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Astron. Astrophys. 357, 898-908 (2000)

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2. Observations

As part of the Central Program "MPEXGAL", we have observed the central region of NGC 4945 with the Short Wavelength Spectrometer (SWS) and the spectrophotometer PHT-S on board ISO.

2.1. SWS spectroscopy

SWS grating line profile scans (SWS02 mode) were obtained on 1996 February 6 for 28 spectral lines in the range of 2.42 to 40.34 µm. The spectral resolution in this range varies between R=900 and 2000, corresponding to a velocity resolution of 330-150 km s-1. Aperture sizes used range between 14[FORMULA]20" and 20[FORMULA]33". SWS was centered on the 1.4GHz continuum peak from Ables et al. (1987), which coincides with the the position of the L-band peak of Moorwood et al. (1996a). At the time of observation the position angle of the major axis of the SWS apertures was 35.4o, 10o off from that of the galaxy major axis (45o).

The data reduction was performed using the SWS Interactive Analysis system (SIA; Lahuis et al. 1998; Wieprecht et al. 1998), putting special emphasis on tools to improve cosmic ray spike removal, dark current subtraction and flat fielding. The wavelength calibration of SWS is discussed by Valentijn et al. (1996a). We used calibration files as of March 1999. The accuracy of the flux calibration is estimated to be 30% (Schaeidt et al. 1996). The resulting spectra are shown in Fig 1.

In total 17 spectral lines were detected. For another 11 lines we derived upper limits, using gaussian profiles of width equal to other lines of the same (or comparable) species, scaled to a peak height corresponding to approximately 3[FORMULA] of the noise. Both detections and upper limits are presented in Table 1.


Table 1. NGC 4945 results from SWS observations

2.2. PHT-S spectrophotometry

We have obtained two low resolution ([FORMULA]/[FORMULA]90) ISO-PHT-S spectra, on 1996 October 12 and 1997 August 3, respectively. ISO-PHT-S comprises two low-resolution grating spectrometers covering simultaneously the wavelength range 2.47 to 4.87 µm and 5.84 to 11.62 µm. The spectrum is registered by two linear arrays of 64 Si:Ga detectors with a common entrance aperture of 24" [FORMULA] 24". The measurements were carried out in rectangular chopped mode, using a chopper throw of 180". The resulting spectra thus are free of contributions from zodiacal light, that would otherwise affect the spectrum. The pure on-source integration times were 512 and 1024 s.

The ISO-PHT-S data were reduced using PIA 1 (Gabriel et al. 1997) version 8.1. Steps in the data reduction included: 1) deglitching on ramp level. 2) subdivision of ramps in two sections of 32 non destructive read-outs. 3) ramp fitting to derive signals. 4) masking of bad signals by eye-inspection. 5) kappa sigma and min/max clipping on remaining signal distribution. 6) determination of average signal per chopper plateau. 7) masking or correction of bad plateaux by eye-inspection. 8) background subtraction using all but the first four plateaux. 9) finally, flux calibration, using the signal dependent spectral response function of Acosta-Pulido (1999). This spectral response function avoids some deficiencies of the previous PIA response function, in particular in the 3 µm region near the "Ice" feature. The absolute calibration is accurate to within 20%.

The two resulting spectra were obtained at slightly different position angles about the nucleus. For the first, the square aperture was aligned with the galaxy major axis (45o). For the second, the position angle was 31.1o. Fig. 2 shows the averaged ISO-PHT-S spectrum. The on-source integration times were used as weight factors in the computation of the average spectrum.

[FIGURE] Fig. 2. The average ISO-PHT-S spectrum of NGC 4945. Upper inset: comparison with the ISO-SWS spectrum of the line of sight towards the Galactic center. Lower inset: the ISO-PHT-SL spectrum in linear flux scale

A number of emission lines can be identified in the ISO-PHT-S spectrum. These include 9.66 µm H2 (0-0) S(3), the unresolved blend of 6.99 µm [ArII ] and 6.91 µm H2 (0-0) S(5), and 4.05 µm H Br[FORMULA]. For the 9.66 µm H2 (0-0) S(3) line, not included in the SWS02 line scans, we measure a flux of 5.4[FORMULA]10-20 W/cm2, with an uncertainty of 30%.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000