The Faintest Radio Source Yet: Expanded Very Large Array Observations of the Gravitational Lens SDSS J1004+4112

Abstract

We present new radio observations of the large-separation gravitationally lensed quasar SDSS J1004+4112, taken in a total of 6 hr of observations with the Expanded Very Large Array. The maps reach a thermal noise level of approximately 4 μJy. We detect four of the five lensed images at the 15-35 μJy level, representing a source of intrinsic flux density, after allowing for lensing magnification, of about 1 μJy, intrinsically probably the faintest radio source yet detected. This reinforces the utility of gravitational lensing in potentially allowing us to study nJy-level sources before the advent of the Square Kilometre Array. In an optical observation taken three months after the radio observation, image C is the brightest image, whereas the radio map shows flux density ratios consistent with previous optical observations. Future observations separated by a time delay will give the intrinsic flux ratios of the images in this source.