Type Ia supernovae are dramatic explosions of white dwarf stars at the ends of their lives. Approximately 2 weeks later, each single event reaches a peak luminosity of 10 millions sun-like stars with a remarkable consistency between different events. These objects are known as ‘standard candles’ in astrophysics. Today, the Zwicky Transient Facility survey (ZTF) publishes 3628 highly sampled type Ia Supernovae data collected by a single instrument between March 2018 and December 2020. The dataset is published alongside a special issue of 21 articles in the journal Astronomy and Astrophysics.
This release opens the era of high precision in supernovae cosmology. The team, in which the Institute of Space Sciences (ICE-CSIC) is involved, discovered a new effect that could change how the expansion history of the Universe is measured and may have important consequences for current deviation observed in the standard model of cosmology. One of the key outcomes of the studies published is that type Ia supernovae intrinsically vary depending on their host environment, more so than expected before, and the correction mechanism assumed so far has to be revisited. The ICE-CSIC participates through some members of the supernovae research group: Lluís Galbany, researcher from the ICE-CSIC and the Institute of Space Studies of Catalonia (IEEC), and ICE-CSIC predoctoral researchers Kim Phan and Alaa Alburai.
“For the past five years, a group of thirty experts from around the world have collected, compiled, assembled, and analysed this data. We are now releasing it to the entire community. This sample is so unique in terms of size and homogeneity, that we expect it to significantly impact the field of Supernovae cosmology and to lead to many additional new discoveries in addition to results we have already published”, says Mickael Rigault, researcher at the Institut des deux Infinis de Lyon (CNRS / Claude Bernard University) and head of the ZTF Cosmology Science working group.
Cosmologists have learned to use these complex standard candles to probe distances across the Universe by comparing their fluxes, as further objects appear dimmer. The acceleration of the Universe expansion, whose discovery was awarded the Nobel prize in 2011, was first detected in the late 90s using around 100 of these supernovae. Since then, cosmologists have been investigating the reason for this acceleration caused by the dubbed dark energy that plays the role of an anti-gravity force across the Universe.
Recent state-of-the-art type Ia supernovae datasets compile around 2000 objects gathered from many different telescopes and acquired for the last two decades. Analyses of these samples suggest that dark energy may be more complicated than a simple mathematical constant in Einstein’s equation as assumed since its first discovery. Our sparse knowledge of the exact physics responsible for the type Ia supernovae astrophysical phenomena also affects our ability to derive precision distances to probe the fundamental physics of the Universe.
"The uniformity of this dataset sets a new standard for nearby supernova observations, surpassing all those collected over the past decades. It will serve as the reference for future high-redshift supernova studies, including those from next-generation projects like the Legacy Survey of Space and Time (LSST) and the Roman Space Telescope, bringing us closer to understanding the true nature of dark energy that drives the accelerating expansion of the Universe" concludes Lluís Galbany, ICE-CSIC and IEEC researcher, and member of the ZTF Cosmology group.
A unique instrument for a groundbreaking dataset
The ZTF camera, installed on the Samuel Oschin telescope at Palomar Observatory (United States of America), scans the entire northern sky daily in three optical bands thanks to its 47 square degree camera and rapid 30s acquisition. In that short time, the ZTF camera reaches a depth of 20.5 magnitude, so one million times fainter than the dimmest stars visible to the naked eye. This sensitivity allows ZTF to detect nearly all supernovae within 1.5 billion light-years of Earth. This is the first time that astrophysicists have access to such a large and homogeneous dataset. Type Ia supernovae are rare, occurring approximately once per thousand years in a typical galaxy, but ZTF’s depth and survey strategy enable researchers to detect nearly four per night.
Previous supernova samples covering this distance range contained fewer than 200 events. The published dataset increases this number by an order of magnitude, enabling much more accurate analyses, study of rare events, comparing similarities and differences between many sub-groups, etc. This breakthrough allows researchers to address fundamental questions previously hindered by limited sample sizes. “This release provides a game-changing dataset for supernova cosmology,” says Mathew Smith, co-leader of the current dataset release and Lecturer in Astrophysics at Lancaster University. “It opens the door to new discoveries about both the expansion of the universe and the fundamental physics of supernovae," he adds.
“We are providing the community with thousands of density sampled and well calibrated type Ia supernovae light curves, spectra, host properties and observing logs. These resources will enable researchers to refine models and develop new techniques to enhance the precision and accuracy of distances derived from SN Ia observations,” summarizes Jakob Nordin, head of the Berlin ZTF Cosmology Group.
Dark energy and supernovae
For decades, type Ia supernovae have played a pivotal role in the discovery and study of dark energy: the mysterious force responsible for the accelerating expansion of the Universe. To do so, astronomers compare the redshift, i.e., the photon’s wavelength stretch caused by the expansion of the Universe, and the supernovae brightness, which indicates the distance of the light source and thus the time it took for the photon to reach us. Together, these quantities enable us to probe the expansion history of the Universe and consequently the fundamental physics responsible for it.
Yet, the exact nature of type Ia supernovae remains unknown. We know from studying spectra and rate of brightness evolution with time, aka. lightcurves, that this phenomenon arises from the thermonuclear explosion of a Carbon/oxygen white dwarf, very likely from a binary system (or more). But why this white dwarf exploded, how its composition or the one from its companion star influenced the explosion mechanism remains unclear. There could even be multiple channels leading to similar events. Hence the question for cosmology: how accurate are distance measurements derived from type Ia supernovae light curves despite our limited knowledge on what these objects truly are?
This question has been the focus of the current ZTF release: “With this large and homogeneous dataset, we can explore type Ia supernovae with an unprecedented level of precision and accuracy,” comments Mickael Rigault. “This is a crucial step toward honing the use of type Ia supernovae in cosmology and assess if current deviations in cosmology are due to new fundamental physics or unknown problem in the way we derive distances”, he adds.
Measuring distances with complex standard candles
The team studied the diversity of the type Ia supernovae events, unveiling sub-populations and extreme objects to probe the homogeneity of the core sample. This led to refining which supernovae should be used for their study in cosmology.
Researchers explored how type Ia supernovae vary based on their environments, be it made of young or old stars, having lots of interstellar dust or stripped from any gas. This informs us about the origin of observed variabilities: whether they are connected to the progenitor system material or not, the speed of evolution from the progenitor star formation to the resulting white dwarf explosion as a supernovae and if the observed variations are more directly connected to line-of-sight influence of host galaxy materials such as interstellar dust or not. By doing so, the team attends to best correct observed variabilities to obtain accurate distances and better understand the supernovae nature.
Studying the early and late lightcurves points and spectra uniquely acquired with ZTF to directly probe the physics of the progenitor system was another focus of the study. “Thanks to ZTF’s unique ability to scan the sky rapidly and deeply, we have captured multiple supernovae within days—or even hours—of explosion, providing novel constraints on how they end their lives”, highlights Kate Maguire from Trinity College Dublin (Ireland), co-author of the study.
Co-author Ariel Goobar, director of the Oskar Klein Centre in Stockholm (Sweden), one of the founding institutions of ZTF, and also member of the team that discovered the accelerated expansion of the Universe in 1998 says: “Ultimately, the aim is to address one of our time’s biggest question in fundamental physics and cosmology, namely what is most of the Universe made of? For that we need the ZTF supernova data.”
