Magellan/IMACS Spectroscopy of the UDS and COSMOS Fields

 

Here, we describe the basic parameters of the Magellan/IMACS spectroscopic observations, including details regarding the target selection and data reduction. Further information will be added to the webpage as time permits.

UDS Observations:


In the UDS field, spectroscopic targets were selected from the Subaru Rc-band imaging catalog of Furusawa et al. 2008. The sample is magnitude limited at Rc < 23.5 (AB), with sources brighter than Rc = 18 excluded from the target selection process. The spectroscopic targets were prioritized according to a BRci' color selection that was designed to preferentially select sources at intermediate redshift. A color-cut, as shown in the following plot, was defined according to the public redshifts of Akiyama, Simpson, Croom, Geach, Smail and van Breukelen.

The black points in the preceding plot show the color distribution for all sources with Rc < 23.5 in the Subaru imaging catalogs, while the cyan and red points correspond to those objects with spectroscopic redshifts in the ranges z < 0.7 and 0.7 < z < 1.4, respectively. The solid black lines show the color cuts employed to prioritize galaxies at z > 0.7. In addition, sources with high stellarity index (CLASS_STAR) in the Rc band (as given in the Subaru catalogs of Furusawa et al. 2008) were downweighted to avoid overpopulating the slitmasks with M stars.

We utilized the IMACS multi-object spectrograph on the Magellan Baade 6.5-m telescope to collect low-resolution spectra of 502 sources in the UDS field. A total of 4 slitmasks were observed on the nights of December 30-31, 2010 (UT). Each mask contains roughly 125 slitlets, with a fixed slitlength of 8" and slitwidth of 1". Spectroscopic observations used the 300 lines/mm grism (blaze angle = 26.7°) with the clear (or "spectroscopic") filter, which yields a resolution of R ~ 1200 (at 7500Å). Exposures times were roughly 1.5 hours per slitmask (3 x 30min).

The 4 slitmasks in the UDS share a common position and orientation on the sky and were placed so as to maximize the overlap with the CANDELS HST/WFC3-IR pointings in the field. The following image shows the location of the targeted objects in relation to the WFC3-IR and ACS imaging regions:

COSMOS Observations:


In the COSMOS field, spectroscopic targets were selected from the public COSMOS Intermediate and Broad Band Photometry Catalog (2008), primarily utilizing the broad-band (Bjr+i+) photometry from the Subaru telescope. The sample is magnitude limited at r+ < 23.5 (AB), with sources brighter than r+ = 18 excluded from the target selection process. The spectroscopic targets were prioritized according to a Bjr+i+ color selection that was designed to preferentially select sources at intermediate redshift. A color-cut, as shown in the following plot, was designed using the public (DR2) spectroscopic redshifts from the zCOSMOS program.

The black points in the preceding plot show the color distribution for all sources with r+ < 23.5 in the Subaru imaging catalogs, while the cyan, orange, and red points correspond to those objects with spectroscopic redshifts in the ranges z < 0.5, 0.5 < z < 0.7, and 0.7 < z, respectively. The solid black lines show the color cuts employed to prioritize galaxies at z ~ 1. In addition, sources with high stellarity index in the I814 band were downweighted to avoid overpopulating the slitmasks with M stars.

A total of 4 slitmasks were observed on the nights of December 30-31, 2010 (UT). Each mask contains roughly 125 slitlets, with a fixed slitlength of 8" and slitwidth of 1". Spectroscopic observations used the 300 lines/mm grism (blaze angle = 26.7°) with the clear (or "spectroscopic") filter, which yields a resolution of R ~ 1200 (at 7500Å). Exposures times were roughly 1.5-2 hours per slitmask (3-4 x 30min). Real estate on each of the 4 slitmasks was shared with a competing observing program (PI Rhoads). As such only a subset of each slitmask is included in this data set. Furthermore, the location of the 4 slitmasks are the result of a compromise between the two science programs, with 2 of the 4 slitmasks overlapping the CANDELS footprint to a large degree and the other 2 yielding significantly less overlap. The following plot shows the distribution of targets

with respect to the CANDELS WFC3-IR and ACS regions: