The Arizona CDFS Environment Survey


Here, we describe the basic parameters of the survey, including details regarding the target selection, spectroscopic observations, and (ongoing) data reduction. Further information will be added to the webpage as time permits. A more current and complete description of ACES is presented in Cooper et al. 2012.

Spectroscopic targets were selected from the COMBO-17 R-band imaging catalog of Wolf et al. 2004 (see also Wolf et al. 2008). The survey's main sample is selected down to RAB = 23, though there is a significant tail of fainter targets down to RAB = 24.1. Sources brighter than RAB = 18 were excluded from the target selection process. In addition, we down-weighted stars (identified based on a multi-color photometric selection) as well as sources for which a redshift already existed in the public domain (e.g., from the VVDS or VLT/FORS2 campaigns) or from private data sets.

Over the past 3+ years, ACES has utilized the IMACS multi-object spectrograph on the Magellan Baade 6.5-m telescope to collect low-resolution spectra of >7000 galaxies in the ECDFS. To date, ACES has observed 40 IMACS slitmasks in the 30' x 30' ECDFS, with each slitmask containing >300 targets. Typical slitlengths are >7" with a standard slitwidth of 1". Spectroscopic observations use the 200 lines/mm grism with the WB5650-9200 order-blocking filter, which gives a spectral range of 5650-9200Å (as the filter name suggests) at a resolution of R ~ 750. Exposures times are roughly 1.5-2 hours per slitmask (3-4 x 30min). Fainter objects are placed on multiple slitmasks to accumulate greater net exposure time. However, to this point, data across different slitmasks have not yet been combined. Software is in development to enable this, which should increase the redshift success rate for fainter targets.

The large field-of-view of IMACS allows ACES to efficiently cover the 30' x 30' ECDFS field. As shown in the following plot, the completeness for the main sample of spectroscopic targets (RAB < 23) is high (~75%) and relatively uniform across the entire field.

The above map shows the completeness for ACES targets at RAB < 23 as a function of spatial position in the ECDFS, as computed in a sliding box with height and width of ~35’’ (see box in upper right-hand corner). The black dashed line shows the extent of the COMBO-17 parent R-band catalog (roughly 30' x 30'), while the magenta box designates the location of upcoming HST/WFC3-IR observations as part of CANDELS. The red %s to the right of the color bar gives the portion of the CDFS area with a target completeness greater than the corresponding level in the color bar.

The IMACS data are being reduced using the COSMOS data reduction package, developed at OCIW. COSMOS produces a co-added, sky-subtracted 2-d spectrum for each slitlet. Separate reduction routines, developed as part of the DEEP2 Galaxy Redshift Survey for processing Keck/DEIMOS data, were adapted for IMACS by Renbin Yan (NYU) and Michael Cooper (UC Irvine). These routines extract 1-d spectra, measure redshifts via cross-correlation, and enable user identification of the correct redshift value for each target.