaxies as it is the second most abundant molecule after H2 and has much lower excitation requirements. Moreover, its rest frequency of around 230MHz means that it is easily observable from the ground using telescopes such as APEX.
The team at APEX worked really hard to supply us with clean, low-noise raw data, which was helped by favourable weather conditions and a new receiver that was working better than expected. As a result, we ended up with very high-quality spectra once we had reduced the raw data. We used these spectra to measure the luminosity of the CO(2-1) transition, which was then converted into the CO(1-0) luminosity often used as a proxy for molecular gas mass. Of our total sample of 92 galaxies, we found around 30 galaxies with significant (signal-to-noise more than three) CO(2-1) detections, implying that around a third of our galaxies had significant reservoirs of molecular gas. These detections were independent of the position of the galaxy in the cluster; some detections were found in the cluster core while others were found further towards the outskirts.
We correlated our observations of the molecular gas reservoirs of our galaxy sample with their stellar masses, star formation rates and atomic hydrogen (HI) contents. We found strong correlations between star formation rate and CO(2-1) luminosity which, as expected, implies that galaxies with larger molecular gas reservoirs tend to be more star-forming. More surprisingly, we found little correlation between HI mass and CO(2-1) luminosity, and only a small fraction of galaxies with significant HI detections also have significant CO(2-1) detections. This implies that galaxies with significant reservoirs of atomic Hydrogen don’t necessarily have significant reservoirs of molecular gas in disturbed cluster environments. Finally, we found that our galaxies in the Antlia cluster have comparable molecular gas masses to a similar sample of galaxies residing in the field (these field galaxies make up the ALLSMOG survey), implying that galaxies residing in disturbed cluster environments are able to hold on to their reservoirs of molecular gas and avoid can avoid significant stripping. This agrees with the interpretation that enhanced star formation rates in disturbed cluster environments are a result of the interaction between gas-rich cluster members and the large shocks created by the galaxy cluster mergers. We also looked for evidence of stripping in these disturbed cluster environments by observing optical images of the galaxies with interesting, often asymmetric, CO line profiles. This allowed us to highlight potential candidates for follow up observations using more powerful telescopes.
To anyone considering taking part in a similar research project abroad, I could not recommend these opportunities more strongly. Personally, I believe that taking part in a real research project is the most exciting way to learn new things in your chosen field, but there are many other factors that make these experiences so important. These research projects give students an insight into the world of academia, showing them whether they love their chosen field enough to make a career out of it. For me, talking to the staff and students at ESO about their experiences and interests also helped to cement my interest in a career in academia. While these opportunities are academically rewarding, I have also found it so personally rewarding to throw myself into a different culture and experience new and exciting things that I would not have been able to experience had I stayed in England.