High-resolution radio observations of Seyfert galaxies in the extended 12-micron sample - II. The properties of compact radio components

We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-micron AGN sample of Rush et al.(1993). Our main results can be summarised as follows. Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extra-nuclear emission regions; extra-nuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of `ionisation cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-micron flux densities have well matched properties

A Radio Study of the Seyfert galaxy Markarian 6: Implications for Seyfert life-cycles

We have carried out an extensive radio study with the Very Large Array on the Seyfert 1.5 galaxy Mrk 6 and imaged a spectacular radio structure in the source. The radio emission occurs on three different spatial scales, from ~7.5 kpc bubbles to ~1.5 kpc bubbles lying nearly orthogonal to them and a ~1 kpc radio jet lying orthogonal to the kpc-scale bubble. To explain the complex morphology, we first consider a scenario in which the radio structures are the result of superwinds ejected by a nuclear starburst. However, recent Spitzer observations of Mrk 6 provide an upper limit to the star formation rate (SFR) of ~5.5 M_sun/yr, an estimate much lower than the SFR of ~33 M_sun/yr derived assuming that the bubbles are a result of starburst winds energized by supernovae explosions. Thus, a starburst alone cannot meet the energy requirements for the creation of the bubbles in Mrk 6. We show that a single plasmon model is energetically infeasible, and we argue that a jet-driven bubble model while energetically feasible does not produce the complex radio morphologies. Finally, we consider a model in which the complex radio structure is a result of an episodically-powered precessing jet that changes its orientation. This model is the most attractive as it can naturally explain the complex radio morphology, and is consistent with the energetics, the spectral index and the polarization structure. Radio emission in this scenario is a short-lived phenomenon in the lifetime of a Seyfert galaxy which results due to an accretion event.Comment: Accepted for publication in Ap

Quasars, their host galaxies, and their central black holes

We present the final results from our deep HST imaging study of the hosts of radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs). We describe new WFPC2 R-band observations for 14 objects and model these images in conjunction with the data already reported in McLure et al (1999). We find that spheroidal hosts become more prevalent with increasing nuclear luminosity such that, for nuclear luminosities M_V < -23.5, the hosts of both radio-loud and radio-quiet AGN are virtually all massive ellipticals. Moreover we demonstrate that the basic properties of these hosts are indistinguishable from those of quiescent, evolved, low-redshift ellipticals of comparable mass. This result kills any lingering notion that radio-loudness is determined by host-galaxy morphology, and also sets severe constraints on evolutionary schemes which attempt to link low-z ULIRGs with RQQs. Instead, we show that our results are as expected given the relationship between black-hole and spheroid mass established for nearby galaxies, and apply this relation to estimate the mass of the black hole in each object. The results agree very well with completely-independent estimates based on nuclear emission-line widths; all the quasars in our sample have M(bh) > 5 x 10^8 solar masses, while the radio-loud objects are confined to M(bh) > 10^9 solar masses. This apparent mass-threshold difference, which provides a natural explanation for why RQQs outnumber RLQs by a factor of 10, appears to reflect the existence of a minimum and maximum level of black-hole radio output which is a strong function of black-hole mass. Finally, we use our results to estimate the fraction of massive spheroids/black-holes which produce quasar-level activity. This fraction is \~0.1% at the present day, rising to > 10% at z = 2-3.Comment: Revised version accepted for publication in Monthly Notices of the Royal Astronomical Society. 46 pages, the final 19 of which comprise an Appendix. 15 figures in main text. A further 14 4-panel greyscale plots and 14 line plots which appear in the Appendix have been reproduced here with reduced quality due to space limitations. A full resolution copy of the manuscript can be obtained via ftp://ftp.roe.ac.uk/pub/jsd/dunlop2002.ps.g

