How Standard Are Cosmological Standard Candles?

1. How Standard Are Cosmological Standard Candles? Mathew Smith, Bruce Bassett and the SDSS-II Collaboration UCT SKA Postgraduate Bursary Conference – 7th Dec 09

2. Standardisable Candles • Type Ia SNe are not standard candles • There is scatter in their peak brightness • Without correction they provide reasonable distance measures • After correction – their light-curves are extremely homogeneous • We investigate any diversity within the Ia population and determine its effect on cosmological measurements

3. Standardisable Candles • Type Ia SNe are not standard candles • There is scatter in their peak brightness • Without correction they provide reasonable distance measures • After correction – their light-curves are extremely homogeneous • We investigate any diversity within the Ia population and determine its effect on cosmological measurements

4. The Setup The SDSS-II Supernova Survey Two datasets: 340 SNe. Principally designed to investigate the SNe Ia rate z < 0.25, incompleteness corrected (i.e. contains non-spectroscopically identified Ia’s) 372 SNe. Determine how diversity affects cosmological parameters z < 0.4, all spectroscopically confirmed • Derived Information: • Host Galaxies of each object visually determined • Using the magnitude and redshift information the host type, mass and SFR of each galaxy is determined using the PEGASE SEDs • Sample corrected for the efficiency of the the survey • Large comparison field sample created • Sample determined from 2 light-curve fitters (MLCS and SALT)

5. Rates • The rate of SNe Ia’s varies as a function of host galaxy type and • More Ia’s are found in star-forming galaxies • This result is independent of redshift • More SNe are found in high mass systems • Type Ia’s in star-forming galaxies follow a different law • Recent star-formation activity drives and dominates the SNe Ia rate

6. Properties • The stretch – brightness correction varies as a function of host galaxy type • Bright SNe are primarily seen in star-forming galaxies – caused by recent SF activity? • The distribution of extinction / colour in SNe is not dependent on the host galaxy type. • This is independent of choice of prior used

7. Cosmology MLCS Standard MLCS No Prior SALT

8. LC Fitter – Or Diversity – Or both? • There is a population of “passive” SNe that are treated differently between MLCS and SALT • Due to a lack of “passive” SNe at low redshift? • These SNe do not drive the result found previously, since, when fitted separately SALT sees the same!

9. Conclusions • There is considerable diversity in the type Ia supernova population, especially with respect to host galaxy properties. • However, the situation is difficult to disentangle • The SALT and MLCS light-curve fitters treat “passive” SNe differently, but not “star-forming” SNe – a training artefact? • The MLCS light-curve fitter implies that “passive” type Ia’s have different absolute magnitudes from “star-forming” SNe. • A 2nd parameter? • A training issue, but so does SALT? (Maybe related to Rv?)