My Research
Supernovae are the powerful explosions of stars at the end of their life-cycles. There are many different types of supernova, depending on what type of star exploded. Perhaps the most famous are those categorised as “Type Ia”.
Type Ia Supernovae
Type Ia supernovae (SNe Ia) result from the thermonuclear explosion of a white dwarf star, which releases energy through the radioactive decay of Nickel-56. This provides a constraint on possible brightnesses of the explosion, enabling the use of SNe Ia as “standardisable candles” for measuring distances in the universe and uncovering the accelerating expansion of the universe.
Quite simply, if the absolute brightness of SNe Ia is known, their distance away from an observer can be measured by comparing the brightness observed to the known brightness.
Host Galaxy Properties
However, in reality, things are not so straight-forward. SNe Ia in high-mass galaxies are brighter than SNe Ia in low-mass galaxies. This suggests additional physics at play, such as unknown progenitor systems or explosion mechanisms, and raises questions about the validity of SNe Ia as “standardisable candles”. It is important to take the properties of the host galaxies, and the local environments of the SNe, into account in cosmological analyses. This is the particular focus of my research, with highlights in Kelsey et al. (2021) and Kelsey et al. (2023).
Local versus Global Environments
The host galaxy correction used in cosmology is usually built on stellar mass from optical-only SED fitting. In Kelsey (2026), I used UV-to-far-infrared photometry from the DustPedia galaxy catalogue to compare the local environments of 90 nearby SNe Ia with the global properties of their hosts. The far-infrared coverage gives much better constraints on dust, and helps break the age–dust degeneracy that limits optical-only studies. Local colour, specific star-formation rate and dust attenuation all differ from the host galaxy averages by more than the measurement uncertainties allow, while mass-weighted age is the only property consistent locally and globally. Galaxy-integrated properties don’t describe the conditions where the SN actually exploded.
Supernova Siblings
SNe that share the same host galaxy (known as “siblings”) can help us with this. Given siblings share host galaxies, they share global environmental properties and associated systematic uncertainties. By comparing cosmological properties between sets of siblings, we could determine if sub-galactic environmental dependencies are the dominant factor in SNe Ia cosmology.
I have built the largest sample of SNe Ia siblings to date (Kelsey 2024), which I am using to investigate this further, with my Hubble Space Telescope program (#17194) studying their local environments. The siblings in my 2026 paper show this effect: those sampling the same galaxy structure, such as two SNe in spiral arms, have consistent environments, whilst others diverge.
