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Astron. Astrophys. 318, 997-1002 (1997)

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2. Observations

The HST archival images of NGC 5929 (see Bower et al., 1994) were obtained on September 27, 1992, using the WF/PC-I in the PC mode, with a pixel size of 0.043 arcsec. The four 800 x 800 pixels CCD cover a field of view of [FORMULA] x [FORMULA]. Two images obtained with the filters F664N and F718M were used, the first including the emission from the [FORMULA] and [N II] lines, the second covering the spectral region 6700 Å - 7600 Å. The exposure times were 1800 and 900 sec respectively. These images were deconvolved using 50 iterations of Lucy's (1974) iterative algorithm and two model Point Spread Functions built with the Tiny Tim software (Krist, 1992) centered respectively on NGC 5929 and its companion NGC 5930, which is located [FORMULA] NW of NGC 5929. The continuum emission was then removed from the on-band filter F664N by properly scaling the F718M image.

The star density of the fundamental catalogs presently available is generally too low for the direct calibration of the relatively small fields covered with CCDs. Therefore, two different kinds of ground based images were necessary. Two plates centered on NGC5929 were taken with the 38cm photographic refractor (scale [FORMULA] 30"/mm, field-of-view= [FORMULA] 5 [FORMULA] [FORMULA] 5) of Torino Observatory (Table 1). The relatively large FOV of the refractor ensures that a sufficient number of astrometric reference stars is available for the tie to the optical reference frame (see next section). However, direct astrometry of NGC5929 on those plates would not be adequate, as exposure time was optimized for measuring with comparable accuracy both the bright primary and the fainter secondary reference stars. Astrometry of the Seyfert galaxy was done on frames taken in the Summer of 1994 with the EEV 1242 [FORMULA] 1152 pixel CCD (scale [FORMULA] 0."455/pixel) attached at the f/10 105cm astrometric reflector of Torino Observatory (Table 1). The 600-sec image was selected for the final registration with the HST WF/PC-I R-band image. The FWHM seeing was 1."5, which is average for the Pino Torinese site.


Table 1. Photographic plates and CCD frames centered on NGC 5929

The two photographic plates were digitized with one of the two PDS-type measuring machines available at ST ScI (Lasker et al. 1990). Scanning utilized the raster-type sampling with pixels of 15 µm on a side ([FORMULA] 0."45/pixel). The metrological and stability properties of the digitizer are quite adequate for accurate astrometry, as discussed in Lattanzi et al. (1991). This article also discusses the image centering technique used to measure both the digital copies of the plates and the CCD frames. The centering precision is of the order of, or better than (depending on Signal-to-Noise of each individual image), 1/15 pixel for the photgraphic images, and [FORMULA] 1/50 pixel for the CCD images.

HST and ground based images are shown in Fig. 1. The WF/PC-I R band image of NGC 5929 shows a bright central component located immediately North West of a dust lane which crosses its nuclear region along PA [FORMULA]. A marginal contamination of line emission in this image produces a knot SW of the peak of emission. The line emission is extended and diffuse. Two compact features dominate the [FORMULA] image located symmetrically with respect to the peak in the continuum image. They are connected by lower brightness emission.

[FIGURE] Fig. 1. a HST WF/PC-I and b ground based R band images of NGC 5929 and its companion NGC 5930; c continuum and d line emission ([FORMULA] and [N II]) structure of NGC 5929

Su et al. (1996) recently presented radio images of NGC 5929 obtained with MERLIN with a resolution of [FORMULA].06. At radio wavelengths NGC 5929 shows a triple structure elongated along PA  [FORMULA] [FORMULA]. The central component has a flat spectrum which leads to its identification with the self-absorbed radio nucleus (Wilson & Keel, 1989). The two lobes are located at [FORMULA] SE and [FORMULA] NW of the core, respectively.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998