The Changing Fractions of Type Ia Supernova NUV-Optical Subclasses with Redshift

Understand

UV and optical photometry of Type Ia supernovae (SNe Ia) at low redshift have revealed the existence of two distinct color groups, NUV-red and NUV-blue events.

• The color curves differ primarily by an offset, with the NUV-blue u- color curves bluer than the NUV-red curves by 0.4 mag.
• For a sample of 23 low-z SNe~Ia observed with Swift, the NUV-red group dominates by a ratio of 2:1.
• We compare rest-frame UV/optical spectrophotometry of intermediate and high-z SNe Ia with UVOT photometry and HST spectrophotometry of low-z SNe Ia, finding that the same two color groups exist at higher-z, but with the NUV-blue events as the dominant group.