Host Galaxies of Active Nuclei

Some relation between galaxy and AGN properties might be expected, if only due to the depth of the stellar potential well and the amount of gas available for feeding the monster. Also important are connections between such properties as radio emission and its structure and the type of surrounding galaxy.

Seyfert galaxies are, almost by definition, mostly in spirals and S0s (Simkin et al 1980 ApJ 237, 404; Heckman 1980 PASP 90, 241; Adams 1977 ApJSuppl 33, 19). Companions and distortions are often present (see p. 83). Outer faint rings are more common than in spirals overall, but as Seyferts concentrate to earlier types Sa,Sb, so do outer rings (R), so this may not be significant. There is little correlation among Seyferts between galaxy and nuclear properties, except that the most luminous AGN seem to prefer overluminous galaxies. The nuclear line widths (NLR) do not correlate with disk inclination, so they are not driven by galaxy kinematics (Yee 1983 ApJ 272, 483). However, they do correlate in part with measures of the three-dimensional virial velocity (Whittle 1992 ApJSuppl, 79, 49), so there is a less direct connection. The probability of having a Seyfert nucleus rises for brighter galaxies (Meurs and Wilson 1984 A&A 136, 206). A few dwarf systems (like the SMC or even fainter) have Seyfert nuclei, down to objects with luminosity not much more than the brightest supergiant stars (Filippenko and Sargent 1989 ApJLett 342, L11). There is no obvious connection with overall H I content (Heckman et al 1978 ApJ 224, 745) except the profile anomalies associated with interactions. There have been reports, and considerable controversy, as to whether Seyfert hosts have higher star-formation rates near the nuclei than otherwise expected. SDSS spectra indicate that SFR does correlate with level of AGN activity, whioch may require that the bulge stars are built more or less contemporaneously with episodes of AGN activity.

Radio galaxies, again almost by definition, favor elliptical galaxies or their relatives, when close enough for us to see in detail. Many of these are disturbed if looked at carefully (don't poke that galaxy; it's violent if disturbed...). They are predominantly luminous systems, not dwarfs (Matthews, Morgan and Schmidt 1964 ApJ 140, 35) so that their Hubble diagram is better behaved than that for all optically-selected ellipticals. They are more likely to have companions or be in groups or clusters than optically similar radio-quiet galaxies (Heckman et al 1985 ApJ 228, 122), but the magnitude of this effect depends on just what sample is examined (Dressel 1981 ApJ 245, 25; Stocke 1978 AJ 83, 348; Adams et al 1980 AJ 85, 1010; Zirbel 1997 ApJ 476, 489). The highest-power radio galaxies (those close enough for detailed study anyway) almost universally show close companions or evidence of a merger (Heckman et al 1986 ApJ 311, 526; Smith and Heckman 1989 ApJ 341, 658; 1990 ApJ 348, 38; Smith et al 1990 ApJ 356, 399). While eating a gas-rich galaxy may help produce a radio source, the large amount of cool ISM may also disrupt such a source, as seen in 3C 293 and 3C 305 (van Breugel et al 1984 ApJ 287, 82; Heckman et al 1982 ApJ 262, 529). FR I and FR II sources occur in somewhat different galaxies, with the FR II galaxies not centered in clusters and perhaps a bit less luminous optically; Zirbel suggests interactions versus cluster cannibalism to account for this. This montage shows HST images of radio galaxies at redshifts z=0.6-1.5, showing the range of morphologies and strange structures at higher redshifts.

An important question is why ellipticals make double radio sources and spirals don't. This is a very general rule, broken (as far as I know) only by the source 0313-192 in Abell 428. The most promising possibility is that the dense ISM in spirals disrupts formation of extended structure, and the energy is soaked up into heating the ISM. Something of the kind seems to be happening in 3C 293 and 3C 305. Numerous studies have now shown that Seyfert galaxies have radio jets on rather small (hundred-parsec) scales, and that interactions with the surrounding ISM are important. Jets are energetic, spectacular, and apparently fragile. Recall that the emission-line regions are very similar in radio galaxies and Seyferts.

As one looks at higher redshift, and simultaneously at earlier cosmic times and at phenomenally powerful and rare galaxies, bizarre morphologies dominate. There is often little evidence of a single symmetric concentration, but rather distinct knots which often line up with pieces of the radio structure. It is the bane of evolutionary studies involving radio galaxies that luminosity and evolution can be difficult to separate in flux-limited samples, but there are now studies of lower-power radio galaxies at large redshift which suggest that much of the difference is in fact linked to cosmic time; these have the same diseases, generally at a lower level. An especially noteworthy property is the alignment effect - a tendency for the host galaxy isophotes to align with the radio axis at redshifts z > 0.5. In part this is a passband effect - the aligned components are generally blue, so at higher redshifts they become more prominent. Possible interpretations include AGN light scattered by dust in the unobscured cone (borne out by polarimetry in 3C 368), star formation triggered by the initial advance of jets, or a relation between visibility of the jets and the amount of ISM in their path without necessarily invoking local star formation. For more details, start with McCarthy et al. 1987 (ApJL 319, L39) and Dunlop and Peacock (1993 MNRAS 263, 936).

