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Selecting Supernovae for Cosmology
Cosmic Co-Motion, Courant Cove, September 2010
Troels Haugbø[email protected]
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