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Explore the diverse types of active galaxies and their unified model, radio lobes, and jets as probes. Uncover the evolution from Seyfert galaxies to radio galaxies with significant spectroscopic variations.
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28. Active Galaxies Goals: 1. Describe and characterize the various types of galaxies that display high activity relative to “normal” (non-active) galaxies. 2. Examine the unified model used to explain the origin of activity in active galaxies and describe its features. 3. Discuss the nature of radio lobes and jets and how they are used to probe the universe.
Observations of Active Galaxies: The first indication that not all galaxies were quiescent conglomerations of stars with gas and dust mixed in was made by Fath in 1908 when he was examining spectra of spiral nebulae visually with a spectroscope. The galaxy NGC 1068 displayed a few bright emission lines rather than the expected stellar absorption-line spectrum.
Seyfert Galaxies Hubble later recorded emission line spectra for two other galaxies in 1926, but it was Carl Seyfert who was able to characterize the phenomenon when he noticed that a small percentage of galaxies have very bright nuclei that are sources of broad emission lines. Today they are designated as Seyfert galaxies, with further spectroscopic subdivision into Seyfert 1 and Seyfert 2, with Seyfert 1s displaying broad emission lines (permitted) and narrower forbidden lines, and Seyfert 2s displaying only narrow lines (permitted and forbidden). The spectrum of Markarian 509, a Seyfert 1 galaxy.
Spectra of Seyfert 1 and Seyfert 2 galaxies (left) are compared with the spectra of a normal galaxy (top right) and that for BL Lacertae (bottom right). Intermediate types (Seyfert 1.5) are also recognized, and the spectroscopic characteristics can be temporally variable. Seyfert 1 galaxies are frequently X-ray sources, Seyfert 2 galaxies less so, with huge column densities of hydrogen accounting for the difference for the latter.
Flux Distributions in Active Galactic Nuclei The spectral energy distribution (SED) of the Seyfert 1 galaxy NGC 3783 (Alloin et al. 1995) from radio to gamma-rays. Also shown is the SED for a normal (type Sbc) galaxy from Elvis et al. (1994). The flux scale of the normal galaxy spectrum has been adjusted to give the correct relative contribution of the AGN component and starlight for NGC 3783 (in mid-1992) at 5125 Å through a 5" 10" spectrograph aperture.
The spectra of Active Galactic Nuclei (AGNe) tend to be fairly flat, so they were initially described by flux distributions of the type: where α 1. Typical SEDs contain a Big Blue Bump as well as a smaller Infrared (IR) bump. See typical SEDs at right. Big Blue Bump IR Bump
A pure power-law spectrum with α constant is the signature of synchrotron radiation: where α 1. An inverse Compton spectrum is similar, but with different breaks.
Radio Galaxies Radio astronomy is a relatively new area of astronomy that began following World War 2 with the availability of radio dishes, originally used for radar, in countries like The Netherlands (Holland), the United Kingdom, Australia, Canada, and the U.S.A. Radio sources had been detected prior to and during the war, but it was not until after that it was possible to begin the science in earnest. Radio surveys of the sky at various frequencies were done at places like Cambridge University (the various Cambridge catalogues), and the detection of the postulated 21-cm radiation from neutral hydrogen gas in the Galaxy was used by astronomers in Australia and Britain to map the spiral structure of the Milky Way. Early radio maps were of low resolution, so it was rarely possible to identify prominent radio sources with objects in the optical sky. The exceptions were sources in the ecliptic band that were regularly occulted by the Moon. Such events enabled precise positions to be established.
One of the first radio galaxies identified with an optical object was Cygnus A, a peculiar cD galaxy crossed by a ring of dust with two mammoth ejected radio lobes.
