For over four decades SNe Ia (see Exploding Stars) have been of interest to Cosmologists as distance indicators. There are two reasons for this, firstly because SNe Ia are very bright (they have luminosities more than a billion times the luminosity of our Sun). Secondly, SNe Ia only have a small spread in their maximum brightness and this small spread can be further reduced by using information about the colour and width of the supernova light curve.
The reduction in the spread of the maximum magnitude in the light curve makes the SNe Ia standardisable candles in the Universe. The brightness of the SNe Ia can then be used to accurately measure distances in the Universe. Comparing the distance calculated from the brightness to that determined from the redshift of the supernova enables scientists to see that the Universe must be accelerating in its expansion, because these distances do not match, i.e. the SNe Ia are too faint for a given redshift, so must actually be further away than previously realised. The discrepancy in these measurements can be used to work out how much Dark Energy there is in the Universe.
SNe Ia are by now well-proven cosmological probes which provide convincing evidence for an accelerating Universe and can be used to map the geometry of the Universe to a high precision. In fact, SNe Ia were the first astronomical tool used to discover the accelerated expansion of the Universe and won three scientists, Saul Perlmutter, Brian P. Schmidt and Adam G. Riess the Nobel prize for physics in 2011.
However, there remain some unanswered questions in our understanding of supernovae, and SNe Ia in particular, with one of the most fundamental being what is it that triggers them. We know that a SN Ia is the explosion and complete destruction of a white dwarf in a binary (two star) system which happens when its mass gets too large for the star to sustain (known as Chandrasekhar limit, equal to 1.44 solar masses). But does the increase in mass come from the collision of two binary white dwarfs, or from the transfer of material onto a white dwarf from its binary companion? In the latter case the companion may be either a Main Sequence star, like our sun, or a Red Giant. Researchers have seen evidence for both of these possibilities in the last few years.
Gaia is expected to detect an average of three supernovae per day, adding up to a total of approximately 6300 supernovae over the five year life span of the Gaia mission. About 70% of these (~4400) are expected to be SNe Ia below redshift 0.1 (i.e. in the nearby universe). Each SN Ia which is brighter than 19th magnitude will have a low resolution spectrum from the blue photometer (BP) or red photometer (RP) spectrographs, which will help in determining whether it is a SNe Ia or not.
Here is an animation of some of the SN which have been previously discovered and gives an idea of what we might expect from Gaia (animation courtesy of Sergey Koposov, Institute of Astronomy, Cambridge) (see History of Supernovae Detections page for description; skip video):
There are many reasons why the supernovae sample detected with Gaia will be exciting and invaluable to astronomers:
1. SNe Ia and their host galaxies
- Supernovae appear to be affected by the galaxies within which they explode. Massive, old, red galaxies appear to have brighter or fainter explosions after scientists correct for the colour and shape of the light curve. These differences can affect the cosmological parameters that scientists are constraining and which tell us about the evolution of the universe, so understanding the relationship between the host galaxy and its SNe improves the cosmological measurements. There could be many reasons for this relationship :
a) The two stars which cause the supernova explosion could vary between different types of galaxy. This difference in origin is known as different “progenitor channels”. For example, in massive red galaxies SNe Ia may be caused by a white dwarf drawing material from a companion star. This is called the “prompt channel”. In less massive blue galaxies, SNe Ia could be triggered by the collision of two binary white dwarfs. This is known as the “delayed channel” as it takes longer to form two progenitor white dwarfs than it does to form just one.
b) There could be more dust blocking our view of the SNe Ia in some galaxies than in others - either dust within the host galaxy or material between the supernova host galaxy and us.
c) Some astronomers think that the amount of heavy elements or metals in the host galaxy may be driving the difference between supernova explosions.
d) Something else is happening which we haven't even thought about yet...
If astronomers can understand the relationships between the SNe Ia and the galaxies in which they occur then the accuracy of SNe Ia as standard candles can be improved even further. This will refine the distance measurements and give us a better measurement for the amount of Dark Energy in the Universe.
The Gaia SNe Ia sample is closer to us than most large SN surveys (redshift<0.15) so it will be easier to study relationships with the host galaxy, as a detailed study of the host galaxies is easier at closer distances. Then, relationships between the host galaxy and the SN characterised at low redshift will help to improve SNe Ia as distance indicators.
