A young Roderik Overzier at the National Air & Space Museum in Washington DC, standing in front of a press release about his PhD research with the Hubble Space Telescope (2004).

The question of how the galaxies and the large-scale structure formed is extremely complex. The answers involve a great many details related to star formation, dark matter, supernova feedback, active galactic nuclei, gas accretion, and cosmology that are still unknown. We study how galaxies formed and evolved as a function of time and place in the cosmic web. We use multi-wavelength observations from the X-rays to the radio from the world’s main ground- and space based observatories (for example, VLT, Hubble Space Telescope, Chandra, Spitzer, etc.). We also use detailed theoretical predictions and cosmological simulations in order to better understand the observational results and in order to create better models. We are performing small, targeted research programs, as well as participating in a number of large international collaborations in order to find the answers to the many intriguing questions.

Motivation for doing Extra-galactic Astrophysics in Brazil

Originally from The Netherlands, I have been very fortunate to work in several astrophysics departments around the world (Leiden Observatory in Holland, Johns Hopkins University and the University of Texas in the United States, and the Max-Planck Institute for Astrophysics in Germany). I joined the Observatório Nacional in Rio de Janeiro in 2013 in order to contribute to the development of extra-galactic astrophysics and astronomy in general in Brazil. Today is a very exciting time for young people to be interested in astronomy in Brazil, with some amazing opportunities to do fore-front research with the world’s best telescopes (Gemini, Subaru, SDSS, ALMA, VLT, HST, JWST, GMT, etc.). Especially in the field of extragalactic research, we are benefiting from the many new exciting international projects and telescopes that we are part of.

Some areas of activities by our group

The formation of galaxy clusters – Galaxy clusters are the largest gravitationally-bound objects in the universe. We study how to find them at high redshifts, and how to study the cluster formation process by comparing clusters and the progenitors of clusters (“protoclusters”) at high and low redshift. For a recent review on the topic, see my paper The realm of the galaxy protoclusters (Overzier, A&ARv, 24, 14, 2016).

The formation and evolution of galaxies – At high redshift, the Hubble Space Telescope has found small galaxies that must be the building blocks of the present-day population of spiral and elliptical galaxies. How did this process of galaxy assembly exactly happen? By using detailed observations of both high redshift and nearby galaxies, we are trying to piece together this complicated puzzle. In the process, we are learning many new details about a variety of interesting astrophysical processes including star formation, supernovae, winds, galaxy merging, black hole formation, and chemical enrichment.

Active galaxies and black holes – Active galactic nuclei (AGN) are extremely energetic phenomena occurring in the cores of some galaxies. They are the result of the inflow of matter onto a supermassive black hole. We are especially interested in the role of AGN in the formation of galaxies, and their relation with the large-scale structure.

Properties of the first galaxies – Telescopes can now quite easily select galaxies at redshifts z > 6, corresponding to only ~0.5-1.0 Gyr after the Big Bang. We are trying to measure the properties of these galaxies in order to get an idea of what this first population looked like and what it may have evolved into at later times. They also offer an important probe of the epoch of cosmic reionization, as these galaxies produced the hydrogen ionizing photons that were needed to ionize the neutral intergalactic medium.

Virtual theoretical observatory and cosmological simulations – We make frequent use of so-called theoretical universes and virtual observatories. A virtual observatory can be used to create simulated telescope images and spectra based on cosmological simulations of galaxy formation. When we developed our virtual observatory, The Millennium Run Observatory, this was a very new concept in astrophysics. Today, such techniques have become a standard tool in many new survey projects.

The Sloan Digital Sky Survey – ON is a member of the SDSS-IV survey project, which includes a large integral field survey of 10,000 nearby galaxies (MaNGA) and a high redshift spectroscopic survey that will target 1 million distant galaxies and quasars (eBOSS).

The S-PLUS Survey – I am the Project Scientist for the Southern Photometric Local Universe Survey (S-PLUS). This is the largest brazilian-led survey project undertaken to date, and aims to map 9000 square degrees of the sky in 12 broad and narrow band filters using a small (0.8m) robotic observatory at CTIO. An overview of the survey along with the first public data release can be found in the paper by Mendes de Oliveira and the S-PLUS collaboration (MNRAS, 2019, in press).

The J-PAS Survey – The Javalambre Physics of the Accelerating Universe Survey (J-PAS) is the big sister of S-PLUS. It is a Spanish-Brazilian project that will image 8000 square degrees of the sky in 56 narrow optical filters to measure dark energy, galaxy evolution, stars, and solar system bodies. The survey is expected to start in 2019/2020, but has performed a small pathfinder survey that is currently being analyzed by the collaboration. 

The Subaru HyperSuprimeCam SSP – I am an external member of the team that is exploiting the HSC-SSP to study the formation of galaxy clusters. This project has already led to some amazing results, such as ~200 new protoclusters at z~4 (this is 5 times more than have been found in the past 20 years!). Read all about it in this NAOJ press release.

The Subaru Prime Focus Spectrograph Survey – Through IAG/USP and FAPESP, I am a member of the collaboration to build a revolutionary new multi-object spectrograph for the Subaru Telescope. The goal is to measure dark energy, galaxy evolution and AGN at high redshifts. The collaboration is led by IPMU in Tokyo with partners Princeton University, Caltech Institute of Technology, Johns Hopkins University, Laboratoire d’Astrophysique Marseille, ASIAA/Taiwan, MPA, and IAG/USP and LNA in Brazil. With PFS, our group aims to study galaxies, galaxy clusters, and AGN at high redshifts to a level of detail provided by no other survey starting around 2021.

Giant Magellan Telescope – I am a member of the Scientific Advisory Committee of the GMT project for Brazil. The project is absolutely fascinating, and will offer us a chance to be among the first astronomers in the world working with the next generation of extremely large telescopes. 

Other projects – The universe is a very big place and extragalactic astrophysics is a very broad area of research that is always changing! The above list offers only a glimpse of the possibilities. Other active projects in my group include observations with the Hubble Space Telescope and the future James Webb Space Telescope and the Gemini Observatory.