Sky subtraction at the Poisson limit with fibre-optic multi-object spectroscopy

We report on the limitations of sky subtraction accuracy for long duration fibre-optic multi-object spectroscopy of faint astronomical sources during long duration exposures. We show that while standard sky subtraction techniques yield accuracies consistent with the Poisson noise limit for exposures of 1 hour duration, there are large scale systematic defects that inhibit the sensitivity gains expected on the summation of longer duration exposures. For the AAOmega system at the Anglo-Australian Telescope we identify a limiting systematic sky subtraction accuracy which is reached after integration times of 4-10 hours. We show that these systematic defects can be avoided through the use of the fibre nod-and-shuffle observing mode, but with potential cost in observing efficiency. Finally we demonstrate that these disadvantages can be overcome through the application of a Principle Components Analysis sky subtraction routine. Such an approach minimise systematic residuals across long duration exposures allowing deep integrations. We apply the PCA approach to over 200 hours of on-sky observations and conclude that for the AAOmega system the residual error in long duration observations falls at a rate proportional to t^-0.32 in contrast to the t^-0.5 rate expected from theoretical considerations. With this modest rate of decline, the PCA approach represents a more efficient mode of observation than the nod-and-shuffle technique for observations in the sky limited regime with durations of 10-100 hours (even before accounting for the additional signal-to-noise and targeting efficiency losses often associated with the N+S technique).[abridged]

The Nuclear Ring in the Barred Spiral Galaxy IC 4933

We present infrared imaging from IRIS2 on the Anglo-Australian Telescope that shows the barred spiral galaxy IC 4933 has not just an inner ring encircling the bar, but also a star-forming nuclear ring 1.5 kpc in diameter. Imaging in the u band with GMOS on Gemini South confirms that this ring is not purely an artifact due to dust. Optical and near-infrared colours alone however cannot break the degeneracy between age, extinction, and burst duration that would allow the star formation history of the ring to be unraveled. Integral field spectroscopy with the GNIRS spectrograph on Gemini South shows the equivalent width of the Pa-beta line to peak in the north and south quadrants of the ring, indicative of a bipolar azimuthal age gradient around the ring. The youngest star-forming regions do not appear to correspond to where we expect to find the contact points between the offset dust lanes and the nuclear ring unless the nuclear ring is oval in shape, causing the contact points to lead the bar by more than 90 degrees.