Variable extended objects in Selected Area 57

We have isolated a sample of 14 candidate variable objects with extended image structure to B-J = 22.5 in 0.284 deg(2) of Selected Area 57. The majority of candidates are blue (U-B < 0) and relatively compact. At fainter magnitudes, there is a steep rise in the number of variable objects. These objects are also compact and blue, and some of them are likely to be truly stellar. Twelve of the B-J less than or equal to 22.5 candidates have been observed spectroscopically over limited ranges of wavelength and a variety of resulting signal-to-noise ratios. Three of the four brightest have broad emission lines characteristic of Seyfert I galaxies. The fourth has a highly variable spectrum consistent with Seyfert-like activity. A fifth candidate has emission-line ratios nominally consistent with a Seyfert 2 galaxy. In most cases where we have not been able to confirm a Seyfert spectroscopic type, the spectra are of insufficient quality or coverage to rule out such a classification. Based on spectroscopic and photometric redshift information, the majority of candidates have luminosities less than 10% of the nominal demarcation between QSOs and active galactic nuclei (M-B = -23, H-o = 50 km s(-1) Mpc(-1), q(o) = 0.5). The surface density of confirmed M-B > -23 active galactic nuclei (AGNs) to B-J = 22, including stellar sources, is similar to 40 deg(-2), in good agreement with other surveys at this depth. The confirmed AGNs in extended sources make up 36% of this population. Thus, the application of a variability criterion to images with extended structure enhances the completeness of the census of active nuclei. If the majority of our candidates are bona fide AGNs, the surface density could be as high as 82 deg(-2) for M-B > -23 and 162 deg(-2) for all luminosities to B-J = 22, with extended sources contributing up to 33% of the total.