Astron. Astrophys. 343, 943-952 (1999)

2. Observations and results

2.1. ISO spectroscopy

Complete SWS01 (speed 4, 6700 sec total integration time) and LWS01 (4400 sec total integration time, no background spectrum subtracted) spectra centered at the peak of [FeII]1.644 µm line emission (cf. Fig. 7) were obtained on February 6 and February 20, 1996. These were complemented by a quick PHT-S spectrum (448 sec, February 20) and deeper SWS02 observations at selected wavelengths (total 7000 sec, obtained on August 15, 1996 and February 17, 1997), always centered at the same position. The short wavelength section (2.5-5 µm) of the PHT-S spectrum was strongly contaminated by detector memory effects (a very bright source was observed just before RCW103), the PHT-SS results are therefore unreliable and not presented here.

The SWS data were reduced using standard routines of the SWS interactive analysis system (IA) using calibration tables as of September 1997. Reduction relied mainly on the default pipeline steps, plus removal of signal spikes, elimination of the most noisy band 3 detectors, and flat-fielding. The LWS spectrum is based on the end-product of the automatic pipeline as of April 1997 (i.e. OLP 6). A post-processing was performed within the ISO Spectral Analysis Package (ISAP) ¹, Version 1.2, with special emphasis on removal of signal spikes and memory effects, averaging of the different scans, and flat-fielding of the 10 detectors.

The final rebinned spectra are displayed in Figs. 1 to 4 and the derived line fluxes are listed in Table 1 together with additional information. Note that most of the lines in the 2.4-40 µm range were observed twice, i.e. in the complete SWS01 spectrum (Fig. 2) and in the deeper SWS02 line scans (Fig. 1). The derived fluxes were always within 30% and in most cases equal to much better than 20%. The errors quoted in Table 1 also include the differences between the two measurements. It should be noted that, to the best of our knowledge, the transitions of [ClII]14.36 and [PII]32.8 are newly detected astronomical lines. Of interest is also the marginal detection of an unidentified feature at 74.2 µm whose position and flux are remarkably similar to those found in the spectrum of NGC7027 (Liu et al. 1996).

Fig. 1. Individual line scans with SWS02. Wavelengths are in µm and fluxes in Jy. Arrows mark the positions of undetected lines.

Fig. 2. Complete 2.4-45 µm SWS01 spectrum with line identification. Note that each spectral segment has been "continuum subtracted" to remove instrumental offsets and drifts. The continuum level can be estimated from the PHT-S spectrum (Fig. 4) and from the deeper SWS02 line scans (Fig. 1).

Fig. 3. Complete LWS01 spectrum with lines identification. Note that RCW103 lies close to the the Galactic plane (). Therefore, the 100 µm continuum and the [OI], [CII], [NII] lines are strongly contaminated, and most probably dominated by fore/background emission.

Fig. 4. Complete PHT-SL spectrum. The PAH features are most probably dominated by fore/background emission from the Galaxy disk.

Table 1. Observed ISO line fluxes.
Notes:
(1) Observed line flux, units of W cm^-2, errors are given in parenthesis
(2) Extinction (mag) extrapolated from =0.75 (OMD90) using up to 9 µm. Reddening is assumed to be negligible beyond 12 µm
(3) Size (in arcsec) of the SWS ( µm) and LWS ( µm) apertures
(4) Correction factor to account for different apertures, based on the IRSPEC map of [FeII]1.644 (see caption of Fig. 7).
a) Only SWS01 measurement available
b) Adopting a "standard" A(silicate feature)/ ratio
c) Line flux is probably contamined by back/foreground galactic emission
d) Unidentified feature also seen in NGC7027 (Liu et al. 1996)

The SWS spectrum (Figs. 1, 2) is characterized by prominent lines over a faint continuum (about 0.5 Jy at 10 µm) while emission by cold dust is evident in the LWS spectrum (Fig. 3). The level of the 100 µm continuum is similar to the background IRAS level reported by Arendt (1989) and the continuum seen in the LWS spectrum is, therefore, probably dominated by back/foreground emission from the galaxy disk. The same applies to the PAH's features visible in the PHT-S spectrum (Fig. 4) and to the [OI]63.2, [NII]121 and [CII]157 lines which are likely to be strongly contaminated by emission from the diffuse ISM.

