Herschel offers two types of spectroscopic capability. PACS and SPIRE offer low to intermediate resolution spectroscopy covering the full Herschel wavelength range. HIFI offers high-resolution spectroscopy over the range from 157-625 μm (480-1910 GHz) using heterodyne techniques, although there is a small gap in coverage from 213-240 microns (1272-1430 GHz), between the HIFI 5b and 6a sub-bands. Users can thus be able to select a wide range of resolutions from Δλ/λ=20 to Δλ/λ=10 000 000 according to the brightness of their source and the science that is required. The main spectroscopic capabilities are summarised in Table 3.2.
In its highest resolution mode Herschel offers a velocity resolution as high as 0.3km/s. The wavelength range covered by Herschel has many thousands of lines of water, atomic transitions and organic molecules. This allows Herschel to study the chemistry of the interstellar medium, tracing water and organic molecules in molecular clouds. Herschel is also be able to study the chemistry of solar system bodies such as the atmosphere of Mars and the comas of comets in unprecedented detail.
All three instruments have a mapping capability in spectroscopic mode, even though HIFI's is somewhat limited, although by no means negated, by the fact that its detector has only a single pixel. PACS and HIFI can scan the detectors across the sky, accumulating spectroscopic data along the length of the scan. SPIRE cannot do that, instead it uses a beam-steering mirror to make filled maps. All three instruments can make a raster map in spectroscopic mode. This allows a spectroscopic survey to be made either of a region that has been mapped in imaging mode, such as a cluster of galaxies, or across a known extended source such as a molecular cloud.
Table 3.2. The main spectroscopic capabilities of PACS, SPIRE and HIFI. For more details please check the relevant instrument manual.
|Wavelength range||55-210 μm||194-313 and 303-671 μm||157-213 and 240-625 μm (with gap)|
|Field of view||47x47"||2.0' (unvignetted)||Single pixel (see below)|
|Pixel size||9"||17", 29" (varies across the bands)||39" (488GHz), 13" (1408GHz)|
|Sensitivity (5σ/1hr, point source)||2x10-18 Wm-2 (130 μm, 1st order), 5x10-18 (70 μm, 3rd order). Continuum: 100 mJy (1st order), 250 mJy (3rd order)||1.0-2.2x10-17 Wm-2 (high resolution), 40-88mJy (low resolution) [5 sigma/1 hr]||"A few" mK (Band 1a) to 100mK (Band 7b), 1 sigma/1hr|
|Resolution||900-2100 (1st order, 102-210 μm), 1800-3000 (2nd order, 72-98 μm), 2600-5400 (3rd order, 55-72 μm)||20-1000||1000-107|
For the latest information on instrument sensitivities please check the Herschel website at http://herschel.esac.esa.int/.
Note also that the PACS sensitivity below 57 microns is very low, although HSpot permits the entry of line observations at shorter wavelengths.