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Open Access Publications from the University of California

A Monte Carlo Method for Identifying Imaging Systematics in Galaxy Surveys

  • Author(s): Burleigh, Kaylan
  • Advisor(s): Nugent, Peter E
  • et al.

The Dark Energy Spectroscopic Instrument (DESI) will soon start to obtain optical spectra for tens of millions of galaxies and quasars, constructing a 3-dimensional map spanning the nearby universe to 10 billion light-years. DESI aims to use the fossil imprint of sound waves from the first 380,000 years of the universe, which is still detectable as a pattern of temperature variations in the cosmic microwave background radiation (CMB), to measure how the universe has evolved since then the Baryon Acoustic Oscillations (BAO) technique. The early CMB temperature differences map early variations in density (the sound waves) that subsequently evolved into the clustering of galaxies and intergalactic gas (the baryons), as well as dark matter at recurrent intervals throughout space. These regularly spaced clusterings are consistent over time, very much like a ruler to measure the universe, with the CMB at one end. This allows one to measure the effect of dark energy on the expansion of the universe.

This thesis presents my work as a member of the DESI Imaging Team, for which I received DESI Builder status. We transform images of the night sky into a catalog of positions properties of automatically detected and measured astrophysical sources. This catalog will contain billions of astrophysical sources, but just a subset (tens of millions) of sources will be selected for spectroscopic observation with DESI. I was involved in all stages of the Legacy Surveys, from carrying out observations to building the large–scale-structure catalogs.

A major challenge for they Legacy Surveys is understanding the inevitable biases and systematics in their galaxy samples. The key product of my thesis is a Monte Carlo method, called Obiwan, that adds simulated sources to random locations in astronomical images and then performs source detection and measurement, characterizing the complex selection inherent in large-scale-structure catalogs. The process is repeated until the injected source density is high enough to satisfy one’s science objectives. For instance, the DESI target density for emission line galaxies (ELGs) is 2400 deg2, so simulated ELGs should be injected at more than 10 times this density.

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