BUILDUP by EU Research

During the peak activity epoch,

galaxies were intensively forming stars across the universe.

While there are more galaxies today, the star

Tracing the first steps of galaxy evolution Galaxy surveys are a central tool in investigating the history of the universe, allowing researchers to look far back in cosmic time and study different stages of galaxy evolution. Researchers in the BUILDUP project aim to reconstruct galaxy assembly and evolution when the universe was young, as Professor Karina Caputi explains. The universe is

thought to be around 13.8 billion years old, and researchers continue to probe ever deeper into its history and evolution. While in the first billion years after the Big Bang the universe was comprised solely of gas and light, gravitational collapse then led to the formation of stars and galaxies. “That period started less than one billion years after the Big Bang, it’s called reionization. That’s because the first stars and galaxies produced copious amounts of ultraviolet photons that were able to ionize the surrounding gas,” explains Professor Karina Caputi, the Principal Investigator of the BUILDUP project. This is considered to be the starting point of galaxy evolution, now Professor Caputi and her colleagues in the project aim to build a fuller picture of how galaxies formed over the subsequent few billion years after reionization. “Our aim is to reconstruct galaxy evolution over that epoch, just after the formation of the first stars and galaxies. This project is about the first steps of galaxy evolution,” she explains.

Reaching the peak activity epoch The peak activity epoch of the universe, during

which star formation activity reached a peak, happened long after the reionization period, around 10 billion years ago. “How that peak activity epoch was reached, from the beginning of the reionization period, is quite unknown. That period is still relatively unexplored,” says Professor Caputi. This period is the main focus of the project. “The big goal of the project is to look at galaxy evolution, after the formation of the first galaxies. So, the period between the formation of the first galaxies, until the universe reached that peak activity epoch,” continues Professor Caputi. “During that epoch, galaxies were intensively forming stars across the universe. While there are more galaxies today, the star formation rates and the overall levels of activity are much lower.” By way of comparison the Milky Way currently forms one or two new stars a year, whereas star formation rates during the peak activity epoch were well into double figures, and sometimes even reached the hundreds. Professor Caputi and her colleagues in the project are analysing data gathered from the Spitzer Space Telescope to investigate the formation of galaxies from right back beyond the peak activity epoch. “Our

galaxy survey starts with data from Spitzer – but we also make use of other sources of data,” she outlines. The survey has a unique combination of area and depth, from which researchers hope to learn more about how galaxies developed during the young universe. “Sometimes you have very deep images of a small region of the sky, while on other occasions we have large images which are not very deep, so we cannot see that far,” says Professor Caputi. “The advantage of our survey is that it has a very nice combination of area and depth.” The Spitzer Space Telescope itself has been operating for almost 15 years now, sending back images of vast numbers of stars and galaxies at infrared wavelengths. While there is clearly enormous scope for observation from a telescope like Spitzer, Professor Caputi focuses her attention on a particular patch of sky. “If you want to see unknown galaxies then you need to stay for a long time in the same blank piece of the sky,” she explains. By devoting more time to observing the same patch of the sky, researchers hope to learn about previously unknown galaxies. “Certain galaxies are clearly visible – but if you don’t

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formation rates and the overall levels of activity are much lower. stay long enough then you would never see new galaxies,” points out Professor Caputi. “If you stay for a long time in a particular part of the sky, and you keep the telescope there, then you would collect enough photons to eventually see them.” This is allowing researchers to look back deep into cosmic time and collect statistics and data on around 300,000 galaxies, all present in the patch of sky that Professor Caputi and her colleagues are studying. These galaxies were formed at different points in time, so there are a number of factors to consider in studying them. “We study the spectra of these galaxies by looking at many images from different wavelengths. We look at the shape of these spectra, and the way these have shifted into the red,” explains Professor Caputi. This shift is produced by the Doppler effect in astronomy, which is caused by the expansion of the universe. “It’s the same kind of effect that you experience with sound, for instance as an ambulance approaches with sirens wailing,” outlines Professor Caputi. “As it approaches, the tone will be quite high, then as it moves away, the pitch of the sound will be much lower.”

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Galaxy formation models Researchers are also working with galaxy formation models, based on the Cold Dark Matter (CDM) framework, which generate predictions of how cold dark matter was distributed in the universe at different periods. While a number of other cosmological models have been developed, the CDM model has been the most successful in terms of predicting the general properties of galaxies that we see today. “The CDM makes specific predictions of how galaxies are distributed in the universe spatially, and the large-scale structures of the universe that we see today. If you compare those predictions to the data that we see in nearby galaxies, the CDM framework is the best,” says Professor Caputi. This does not mean that models are entirely accurate however, and Professor Caputi says there are some limitations. “Sometimes the models cannot correctly predict how observed galaxies behave, and their properties at different cosmic times,” she acknowledges. By comparing observed data with predictions from models, researchers can then assess whether those predictions are correct or not, and learn more about the galaxies they are observing. Professor Caputi and her colleagues are working on a number of papers, and significant progress is being made. “We are really understanding much better how galaxy evolution proceeded in the time before the peak activity epoch of the universe,” she says. The development of the James Webb Telescope will also open up new observational possibilities; Professor Caputi is keen to work with data from this telescope in future. “This new telescope will be extremely powerful, but there will also be limitations. We need to understand that preferentially before the data arrives, in order to understand how we are going to work with that data and make it suitable for analysis,” she outlines.

Buildup Galaxy Buildup in the Young Universe: from the First Billion Years through the Peak Activity Epoch Project Objectives

The aim of the BUILDUP project is to reconstruct the history of galaxy assembly and evolution from the first billion years of cosmic time through the peak activity epoch, which occurred 10 billion years ago, in order to provide fundamental constraints for galaxy evolution models. BUILDUP involves the scientific exploitation of one of the largest observing programmes ever conducted with the Spitzer Space Telescope, and constitutes a bridge between current and future generations of infrared galaxy surveys.

Project Funding

This Project is exclusively funded by the European Research Council and the only beneficiary is the University of Groningen.

Contact Details

Professor Karina I. Caputi Kapteyn Astronomical Institute University of Groningen P.O. Box 800 9700 AV Groningen The Netherlands T: +31 50 363 8325 E: karina@astro.rug.nl W: https://www.astro.rug.nl/~karina/ ERC_BUILDUP.html Cordis: https://cordis.europa.eu/project/ rcn/200775_en.html - Caputi, K.I. et al., Star Formation in Galaxies at z~4-5 from the SMUVS Survey: A Clear Starburst/Mainsequence Bimodality for Hα Emitters on the SFR-M* Plane, The Astrophysical Journal, 849, 45 (2017). - Cowley, W. I. et al., The Galaxy–Halo Connection for z=1.5-5 as Revealed by the Spitzer Matching Survey of the UltraVISTA Ultra-deep Stripes, The Astrophysical Journal, 853, 69 (2018). - Bisigello, L. et al., The Impact of JWST Broadband Filter Choice on Photometric Redshift Estimation, The Astrophysical Journal Supplement, 227, 19 (2016). - Ashby, M.N.L. et al., Spitzer Matching survey of the UltraVISTA ultra-deep Stripes (SMUVS): Full-mission IRAC Mosaics and Catalogs, The Astrophysical Journal Supplement, in press (2018)

Professor Karina Caputi

Karina Caputi is Associate Professor of Astronomy and ERC Consolidator Grant Laureate at the Kapetyn Astronomical Institute, University of Groningen. She previously held research positions in France, the UK and Switzerland. She has a combined background in physics and astronomy.