Supernovae are very rare phenomena in the Universe and their transient nature made them difficult to find for a long time. So, it is not surprising that the discovery rate was around two supernovae per month 30 years ago. Today, we are able to find supernovae daily. For example, the Intermediate Palomar Transient Factory, in which our group at the Oskar Klein Centre is involved, has discovered almost 3000 supernovae in the last few years. However, these supernovae are all relatively nearby, since the survey is not sensitive to the very distant ones.
Supernova rates, particularly at high distances, are important for several reasons. For example, core-collapse supernovae originate from the deaths of massive stars, and their rate can be used to trace the history of star formation in galaxies. Also, supernovae are the one of the major producers of metals in the Universe, so measuring supernova rates informs us about the chemical enrichment of galaxies over time.
However, measuring the supernova rate in the distant Universe is difficult. Even though supernovae are one of the brightest explosions that exist, at distances bigger than four billion light years they are hard to find simply because they become too faint. This has been especially problematic for the study of the rate of core-collapse explosions since they are on average the faintest objects in the supernova family and often embedded in dusty environments. Furthermore, due to the expansion of the Universe, the visual light from all distant objects is shifted to longer wavelengths. From the ground, near-infrared observations are particularly challenging due to the brightness of and variability of the atmosphere at these wavelengths.
Instead of waiting for more powerful telescopes to come online, we used the existing facilities and the magnification power of galaxy clusters as gravitational telescopes to take a peek at the supernovae in the early Universe. Galaxy clusters are the most massive gravitationally bound objects in the Universe, distorting and magnifying objects behind them. As predicted by Einstein’s general relativity, gravitational lensing magnifies both the area and the flux of background objects, thereby increasing the depth of the survey. In this way, the ability to find very distant supernovae is enhanced. It was Fritz Zwicky who suggested the use of gravitational telescopes nearly 80 years ago, but it is only recently that systematic supernova searches have been performed in background galaxies behind clusters. The supernova group here in Stockholm was the first one to explore this possibility using ground-based facilities in 2003.
As a continuation of this effort, during 2008-2014, we surveyed the galaxy cluster Abell 1689, which is one of the most powerful gravitational telescopes that nature provides. The results are presented in a recent publication in the journal Astronomy & Astrophysics. We used a near-infrared instrument on the Very Large Telescope in Chile, with obtaining supporting optical data from the Nordic Optical Telescope at La Palma. Our search resulted in the discovery of five very distant and magnified supernovae. Notably, we discovered a supernova located nearly 10 billion lightyears away that was magnified four times by the galaxy cluster, which makes it among the most distant supernovae yet observed. Using these discoveries, we measured the supernova rates up to the time when the Universe was only two billion years old, without requiring any expensive space-based follow-up facilities.
Galaxy cluster Abell 1689 observed through the years 2008-2014 where the position of the discovered supernovae are shown. The red contours are the magnifications of the background sources from the cluster. Prepared by Rahman Amanullah.
Monitoring the foreground galaxy cluster also offers the opportunity to detect supernovae that originate from galaxies which are cluster members. Since clusters are dominated by galaxies where star formation has ceased, these are not core-collapse supernovae, but so-called thermonuclear or supernova Type Ia. We discovered two of this kind in Abell 1689 which allowed us to estimate the rate of supernova explosions in the cluster. Cluster rates are important since they have can be used to study the origin of the mysterious progenitors of supernovae Ia (read also A shocked neighbour) and are essential in understanding the iron abundance in medium between the cluster galaxies.
Another prediction of Einstein’s relativity is that strong gravitational lensing like the one from galaxy clusters, can give multiple images of the same background galaxy. If a supernova explodes in one of these multiply-imaged galaxies, its images will appear with certain time delays relative to each other due to the fact that the light from each image has to travel a different path. Given this event is very very rare and that our observing program was modest, we did not discover such an event. However, in the paper, we estimated the number that can be expected for upcoming transient surveys. We found that LSST , and in particular WFIRST , can be expected to find tens of strongly lensed supernovae that would allow the time delays between the multiple images to be measured, which can be measure the Hubble constant but also other cosmological parameters.
– Tanja Petrushevska (firstname.lastname@example.org)
and Rahman Amanullah (email@example.com)
Preprint of the manuscript: http://arxiv.org/pdf/1607.01617.pdf