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This paper presents a sensitive and comprehensive IRAC 3-8 $\mu$m photometricsurvey of white dwarfs for companions in the planetary mass regime withtemperatures cooler than the known T dwarfs. The search focuses on descendentsof intermediate mass stars with $M\ga3$ $M_{\odot}$ whose inner, few hundred AUregions cannot be probed effectively for massive planets and brown dwarfs byany alternative existing method. Furthermore, examination for mid-infraredexcess explores an extensive range of orbital semimajor axes, including theintermediate 5-50 AU range poorly covered and incompletely accessible by othertechniques at main sequence or evolved stars. Three samples of white dwarfs arechosen which together represent relatively young as well as older populationsof stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17metal-rich field white dwarfs. In particular, these targets include: 7 Hyadsand 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs ofsimilar age; van Maanen 2 and 16 similarly metal-rich white dwarfs with agesbetween 1 and 7 Gyr. No substellar companion candidates were identified at anystar. By demanding a 15% minimum photometric excess at 4.5 $\mu$m to indicate acompanion detection, upper limits in the planetary mass regime are establishedat 34 of the sample white dwarfs, 20 of which have limits below 10 $M_{\rm J}$according to substellar cooling models. Specifically, limits below the minimummass for deuterium burning are established at all Pleiades and Hyadeswhitedwarfs, as well as similarly young field white dwarfs, half a dozen of whichreceive limits at or below 5 $M_{\rm J}$. Two IRAC epochs at vMa 2 rule out$T\ga200$ K proper motion companions within 1200 AU.

Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates