NGC 6946 serves as our test case. The five 54.6"x151.3" rectangles overlayed on the 15 micron image outline the fields we could cover in one set of spectral mapping observations. The arrangement of the fields is dictated by the physical layout of the IRS instruments: the two short-low subunits (5.3 to 7.5 and 7.5 to 14.2 microns) are 54.6" long and separated by 23", whereas the two long-low subunits (14.2 to 21.8 and 20.6 to 40.0 microns) are 151.3" in length with a separation of 43". Here the central field would be mapped by all four subunits, while each of the four `satellite' fields would be mapped by a different subunit.
The raw surface brightness noise levels for a variety of elementary integrations are computed from quoted 1-sigma continuum levels for point source observations. More realistic noise levels are obtained after combining data from the two `ramps,' the two pixels that fill the slit width, and after smoothing the short-low data to the spatial resolution of the long-low data.
Spectra for four potential regions of NGC 6946 are displayed. The four regions are indicated by plus symbols in the first figure of NGC 6946. The spectra are constructed from the 15 micron and 7/15 micron maps, and an IR SED model that was developed for normal galaxies using IRAS and ISO data (Dale et al. 2000). The overall flux levels are set by the 15 micron fluxes, and the long wavelength shapes are determined by the 7/15 colors and the model predictions.
Signal-to-noise estimates show that we can recover adequate data using a 14 second elementary (scan) integration. Estimates are also provided for 30, 60, 120, and 240 second (scan) integrations.
The following table shows the integration times necessary to completely map the central rectangular field (the partial maps of the four satellite fields come for free).
Exptime Total integration
Short-low 6 sec
2x 18.2 min (factor of two to cover both subunits).
14
2x 29.4
60
2x103.7
240
2x414.7
Long -low 6 sec
2x 2.4 min
14
2x 3.9
30
2x 6.9
120
2x27.8
Danny Dale
1 May 2000