- The number of SNe Ia explosions in galaxies forming stars (mostly blue) and galaxies which have stopped forming stars, which astronomers call “passive” galaxies (mostly red), appear to be quite different. It fact, only a quarter of SNe Ia explosions are found in passive galaxies. This effect has only been measured at higher redshift and astronomers are interested to see if the same differences are present in the local Universe. The rates of SNe Ia in different host galaxies can be used to help investigate how galaxies evolve over time. Additionally, the rates can show us similarities between galaxies in the nearby Universe and their more distant relatives.
- Gaia provides extremely accurate positions and spatial resolution, which will allow for the detailed study of the spatial distribution of SNe Ia and core-collapse SNe within different galaxy types and environments. We might even be able to see some of the stars which exploded as supernova by going back and looking at older images of that patch of sky from, e.g. the Hubble Space Telescope, or even Gaia itself.
2. The Hubble Constant
- The results from the recent Planck satellite, measuring the Cosmic Microwave Background, highlighted a problem between the measurement of the Hubble constant in the distant Universe (using the Cosmic Microwave Background) and the local Universe (measured using tens of SNe Ia in the nearby Universe). The large sample of nearby SNe Ia detected by Gaia will greatly help in understanding why these two measurements do not match. By combining the large sample of nearby supernova detected by Gaia and more distant supernovae samples we may be able to detect the changes in Hubble constant from dark energy.
3. Large sample 'anchors' the Hubble diagram.
- Supernovae are used to constrain cosmological parameters using a Hubble diagram. A Hubble diagram is a plot of the distance modulus against redshift. The larger the range of redshift covered by the supernovae, i.e., the longer the x-axis on the Hubble diagram, the easier it is to constrain the cosmological parameters. Current SNe Ia cosmology studies use a range for local SNe Ia gathered from different telescopes each with different observational characteristics and potential inconsistencies, which will be vastly improved using the large low redshift sample from Gaia.
4. Super-Luminous Supernovae
- In the last five years a new type of supernova has been discovered called a “Super-Luminous supernova” . At the moment fewer than 20 have been discovered. Super-Luminous supernovae are ten to a hundred times more luminous than traditional SNe and are not yet understood. These super-luminous SNe change more slowly over time than 'normal' SNe and they have only been found at higher redshift. These new SNe seem to be about a thousand times rarer than 'normal' SNe. Local SNe searched have, in the past, usually been confined to small regions of sky, so it is not surprising we have not yet spotted any of these incredibly bright explosions nearby. Gaia is expected to find lots of these unusual objects and to detect the closest of these that we have ever seen. Gaia will be able to confirm the nature of these super-luminous SNe, and make the first measurement of the rate at which they occur.
5. Problems caused by our movement in the galaxy
- The Sun is moving as it orbits the centre of our Galaxy. Stars in other galaxies are similarly moving, as are the galaxies themselves (with nearby galaxies moving in similar directions). These movements, if not properly corrected for, could affect the measurements of nearby supernovae, which in turn could affect the cosmological parameters. As Gaia will detect SNe Ia over the entire sky these movements can be comprehensively investigated in order to minimise their effect on the SNe measurements.
6. SN rates in the local Universe
- During the last seventy years, many studies on the SN rate in the local Universe have been carried out. However, most previous nearby SN surveys have been confined to a sample of known galaxies as SN are relatively rare in the local Universe and it was not possible to observe SNe Ia over the whole sky. Working out the total rate of SNe from such surveys is very complex, since it is such a small sample and has large uncertainties. Gaia will allow us to measure rates of SNe over the whole sky and there will be enough SNe to investigate their rates in different types of galaxies. Comparing the rates of different types of SN at low and higher redshift will allow astronomers to investigate the different progenitor channels for the SN (e.g. two white dwarfs or a white dwarf and a companion).
In addition, measuring the rates of core-collapse SNe will help astronomers investigate star formation in galaxies.
For some additional information about the supernova Gaia expects to detect see Transient astronomy with the Gaia satellite.
To help understand the diversity of the SNe Ia observations the Canada-France-Hawaii Telescope Legacy Survey have turned the 241 SNe Ia they detected into a sound animation (created by Alex H. Parker (University of Victoria) and Melissa L. Graham (University of California Santa Barbara / LCOGT); original post: http://www.astro.uvic.ca/~alexhp/new/supernova_sonata.html). Here the volume represents the distance to the SNe Ia (more distant SNe Ia are quieter), the pitch represents the stretch (wider light curves are played on higher notes) and the instrument represents the mass of the galaxy which hosted the SNe Ia (massive hosts were played on the stand-up bass and the less massive hosts played by the grand piano). (Skip video.)
Page last updated: 01 May 2016