A Deep Keck/NIRC2 Search for Thermal Emission from Planetary Companions Orbiting Fomalhaut

We present deep Keck/NIRC2 1.6 and 3.8 $\mu m$ imaging of Fomalhaut to constrain the near-infrared (IR) brightness of Fomalhaut b, recently confirmed as a likely planet (Currie et al. 2012a), and search for additional planets at r_proj = 15--150 AU. Using advanced/novel PSF subtraction techniques, we identify seven candidate substellar companions at projected separations comparable to Fomalhaut b. However, multi-epoch data shows them to be background objects. We set a new 3-$\sigma$ upper limit for Fomalhaut b's H-band brightness of m(H) $\sim$ 23.15 or 1.5 to 4.5 M_J, depending on the system age and planet cooling models used. We do not recover the possible point source reported from Spitzer/IRAC data: at its location detection limits are similar to those for Fomalhaut b. Our data when combined with other recent work rule out planets with masses and projected separations comparable to HR 8799 bcde and planets with masses M $>$ 3 M_{J} at r_proj $>$ 45 AU. The James Webb Space Telescope will likely be required to shed substantial further light on Fomalhaut's planetary system.