How Standard are Cosmological Standard Candles?

1. How Standard areCosmological Standard Candles? Mathew Smith and Collaborators (UCT, ICG, Munich, LCOGT and SDSS-II) SKA Bursary Conference 02/12/2010

2. Introducing Type Ia Supernova • One of the (optically) brightest astrophysical phenomena, so can be seen to large distances • Categorised through their spectral features • A limited understanding of their nature / origin

3. Standardisable Candles • Type Ia SNe are observed to be an extremely homogeneous population • Both spectra and light-curves show little variation • Using an empirical correction (Phillips 1993), the scatter is reduced • Peak brightness correlated with decline rate (“stretch”) • After correction ~ 0.15 mag, or cosmological distances to 7% • Correction reduces scatter on your Hubble diagram • Used to infer the apparent acceleration of the Universe, and thus “Dark Energy” • However, scatter is still seen, and needs to be improved for future surveys

4. Standardisable Candles • Type Ia SNe are observed to be an extremely homogeneous population • Both spectra and light-curves show little variation • Using an empirical correction (Phillips 1993), the scatter is reduced • Peak brightness correlated with decline rate (“stretch”) • After correction ~ 0.15 mag, or cosmological distances to 7% • Correction reduces scatter on your Hubble diagram • Used to infer the apparent acceleration of the Universe, and thus “Dark Energy” • However, scatter is still seen, and needs to be improved for future surveys

5. In order to reduce SNe scatter we need to understand their properties. • Most previous studies focus on local or high-z SNe, so obtain biased samples • Need to obtain a homogeneous and representative sample, independent of galaxy type • Obtained high-quality light-curves for SNe with 0 < z < 0.5 • Spectroscopically confirmed over 500 SNe Ia, with ~1,000 photometric SNe Ia – key for future surveys • Able to constrain cosmological parameters

6. B. Dilday

7. Determining Host Galaxy Properties • Sample Information: • Produce a uniform sample, independent of observational “issues” (filter transmissions, etc) • Host Galaxies of each object determined from deep stacks • Use Spectral Energy Distributions to determine galaxy properties from magnitude and redshift information • Sample split in two groups; • ‘passive and star-forming’ • Large study of the systematics of template fitting (another talk!) z<0.45 357 galaxies, 30% passive z<0.21 135 galaxies, 26% passive

8. Do SNe know where they come from? - Sort of • 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. • True for two SNe light-curve fitters

9. Cosmology – Hubble Diagrams …

10. Cosmology – Hubble Diagrams …

11. Cosmology – Galaxy Type Standard No Prior

12. Cosmology – Galaxy Type Standard No Prior

13. What’s causing this? • There is a clear correlation between: • Galaxy type • Stellar mass • Star-formation rate • “Stretch” / Delta • Residuals from the Hubble diagram seen for several galaxy properties (independent of redshift) • Is there a “higher-order” parameter governing all of this • - Metallicity?? • Galaxy properties can be used for cosmological constraints SALT2 Distances determined using:

14. What’s causing this? • There is a clear correlation between: • Galaxy type • Stellar mass • Star-formation rate • “Stretch” / Delta • Residuals from the Hubble diagram seen for several galaxy properties (independent of redshift) • Is there a “higher-order” parameter governing all of this • - Metallicity?? • Galaxy properties can be used for cosmological constraints SALT2 Distances determined using:

15. Other implications • The dispersion on the Hubble diagram is smaller for passive galaxies • Is this telling us that they are better distance estimators • Or about “intrinsic dispersion”? • Important for the next generation of surveys • There are indications that SNe in different environments obey a different colour law. • An additional parameter (such as host galaxy mass) “improves” the Hubble diagram.

16. An aside: SNe Ia Rates • The SN Ia rate is dependent on the specific star-formation rate – the proportion of a galaxy’s mass that is used to form stars • There is a dependence on host galaxy mass – that differs for different galaxy types • The star-formation rate drives the SN Ia rate

17. Currently - The Maraston Models • The Maraston models give us a handle on metallicity • - The possible hidden parameter / correlation? • PEGASE has issues with accurately determining galaxy properties • However, template selection is far more complex – need to select on colour?

18. Metallicity (tentatively) • HIGHLY PRELIMINARY • 4 estimates of metallicity • Not a continuous parameter • An offset seen with Hubble residual? • Different populations of “stretch” • Color distribution ‘uncertain’ • Need to consider systematics • Metallicity estimated from the colours • Able to estimate for each galaxy, but less accurate • Degenerate with age / extinction

19. The Summary • SNe Ia are important cosmological probes – we are now at the stage where we are seriously considering systematics. • The SDSS-II Supernova Survey is complete – we have over 500 SNe with z<0.5 • We have produced a large, homogeneous and representative sample with which to study the SN Ia population • SNe in different galaxies have different absolute magnitudes (modulo template fitting issues) • The colour law may be different for different galaxy types • ‘passive’ SNe show a lower dispersion from the best-fitting Hubble diagram – target them for future surveys? • Metallicity could be the hidden parameter • Need to test this with more advanced models and spectral information • Combining SDSS and SNLS we will be able to study this effect with redshift • Host Galaxy information maybe useful for cosmological analyses