Thermal infrared 3-5 micron colors of obscured and unobscured active galactic nuclei

Thermal infrared photometry in the L and M' bands and L-M' colors of type 1 and type 2 active galactic nuclei (AGNs) are presented. After combining ou r observations with photometric data at similar wavelengths taken from the literature, we find that the excess of L-M' colors of type 2 AGNs (37 sourc es, 50 data points) relative to type 1 AGNs (27 sources, 36 data points), d ue to dust extinction, is statistically detectable but very small. We next investigate the L-M' colors of type 2 AGNs by separating less dust obscured type 2 AGNs and highly dust obscured type 2 AGNs. In both cases, the L-M' colors are similar to the intrinsic L-M' color of unobscured AGNs, and the L-M' color excess of the latter highly dust obscured type 2 AGNs, due to du st extinction, is much smaller than that expected from the Galactic dust ex tinction curve. Contamination from starbursts and the time lag of flux vari ation are unlikely to explain this small L-M' color excess, which is best e xplained if the dust extinction curve in the close vicinity of AGNs is fair ly flat at 3-5 mum, as a result of a size increase of the absorbing dust gr ains through coagulation.