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Astron. Astrophys. 322, 38-40 (1997)

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2. Observations and results

The observations were made in November 1991 using the ESO Faint Object Spectrograph and Camera (EFOSC) at the Cassegrain focus of the ESO 3.6 m telescope on La Silla. The Tektronix 512 x 512 chip used had a readout noise of 8.8 e [FORMULA] and pixel size corresponding to 061. The 2-minute Gunn-r image is shown in Fig. 1, and the red object is clearly visible superimposed on the image of the spiral galaxy, 25 south-west of its optical centre.

[FIGURE] Fig. 1. Gunn-r of PKS 0400-181. The horizontal box indicate the position of the slit used for the spectroscopic observations. The frame size is 62 arcmin square. North is at the top, east to the left.

Three spectra using the B300 grism (3600-7000Å) totaling 98 minutes were obtained. The slit width was 15. The spectra were reduced and calibrated with MIDAS. Following the bias and flat field corrections we used the LONG context for the wavelength and flux calibrations. Cosmic hits were removed by applying a median filter in the spatial direction.

Fig. 2 shows two scans along the slit, at the blue and red ends of the two-dimensional B300 spectrum. These clearly reveal the position of the red object, and were used to select the scan lines extracted for the object and reference spectra. The western end of the spectrum is bluer than the eastern end (the two scans were scaled to have the same flux at R [FORMULA]). The S1 spectrum (c.f. Fig. 2) is quite flat redwards of the Balmer break and shows no emission. The S2 spectrum is bluer and shows [OII] [FORMULA] 3727 and H [FORMULA] in emission. The H [FORMULA] equivalent width is about 11 Å. Both spectra show H [FORMULA] in absorption with an equivalent width of [FORMULA] 7 Å. Together with the spectral shape this indicate that the stellar population consists mainly of main sequence F and giant G type stars. On the whole the spectra are typical for Sb type galaxies (Kennicutt, 1992). The cause of the colour difference along the slit we attribute to the presence of HII regions in the western part of the foreground galaxy, although we cannot exclude some contribution from differential extinction.

[FIGURE] Fig. 2. Scans along the slit, obtained from the red (solid line) and blue (dashed line) ends of the two-dimensional spectrum. The ranges used for the red object (O) and reference spectra (S1 and S2) are indicated. R is the distance to the centre of the red object in arscec. The eastern (E) and western (W) directions are marked.

We then subtracted S1 and S2 from the red object spectrum with weights based on the relative flux levels in the blue part of the spectra. As discussed above, the spectra corresponding to scans S1 and S2 have different shapes so we should actually use wavelength dependent weights. In fact the red object is located on the spot where the spiral galaxy show the strongest emission which probably make the final spectrum of the red object somewhat bluer that it actually is. However, as we are primarily interested in the absorption lines features and not the general spectral shape we use wavelength independent weights here. We also note that the sky subtraction was automatically taken care of in this procedure. A few of the stronger sky lines, e.g. [OI] [FORMULA] 5577 Å and Na I at 5892 Å, are not perfectly subtracted as seen in Fig. 3. There are no strong sky lines at the positions of the absorption lines that we identify with the red object, however.

[FIGURE] Fig. 3. Spectrum of the background elliptical galaxy (heavy middle line). The lower spectrum shows a redshifted ([FORMULA]) template E galaxy spectrum (Kennicutt, 1992) and the upper spectrum shows a redshifted model E spectra (Olofsson, private communication). Some of the major identifying spectral features are marked.

The final spectrum of the red object is shown in Fig. 3. The thin absorption feature seen in the spectrum redward of the H line as well as the dip just blueward of the K line we trace back to sharp noise features in the reference spectra used to subtract the spiral galaxy contribution (they are only present in individual integrations).

For comparison we also show in Fig. 3 the spectra of NGC 3379 (Kennicutt, 1992), which is a typical E galaxy, and a model spectrum of an old (12 Gyr) stellar population (K. Olofsson, private communication) derived from spectral evolutionary models. These spectra have been redshifted to [FORMULA]. Comparing this with our data we note two features: 1) the general similarity of the spectral energy distributions and 2) the coincidence of some absorption lines: the Calcium H & K absorption and spectral break, the Mg G band and possibly a broad H [FORMULA] with some filled in emission. The redshift obtained from these features is [FORMULA].

The redshift of the spiral galaxy obtained from the observed [OII] [FORMULA] 3727 and H [FORMULA] emission lines lying closest to the centre was measured to be [FORMULA], in agreement with that obtained by Shaver et al. (1983). Rotation is clearly seen, with a total velocity range of a few hundred km s-1, consistent with the properties of this spiral.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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