Apertif is an innovative focal-plane array system on the Westerbork synthesis radio telescope in Dwingeloo that will allow wide-field HI surveys out to cosmological distances (z~0.2). With commissioning expected to finish in late 2017, Apertif will provide radio-source targets to WEAVE for optical follow-up of 100,000 massive gas-rich galaxies in a timely fashion. Apertif is the first working focal-plane array capable of full Westerbork resolution (~15”x15” beam) over a single, full 8 deg2 pointing. Operating in the frequency range 1000-1750 MHz with nearly the sensitivity of the present single-pixel WSRT frontend. With Apertif, the WSRT can image an area on the sky about 25 times the size of the full moon.
The current survey program for Apertif calls for a two-tiered imaging survey that will provide strong synergy with WEAVE, comprising a blind medium-deep survey of ~300 deg2 and a shallow survey of ~3000 deg2. Both surveys will be largely restricted to the SDSS imaging footprint and to declinations above δ=+27o, with an emphasis on surveying the supergalactic plane with its enhanced density of nearby galaxies. The medium-deep survey aims at targeting areas with exhaustive ancillary data such as the HETDEX and Herschel-ATLAS areas, or volumes of particular environmental value such as the nearby CVn group and the Perseus-Pisces supercluster. Together, both surveys are expected to yield ~105 HI-detected galaxies of which ~104 galaxies will be spatially resolved by Apertif and of those, ~5000 will have optical dimensions that fit the LIFU field-of-view of WEAVE. The Apertif surveys will provide redshifts and the neutral gas content, morphologies, dynamics, and dynamical masses at the resolution described above. Furthermore, Apertif will also detect and measure HI absorption against radio-loud AGN yielding information on gas accretion and outflows, provide spatially-resolved, extinction-free star-formation maps from the radio continuum emission, and identify OH megamasers as locations of intense star formation.
The WEAVE-Apertif survey will be able to harness WEAVE's dual integral-field-unit (IFU) capability, as the large IFU will be ideally suited for large nearby galaxies, while the multiple small IFUs will be perfect for small and distant galaxies. A large-scale integral-field survey with WEAVE creates a strong synergy with the Apertif imaging surveys, significantly increasing the power of each instrument on its own. We will pursue three primary science cases:
WEAVE-Apertif will study transformations in galaxies by comparing the optical and ISM properties of galaxies transitioning from the main star-forming sequence into the Red sequence. The unique aspect of this survey (compared to other large IFU surveys) resides in the HI selection of targets, which enables the detailed analysis of galaxies in different stages of the transformation process.