Survey Design

Depth & Target Selection

To characterise sub-Mpc structure to z < 0.8, DEVILS will reach the lowest possible stellar mass galaxies, while remaining within the constraints of the AAT+AAOmega. To meet our science goal the DEVILS selection depth is designed to: i) identify multiplicity >3, >10¹³M_⊙ groups to z = 0.7 , ii) measure the galaxy stellar mass function to M^∗_z=0 at z = 1 (Fig 1), and iii) measure the major merger rate for M∗z=0 galaxies to z = 0.7 by identifying all systems at < 3:1 mass ratio (> 0.3 M^∗_z=0, grey band Fig 1). A Y < 21.2 selection will meet these constraints. DEVILS will use the Y -band as we wish to i) be close to a single band measurement of stellar mass (NIR), ii) select in flux at rest-frame > 4000 ̊A to z ∼ 1 (> 8000 ̊A observed), iii) select from a single instrument/facility (VISTA has the only contiguous observations across our fields) and iv) meeting previous conditions, be as close to the observing wavelength of AAOmega as possible. The VISTA Y -band at ∼ 1 μm meets all of these criteria, and is the deepest band in VIDEO (5σ point source depth is Y ∼ 25). Observations with the AAT will cover ∼ 3700-8800 ̊A at R ∼ 1000-1600, allowing identification of key spectral features across our redshift range; [OII] and Ca H & K to z ∼ 1.

The robustness of any evolution and structure survey increases with volume. Thus, we wish to maximise area, while once again remaining within observing constraints and retaining maximum legacy value. We require robust number statistics for both structure tracers (groups/pairs) and individual galaxies. Given our depth and key science goals we require ~6deg2 to both parametrise the evolution of the high mass end of the halo mass function and determine the effect of environment on galaxy evolution at intermediate redshifts. To maximise legacy science with auxiliary data, DEVILS will target deep, well studied extragalactic fields.  There are a number of optimal 2dF (∼ 2-3 deg²) deep fields with diverse multi-wavelength data accessible from the AAT. We have selected three (Fig 2, Table 1) which have the most extensive existing legacy data and are soon to be targeted with the next generation of instruments: each field has/will have: deep x-ray (XMM, Chandra), optical (HSC, DES, LSST deep-drill), NIR (VIDEO, UltraVISTA), MIR (Spitzer), FIR (Herschel) imaging, high resolution imaging (Euclid, HST), and radio imaging/spectra (ATCA, MeerKAT, JVLA), as well as existing spectroscopy (e.g. OzDES, zCOSMOS…). As such, these are key legacy fields which would benefit immensely from a highly complete spectroscopic survey – a unique database for the astronomy community. We opt to target a 2 × 1.5 deg² contiguous region in XMM-LSS to maximise legacy science with x-ray observations of DEVILS groups and large-scale structure. We will however, prioritise a single 1.5deg² region in XMM-LSS before observing the second, to ensure we obtain the completeness for our science goals.