The Serpens Molecular Cloud in ¹²CO and ¹³CO: High Resolution and Large Field of View

We used ¹²CO and ¹³CO J = 2-1 emission, to map 1.04 deg² of the Serpens molecular cloud with 38" spatial and 0.3 km s⁻¹ spectral resolution. Our molecular tracer study is important because our maps resolve kinematic properties for the entire Serpens cloud. We compare integrated intensity, velocity centroid, and velocity dispersion maps to positions of Young Stellar Objects (YSOs) and 1.1 mm emission from previous work. Our final result is a total H-nuclei (HI + 2 H₂) column density map, referred to as the N(H) map, which we find using an escape probability statistical equilibrium model that estimates ¹²CO column density (to infer the H₂ column) and the radiation field (to infer the HI column) from our ¹²CO and ¹³CO data. The average column densities in the Serpens core, Serpens South, and VV Ser regions are 8.6 x 10²¹, 10.0 x 10²¹, and 6.7 x 10²¹ cm⁻², respectively. Our final measurement for the mass of Serpens is 1860 M(sun). We conclude that this is a lower bound because ¹²CO and ¹³CO are self-absorbed in the Serpens core and Serpens South regions. Our N(H) map provides an observational test for numerical models of low-mass star forming clouds

A Monte Carlo Method for Identifying Imaging Systematics in Galaxy Surveys

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

Removing imaging systematics from galaxy clustering measurements with Obiwan: application to the SDSS-IV extended Baryon Oscillation Spectroscopic Survey emission-line galaxy sample

This work presents the application of a new tool, Obiwan, which uses image simulations to determine the selection function of a galaxy redshift survey and calculate three-dimensional (3D) clustering statistics. Obiwan relies on a forward model of the process by which images of the night sky are transformed into a 3D large-scale structure catalogue, and offers several advantages over more traditional map-based techniques - such as operating on individual exposures and adopting a maximum likelihood approach. The photometric pipeline automatically detects and models galaxies and then generates a catalogue of such galaxies with detailed information for each one of them, including their location, redshift, and so on. Systematic biases in the imaging data are therefore imparted into the catalogues and must be accounted for in any scientific analysis of their information content. Obiwan simulates this process for samples selected from the Legacy Surveys imaging data. This imaging data will be used to select target samples for the next-generation Dark Energy Spectroscopic Instrument (DESI) experiment. Here, we apply Obiwan to a portion of the SDSS-IV extended Baryon Oscillation Spectroscopic Survey emission-line galaxies (ELGs). Systematic biases in the data are clearly identified and removed. We compare the 3D clustering results to those obtained by the map-based approach applied to the complete eBOSS Data Release 16 (DR16) sample. We find the results are consistent, thereby validating the eBOSS DR16 ELG catalogues, which is used to obtain cosmological results

Dynamic Observing and Tiling Strategies for the DESI Legacy Surveys

International audienceThe Dark Energy Spectroscopic Instrument Legacy Surveys, a combination of three ground-based imaging surveys, have mapped 16,000 deg2 in three optical bands (g, r, and z) to a depth 1–2 mag deeper than the Sloan Digital Sky Survey. Our work addresses one of the major challenges of wide-field imaging surveys conducted at ground-based observatories: the varying depth that results from varying observing conditions at Earth-bound sites. To mitigate these effects, the Legacy Surveys (the Dark Energy Camera Legacy Survey, or DECaLS; the Mayall z-band Legacy Survey, or MzLS; and the Beiijing-Arizona Sky Survey, or BASS) employed a unique strategy to dynamically adjust the exposure times as rapidly as possible in response to the changing observing conditions. We present the tiling and observing strategies used by the first two of these surveys. We demonstrate that the tiling and dynamic observing strategies jointly result in a more uniform-depth survey that has higher efficiency for a given total observing time compared with the traditional approach of using fixed exposure times

Overview of the DESI Legacy Imaging Surveys

The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing–Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project