VIRUS-W: commissioning and first-year results of a new integral field unit spectrograph dedicated to the study of spiral galaxy bulges

Abstract

In November and December 2010 we successfully commissioned a new optical fibre-based Integral Field Unit (IFU) spectrograph at the 2.7m Harlan J. Smith Telescope of the McDonald Observatory in Texas. Regular science observations commenced in spring 2011. The instrument achieves a spectral resolution of λ/Δλ = 8700 with a spectral coverage of 4850A – 5480A and a spectacular throughput of 37% including the telescope optics. The design is related to the VIRUS-P instrument that was developed for the HETDEX experiment, but was modified significantly in order to achieve the large spectral resolution that is needed to recover the dynamical properties of disk galaxies. In addition to the high resolution mode, VIRUS-W offers a stellar population mode with a resolution of λ/Δλ = 3300 and a spectral coverage of 4340A – 6040A. The IFU is comprised out of 267 150 μm-core optical fibers with a fill factor of 1/3. With a beam of f/3.65, the core diameter translates to 3.2 on sky and a large field of view of 105 x 55 that is ideally suited to study the bulge regions of local spiral galaxies. The large throughput is due to a design that operates close to the numerical aperture of the fibers, a large 200mm aperture refractive camera with no central obscuration, highly efficient volume phase holographic gratings, and a high-QE CCD. We will discuss the design, the performance and briefly present an example for the very up-to-date science that is possible with such instruments at 2m class telescopes.