Quasars have been a much harder problem - if you could see the host galaxy, it wouldn't be quasistellar. Fuzz has long been observed around low-redshift QSOs with about the right size and luminosity for a surrounding galaxy (Kristian 1973 ApJLett 179, L129; Gehren et al 1984 ApJ 278, 11; Hutchings et al 1984 ApJSuppl 55, 319; Malkan 1984 ApJ 287, 555). Such fuzz is detectable for most QSOs at z < 0.4 by now. Spectroscopic confirmation that these are galaxies containing (more or less) normal stars has been difficult, because the fuzz is of low surface brightness and close to a strong point source. First attempts found extended emission lines (Wampler et al 1975 ApJ 198,249, Richstone and Oke 1977 ApJ 213, 6; Stockton 1976 ApJLett 205, 115...) Stellar absorption features (from a fairly young population) were clearly found around 3C 48 by Boroson and Oke (1982 Nature 296, 391). Several more cases have now been observed, all at the QSO redshift (Boroson and Oke 1984 ApK 281, 535; Boroson and Green 1982 ApJ 263, 32; Balick and Heckman 1983 ApJLett 265, L1; Boroson et al. 1985 ApJ 293, 120). Boroson et al. suggest that there are two kinds of QSO host galaxies: those with young stellar populations (often radio-loud) and redder fuzz with emission lines lit up by the QSO (radio-quiet). This contravenes the conventional wisdom that radio-loud objects live in normal ellipticals. A QSO could have significant impact on the surrounding galaxy, so maybe the conventional wisdom expected too much.

HST imaging has, as long expected, produced spectacular results on quasar host galaxies, even if not the results that many people expected. QSOs can live in normal spirals or ellipticals, and in many cases in quite faint or small (compact) galaxies. Many show spectacular tidal distortions, and most have close compact companions. The references are given back in the interaction and merger lecture. It is still striking that there is not a very good correlation between the core power and galaxy luminosity. There is a limit or envelope in that the brightest AGN come in unusually luminous galaxies, and the probability of hosting an AGN increases for brighter galaxies.

Note that there is an overlap in luminosity between luminous Seyferts and QSOs depending on who is writing - at z < 0.2 it's not clear one is dealing with traditional QSOs in many cases. However, there is no doubt that the fuzz around 3C 48 and 3C 273 represents galaxies around bona fide QSOs. Some lower-luminosity objects discussed in this context (Malkan et al 1984 ApJ 280, 66; Bothun et al 1982 ApJ 257, 46) are distant Seyferts and not really suitable as "proof" that QSOs are in some kind of galaxy or other. A supernova has been reported in the galaxy around 1059+730 at z=0.039 by Campbell et al 1985 (ApJLett 291, L37). Some resolved structure has been reported around QSOs at z< 2, which cannot be nornal galaxies because the cosmological (1+z)4 surface-brightness dimming (or Tolman dimming) would drop their diffuse flux beyond detection. In the local Universe, such objects as PG 1351+640 have such low limits on host-galaxy lyuminosity or compactness as to require that the host galaxy is no larger or brighter than M32. In the other direction, there are signs that some high-redshift QSOs must live in extremely luminous "superhosts". This may be attributable to a starburst-AGN connection, if the timescales are right.

There have been numerous reports of clusters of galaxies around QSOs (Phillips 1980 ApJLett 236, L45; Kriss and Canizares 1982 ApJ 261, 51; Hintzen et al 1981 ApJLett 246, 21; French and Gunn 1983 ApJ 269, 29; Yee and Green 1984 ApJ 280, 79 and ApJSuppl 54, 495). The Yee and Green survey produced evidence that the fraction of QSOs in rich clusters has dropped steeply between z=0.5 and now.

Even using HST, detailed work on QSO host galaxies is extremely difficult -- some objects have surface-brightness profiles consistent with normal E or S galaxies but this is not unique because one is limited to the outer region (by the wings of the bright nuclear image and uncertainties in its radial profile) and thus to regions of low surface brightness. Are the host galaxies of luminous AGN always disturbed? How much alteration has been produced by star formation either contemporary with or triggered by the AGN?

Evidence of a relation between black-hole mass and bulge properties (sometimes known as the Magorrian relation) has focussed attention on the co-evolution of nuclei and bulges. Correlations exist with bulge absolute magnitude (often expressed as stellar mass, in the half-percent rule) and velocity dispersion:

M = 1.48 ± 0.24 x 108 (σ/200 km/s) 4.65 ± 0.48

following papers by Ferrarese and Merritt and Gebhardt et al. (both 2000). A relation of this kind likely means that the bulge initially regulates the formation of a massive black hole, which grows until its dynamical influence reshapes stellar orbits in its vicinity. It is not ruled out that the black holes came first, but the tightness of the correlation (± 50% or so) is suspiciously good for this. The return of STIS promises to give us more points testing this relation in different and more extreme environments, after a frustrating hiatus while it was out of service.

Recognition of the dichotomy in galaxy color has led to another fascinating aspect to AGN hosts. Fro the SDSS and GALEX surveys, it is clear that the fraction of AGN peaks in the "green valley" between the major sequences of star-forming and passive galaxies. There are too few objects in the valley for there to be many starbursts superiposed on red galaxies, so most of the population there must be involved in a one-time transfer from blue to red sequences. What happens then to make feeding the AGN more efficient? There is also evidence that disk galaxies tend to host less massive black holes, seen in AGN acreting at a higher Eddington ratio than more massive ones in elliptical galaxies. This could also be connected to the major transformations in the green valley.

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