Radio galaxies can also be classified into subgroups: broad-line radio galaxies (BLRGs), like Seyfert 1s, and narrow-line radio galaxies (NLRGs), like Seyfert 2s. Optically, BLRGs have bright starlike nuclei surrounded by hazy envelopes, while NLRGs are giant or supergiant ellipticals (cd, D, and E) like Cygnus A. But Seyfert galaxies are relatively quiet at radio wavelengths, compared with radio galaxies. Radio Lobes and Jets Double-lobed structures and jets are fairly common among radio galaxies. Some have rather enormous dimensions. Where double jets are observed with one stronger and the other weaker, the stronger jet is probably directed more towards us than is the weaker. Solitary jets are believed to be double with the weaker counterjet too weak to be detected.
The twin radio galaxies 3C 75 with their radio lobes (blue) and optical appearance (red).
M87 (NGC 4486) at optical wavelengths, deep and shallow exposures. Jet
Wu & Tremaine (2006) estimate a mass of 2.4 1012M (2 trillion solar masses ~ 10 Milky Way galaxies) within 32 kpc of M87 according to its globular cluster system. Note the jet of high-speed particles emanating from the nucleus of M87.
3C 236 is one of the largest radio galaxies in terms of size. It has a redshift of z = 0.0988 and radio lobes extending to 40 arcmin (larger than the Moon’s apparent size), corresponding to 2 Mpc or more.
Many radio galaxies have jets of high-speed gas ejected symmetrically about the centre of the galaxy, well away from the optical galaxy. Many such galaxies are referred to as having active galactic nuclei, and are termed AGN galaxies.
Quasars and QSOs Two radio sources with precise locations derived in the late 1950s via lunar occultations were 3C 48 and 3C 273, where “3C” denotes the Third Cambridge Catalogue.
The spectrum of 3C 273 obtained by Maarten Schmidt revealed an emission-line spectrum that corresponded partly with the hydrogen Balmer lines redshifted by z = 0.158.
3C 48 proved to be even more unusual, appearing starlike on deep images, yet also displaying an emission-line spectrum with a large redshift, in this case of z = 0.367.
Such radio sources were classified as quasi-stellar radio sources, or QSRs, and became known as quasars. Studies later revealed that quasars had significant ultraviolet excesses (U−B < −0.4), undoubtedly because of emission lines in their spectra redshifted from the ultraviolet into the optical region of the spectrum. Following that discovery, searches for additional QSRs were made optically by searching for high-latitude stars with ultraviolet excesses. Many such objects were found, and also proved spectroscopically to be quasars, but without strong radio emission: radio-quiet quasars. They were referred to as quasi-stellar objects, or QSOs. Today both types, QSRs and QSOs, are referred to collectively as quasars. Spectroscopically, quasars are similar to broad-line radio galaxies and Seyfert 1 galaxies, but with large redshifts implying large distances. Chip Arp has published papers claiming connections of some quasars with nearby galaxies, but the evidence is open to interpretation.
Examples of (left) quasars = quasi-stellar radio sources (i.e., lots of radio noise) and (right) QSOs = quasi-stellar objects (very little radio noise, if any). Double QSO 0957+561
The Sloan Digital Sky Survey (SDSS) has catalogued some 46,420 quasars, some at extremely large implied distances. For example, SDSS 023137.65−07286 54.4 has a redshift of z = 5.4135, implying a recessional velocity of 0.95c. Note that the dimensions of the universe depend critically upon look-back time given the constant expansion occurring, i.e.: Therefore: So the universe is presently 6.4135 times larger than it was when the light from SDSS 023137.65−07286 54.4 was emitted.
Evidence for Quasar Evolution ... can be found when one compares the luminosities derived for quasars at different redshifts, which represent different epochs of the past. For example, intrinsically bright quasars were more common at earlier epochs than in the present era. Statistical studies indicate that there are more than 1000 times as many quasars per Mpc3 (for a comoving space density) brighter than MB = −25.9 at z = 2 than today at z = 0. Yet the total number of quasars does not appear to have changed significantly between the present and the past epoch at z = 2. But changes are apparent in more distant epochs, z > 3, and it appears possible to study the birth and evolution of quasars in the early stages of the universe. That is the field of observational cosmology. Like AGNe, quasars are believed to be powered by a supermassive black hole lying at the centre of a galaxy.
