SHELS: Optical Spectral Properties of WISE 22 \mu m-selected Galaxies

Abstract

We use a dense, complete redshift survey, the Smithsonian Hectospec Lensing Survey (SHELS), covering a 4 square degree region of a deep imaging survey, the Deep Lens Survey (DLS), to study the optical spectral properties of Wide-field Infrared Survey Explorer (WISE) 22 \mu m-selected galaxies. Among 507 WISE 22 \mu m-selected sources with (S/N)_{22\mu m}>3 (\simS_{22\mu m}>2.5 mJy), we identify the optical counterparts of 481 sources (\sim98%) at R<25.2 in the very deep, DLS R-band source catalog. Among them, 337 galaxies at R<21 have SHELS spectroscopic data. Most of these objects are at z<0.8. The infrared (IR) luminosities are in the range 4.5x10^8 (L_sun) < L_{IR} < 5.4x10^{12} (L_sun). Most 22 \mu m-selected galaxies are dusty star-forming galaxies with a small (<1.5) 4000 \AA break. The stacked spectra of the 22 \mu m-selected galaxies binned in IR luminosity show that the strength of the [O III] line relative to H\beta grows with increasing IR luminosity. The optical spectra of the 22 \mu m-selected galaxies also show that there are some (\sim2.8%) unusual galaxies with very strong [Ne III] \lambda 3869, 3968 emission lines that require hard ionizing radiation such as AGN or extremely young massive stars. The specific star formation rates (sSFRs) derived from the 3.6 and 22 \mu m flux densities are enhanced if the 22 \mu m-selected galaxies have close late-type neighbors. The sSFR distribution of the 22 \mu m-selected galaxies containing active galactic nuclei (AGNs) is similar to the distribution for star-forming galaxies without AGNs. We identify 48 dust-obscured galaxy (DOG) candidates with large (\gtrsim1000) mid-IR to optical flux density ratio. The combination of deep photometric and spectroscopic data with WISE data suggests that WISE can probe the universe to z\sim2.