Dark Energy Survey Year 1 Results: Photometric Data Set for Cosmology (1708.01531v2)

Abstract: We describe the creation, content, and validation of the Dark Energy Survey (DES) internal year-one cosmology data set, Y1A1 GOLD, in support of upcoming cosmological analyses. The Y1A1 GOLD data set is assembled from multiple epochs of DES imaging and consists of calibrated photometric zeropoints, object catalogs, and ancillary data products - e.g., maps of survey depth and observing conditions, star-galaxy classification, and photometric redshift estimates - that are necessary for accurate cosmological analyses. The Y1A1 GOLD wide-area object catalog consists of ~137 million objects detected in coadded images covering ~1800 deg$^2$ in the DES grizY filters. The 10{\sigma} limiting magnitude for galaxies is g = 23.4, r = 23.2, i = 22.5, z = 21.8, and Y = 20.1. Photometric calibration of Y1A1 GOLD was performed by combining nightly zeropoint solutions with stellar-locus regression, and the absolute calibration accuracy is better than 2% over the survey area. DES Y1A1 GOLD is the largest photometric data set at the achieved depth to date, enabling precise measurements of cosmic acceleration at z $\lesssim$ 1.

Overview of the Dark Energy Survey Year 1 Photometric Data Set for Cosmology

The publication titled "Dark Energy Survey Year 1 Results: Photometric Data Set for Cosmology" details the methodology, construction, and validation of the Dark Energy Survey (DES) Year 1 (Y1) cosmology data set, known as Y1A1 GOLD. This essay presents a comprehensive summary of the key aspects of this data set and its implications for cosmological research.

The DES utilizes the Dark Energy Camera (DECam) mounted on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory to survey 5000 square degrees of the southern sky using five broadband filters: $g$, $r$, $i$, $z$, and $Y$. This survey aims principally to elucidate the nature of cosmic acceleration through four primary probes: weak lensing, large-scale structure, galaxy clusters, and Type Ia supernovae. The Y1A1 GOLD data set supports these objectives by providing a rigorously calibrated photometric resource that includes object catalogs and comprehensive ancillary data products necessary for robust cosmological analysis.

Key Components

1. Photometric Calibration and Uniformity: The photometric calibration of Y1A1 GOLD was achieved by integrating nightly zeropoint solutions with a global calibration process and supplemented with adjustments from stellar locus regression. The calibration achieved an impressive uniformity with errors under 2% across the survey area, key for reducing systematic biases in cosmological parameter estimation.
2. Catalog Construction and Data Quality: The Y1A1 GOLD catalog consists of 137 million objects detected in coadded images covering 1800 square degrees, with a median magnitude limit of $g = 23.4$, $r = 23.2$, $i = 22.5$, $z = 21.8$, and $Y = 20.1$. Comprehensive quality cuts and catalogs of photometry, shape measurements, and photometric redshifts were developed to support precise scientific inquiries. A secondary selection focused on reducing spurious object inclusions without significantly reducing completeness.
3. Survey Footprint and Systematic Characterization: Ancillary maps that detail the survey coverage, depth, observing conditions, and potential systematic effects were produced. These maps are crucial for characterizing spatial variations that could otherwise introduce biases into cosmological studies such as galaxy clustering or shear measurements. The area managed under consistent and rigorous data quality standards was reduced to an effective 1506 square degrees.
4. Star-Galaxy Separation and Photometric Redshifts: The paper discussed advanced methods for distinguishing stars from galaxies using the SPREAD MODEL statistics alongside photometric redshift estimation strategies. These methodologies were tailored for cosmological relevance, optimizing redshift accuracy for specific subsamples integral to DES's scientific objectives. The average photometric redshift uncertainty was shown to be controlled within acceptable limits for cosmological analyses.
5. Astrometric Precision: The calibration and correction processes in Y1 ensured astrometric precision, with internal alignment discrepancies kept to within 25 milliarcseconds and external alignment to within 200-350 milliarcseconds when compared to 2MASS, showcasing DES's capability in both relative and external astrometric accuracy.

Implications and Future Directions

The Y1A1 GOLD data set represents a substantial step forward in large-scale photometric survey capabilities. It sets a rigorous precedent for future DES data releases and serves as an important reference for forthcoming surveys such as the LSST. The methodologies developed for photometric calibration and systematic effect mitigation have broader applications beyond DES, providing frameworks adaptable to various upcoming astronomical surveys.

Practically, the improved precision in photometric redshift determination and galaxy separation fosters enhanced accuracy in measuring cosmic shear and the distribution of large-scale structures, directly impacting our understanding of cosmological parameters. Theoretical advancements may arise from integrating these results into models that refine our comprehension of dark energy and its role in the universe’s expansion.

In conclusion, the DES Y1A1 GOLD data set exemplifies the convergence of collaborative efforts in astronomical data gathering and processing. It informs current cosmological discourse while laying the groundwork for enriched understanding through future exploratory missions.