The origin of the Narrow Line Region of Mrk 3: an overpressured jet cocoon

We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the radio-jet the velocity field is highly perturbed and shows two velocity systems separated by as much as 1700 km/s. We interpret this to be the consequence of the rapid expansion of a cocoon of hot gas, shocked and heated by the radio-emitting outflow, which compresses and accelerates the ambient gas. The NLR itself is essentially a cylindrical shell expanding supersonically. From the size and velocity of the expanding region, we derive an upper limit to the radio-source age, ~ 2 E42 erg/s required to inflate the cocoon and estimate that the jet minimum advance speed is 3 E-3 pc per year. The total kinetic energy of the high velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy carried by the jet over its lifetime and this quantitatively supports the idea that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is representative of the Seyfert population then these sources must also be short lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2 and 3 orders of magnitude smaller than that derived for radio-loud AGNs with similar emission-line luminosity. On the other hand, the fraction of jet power dissipated in radio-emission is similar. We speculate that the main distinction between radio-quiet and radio-loud AGN is ascribed to a difference in jet power rather than to a different efficiency in synchrotron emission production.Comment: 13 pages, 8 figures, Astrophysical Journal in pres

Radio Variability in Seyfert Nuclei

Comparison of 8.4-GHz radio images of a sample of 11 early-type Seyfert galaxies with previous observations reveals possible variation in the nuclear radio flux density in 5 of them over a 7-yr period. We find no correlation between radio variability and nuclear radio luminosity or Seyfert nuclear type, although the sample is small and dominated by type 2 Seyferts. Instead, a possible correlation between the presence of nuclear radio variability and the absence of ~100-pc-scale radio emission is seen. NGC2110 is the only source with significant extended radio structure and strong nuclear variability (>38% nuclear decline over seven years). Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognised in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio-jet emission within the central ~50 pc. If flares in radio light curves correspond to ejection of new relativistic components or emergence of shocks in the underlying flow, we suggest that radio jets may be intrinsically non-relativistic during quiescence, but that Seyferts, as black-hole driven AGN, have the capacity to accelerate relativistic jets during radio flares. Taken together with the increased detection rate of flat spectrum radio nuclei in Seyferts imaged at VLBI resolutions and the detection of variable water megamaser emission, our results support the paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population. (Abridged).Comment: Accepted for publication in Astrophysical Journal; 15 pages, 7 figures and 3 table

The Geometry of Mass Outflows and Fueling Flows in the Seyfert 2 Galaxy Mrk 3

We present a study of the resolved emission-line regions and an inner dust/gas disk in the Seyfert 2 galaxy Mrk 3, based on Hubble Space Telescope observations. We show that the extended narrow-line region (ENLR), spanning ~4 kpc, is defined by the intersection of the ionizing bicone of radiation from the AGN and the inner disk, which is not coplanar with the large-scale stellar disk. This intersection leads to different position and opening angles of the ENLR compared to the narrow-line region (NLR). A number of emission-line arcs in the ENLR appear to be continuations of dust lanes in the disk, supporting this geometry. The NLR, which consists of outflowing emission-line knots spanning the central ~650 pc, is in the shape of a backwards S. This shape may arise from rotation of the gas, or it may trace the original fueling flow close to the nucleus that was ionized after the AGN turned on.Comment: 22 page, 7 figures, accepted for publication in the Astronomical Journa

Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231

The VLBA and the VLA have been used to image the continuum radio emission from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local universe. The smallest scales reveal a double source less than 2 pc in extent. The components of this central source have minimum brightness temperatures of 10^9 to 10^{10} K, spectral turnovers between 2 and 10 GHz, and appear to define the galaxy nucleus plus the inner regions of a jet. The components may be free-free absorbed or synchtrotron self-absorbed. On larger scales, the images confirm a previously known north-south triple source extending 40 pc and elongated perpendicular to a 350-pc starburst disk. Both lobes show evidence for free-free absorption near 2 GHz, probably due to ionized gas with a density of 1-2 X 10^3 cm^{-3} in the innermost parts of the starburst disk. The absorbing gas may be ionized by the active nucleus or by local regions of enhanced star formation. The elongation of the 40-pc triple differs by 65 deg from that of the 2-pc source. The different symmetry axes on different scales imply strong curvature in the inner part of the radio jet. The radio continuum from the 350-pc disk has a spectral index near -0.4 above 1.4 GHz and is plausibly energized by a massive burst of star formation. On VLA scales, asymmetric and diffuse emission extends for more than 25 kpc. This emission has a steep spectrum, linear polarization exceeding 50% at some locations, and shares the symmetry axis of the 40-pc triple. The diffuse radio source is probably generated by energy deposition from a slow-moving nuclear jet, which conceivably could help energize the off-nuclear starburst as well.Comment: 34 pages, 7 Postscript figures, LaTeX file in AASTeX format, accepted in ApJ, Vol. 516, May 1, 199