Astron. Astrophys. 329, L21-L24 (1998)

2. The data

The spectra were collected on March 7 1997 using EFOSC1 mounted on the ESO 3.6m telescope. The detector was a Tek 512x512 CCD array with 27 m (0.61" sky projected angle) pixels and the instrument setup included a rotating /2 plate and a Wollaston prism in the collimated beam, and the disperser The 2" broad slit was aligned at PA=318 , roughly along the [OIII] cone axis and perpendicular to the galaxy disk (Fig. 1). Twenty exposures with a total integration time of 5 hours were collected, and consisted of five cycles of four 15 minutes exposures with the plate rotated by 0, 22.5, 45 and 67.5 degrees. Measurements of polarized (HD126593, HD298383), unpolarized (HD64200) and spectroscopic (Hiltner 600) standard stars were also performed for calibration purposes. Standard reduction of the 2D frames was applied and three spectra were extracted at different positions along the slit (cf. Fig. 1 and the caption of Fig. 2), the total flux spectra are shown in Fig. 2.

Fig. 1. The position of the 2" slit is overlaid onto a R frame taken with EFOSC1 (left panel), a [OIII] line image (central panel) and on a `true colour' (red=[SII], green=H +[NII], blue=[OIII]) representation of the Circinus galaxy. North is up, east is left and the numbers are arcsec offsets from the nucleus. The line images are from M94 and are also available at http://www.arcetri.astro.it/~oliva.

Fig. 2. Extracted spectra along the slit (cf. Fig. 1), `nucleus' is 2" x5" centered on the optical peak which is coincident within 0.3" with the IR nucleus (M94), `SE' is 2" x6" centered 7" SE of the nucleus and toward the dust lane (cf. Figs. 1, 2 of M94) and `NW' is 2" x5" centered 5" NW of the nucleus and within the [OIII] cone (cf. Fig. 1). Wavelengths are in Å and is in units of 10-15 erg cm-2 s-1 Å-1. The observed linear polarization degree and angle are and while , and , are the values corrected for local polarization using two different estimates of the galactic polarization vector (cf. Sect. 2). Note that the spectra have the original resolution.

The polarimetric reduction of the 1D spectra was performed using the software written by J.R. Walsh under the MIDAS environment and the resulting linear polarization degree () and position angle () are displayed in Fig. 2. These quantities must be corrected for the polarization by our Galaxy whose polarization vector can be first estimated from the spectra of the foreground star (Fig. 1) which yield =1.8% at =68 , an angle equal to that found in the NW spectrum. The corrected spectra are and which are also plotted in Fig. 2 where the polarization degree of NW decreased to 0.8% while its angle remained 68 and equal to the galactic polarization angle. This indicates that the NW spectrum may be intrinsically unpolarized and the correct amount of Galactic polarization could therefore be foreground star which is probably too close to properly sample the whole disk of our Galaxy. Using =2.6% the corrected spectra become and which are plotted in the bottom rows of Fig. 2. The true polarization degree of the Circinus spectra is somewhere between and . Noticeably, the P spectra of the nucleus and of the SE region are rather independent on the details of the correction for local polarization, i.e. and are virtually equal in the nuclear and SE spectra. However, the polarization angle does depend on the correction applied but is basically independent on wavelength in both and (cf. Fig. 2).

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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