Btl

STIS Optical

Fig 19.14.

The Seyfert galaxy NGC l566.This is at a distance 15 Mpc.The active region in the center is found to vary on a time scale of less than a month. (a) HST Wide Field Planetary Camera 2 (WFPC2) image of the oxygen emission (5007 A) from the gas at the heart of NGC 4151. Though the twin cone structure can be seen, the image does not provide any information about the motion of the oxygen gas. (b) In this STIS (imaging spectrometer) spectral image of the oxygen gas, the velocities of the knots are determined by comparing the knots of gas in the stationary WFPC2 image to the horizontal location of the knots in the STIS image. (c) This STIS spectral image shows the velocity distribution of the carbon emission from the gas in the core of NGC 4151. It requires more energy to make the carbon gas glow (CIV at 1549 A) than it does to ionize the oxygen gas seen in the other images. (d) In this false color image the two emission lines of oxygen gas (the weaker one at 4959 A and the stronger one at 5007 A) are clearly visible.The horizontal line passing through the image is from the light generated by matter falling into the black hole at the center of NGC 4151. [STScI/NASA]

disk of a spiral galaxy. This suggests that we are seeing spiral galaxies with unusually bright nuclei.

The optical spectra (Fig. 19.15) are characterized by strong, broad emission lines. The lines are more than 103 km/s wide. This is more than ten times the width of lines in normal galaxies. If the broadening is thermal Doppler broadening, this would imply a temperature in excess of 107 K (see problem 19.10).

There are some differences among Seyferts in the appearance of lines called 'forbidden lines'. These are spectral lines that are not strong under normal circumstances. In some Seyferts, the forbidden lines are broad, and in others they are narrow. The ones with narrow lines are called type I, and the others are called type II. In addition, infrared emission from type I Seyferts is non-thermal, while that from type II Seyferts is thermal emission from dust. Also, type I Seyferts are weak radio sources, while half of the type II have moderate radio emission. Type I Seyferts also have strong X-ray emission, with a correlation between optical and X-ray luminosity. In Fig. 19.15, we see that the Seyfert spectra have similarities with other types of active galaxies.

The brightest Seyfert is shown in Fig. 19.14. It is an 11th magnitude galaxy in Coma Venatici. Ultraviolet emission lines from this galaxy show rapid variations in strength and width. To explain this phenomena, a nucleus with a black hole of mass 109 M0 has been proposed.

Another type of active galaxy is called a BL Lac object, named after the first one of this type observed. A spectrum of a BL Lac object is shown in Fig. 19.16.

19.4 I Quasars

19.4.1 Discovery of quasars

In our discussion of radio galaxies we mentioned the importance of predicting accurate radio positions for the purposes of finding optical counterparts. Before the use of interferometry, some sources were studied by lunar occultation. In such an experiment, the source is observed as the Moon passes in front of the source. Since we know the position of the edge of the Moon very accurately, we can determine the location of a radio source by noting the times at which the source disappears and reappears.

This technique was used at the Parkes radio telescope in Australia to study the radio source 3C273. (The designation means that it is the 273rd

3500 4000 4500 5000 5500 6000 Wavelength, Angstroms

Seyfert 1 NGC 4151

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Seyfert 2 NGC 7682

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