How Standard Are Cosmological

Standard Candles?

Mathew Smith, Bruce Bassett and the SDSS-II Collaboration

UCT

SKA Postgraduate Bursary Conference – 7th Dec 09

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

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

The SetupThe SDSS-II Supernova Survey

Two datasets:1. 340 SNe. Principally designed to investigate the SNe Ia rate

z < 0.25, incompleteness corrected (i.e. contains non-spectroscopically identified Ia’s)

2. 372 SNe. Determine how diversity affects cosmological parametersz < 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)

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

Properties

• The distribution of extinction / colour in SNe is not dependent on the host galaxy type.• This is independent of choice of prior used

• 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?

Cosmology

MLCS Standard

MLCS No Prior

SALT

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!

Passive SFingM0 (~H0) 30.22 30.14

alpha 0.17 0.13

beta (~Rv + 1) 2.5 3.2

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?)