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A broad continuum excess in the near-infrared, peaking in the rest-frame at2-5 micron, is detected in a spectroscopic sample of 88 galaxies at 0.5<z<2.0taken from the Gemini Deep Deep Survey. Line emission from polycyclic aromatichydrocarbons (PAHs) at 3.3 micron alone cannot explain the excess, which can befit by a spectral component consisting of a template of PAH emission linessuperposed on a modified blackbody of temperature T~850 K. The luminosity ofthis near-infrared excess emission at 3 micron is found to be correlated withthe star formation rate of the galaxy. The origin of the near-infrared excessis explored by examining similar excesses observed locally in massive starforming regions, reflection and planetary nebulae, post-asymptotic giant branchstars and in the galactic cirrus. We also consider the potential contributionfrom dust heated around low-luminosity active galactic nuclei. We conclude thatthe most likely explanation for the 2-5 micron excess is the contribution fromcircumstellar disks around massive young stellar objects seen in the integratedlight of high-redshift galaxies. Assuming circumstellar disks extend down tolower masses, as they do in our own Galaxy, the excess emission presents uswith an exciting opportunity to measure the formation rate of planetary systemsat cosmic epochs before our own solar system formed.

A near-infrared excess in the continuum of high-redshift galaxies: a tracer of star formation and circumstellar disks?