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How do galaxies get their gas?

Final Report Summary - GALGASSKA (How do galaxies get their gas?)

The main goal of the GalGasSKA project is to address the question of where galaxies get their gas from. The amount of gas that is currently observed in galaxies is only enough for galaxies to form stars for another 2 billion years. This is much shorter than the lifetimes of these galaxies, and implies that they are being fuelled from an external source. The most likely source, as also indicated by cosmological computer simulations, is hydrogen from the intergalactic medium (IGM; also known as the “cosmic web”). One might thus expect to observe a steady stream of cool gas clouds raining onto galaxies. Recent numerical simulations support this. However, a recent inventory of observations of nearby galaxies found that only 10 to 20% of the needed gas is found in this form. This poses a major problem. We know that galaxies must get their gas from somwhere, or else they would not form stars, but we are unable (so far) to observe this process.

GalGasSKA is preparing for and carrying out the deepest radio observations ever taken of nearby galaxies, to see if this so-called accretion process can still be observed. Some of this work is done with current telescopes, but a large part of the project is also to prepare for the arrival of new, next-generation telescopes that will be able to carry out much more sensitive observations. This obviously carries some risks, as major international projects can get delayed, but also offers a great opportunity to help prepare the science cases for these new large facilities.

Apart from the valuable scientific work, the GalGasSKA project therefore had as a second goal to help define the science cases for MeerKAT in South-Africa, Apertif in the Netherlands, and, in the longer term, the Square Kilometre Array (SKA) in South Africa and Australia. For these telescopes to be a success and justify their funding, the science case needs to be compelling. GalGasSKA addresses one of the current key questions in the field of radio astronomy and de Blok has been successful in getting his science be part of the Key Science Projects on the MeerKAT SKA-precursor telescope in South Africa, and to have it be one of the High Priority Science Objectives for the forthcoming SKA telescope.

The project has been very successful when measured against the original milestones, taking into account the revised timelines on some of these new large international facilities. The risk mitigation strategy as defined in the original proposal has in that sense worked well.

For the MeerKAT SKA-precursor telescope in South Africa, major progress has been made. The 6000h MHONGOOSE Large Survey Project to look at nearby galaxies (of which de Blok is PI and has been guaranteed time on MeerKAT) was further developed by defining a representative sample of galaxies, some of which were subsequently also observed with the KAT-7 MeerKAT precursor telescope. Commissioning observations with MeerKAT are currently taking place. This work has been done in close collaboration with the partners in South-Africa where de Blok still maintains the professional connections he made while working there. His honourary professorship at the University of Cape Town is a plus in that regard.

De Blok has taken on a major role within the Apertif/WSRT project here at ASTRON. This project to equip the Westerbork Synthesis Radio Telescope with new wide-field receivers is now in its commissioning phase and is expected to start delivering scientifically useful data later this year. Two years ago de Blok has taken on the role of Chair of the Apertif Survey Team. This meant spending a significant amount of time on defining the surveys, getting the community involved, coordinating work on the reduction pipeline and archive, maintaining contacts with the engineers and commissioning scientists and coordinating the scientific part of the project in general.

Two years ago de Blok also agreed to co-chair the HI Galaxy Science Working Group of the SKA (together with Dr. Martin Meyer from ICRAR, Australia). Here he has played a large role in promoting HI science within the SKA community, and also to be a liason between the SKA project and the scientific community. This meant a deep involvement with all sorts of aspects of the SKA, from defining surveys to evaluating SKA re-baselining proposals. The fact that out of the 13 SKA High Priority Science Objectives (which cover all of astronomy), three are HI Galaxy Science topics is testament to that. One of them furthermore directly targets the science questions of the GalGasSKA project.

Where science questions could not be directly addressed with these telescopes that are still in the (pre-)construction phase, de Blok has mitigated this by using existing facilities. His long-term involvement in The HI Nearby Galaxy Survey (THINGS; a survey he co-led, which is now almost a decade old, but still the benchmark in this particular field) meant that some aspects of the GalGasSKA questions could be addressed already. New projects were also started on the EVLA telescope in the US. One of these is currently in progress and aims to obtain the deepest, high-resolution radio observation ever taken of a nearby galaxy. These data are being analysed and thus directly address a key question of this project to try and observe the accretion of gas in nearby galaxies.

In summary, de Blok has integrated well here at ASTRON and now has a permanent position here as Senior Research Astronomer. His scientific interests and the questions posed in the GalGasSKA project align very well strategically with ASTRON's priorities as well as with the science cases of the SKA and its precursor telescopes. This is not only interesting scientifically, but also in terms of “Big Data”. These new telescopes will produce orders of magnitude more data than astronomers are used to, and the only way to deal with the data flow is through dedicated multi-node Science Data Centers (SDC). ASTRON has set-up an agreement with South African universities to start proto-typing and testing such a SDC where Apertif/WSRT (Netherlands) and MHONGOOSE/MeerKAT (South Africa) data have been chosen to test the principles of “one data set – multiple sites” data reduction.

Finally, I refer to the publication list attached in this report, showing that in addition to the many commitments to these upcoming projects, de Blok has managed to sustain a good refereed paper publication rate.