The micro-jansky radio source population at 5 GHz

We have imaged a field of area 1.5×10^-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 μJy. Subsequent observations at 1.5 GHz provided spectral information about the sources. The inner part of this field, centered at α = 14^h 16^m and δ = 52.°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 μJy at 8″ resolution and a typical positional accuracy of 0.″2. The integral source count between 16 and 1000 μJy 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 μJy. 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 μJy is - 1.3±0.2. Thus, the slope of the count between 2 and 1000 μJy 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 μJy. Fifteen of the 16 radio sources between 60 and 800 μJy contain only a radio core > 1.″5 in diameter. In contrast, 18 of the 37 sources between 16 and 60 μJy have extended emission with a median size of 4″, often in addition to a radio core. The median spectral index, α, of the μJy sources is 0.38(S∝ν^-α) and 60% of the sources have α<0.5; little change occurs in the average spectral properties between 16 and 1000 μJy. Nearly all sources have optical counterparts brighter than V∼27.5 mag, and most weak radio sources identified with faint blue galaxies with a median magnitude of V∼23. At μJy 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 μJy 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 (≲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 μJy sources are low luminosity starburst galaxies or normal spiral and low luminosity elliptical galaxies will require detailed optical photometry and spectroscopically measured redshifts.