4. Deep Imaging of Five Clusters of Galaxies at 3.6 - 8.0 micron using IRAC

In this chapter (and the next), we document the full procedure one might follow in developing a Spitzer observing proposal. We will design a deep imaging observation that covers a relatively small area of sky. This example will illustrate how to perform a deep, small area imaging campaign, coordinating observations between two Spitzer instruments: IRAC $(3.6 - 8.0 \; \mu {\rm m})$, and MIPS $(24 - 160 \; \mu {\rm m})$. The IRAC observations are described in this chapter, and the complementary MIPS program is developed in §5. You may notice slight differences between what appears on your screen and the screen-grabs that appear in these chapters, since some of the screen-grabs were generated using an earlier version of Spot. You may also notice slight differences in Spot's estimation of the duration of the observations or in the instrument sensitivities from the values quoted here.

In order to mimic more closely the complete procedure an observer might follow, we develop a test case ``real observation.'' The science background is as follows:

The goal is to survey selected regions of the sky to study the formation and evolution of normal galaxies to redshifts $z \gtrsim 3$, and probe the high-z population of star forming galaxies. We will choose well-studied regions of the sky, where deep spectroscopy has been obtained from 10m class telescopes, and observations are extant in the radio, optical and X-ray. The objective will be to obtain deep IR imaging to $\simeq 1.7 \; \mu {\rm Jy}\; (1 \sigma)$ at 8 microns. This will enable us to address a number of science goals. For example, Spitzer imaging to this depth, combined with spectroscopic data (obtained elsewhere), would allow us to construct a luminosity function for normal galaxies at $z \simeq 3$.

Such a program has been performed, e.g., in the Spitzer GTO program (``The IRAC Deep Survey,'' PI: G. Fazio^4.1) by imaging a wide area (multiple fields) in the Groth strip. Here we describe a complementary approach to image a smaller total area of the sky, but to greater depth, using the magnification by foreground clusters to probe deeply into the IR sky. We will coordinate two Spitzer instrument campaigns to obtain photometry from $3.6 - 8.0 \; \mu {\rm m}$ (§4) and $24 - 160 \; \mu {\rm m}$ (§5).

The gravitational lensing by a massive, low-z cluster magnifies background sources, and hence galaxies that would fall below the sensitivity limit of blank field observations are boosted above the detection threshold. For simplicity we ignore this effect for this example.

In the remainder of this chapter, we develop the observations in the 3.6 - 8.0 micron range using the InfraRed Array Camera (IRAC). The longer wavelength $(24 - 160 \; \mu {\rm m})$ observations using MIPS are discussed in §5. IRAC is the four-channel camera that provides simultaneous $\sim 5\hbox{$.\!\!^{\prime}$}2 \times 5\hbox{$.\!\!^{\prime}$}2$ images at 3.6, 4.5, 5.8, and 8 microns. The pixel size is $\simeq 1\hbox{$.\!\!^{\prime\prime}$}2$ in all bands and all four detector arrays in the camera are 256 $\times$ 256 pixels in size. IRAC is the instrument for imaging observations at the ``bluest-end'' of the Spitzer wavelength coverage.