Galactic Plane
S. H. Beer and A. E. Vaughan, PASA, 16 (2), in press.
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The Seyfert Galaxy
The spectrum of this object is shown in Figure 6, which is the average of four separate CCD exposures. The radial velocity of 8730 km s-1 and clearly defined emission lines from a small area 1.5 arcseconds in diameter are typical of an Active Galactic Nucleus (AGN). The emission line source is at the centre of a larger optical source of size
, and a still larger radio source of size
in diameter. A radio image of the object is shown in Figure 7, taken from the VLA NVSS at 1.4 GHz. This image is approximately
in size.
Distance is estimated at 120 Mpc using a Hubble constant of 75 km s-1 Mpc-1. Linewidths are km s-1, and the 5 GHz power output, calculated from the PMN flux and a distance of 120 Mpc, is
2 x 1023 W Hz-1. Classification of this object as a Seyfert 2 galaxy is based on its known properties and the properties of the various AGN classes discussed by Woltjer (1990).
The presence of the 3727 [OII] line is typical of PN and of many AGN types, including Seyfert galaxies. However in PN, the excitation is produced by a central star, with an approximately black-body continuum, in a relatively narrow range of wavelengths. The power-law spectra of AGN is broader, and allows a wider range of excitation states to be seen. In this object, the 3727 [OII] and the [OIII] lines are all much stronger than H, a feature not typically seen in nebulae excited by starlight. Woltjer (1990) quotes typical values for the flux ratios 5007 [OIII]/H and 3727 [OII]/H for Seyfert 2 galaxies as 13 and 2 respectively. For IRAS 07245-3548 these ratios are 16 and 5 respectively, which may suggest a broader than average range of ionisation states.
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© Copyright Astronomical Society of Australia 1997