CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (1105.3753v3)

Abstract: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8--1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z>1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_\odot to z \approx 2, reaching the knee of the ultraviolet luminosity function (UVLF) of galaxies to z \approx 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5\sigma\ point-source limit H=27.7 mag) covers \sim 125 arcmin^2 within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (EGS, COSMOS, and UDS) and covers the full area to a 5\sigma\ point-source limit of H \gtrsim 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

• The paper presents the CANDELS survey design, dividing deep and wide fields to document galaxy evolution from redshift 8 to 1.5.
• It employs HST’s multi-wavelength data to probe stellar mass distributions, low-luminosity galaxies, and star formation rates across cosmic time.
• The study refines cosmological models by assessing Type Ia supernovae as standard candles and providing a foundation for future JWST research.

Overview of the CANDELS Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) represents a significant effort to document the history of galactic evolution over an extensive redshift range of 8 to 1.5. This substantial survey builds upon data obtained from the Hubble Space Telescope (HST), utilizing its capabilities to gather information across a broad spectrum, from the ultraviolet to the near-infrared. The primary objectives of CANDELS include understanding galactic assembly, tracking star formation, and testing the consistency of Type Ia supernovae as standard candles for cosmology.

Methodology and Survey Design

CANDELS employs a strategic division of its survey area into deep and wide regions. The survey incorporates five premier fields: GOODS-N, GOODS-S, COSMOS, EGS, and UDS, ensuring large-scale statistical robustness by mitigating cosmic variance through this spatial distribution.

1. CANDELS Deep:
   • It targets regions within the GOODS-N and GOODS-S fields, covering approximately 125 arcmin² with a depth sufficient to probe stellar mass distributions down to $10^{9} M_\odot$ at redshift $z \sim 2$.
   • The deep component achieves an AB magnitude depth of $H = 27.7$ over its survey area, allowing the identification of low-luminosity galaxies and their contributions to the UV luminosity function up to $z \sim 8$.
2. CANDELS Wide:
   • Encompasses multiple fields, expanding coverage to additional sky regions with a collective area of about 800 arcmin².
   • This portion targets a broader cosmic volume with shallower depth ($H \gtrsim 27.0$) compared to the deep fields, thus sampling a larger number of galaxies and rare, massive objects.

Scientific Goals and Preliminary Findings

CANDELS is designed to address several pivotal scientific goals:

• Cosmic Dawn (CD): This component examines early galaxies and AGNs, aiming to improve constraints on the galaxy luminosity function (LF) at $z \sim 7$ and 8. Such efforts are crucial for understanding the universe’s reionization. The survey uses clustering statistics to estimate the dark-halo masses of high-redshift galaxies and limits AGN populations at these epochs.
• Cosmic High Noon (CN): Focusing on the peak epoch of star formation and AGN activity around $z \sim 2$, CANDELS investigates the morphological evolution of galaxies and measures key properties such as stellar mass, star-formation rates, and morphological types.
• Supernovae (SN): The presence of high-redshift Type Ia supernovae within the survey fields allows for the refinement of cosmological parameters and the examination of SN Ia evolutionary effects over cosmic time.
• Ultraviolet Observations (UV): Specific CANDELS observations, particularly in the GOODS-N field, facilitate the paper of the Lyman-continuum escape fraction from galaxies at $z \sim 2.5$, crucial for understanding the ionizing background.

Implications and Future Prospects

CANDELS provides a rich dataset for a variety of cosmological inquiries. Its broad wavelength coverage helps disentangle the many factors influencing galactic evolution. The survey’s multicycle approach allows for a dynamic observation strategy that adapts as new findings emerge.

Theoretical implications include refined models of dark matter halo growth and predictions concerning the distribution of baryonic matter during different cosmic epochs. Practically, CANDELS data serve as a fundamental reference point for subsequent studies using current and future observatories, such as the James Webb Space Telescope (JWST). The open-data policy further invites comprehensive analyses by the wider scientific community, potentially leading to insights beyond the original scope of CANDELS.

In conclusion, CANDELS stands as a cornerstone program that enhances our knowledge of the universe's evolution, laying the groundwork for future exploration and discovery in extragalactic astronomy.