Quasar and AGN Variability A surprise from the initial studies of both quasars and AGNe was that both types of objects are light variable on fairly short time scales. The light curves below are from the Magellanic Quasars Survey, a study of QSOs near the LMC and SMC. X-ray fluctuations are the most rapid.
Spectroscopic characterization of quasars in the MQS.
Properties of quasars in the MQS according to their measured magnitudes and redshifts.
BL Lacertae Objects BL Lacertae, as the name implies, is a starlike object originally included in the General Catalogue of Variable Stars because it displayed light variability. The spectrum of the star was difficult to classify, since it displayed few features on an almost featureless continuum. It was Oke and Gunn (1974, ApJ, 189, L5) who were first able to record the faint spectrum from the object with the bright central regions obscured by an occlusion disk. The annulus spectrum for BL Lac displayed a few absorption lines redshifted by z = 0.07, implying quasarlike properties. Thereafter the object was considered to be a quasar with the main optical jet directly pointing towards the observer, producing the featureless continuum of synchrotron radiation with a power law spectrum of α = −1.55. Many other such objects have since been detected, and are sometimes referred to as Blazers.
Oke and Gunn’s results for BL Lac. The annulus spectrum originates from those galaxy portions not obscured by the jet. It appears to be the spectrum of a quasar although without much emission.
LINERs A final class of objects consists of galaxies with low luminosity nuclei displaying low ionization (i.e., forbidden lines) nuclear emission-line regions (LINERs). LINERs are similar to Seyfert 2 galaxies, but it is not clear that they represent the low luminosity tail of Seyferts. Forbidden-line radiation is also seen in starburst galaxies and H II regions, for example.
A summary of the AGN menagerie is given in Table 28.1 of the text.
So a rough sketch of the unified model for AGNe consists of a central engine comprising an accretion disk orbiting a rotating, supermassive black hole. The AGN is powered by the conversion of gravitational potential energy into synchrotron radiation, although the rotational kinetic energy of the black hole may also serve as an important energy source. The structure of the accretion disk depends upon on the ratio of the accretion luminosity to the Eddington limit (the most rapid rate that energy can escape into space without leading to mass loss). To supply the observed luminosities, the most energetic AGNe must accrete between ~1 and 10 M yr−1. The perspective of the observer, together with the mass accretion rate and mass of the black hole, largely determines whether the AGN is called a Seyfert 1, a Seyfert 2, a BLRG, a NLRG, or a radio-loud or radio-quiet quasar.
Perspectives of Radio Lobes and Jets Jets are generated by charged particles ejected from the central nucleus of an AGN. How they are seen from Earth depends upon a variety of factors, including perspective. The calculations in Fig. 28.36 are intended to explain our view of the Einstein ring MG1131+0456 seen in Fig. 28.37. A simplified picture was first generated by Dyer and Roeder in 1981.
A plexiglass simulation of a gravitational lens created by Charles Dyer and Robert Roeder (1981).
Double quasar, QSO 0957+561. The image of a single background quasar split into two halves by an intervening galaxy.
An image of the galaxy ZW 2237+030, Huchra’s Lens, showing the multiple imaging of a background quasar QSO 2237+0305.
Superluminal Velocities The most compelling argument for relativistic speeds in jets and radio lobes for galaxies involves radio observations of material ejected from the cores of several AGNe with so-called superluminal velocities. The effect is observed within ~100 pc of the AGN’s centre and probably continues further out. The perspective of the knot motion and observer is important for rseolving such situations.
Apparent motion of some galactic jets at speeds exceeding the speed of light are projection effects only. The jets also have motion in the line of sight that produces such an anomaly.
