Astron. Astrophys. 325, 450-456 (1997)

2. Observations

2.1. 1.3mm continuum emission

Continuum emission at 1.3 mm was detected with the 15m ESO/Swedish SEST telescope at La Silla during the nights of 10/11 July 1993 at the nominal galaxy center coordinates of 14h09m17.9, 06'18". The observations were performed with the MPIfR bolometer detector (Kreysa, 1990) in chopping and beam switching mode with a beam separation of 70". Uranus was used as the calibration standard assuming a brightness temperature of 101K and was mapped to determine both the beam shape and the sensitivity of the bolometer. The beam is Gaussian but elongated in elevation. The measured mean aperture was at full width half maximum (FWHM). The atmospheric transmission was determined by sky dips every 30 minutes. Zenith opacities were . Pointing and focus checks were performed typically once per hour. The focus was stable nearly the whole night and the pointing accuracy varied by less than 4" in azimuth and 6" in elevation. As a pointing reference source we have used the Quasar 1424-418. Individual ON-OFF pairs consisted of 10 cycles with 12 sec integration time each.

As cold dust halos have been detected around the star-burst galaxies M82 and N253 (Krügel et al. 1990, Chini et al., 1992a, Hughes et al., 1994) we also searched for a similar halo around Circinus by observing at positions offset by one beam-size () from the center position of the galaxy. The results are summarized in Table 1. There is a clear detection at the center but nothing to a limit of 3 at any of the offset positions.

Table 1. The 1.3mm continuum emission of Circinus

The atmospheric conditions during the first night were sufficiently stable to obtain a 4' 3' map of the 1.3mm continuum emission. The map was centered at the central position of the beam switching observations. A scan velocity of 8"/sec in azimuth and scan separation of 8" was used. Each scan was measured with a beam throw of 70" in azimuth. A total of 16 individual maps were obtained in 3 hours 20 minutes. Observations of the center position in beam switching mode were made after each pointing to check both the focus and tracking. The data have been analyzed using the SEST software package. Fig. 1 shows the final map in which individual exposures have been convolved into a single beam image in which sky noise and chopping effects have been corrected.

Fig. 1. 1.3mm continuum map of the S(b-d) Circinus galaxy. The 1 (= 46mJy), 3 and 5 contours are plotted. The beam is shown as the hashed area in the upper right corner.

Comparison with a map of Uranus shows that Circinus is unresolved and gives a mean calibration factor of 2.4 mJy/count yielding a peak flux of mJy (5.4 ) which is in agreement with the ON - OFF detection.

2.2. 3-5µm imaging

Previously unpublished one dimensional speckle interferometry in the L'(3.8µm) and M(4.8µm) bands was performed at the ESO 3.6m telescope in 1987 using the instrument described by Perrier(1986) together with scanning of the telescope secondary mirror to produce diffraction limited slit scans at PA . The raw scans both have FWHM 0.3" corresponding to the telescope diffraction limit at 5µm. More detailed analysis of the L band visibility curve shows that it can be well fitted by a source emitting 80% of its energy within a 0.3" (FWHM) core and the rest in an extended halo. This is also consistent with the M band data, although fits to the visibility in this case are less reliable due to the lower s/n ratio. As the speckle observations were restricted to scan lengths of 6" they are insensitive to larger scale components e.g. starlight. A subsequent direct L band image obtained with the IRAC1 array camera at the ESO 2.2m telescope confirms that most of the energy is confined to a central spike although the FWHM in this case was seeing limited at 0.7".

2.3. Mid IR imaging

A first attempt to determine the extent of the mid-infrared emission was made by one of us (AM) at the ESO 3.6m telescope in 1987 by performing N(10µm) and Q(20µm) band linear scans with a 1.5" slit. In both cases the profiles are indistinguishable from those obtained for a reference star and show the diffraction rings at 20µm.

Direct images around 10µm were made on May 21/22 1994 with the Thermal Infrared Multimode Instrument (TIMMI) mounted at ESO's 3.6m telescope (Käufl et al., 1994). TIMMI is a cryogenic focal reducer equipped with a 64x64 element Si:Ga detector. We used for the observations the following filters:

• standard N-filter (, )
• narrow band filter centered around the 11.3 PAH transition (, )
• narrow band filter centered around the 8.6 PAH transition (, )

The scale of the images obtained is 0.34 "/pixel and the total field of view arcsec². In order to determine the point spread function, we also imaged the stars and . The size of the point spread function was determined by a two dimensional Gaussian fit to be arcsec².

As the observing nights were not photometric the images have been calibrated photometrically using the aperture photometry obtained by Moorwood & Glass (1984) by simulating their apertures on the images. The flux as a function of aperture for both the Circinus galaxy and the measured PSF where used to estimate the size of the emitting region. We find that Circinus is unresolved at 8.6 µm but appears to be marginally extended with a size of 1.3" (FWHM) at 9.3 µm and 11.3 µm where the s/n is somewhat higher.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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