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Protostellar (class 0/I) disks, having masses comparable to those of theirnascent host stars, and fed continuously from their natal infalling envelopes,are prone to gravitational instability (GI). Motivated by advances innear-infrared (NIR) adaptive optics imaging and mm-wave interferometry, weexplore the observational signatures of GI in disks, using hydrodynamical andMonte Carlo radiative transfer simulations to synthesize NIR scattered lightimages and mm dust continuum maps. Spiral arms induced by GI, located at diskradii of hundreds of AUs, are local overdensities and have their photospheresdisplaced to higher altitudes above the disk midplane, arms therefore scattermore NIR light from their central stars than inter-arm regions, and aredetectable at distances up to 1 kpc by Gemini/GPI, VLT/SPHERE, andSubaru/HiCIAO/SCExAO. By contrast, collapsed clumps formed by diskfragmentation have such strong local gravitational fields that their scatteringphotospheres are at lower altitudes, such fragments appear fainter than theirsurroundings in NIR scattered light. Spiral arms and streamers recently imagedin four FU Ori systems at NIR wavelengths resemble GI-induced structures andsupport the interpretation that FUors are gravitationally unstable protostellardisks. At mm wavelengths, both spirals and clumps appear brighter in thermalemission than the ambient disk and can be detected by ALMA at distances up to0.4 kpc with one-hour integration times at ~0.1 arcsec resolution. Collapsedfragments having masses >=1 MJ can be detected by ALMA within ~10 minutes.

Signatures of Gravitational Instability in Resolved Images of Protostellar Disks