Why supernovae are brighter in old massive galaxies

Type Ia supernovae (SNe Ia) are critical tools for measuring the contents of our Universe, but recent studies found their cosmological brightness showed a bias with respect to the environment in which they are found:   SNe Ia in massive, old, metal-rich galaxies are slightly more luminous than their counterparts in less massive galaxies. This bias hinders efforts to precisely quantify the nature of Dark Energy, and its origin has been a subject of debate for several years. A new study by CAASTRO researchers Michael Childress and Christian Wolf with Harvard colleague Jabran Zahid shows that this bias can be naturally explained by the ages of the stellar systems that explode as SNe Ia, but more importantly that this effect evolves dramatically over cosmic time.

SNe Ia are universally believed to originate from white dwarf stars which reach a critical instability as a consequence of interaction with a binary companion star. These binary systems come in a variety of configurations which result in a distribution of “delay times” between birth of the binary system and final triggering of the SN explosion. Thus SNe Ia have an intrinsic “delay time distribution” (DTD) which is set by the initial distribution of binary system properties, and this DTD is equivalent to the rate of SNe Ia as a function of time following a single instantaneous burst of star formation.

The ages of systems which explode as SNe Ia are influenced not only by this DTD but also by the star formation history (SFH) of the galaxy in which the SNe are born. In this new study, the authors develop a sequence of galaxy evolution models based on observations from large galaxy surveys. These models define the mean SFH for galaxies as a function of their current stellar mass for any epoch of cosmic history.  With these at hand, they then derive the age distribution of systems exploding as SNe Ia for a galaxy of any chosen mass at any epoch of cosmic time.

The results of this calculation show a dramatic change in the age of SNe Ia as a function of their host galaxy mass – one that closely resembles the trend of cosmological SN Ia luminosity bias with host galaxy mass.  More importantly, the team found that this trend evolves with cosmic time, indicating that the SN Ia “host bias” is likely to evolve in a way currently not accounted for by supernova cosmology surveys. This work exhibits the power of theoretical galaxy evolution models for explaining systematic behaviours in supernova cosmology data, and provides key tools for correctly quantifying the nature of Dark Energy.

Figure Caption:
Mean age of SNe Ia as a function of galaxy mass for the local (z=0, solid red line) and distant (z=0.5, solid black line) universe. Dotted red contours represent the full SN age distribution at each galaxy mass, while the dashed blue line shows the earliest epoch of star-formation (and thus oldest possible SN age) for the distant universe.

Publication Details:

Michael J. Childress, Christian Wolf, H. Jabran Zahid in MNRAS, 445 (2014) “Ages of Type Ia Supernova Over Cosmic Time”