Next up - spectra of active galactic nuclei
. These can be broadly defined as galactic nuclei which show large amounts of energy release that we can't account for by the normal processes of stars, ther formation and death. They come in several varieties, originally depending on how they were discovered by subsequently somewhat unified as we learned more.
Quasars - started as an acronym for quasi-stellar radio sources (QSRS), although we know find that most such (also known as quasistellar objects, QSOs) are not very strong radio sources. A quasar is starlike through a typical groundbased telescope, has a significant redshift (say z>0.1, some as high as 6.4), and shows broad emission lines in its spectrum.
Seyfert galaxies - discussed as a class by Carl Seyfert in a 1943 paper. (By the way, he pronounced his name more like See-fert than Say-furt). These are distinguished from normal galaxy cores by their spectra. There are two subtypes. Type 1 show broad emission lines, with widths that imply internal Doppler shifts of several thousand km/s. Type 2 show narrower lines, a thousand km/s or so of Doppler shift, but the relative strengths of these lines are quire different from what we see in star-formng regions such as the Orion Nebula or starburst galaxies. For example, the [N II] line next to H-alpha can be nearly as strong as the hydrogen line. [Semantic digression - the sqare brackets indicate a so-called forbidden transition, one in which the atom takes so long to radiate that it will be left along that long only in a very good vacuum, such as interstellar space. It took a long time to work this out, so there are some old articles which still refer to some of these as unknown elements such as nebulium and coronium.] The conditions in these galaxies also mean that gas with a wide range of oinization states exists in close proximity - the best example, is oxygen, for which we often see strong emission lines from the neutral and one- and twice-ionized states (O I, [O II], and [O III] in spectroscopic parlance) Star-forming galaxies can make plenty ot [O II] and [O III] but not [O I] at the same time. Some very nearby galaxies have Seyfert nuclei if you look closely enough. M81 has a simmering low-level one. The most famous ones are probably NGC 4151 (type 1) and NGC 1068 or Messier 77 (type 2). The spectrum of a type 1 Seyfert is almost undistinguishable from a typical quasar. Both show not only large line widths (which suggests in itself matter anchored in a strong gravitational field), but the kinds of atoms radiating indicate dense gas exposed to radiation rich in far-ultraviolet and X-ray emission (both quite distinct from gas lit up by young stars, which aren't such strong sources of these high-energy bits of the spectrum). This fits with the widely-held idea of a supermassive black hole and surrounding gas which is heated by particle collisions at very high velocities giving rise (through more complex processes than we first expected...) to the observed radiation.
BL Lacertae objects or blazars - their optical spectra are almost competely lacking in features, so getting a redshift is no picnic and may be best done using the faint light from the surrounding galaxy. They seem to be quasars that we happen to see looking right down a jet emerging near the speed of light, so what we see is dominated by the glare of the hard radiation for the jet boosted by effects of relatively.
Radio galaxies - their defining features are obviously strong radio emission, usually in the form of twin blobs or lobes on either side of the galaxy
. The optical spectra may look like Seyfert galaxies or either type, or like an ordinary run-of-the-mill elliptical. Some radio galaxies are pretty clearly quasars seen from such an angle that surrounding dust blocks our view of the bright core (from which careful measurements can sometimes pull out a reflected signal.) The same is true of some type 2 Seyferts - some at least are type 1 objects seen through absorbing material so we don't see the core region directly
LINERs are recognized as sets of emission lines from otherwise normal galaxy cores which may indicate low-level activity, like a Seyfert with the volume turned down. These are seldom powerful enough to show up against the galaxy light in SDSS spectra, so you shouldn't see these often.This graphic
compares the visible-light spectra of various kinds of AGN, all shifted to zero redshift for comparison:
Lots more on AGN can be read here
, with a text intro, glossary, and many images and plots. Points of note here -
- You may well see galaxies with a Seyfert nucleus where you see some of the strong emission lines and also see absorption lines from the stars in the surrounding galaxy.
- The SDSS spectroscopic pipeline is better at picking out type 2 (narrow-line) Seyferts than broad-line type 1 objects, simply because it doesn't have to then guess a line width to identify in what may be a noisy set of data. So you may well see a broad-line spectrum like the Seyfert 1 above in something not autoclassified as a quasar.