THE MICRO-JANSKY RADIO-SOURCE POPULATION AT 5 GHZ

We have imaged a field of area 1.5 X 10(-5) sr with unprecedented sensitivity using the VLA at 5 GHz. The total integration time of 126 hr was spread over the D, C, and B configurations in order to obtain images of faint radio sources with angular scales between 1" and 100" with an rms noise of 2.5-mu-Jy. Subsequent observations at 1.5 GHz provided spectral information about the sources. The inner part of this field, centered at alpha = 14h 16m and delta = 52.-degrees-7 (1950.0) has also been imaged with the Palomar 200 in. 4-shooter CCD array. We have compiled a complete (> 95%) catalog of 62 sources with a peak flux density at 5 GHz greater than 16-mu-Jy at 8" resolution and a typical positional accuracy of 0".2. The integral source count between 16 and 100-mu-Jy is given by N(S) = (23.2 +/- 2.8)S-1.18 +/- 0.19, where N is the number of sources (arcmin)-2 with flux density greater than S mu-Jy. The blending of sources at these low flux density levels was resolved using deep optical identifications and a 2".5 resolution radio image. An analysis of the image intensity distribution below the detection limit indicates that the slope of the integral count between 2 and 16-mu-Jy is -1.3 +/- 0.2. Thus, the slope of the count between 2 and 1000-mu-Jy is nearly constant and close to the Euclidean value of -1.5. The average angular size of sources appears to increase at flux densities below 60-mu-Jy. Fifteen of the 16 radio sources between 60 and 800-mu-Jy contain only a radio core > 1".5 in diameter. In contrast, 18 of the 37 sources between 16 and 60-mu-Jy have extended emission with a median size of 4", often in addition to a radio core. The median spectral index, alpha, of the mu-Jy sources is 0.38(S proportional nu-alpha) and 60% of the sources have alpha < 0.5; little change occurs in the average spectral properties between 16 and 1000-mu-Jy. Nearly all sources have optical counterparts brighter than V approximately 27.5 mag, and most weak radio sources identified with faint blue galaxies with a median magnitude of V approximately 23. At mu-Jy levels, very few double-lobed radio sources are seen, and only a small fraction of the optical identifications are giant elliptical galaxies or stellar objects (stars or quasars). The relative steep slope of the mu-Jy counts suggests that this radio source population-if it is indeed one population-has undergone cosmological evolution similar to radio galaxies and quasars. Our nearly 100% optical identification and the relatively low median redshift (less-than-or-similar-to 1) measured at somewhat brighter radio flux densities suggests that the redshift cutoff of this weak radio source population is of order 2-3. Whether the dominant population of mu-Jy sources are low luminosity starburst galaxies or normal spiral and low luminosity elliptical galaxies will require detailed optical photometry and spectroscopically measured redshifts.