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

We present deep Keck/NIRC2 1.6 and 3.8 μm imaging of Fomalhaut to constrain the near-infrared brightness of Fomalhaut b, recently confirmed as a likely planet, and search for additional planets at r _proj = 15-150 AU. Using advanced/novel point spread function subtraction techniques, we identify seven candidate substellar companions Fomalhaut b-like projected separations. However, multi-epoch data show them to be background objects. We set a new 3σ upper limit for Fomalhaut b's H-band brightness of m(H) ~23.15 or 1.5-4.5 M_J . 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 M > 3 M_J planets at r _proj > 45 AU. The James Webb Space Telescope will likely be required to shed substantial further light on Fomalhaut's planetary system in the next decade.