Observing extended sources with the Herschel SPIRE Fourier Transform Spectrometer

¹ Laboratoire AIM, CEA/DSM − CNRS − Irfu/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: ronin.wu@cea.fr
² RAL Space, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK
³ Institute for Space Imaging Science, Department of Physics & Astronomy, University of Lethbridge, Lethbridge, AB T1K3M4, Canada
⁴ Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁵ Cambridge University, Cavendish Laboratory and the Kavli Institute for Cosmology, CB3 0 HA Cambridge, UK
⁶ University College London, Department of Physics and Astronomy, WC1E 6 BT London, UK
⁷ Blue Sky Spectroscopy, 9/740 4 Ave S, Lethbridge, Alberta, T1J 0N9, Canada
⁸ Cardiff University, The Parade, Cardiff, UK
⁹ Laboratoire d’Astrophysique de Marseille − LAM, Université d’Aix-Marseille & CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁰ Center for Astrophysics and Space Astronomy, 389-UCB, University of Colorado, CO 80303 Boulder, USA
¹¹ Physics Department, Imperial College London, SW7 2, AZ South Kensington Campus, UK
¹² NASA Herschel Science Centre, IPAC, Pasadena, California, USA
¹³ Department of Physical Sciences, The Open University, MK7 6 AA Milton Keynes, UK
¹⁴ European Space Astronomy Centre, Herschel Science Centre, ESA, 28691 Villanueva de la Cañada, Spain

Received: 5 May 2013
Accepted: 25 June 2013

Abstract

The Spectral and Photometric Imaging Receiver (SPIRE) on the European Space Agency’s Herschel Space Observatory utilizes a pioneering design for its imaging spectrometer in the form of a Fourier Transform Spectrometer (FTS). The standard FTS data reduction and calibration schemes are aimed at objects with either a spatial extent that is much larger than the beam size or a source that can be approximated as a point source within the beam. However, when sources are of intermediate spatial extent, neither of these calibrations schemes is appropriate and both the spatial response of the instrument and the source’s light profile must be taken into account and the coupling between them explicitly derived. To that end, we derive the necessary corrections using an observed spectrum of a fully extended source with the beam profile and considering the source’s light profile. We apply the derived correction to several observations of planets and compare the corrected spectra with their spectral models to study the beam coupling efficiency of the instrument in the case of partially extended sources. We find that we can apply these correction factors for sources with angular sizes up to θ_D ~ 17′′. We demonstrate how the angular size of an extended source can be estimated using the difference between the subspectra observed at the overlap bandwidth of the two frequency channels in the spectrometer, at 959 < ν < 989   GHz. Using this technique on an observation of Saturn, we estimate a size of 17.2′′, which is 3% larger than its true size on the day of observation. Finally, we show the results of the correction applied on observations of a nearby galaxy, M82, and the compact core of a Galactic molecular cloud, Sgr B2.