How Standard areCosmological Standard Candles?

Mathew Smithand Collaborators

(UCT, ICG, Munich, LCOGT and SDSS-II)

SKA Bursary Conference

02/12/2010

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

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

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

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

B. D

ilday

Determining Host Galaxy PropertiesSample 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<0.45357 galaxies, 30% passive

z<0.21135 galaxies, 26% passive

Do SNe know where they come from? - Sort of

• The distribution of extinction / colour in SNe is not dependent on the host galaxy type.• True for two SNe light-curve fitters

• 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 – Hubble Diagrams …

Cosmology – Hubble Diagrams …

Cosmology – Galaxy Type

Standard

No Prior

Cosmology – Galaxy Type

Standard

No Prior

What’s causing this?

SALT2 Distances determined using:

• 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

What’s causing this?

SALT2 Distances determined using:

• 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

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.

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

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?

Metallicity (tentatively)

Metallicity estimated from the colours Able to estimate for each galaxy, but less accurate Degenerate with age / extinction

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

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