The DEEP2 Galaxy Redshift Survey: Design, Observations, Data Reduction, and Redshifts

Abstract: We describe the design and data sample from the DEEP2 Galaxy Redshift Survey, the densest and largest precision-redshift survey of galaxies at z ~ 1 completed to date. The survey has conducted a comprehensive census of massive galaxies, their properties, environments, and large-scale structure down to absolute magnitude M_B = -20 at z ~ 1 via ~90 nights of observation on the DEIMOS spectrograph at Keck Observatory. DEEP2 covers an area of 2.8 deg^2 divided into four separate fields, observed to a limiting apparent magnitude of R_AB=24.1. Objects with z < 0.7 are rejected based on BRI photometry in three of the four DEEP2 fields, allowing galaxies with z > 0.7 to be targeted ~2.5 times more efficiently than in a purely magnitude-limited sample. Approximately sixty percent of eligible targets are chosen for spectroscopy, yielding nearly 53,000 spectra and more than 38,000 reliable redshift measurements. Most of the targets which fail to yield secure redshifts are blue objects that lie beyond z ~ 1.45. The DEIMOS 1200-line/mm grating used for the survey delivers high spectral resolution (R~6000), accurate and secure redshifts, and unique internal kinematic information. Extensive ancillary data are available in the DEEP2 fields, particularly in the Extended Groth Strip, which has evolved into one of the richest multiwavelength regions on the sky. DEEP2 surpasses other deep precision-redshift surveys at z ~ 1 in terms of galaxy numbers, redshift accuracy, sample number density, and amount of spectral information. We also provide an overview of the scientific highlights of the DEEP2 survey thus far. This paper is intended as a handbook for users of the DEEP2 Data Release 4, which includes all DEEP2 spectra and redshifts, as well as for the publicly-available DEEP2 DEIMOS data reduction pipelines. [Abridged]

• The paper presents DEEP2's robust methodology using the DEIMOS spectrograph to obtain precise redshifts for more than 38,000 galaxies.
• It details innovative color pre-selection and slitmask techniques that efficiently target galaxies at z > 0.7 for in-depth analysis.
• The extensive dataset confirms key trends in galaxy quenching and clustering, advancing models of galaxy formation and dark matter halo occupation.

Overview of the DEEP2 Galaxy Redshift Survey

The DEEP2 Galaxy Redshift Survey, as detailed in the paper by Newman et al., is one of the most comprehensive redshift surveys that focuses on the large-scale structure and the evolution of galaxies at redshifts around z ~ 1. It leverages the advanced capabilities of the DEIMOS multi-object spectrograph on the Keck II telescope. This survey aims to gather a dense dataset that allows for the detailed study of galaxy properties and clustering, as well as to explore fundamental parameters that influence galaxy formation and evolution.

The DEEP2 survey strategically covers four separate fields, totaling an area of approximately 3 square degrees. This area was meticulously chosen to avoid foreground stars and ensure minimal galactic absorption, which would otherwise complicate data analysis. By targeting galaxies as faint as R_AB = 24.1, the survey is adept at acquiring spectra and redshifts for over 50,000 galaxies, with reliable redshifts secured for more than 38,000 galaxies.

Methodology and Design Considerations

The survey employs a highly efficient color pre-selection process using CFHT 12K photometry in $BRI$ bands, which filters out low-redshift galaxies for three of the four fields. This allows for increased efficiency by focusing DEIMOS' spectroscopic capabilities on galaxies likely situated at z > 0.7. Observations are further optimized by employing slitmask technology, which enables observation of over 100 galaxies simultaneously.

An emphasis was placed on achieving high precision in measuring redshifts, a feat facilitated by the high spectral resolution (R ~ 6000) of DEIMOS. The accurate wavelength calibration and well-characterized instrument response functions ensure that even densely packed OH sky lines, common beyond 7000 Å, are effectively subtracted, providing clean galaxy spectra for analysis.

Results and Scientific Implications

The DEEP2 survey's extensive data set enables a multitude of scientific inquiries. Key results include the confirmation of the doubling of quiescent galaxies since z ~ 1, indicative of ongoing processes that quench star formation in galaxies. This points to an evolving cosmos where galaxy environments and internal processes drive such transitions, reflected in the strong correlation between galaxy color and environment.

Moreover, the DEEP2 survey has enabled detailed studies of the Tully-Fisher and Faber-Jackson relations at high redshift, bolstering models that connect these relations to broader galaxy formation and evolution theories. The survey provides critical insights into clustering statistics, revealing variations in galaxy clustering as a function of type, luminosity, and epoch, which are critical to dark matter halo occupation models.

Future Directions

The data from DEEP2 offer a substantial resource for testing cosmological models and investigating the nature of dark energy through group and cluster counting, employed via redshift-based techniques. As computational techniques in photometric redshift estimation advance, they couple with DEEP2’s spectroscopic data to enhance redshift accuracy, extending insights into even fainter and more distant galaxy populations.

In conclusion, the DEEP2 Galaxy Redshift Survey signifies a pivotal development in understanding the formation and evolution of galaxies. Its legacy extends into ongoing surveys like DEEP3 and precedes future wide-field spectroscopic surveys that aim to probe deeper into the universe's formative years.

Overall, this work exemplifies a major step forward in the ambition to map the cosmos with a level of precision and depth that aligns with the quest to decode cosmic history encoded in the large-scale structure of the Universe.