Selecting Supernovae for Cosmology

Cosmic Co-Motion, Courant Cove, September 2010

Troels Haugbøllehaugboel@nbi.dk

Niels Bohr International Academy – University of Copenhagen

Collaborators: Bjarne Thomsen, Steen Hannestad

Main Points

With upcoming survey telescopes we will discover so many local

supernovae that complete spectroscopic follow up of is unfeasible.

To sample the peculiar velocity field, a regularly spaced

distribution is advantageous, to avoid power leaking.

Obtaining spectra for only a carefully selected subset gives the

best constraints from the least observational investment.

Peculiar Velocity Fields

Velocity trace mass:

v = - H f(m)

where is the density contrast, and f(m) the growth factor

The peculiar velocity field is sourced by the gravitational

potential: It is directly dependent on the dark matter distribution

Connecting the matter and velocity powerspectrum

● Velocity trace mass: v = - H f(m)

● The angular velocity powerspectrum is related to the matter powerspectrum :

Peculiar Velocity Fields● Further away than ~80 Mpc h-1 cosmic variance is small

enough, that we can constrain cosmological models

● Gravity sources the velocity field from density fluctuations on larger scales

● This is why peculiar velocities may be the best measure of 8 at z=0

The velocity field 90 Mpc h-1 away

-1100 1100 km/s

The density field 90 Mpc h-1 away

Upcoming surveys● Lensing/asteroid surveys are better for local supernovae,

than the high-z SNe surveys. They scan the sky continuously, and observe in many bands (typically 6).

● LSST saturates at m < 16-17 or d < 75-120 Mpc h-1

Pan-Starrs

(4x)1.4Gp

2009+

Hawaii

Sky Mapper

256Mp

2010

Australia

LSST3.2Gp

2014

Chile

Pan-Starrs

Goals

● Predict how well we can probe the local velocity field, with upcoming supernovae surveys

● Design the optimal observational strategy to maximize science output

● Use the angular power spectrum of the peculiar velocity field as a tool for constraining cosmology

Goals

● Predict how well we can probe the local velocity field, with upcoming supernovae surveys

● Design the optimal observational strategy to maximize science output

● Use the angular power spectrum of the peculiar velocity field as a tool for constraining cosmology

Goals

● Predict how well we can probe the local velocity field, with upcoming supernovae surveys

● Design the optimal observational strategy to maximize science output

● Use the angular power spectrum of the peculiar velocity field as a tool for constraining cosmology

Forecast● The local supernova rate is approximately

1.2 x 10-4 SN yr-1 h3 Mpc-3

● This gives 60000 potential Type Ia SN per year with distances less than 500 h-1 Mpc (z < 0.17)

● There will be light curves from survey telescopes, but precise redshifts are needed

Forecast● The local supernova rate is approximately

1.2 x 10-4 SN yr-1 h3 Mpc-3

● This gives 60000 potential Type Ia SN per year with distances less than 500 h-1 Mpc (z < 0.17)

● There will be light curves from survey telescopes, but precise redshifts are needed

● A dedicated 1 m telescope would be able to take ~7000 spectra per year, or roughly 25% of the Type Ia SNe, assuming the survey telescopes covers half the sky

Goals

● Predict how well we can probe the local velocity field, with upcoming supernovae surveys

● Design the optimal observational strategy to maximize science output

● Use the angular power spectrum of the peculiar velocity field as a tool for constraining cosmology

Observational Strategy

●The precision we can measure the angular powerspectrum with depends crucially on the geometric distribution on the sphere

●Essentially power can “leak out” if there are big holes on the sky.

●We know where the SNe are before finding the redshift from the surveys

Reconstructing the velocity PS- a geometric detour -

3072 Random Points3072 Glass Points3072 “HealPix” Points12288 Random Points

(figures thanks to Anja Weyant)

Signal

Power Leaking

How to make a supernova survey

Make Nbody sim

Find density and velocity on a spherical shell

Populate with Supernovae

Calculate Angular PS

Size of voids/Max of matter PS

Size of clusters

...but there is more to it

● With a limited amount of SNe, we can only measure a limited part of the powerspectrum

● Algorithm:● Given a set of Supernovae. Calculate powerspectrum

...but there is more to it

● With a limited amount of SNe, we can only measure a limited part of the powerspectrum

● Algorithm:● Given a set of Supernovae. Calculate powerspectrum● Make N mock catalogues with same errors

...but there is more to it

● With a limited amount of SNe, we can only measure a limited part of the powerspectrum

● Algorithm:● Given a set of Supernovae. Calculate powerspectrum● Make N mock catalogues with same errors● Compare the mock powerspectra to the underlying

powerspectrum● This gives theshot noise + window function

...but there is more to it

● With a limited amount of SNe, we can only measure a limited part of the powerspectrum

● Algorithm:● Given a set of Supernovae. Calculate powerspectrum● Make N mock catalogues with same errors● Compare the mock powerspectra to the underlying

powerspectrum● This gives theshot noise +window function

● Subtract the error term from the observed powerspectrum

● There are light curves, but we need precise redshifts● A 1 m telescope can take 1 spectra in ~20 minutes

~7000 spectra per year

● It is not realistic to measure 60000 redshifts per year

● We need to optimize our observation strategy and only select “the right” supernovae

Supernovae on a glass

Goals

● Predict how well we can probe the local velocity field, with upcoming supernovae surveys

● Design the optimal observational strategy to maximize science output

● Use the angular power spectrum of the peculiar velocity field as a tool for constraining cosmology

Connecting the matter and velocity powerspectrum

Small scale amplitude 8

Small scale amplitude or 8

● Amplitude on large scales is fixed by the CMB●8 can be affected by

●Massive neutrinoes less power

256

Mp

c h

-1

Standard CDM 3 x 2.3 eV neutrinoes

Small scale amplitude or 8

● Amplitude on large scales is fixed by the CMB●8 can be affected by

●Massive neutrinoes less power●Features / tilts in the primordial power spectrum

Consequences for cosmology● The overall amplitude depends on

H f(m)8

This combination break

degeneracies,and8 can be constrained: Using 6 redshift bins (3 yrs of data, 23.000 glass Sne), and a simple 2 analysis (with fixed H), we find

a determination of 8 with 95% confidence

● The overall amplitude depends on

H f(m)8

This combination break

degeneracies,and8 can be constrained: Using 6 redshift bins (3 yrs of data, 23.000 glass Sne), and a simple 2 analysis, we find

a determination of 8 with 95% confidence

Consequences for cosmologyGlass SupernovaeAll Supernovae

● Peculiar velocities or bulk flows can be measured using low redshift supernovae

● The peculiar velocity field is important to understand:● It tells out about the structure of the local Universe● It has to be corrected for in the Hubble diagram● We can directly probe the gravitational potential, do

Cosmology, and learn about the bias

● Upcoming survey telescopes will observe thousands of low redshift supernovae - but this potential can only be realized if time at support telescopes is allocated

● Optimizing the window function optimizes the science output

● We forecast that with 3 years of LSST data we can constrain 8 to roughly 5%

Summary