Given the relatively large extinction towards RCW103, i.e. or (cf. Oliva et al. 1990, hereafter OMD90), the reddening corrections are not negligible for the lines at the shortest wavelengths. Therefore, in Table 1 we also list the extinctions values which were derived assuming a `typical' reddening curve, i.e. outside of the silicate band. The largest and most uncertain correction is for [SIV]10.52 which lies within the silicate band and for which we have assumed A(10 µm)/0.1.

Useful dynamical information can be derived from the profiles of the lines between 15 and 19 µm, the wavelength range where SWS02 grating spectra achieve their highest spectral resolution. The instrumental resolution depends on the size of the source along the dispersion direction and varies, for example, between 120 and 175 km s^-1 for a compact and extended [SIII]18.7 source, respectively. Luckily, the SWS slit was almost exactly aligned N-S and was, therefore, roughly uniformly illuminated in the dispersion (E-W) direction (cf. Fig. 7). The observed line profiles are displayed in Fig. 6. The ionic lines are resolved and exhibit similar profiles, within the errors, but are broader than the H₂ line which is unresolved. This agrees well with the higher spatial resolution velocity maps from the NIR data discussed below.

2.2. Ground based near infrared spectroscopy

Near infrared observations were collected in Jan 1992 at the ESO-NTT using the long-slit spectrometer IRSPEC equipped with a 62x58 SBRC InSb array whose pixel size was 2.2" along the slit and 5 Å along the dispersion direction. Line images of [FeII]1.644 and H₂ 2.121 were reconstructed from 57 spectra with the 100" x 2.2" slit aligned E-W and shifted by steps of 2.2" along the N-S direction. Each long-slit spectrum consisted of a single on-chip integration of 30 sec with sky exposures every 10 spectra.

The integrated line images are displayed in Fig. 7 whose caption also include results of [FeII] aperture photometry which has been used to determine the correction factors for the different beams used by SWS (see also Table 1). These assume that all ionized species have spatial distribution similar to [FeII] which is justified by the following arguments.

• The flux of Br seen by ISO is within 10% of the value extrapolated from the ground based measurement (OMD90) assuming a constant [FeII]/Br ratio over the region of interest.

• The morphology of the [FeII] filaments is virtually identical to those seen in optical line images (cf. e.g. Moorwood et al. 1987).

It should be noted, however, that the H₂ lines arise from a totally different region 20" south of the ionized gas and outside the optical/radio/X-ray remnant, as originally found by OMD89. This indicates that H₂ emission traces material which has not yet been reached by the shock, most probably a molecular cloud heated by the soft X-rays from the shock front (cf. OMD90).

Fig. 5 shows images at various velocity bins, each roughly corresponding to the wavelength range covered by 1 pixel. Evident are the high velocity [FeII] filaments whose projected velocities extend up to 250 km s^-1 and are compatible with the idea that this line is produced downstream of the fast shock (cf. the Introduction). The H₂ filaments, on the contrary, do not show evidence of motions larger than the FWHM=130 km s^-1 instrumental resolution.

Fig. 5. Line images at various velocity bins, reconstructed from the IRSPEC spectra discussed in Sect. 2, all frames are cut to the same levels as the v=-60 km s-1 images. Note that [FeII] (1.644 µm) shows bright filaments moving by up to 250 km s-1, while H2 (2.121 µm) is narrow and unresolved (i.e. FWHM130 km s-1). The colour images are also available at http://www.arcetri.astro.it/~oliva

Fig. 6. Normalized profiles of FIR lines falling at wavelengths where the SWS spectrometer achieves its highest spectral resolution. The nominal resolving power for extended sources is 222, 198, 186 and 175 km/s for the [NeIII], H2, [FeII] and [SIII] lines, respectively. The ionic lines are all resolved and significantly broader than H2 which is narrow and unresolved. This result agrees with the ground based velocity maps (Fig. 5) and indicate that the ionized lines are produced downstream of the shock front, while H2 arises from the precursor (see text, Sect. 2).

Fig. 7. Line images of H2 (2.121 µm) and [FeII] (1.644 µm) reconstructed from the IRSPEC spectra described in Sect. 2. Coordinates are arcsec from the `ionized peak' where all the ISO spectra presented here were centered. The sizes of the black rectangles correspond to the smallest and largest slits of SWS. The ground based [FeII]1.644 flux is 70, 82, and 120 ( W cm-2) in the 14x20, 14x27 and 20x33 sqarcsec SWS apertures, respectively. These numbers are used to compute the aperture correction factors listed in Table 1.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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