A Study For Strategic Spectroscopy

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The major current and upcoming probes of dark energy are large astronomical sky surveys that observe billions of galaxies, hundreds of thousands of supernovae, and thousands of galaxy clusters. These include the currently deployed Dark Energy Survey (DES) and the future Large Synoptic Survey Telescope (LSST), both of which have major KIPAC participation. The dark energy constraints from any such survey depend critically on reliable determinations of the redshifts of the objects in question.

Redshifts are a measure of how much an object's light has shifted to longer wavelengths due to the expansion of the Universe, and the redshift of an object is related to how far away it is and how far back in time the light that we see from it was emitted. The traditional technique of redshift determination - taking a full spectrum of each object and observing the precise wavelength of emission and absorption lines - is impossible for sky surveys with billions of galaxies. Instead, the surveys must rely on photometric redshifts, known as "photo-zs", where the redshift of each object is estimated from its brightness in a few broadband colors, defined by filters on the telescope. Because photo-zs are estimated with a relatively small amount of information, and distant galaxies can differ markedly in their properties, errors in photo-z estimation are a major concern, and photo-z estimation techniques are a prominent topic of investigation in the astronomical community.

Because of the intrinsic uncertainties in photo-zs, the inferred redshifts cannot be used for precise cosmological estimates such as required by DES and LSST unless the errors in the estimated redshifts can be modeled with exquisite precision. The most robust way to obtain such accurate models is to have a sample with the more reliable spectroscopic redshifts, which allows one to compare the photo-zs with the actual redshifts and understand where the photo-zs went wrong. The problem is that spectroscopic redshifts are expensive to get, requiring hours of telescope time for each one, and far from perfect themselves.

Carlos Cunha, current Kavli postdoctoral fellow at KIPAC, and KIPAC professor Risa Wechsler, along with collaborators Dragan Huterer of the University of Michigan, Huan Lin of Fermilab, and KIPAC alumnus Michael Busha of the University of Zurich, have developed some of the most detailed simulations of spectroscopic surveys to investigate the requirements on these follow-up spectroscopic samples, in order to most efficiently obtain the necessary spectroscopic samples. The simulations begin with populating the Universe and the observed sky with a realistic distribution of galaxies arising from simulations of the growth of structure in the Universe (so called "n-body simulations") and then carrying them through to what would actually be observed by telescopes such as DES.  The simulated data is then analyzed and cosmological parameters are then estimated and compared with the known cosmological parameters that went into the simulation.

They find that the most important characteristic of a spectroscopic follow-up is accuracy, disproving common assumptions that so-called "completeness" - the degree to which the spectroscopically observed galaxies resembles the total observed population across a variety of characteristics - is the most relevant target. The team was able to quantify that at least 99% of the spectroscopic redshifts need to be correct or else biases in the cosmological parameters become intolerable, necessitating a focus on quality as well as quantity.  Furthermore, they find that there is significant gain in drawing spectroscopic samples from widely separated patches of the sky, and given the size of the field of view of existing spectrographs on the largest telescopes available, this would mean spectroscopic samples would have to be drawn from a few hundred different patches of sky.  These analyses are an important step to establishing a strategy for obtaining spectroscopic samples going forward.

This work is described in part in a paper in the Monthly Notices of the Royal Astronomical Society (MNRAS, 2012, 423, 909), and another paper submitted to that journal and available from astro-ph at arXiv:1207.3347. Further information is available from Carlos Cunha's webpage. Research at KIPAC is supported by the Department of Energy, the Kavli Foundation, the National Aeronautics and Space Administration, the National Science Foundation and Stanford University, as well as private donors. We are grateful to each of these sponsors for their continued interest and support.

Science Contact:
Carlos Cunha
KIPAC
email: ccunha@stanford.edu

Tidbit author: Jack Singal and Carlos Cunha