Astronomy and Astrophysics is the most recent field at ISTA, with a group that was established in 2023 and has since grown to include five research groups. Our research covers a broad range of topics from stars and compact objects all the way to the earliest galaxies and cosmology.
Our understanding of the universe from exoplanets characterization to galactic archaeology strongly relies on our understanding of stellar dynamics. It was recently discovered that stellar evolution models that take into account the rotation of the star and associated hydrodynamic transport processes predict a much faster rotation rate than what is observed. Since then, it is of paramount importance to understand how angular momentum is redistributed inside stars, as it could strongly modify our theory of stellar evolution, and thus the estimation of ages in the universe.
The research activities of the Asterics group led by Lisa Bugnet aim at understanding the evolution of stars and their dynamical processes thanks to Asteroseismology. This recent branch of stellar physics consists of the analysis of oscillations of stars induced by stationary waves.
Stellar magnetic fields, from the core to the envelope of stars, are at the center of the group’s research. They have largely been excluded from stellar evolution models due to a lack of observation and of theoretical prescriptions. To make the necessary breakthrough on our understanding of poorly characterized but essential stellar magnetic fields, the group brings together in synergy observational constraints from Asteroseismology, modeling of stellar evolution, and theoretical prescriptions of the effects of magnetic fields. This synergy allows us to build Magnetoasteroseismology from the main sequence to the late stages of the evolution of stars like the Sun.
Galaxies such as our own Milky Way are the largest bound structures in our Universe and consist of gas, stars, planets, black holes and dark matter. The astrophysical processes that occur in galaxies happened to every atom in our body and therefore teach us about our own cosmic origins. The Matthee group investigates the physical mechanisms that determine how galaxies and their constituents form and evolve using observations of the very distant Universe.
Galaxies form following the collapse of small perturbations in the initial density distribution that formed in the Big Bang. While the physics of gravitational collapse is well understood, detailed feedback processes that happen within galaxies, such as the formation of stars, supernova explosions and the growth of supermassive black holes are poorly understood, while they have significant impact on the fate of galaxies and the stars and planets within them. The Matthee group uses observations of galaxies in the distant Universe to look back in time and probe the properties of the young massive stars in these galaxies responsible for the creation of the majority of heavy elements and the production of most of the ionizing radiation in the early Universe, and their impact on interstellar gas clouds. Matthee uses the largest telescopes on Earth (such as ESO’s Very Large Telescope in Chile) and in space (such as NASA/ESA/CSA’s James Webb Space Telescope) to find and study the first generations of galaxies and focus on empirically mapping the interplay between these galaxies and intergalactic gas in the so called reionization epoch that was the last major phase transition of matter in the Universe and happened only a few hundred million years after the Big Bang. Matthee also uses large cosmological hydrodynamical simulations to investigate the relation between structure formation, galaxies’ growth rates and their imprints on chemical enrichment.
Over the past decades, it has become clear that most galaxies have a massive black hole in their nucleus, with masses ranging from a million to a billion solar masses. The energetic radiation and material jets emanating from these black holes play a significant role in sculpting galaxies over cosmic time, but their origin remains unknown. Observations have shown that these massive black holes already existed in early times, when the universe was less than a billion years old, i.e. they possibly pre-date galaxies.
We also know that galaxies are built up over cosmic time by a series of mergers, from small early protogalaxies with perhaps a million solar masses to present-day galaxies as large as tens of trillions of solar masses. Massive black holes in galactic nuclei are naturally expected to merge and to produce copious gravitational waves in the process. These early massive black hole mergers are the prime targets for forthcoming space-based low-frequency gravitational wave detectors, such as the European Space Agency’s Laser Interferometer Space Antenna (LISA), planned for launch in the 2030s.
Galaxies and their black holes form and evolve in an expanding background universe, whose expansion has recently sped up. This overall cosmic acceleration can be attributed to a hitherto unidentified component of our universe (dubbed “dark energy”), to a finite energy density of the quantum vacuum, or else to our lack of understanding of gravitation deviating from Einstein’s theory on cosmic scales. These different explanations cause subtle differences in how the average universe has evolved over time for the past several billion years, as well as the types of structures that formed and the speed at which these structures grew nonlinear. These differences can be revealed statistically through gravitational lensing, an effect of general relativity, which distorts the apparent shapes of galaxies—so-called weak gravitational lensing.
Zoltan Haiman’s group investigates questions related to the above phenomena through semi-analytic approaches, numerical simulations, and interpreting observations. How and when did the first objects (including stars and black holes) form in the universe, and what impact did they have on the rest of the universe? Are mergers a major contributor to the growth of black holes, and how do such mergers unfold in galactic nuclei in the presence of stars and gas? How can we use observations of both gravitational waves and electromagnetic radiation in tandem to answer these questions? Finally, while weak lensing is a promising cosmological tool, its full potential is yet to be realized. Haiman’s group has been developing new statistical approaches, including through machine learning, to utilize lensing data from large forthcoming surveys that will include measurements of the shapes of up to billions of galaxies.
Massive stars are rare, but so powerful that they can dominate the light from galaxies, provide explosive mechanical feedback to their surroundings and even drive cosmic reionization. Most massive stars spend their lives orbiting so close to a stellar companion that dramatic interaction between the two stars is inevitable as the stars evolve and swell. The perhaps most common outcome from such binary interaction is helium stars stripped of their hydrogen-rich envelopes either through mass transfer or common envelope ejection. These exposed cores are so extremely hot that most of their photons are emitted in the ionizing extreme ultraviolet, making them important contributors of hard ionizing radiation, both nearby and at high redshift. Stripped stars are also considered necessary steps in the creation of binary neutron stars that merge in a gravitational wave event, but can themselves be in so tight orbit with a companion that they should be detectable in the micro-Hz gravitational wave regime.
Despite their importance, hot binary-stripped massive stars remained a theoretical prediction for over half a century, until they recently were discovered in the nearby Magellanic Clouds. We work with the intersection between observation and theory, and with the main goal to use these new observations of stripped stars to improve our theoretical models of interacting binaries and massive stars. Our current computational tools include binary stellar evolution and structure modeling, hot star spectral modeling and spectral binary population synthesis modeling.
Stars and the remnants that represent the last stage in their life — white dwarfs, neutron stars, and black holes — encompass a variety of extremes that cannot ever be achieved on Earth: of gravitation, energy, density, temperature, and magnetic field. The Stars and Compact Objects group at ISTA strives to further our understanding of physics by probing the beautiful and complex laboratories that populate the skies above us.
This is an exciting time for stellar astrophysics as high-cadence time domain surveys (Gaia, PTF, ZTF, ATLAS, Kepler, TESS, and the Vera Rubin Observatory) are revolutionizing the landscape of stellar studies by allowing the exploration of the dynamic sky. Furthermore, spectroscopic surveys are ongoing (SDSS V, DESI, 4MOST, WEAVE etc.), which will provide spectral classifications for millions of stars. Space missions are also opening new windows on stars and their remnants: December of 2021 alone saw the launch of the James Webb Space Telescope, with its unprecedented sensitivity in the infrared, and of the Imaging X-ray Polarization Explorer, the first mission dedicated to X-ray polarimetry. The Laser Interferometer Space Antenna will expand our reach in the field of gravitational waves.
We take advantage of these incredibly rich datasets to explore stellar evolution in star clusters, study compact binaries and their remnants, and investigate neutron stars and black holes in the X-rays. With a mixture of observations, analysis of large datasets and theoretical work, we plan to tackle open questions about stars and their remnants that are important for different fields of astrophysics.