TOOLS OF THE ASTRONOMER’S TRADE

The Diversity of AGN

Active galactic nuclei, or AGN, come in two unique varieties: those with radio jets and lobes (called radio-loud), and those without (called radio-quiet). Radio galaxies, like those in Figure 24-1 and Figure 24-8, are radio-loud AGN. About 10% of quasars are radio-loud, and, when observable, their host galaxies are elliptical. The remaining quasars are radio-quiet and, when observable, their hosts are spiral galaxies. (It is not understood why radio-loud and radio-quiet AGN are each found in ellipticals and spirals, respectively—this remains a mystery.)

The light emitted from jets is focused into a strong beam because the jets’ emitting plasma travels near the speed of light. In cases where the jets are pointed almost directly toward Earth, the jets’ beamed emission swamps the usual AGN emission, and these objects have a very different spectrum (often without any detected emission lines). These objects are called blazars, and while originally thought to be distinct from quasars due to their different appearance, they are now understood as special cases of jet alignment with our line of sight from Earth.

Quite similar to quasars, Seyfert galaxies also show bright, compact nuclei with emission lines. Eventually, it was realized that both quasars and Seyfert galaxies contain the same type of object, and both are examples of AGN. Historically, bright and distant AGN, where the host galaxies were not initially observed, are called quasars; nearby cases where the host galaxies can be easily seen are called Seyfert galaxies.

Seyfert galaxies and quasars also come in two further varieties: those with strong broad hydrogen emission lines (called Type 1) and those with weak narrow hydrogen emission lines (called Type 2). Furthermore, the continuum emission in the Type 1 AGN can vary significantly in brightness, whereas the continuum in the Type 2 AGN does not vary.

Explaining Type 1 and Type 2 AGN in terms of the same central engine was a major advance in understanding quasars, and the explanation is called the unified model. Ultimately, appearing as Type 1 or Type 2 depends on the line of sight, as shown in Figure 24-13. The table below summarizes some properties of AGN; the modern distinction of Type 1 or Type 2 is in parenthesis after the object’s name.