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Observations were made using the LWS in grating mode (L01, L02, 43-197 µm, lambda / Deltalambda ~ 200). The LWS consists of ten detectors with spectral overlap for adjacent detectors. In the grating mode of the LWS, the spectral resolution is about 0.29 µm for the 10 µm-wide short-wavelength detectors (SW1-SW5) and 0.60 µm in the 20 µm-wide long-wavelength detectors (LW1-LW5). The L01 Astronomical Observation Template (AOT) is a range scan of the grating that results in 10 spectra covering a significant range of the LWS. The L02 AOT produces spectra for up to ten wavelengths, specified by the observer. In this mode, data are recorded for all ten detectors while the specified wavelengths are being scanned, producing spectra with significant gaps across the range of the LWS.

All guaranteed and open time observations for 227 galaxies were extracted from the ISO Data Archive and processed through the LWS Pipeline Version 7.0 or 8.7. Slight improvements in the photometric model are made beyond Pipeline Version 7.0, but these changes have minor effects on the calibration of the L01 and L02 grating mode AOTs. These changes yield improvements in the flux accuracies by a few percent but do not significantly alter the line and continuum fluxes that are derived from Pipeline 7.0.

Further data manipulation is then carried out using the LWS Interactive Analysis (LIA; Hutchinson et al. 2001) and the ISO Spectral Analysis Package (ISAP; Sturm et al. 1998). The continuum fluxes in the LWS spectra are significantly affected by the uncertainties in the dark current, which can be of the same order as the source continuum. Many of the galaxies in this sample are in this faint flux regime (fnu(60) < 50 Jy in the 75" LWS beam). As the dark currents are only additive in nature across the whole band, they do not affect the line flux estimates. The dark currents are re-estimated and removed one at a time by hand through visual inspection using the LIA. The data are then corrected detector by detector for any evident instrumental responsivity variations and flux calibrated to the LWS calibration source Uranus, applied using LIA. Glitches due to cosmic rays are removed by hand from the data using ISAP by plotting spectral scans as a function of time and identifying bad data points through the characteristic appearance of falling glitch trails. Depending on the quality of the observation of a galaxy, between 15% and 20% of the data are typically discarded. Spectral scans are co-added and averaged together using a 3sigma clip in spectral bins of about 0.05 µm. For extended sources or for sources that are off-center with respect to the LWS aperture, a sinusoidal fringe associated with internal reflection and interference within the LWS instrument may arise (Gry et al. 2003; Swinyard et al. 1998). The fringes are usually less than 5% of the continuum and do not severely affect the line and continuum measurements. For full-grating L01 observations, these fringes can be removed using a defringing algorithm available within ISAP. The LWS data also suffer from transients. When the grating is scanned between the forward and reverse directions, a small (< 5%) detector memory effect (Gry et al. 2003) may be visible between the two scan directions. This memory effect is due to different response times for the detectors depending on whether the signal increases or decreases with time and is most visible in the SW1, SW2, and LW2 detectors during L01 observations. No correction is applied for these memory effects. When these memory effects are present in the data, each scan direction is averaged separately, and the line and continuum fluxes for each scan direction are measured before estimating the final fluxes and uncertainties (see Sections 4 and 5 for further details). An additional source of uncertainty occurs for extended sources where the variation in the LWS beam from detector to detector might cause a mismatch between adjacent detectors by up to 30% depending on the extent and structure of the galaxy. With the application of an extended source correction, this mismatch can be partially corrected. The data presented in this paper are based on the point source calibration of the pipeline and no correction for extended sources has been applied due to the uncertainty in this correction. See the Appendix for the definition and discussion of the extended source correction.

Through the use of LIA and ISAP, the improvement in the overall quality of the data from the original pipeline Auto-Analysis Result product is substantial. By re-estimating the dark currents, the appearance of negative fluxes in most of these observations is removed. Through the re-estimation of the dark currents and gain corrections and careful glitch removal, the match between overlapping detectors is improved, thus producing more continuous spectra, shown in Figure 7. Any remaining spectral mismatch between adjacent detectors may be the result of residual errors in the dark current subtraction or beam uncertainties from detector to detector. Using LIA and ISAP, the line and continuum calibration uncertainties decrease from 20%-30% to 10%-20%, on average, for faint sources (fnu(60) < 50 Jy) as illustrated in Figure 7.

Figure 7

Figure 7. Two example LWS spectra representing the pipeline product L01 and L02 AOTs displayed before and after using the LIA and ISAP reduction. Improvements from using LIA and ISAP include not only the removal of fringes and glitch removal but also the reduction of negative continuum fluxes and misaligned adjacent detectors.

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