[astro-ph/0502243] Dark Energy Constraints from the CTIO Lensing Survey

Authors:  Mike Jarvis, Bhuvnesh Jain, Gary Bernstein, Derek Dolney
Abstract:  We perform a cosmological parameter analysis of the 75 square degree CTIO lensing survey in conjunction with CMB and Type Ia supernovae data. For Lambda CDM cosmologies, we find that the amplitude of the power spectrum at low redshift is given by sigma_8 = 0.812{+0.146,-0.100} (95% c.l.), where the error bar includes both statistical and systematic errors. The total of all systematic errors is smaller than the statistical errors, but they do make up a significant fraction of the error budget. We find that weak lensing improves the constraints on dark energy as well. The (constant) dark energy equation of state paremeter, w, is measured to be -0.894{+0.156,-0.208} (95% c.l.). Marginalizing over a constant w slightly changes the estimate of sigma_8 to 0.790{+0.170,-0.141} (95% c.l.). We also investigate variable w cosmologies, but find that the constraints weaken considerably; the next generation surveys are needed to obtain meaningful constraints on the possible time evolution of dark energy.
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Antony Lewis
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[astro-ph/0502243] Dark Energy Constraints from the CTIO Len

Post by Antony Lewis » February 17 2005

I was interested to find out some details of how the authors select stars to use for PSF correction. This paper cites astro-ph/0210604 for details, however that paper merely cites (in step 6) a forthcoming "Jarvis 2003" paper for details, which doesn't seem to exist. The paper discussing their nice PCA method for PSF interpolation (astro-ph/0412234) doesn't seem to give an details on star selection either. Is there a reference I've missed?

I'm particularly interested if they have an automated way for selecting stars in the presence of the anisotropic PSF (as they discuss briefly in astro-ph/0210604), and how to avoid diffraction spikes and leakage along CCD readout columns giving spurious ellipticity correlations. Any references appreciated.

Thanks.

Henk Hoekstra
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[astro-ph/0502243] Dark Energy Constraints from the CTIO Len

Post by Henk Hoekstra » February 24 2005

Hi Antony,

Even in the case of an anisotropic PSF the stars stand out in a plot of size vs magnitude, and although all stars have CTE problems and spikes, they are only becoming problematic for saturated stars, which also are easily identified.

You might run into some slight trouble when the anisotropy is varying wildly as a function of position, because this widens the stellar locus, but testing also on a number of other parameters (polarisabilities in my case) allows for a fairly clean separation even then.
But for most data this is a pretty straightforward step.

Alessandro Melchiorri
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[astro-ph/0502243] Dark Energy Constraints from the CTIO Len

Post by Alessandro Melchiorri » March 08 2005

Hi, yes a very interesting paper. It is also one of the few analysis pointing towards
w>-1.
I have a question about the CMB only contour plots in Fig.2 and Fig.3. In Fig.2
we see a degeneracy along one direction, while marginalizing over w seems
to change the degeneracy line in another (I would say opposite) direction.
Why is this ?
For example, models with Omega_m=0.5 and sigma_8 around 1 are ok in
Fig.2 for CMB only. While in Fig3, marginalizing over w seems to rule them out...

Cheers
alessandro

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