Morphological analysis of the centimeter-wave continuum in the dark cloud LDN 1622

The spectral energy distribution of the dark cloud LDN 1622, as measured by Finkbeiner using WMAP data, drops above 30 GHz and is suggestive of a Boltzmann cutoff in grain rotation frequencies, characteristic of spinning dust emission. LDN 1622 is conspicuous in the 31 GHz image we obtained with the Cosmic Background Imager, which is the first centimeter- wave resolved image of a dark cloud. The 31 GHz emission follows the emission traced by the four IRAS bands. The normalized cross- correlation of the 31 GHz image with the IRAS images is higher by 6.6 sigma for the 12 and 25 mu m bands than for the 60 and 100 mu m bands: C12+25 0: 76 +/- 0: 02, and C60+100 0: 64 +/- 0: 01. The mid- IR - centimeter- wave correlation in LDN 1622 is evidence for very small grain ( VSG) or continuum emission at 26 - 36 GHz from a hot molecular phase. In dark clouds and their photon- dominated regions ( PDRs), the 12 and 25 mu m emission is attributed to stochastic heating of the VSGs. The mid- IR and centimeter- wave dust emissions arise in a limb- brightened shell coincident with the PDR of LDN 1622, where the incident UV radiation from the Ori OB 1b association heats and charges the grains, as is required for spinning dust.