Nuclear Radio Research Articles

COLA. III. RADIO DETECTION OF ACTIVE GALACTIC NUCLEUS IN COMPACT MODERATE LUMINOSITY INFRARED GALAXIES

We present results from 4.8 GHz VLA and Global-VLBI observations of the northern half of the moderate FIR luminosity (median L_IR = 10^11.01 L_Sol) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (~10^21 W/Hz) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by AGN. These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100-500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star-formation we conclude that the AGN powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst-AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central super-massive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity.

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).

How I Learned to Stop Worrying and Love the Bomb, the Nuclear Reactor, the Computer, Ham Radio, and Recombinant DNA

How I Learned to Stop Worrying and Love the Bomb, the Nuclear Reactor, the Computer, Ham Radio, and Recombinant DNA

Measuring the kinetic power of active galactic nuclei in the radio mode

We have studied the relationship among nuclear radio and X-ray power, Bondi rate and the kinetic luminosity of sub-Eddington active galactic nucleus (AGN) jets, as estimated from the pdV work done to inflate the cavities and bubbles observed in the hot X-ray-emitting atmospheres of their host galaxies and clusters. Besides the recently discovered correlation between jet kinetic and Bondi power, we show that a clear correlation exists also between Eddington-scaled kinetic power and bolometric luminosity, given by log(L kin /L Edd ) = (0.49 ± 0.07) log(L bol /L Edd )-(0.78±0.36). The measured slope suggests that these objects are in a radiatively inefficient accretion mode, and have been used to put stringent constraints on the properties of the accretion flow. Interestingly, we found no statistically significant correlations between Bondi power and bolometric AGN luminosity, apart from that induced by their common dependence on L kin , thus confirming the idea that most of the accretion power emerges from these systems in kinetic form. We have then analysed the relation between kinetic power and radio core luminosity. Combining the measures of jet power with estimators of the unbeamed radio flux of the jet cores as, for example, the so-called Fundamental Plane of active black holes, we are able to determine, in a statistical sense, both the probability distribution of the mean jet Lorentz factor, which peaks at Γ m ∼7, and the intrinsic relationship between kinetic and radio core luminosity (and thus the jet radiative efficiency), which we estimate as log L kin = (0.81 ± 0.11) log L R + 11.9 +4.1 -4.4 , in good agreement with theoretical predictions of synchrotron jet models. With the aid of these findings, quantitative assessments of kinetic feedback from supermassive black holes in the radio mode (i.e. at low dimensionless accretion rates) will be possible based on accurate determinations of the central engine properties alone, such as mass, radio core and/or X-ray luminosity. As an example, we suggest that Sgr A* may follow the same correlations of radio-mode AGN, based on its observed radiative output as well as on estimates of the accretion rate both at the Bondi radius and in the inner flow. If this is the case, the supermassive black hole in the Galactic Centre is the source of ∼5 x 10 38 erg s -1 of mechanical power, equivalent to about 1.5 supemovae every 10 5 yr.

Jet-cloud collisions in the jet of the Seyfert galaxy NGC 3079

We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN.

The Parsec‐Scale Radio Structure of NGC 1068 and the Nature of the Nuclear Radio Source

We present sensitive, multifrequency Very Long Baseline Array (VLBA) images of the nuclear radio sources of NGC 1068. At 5 GHz and 8.4 GHz, the radio continuum source S1, argued to mark the location of the hidden active nucleus, resolves into an elongated, ~ 0.8 pc source oriented nearly at right angles to the radio jet axis but more closely aligned to the distribution of the nuclear H2O maser spots. S1 is detected at 5 GHz but not at 1.4 GHz, indicating strong free-free absorption below 5 GHz, and it has a flat spectrum between 5 GHz and 8.4 GHz. A 5/8.4 GHz spectral index map reveals an unresolved, inverted spectrum source at the center of the S1 structure which may mark the AGN proper. The average brightness temperature is too low for synchrotron self-absorption to impact the integrated spectrum significantly. In addition, a careful registration with the nuclear H2O masers argues that the S1 continuum source arises from the inner regions of the maser disk rather than a radio jet. The emission mechanism may be direct, thermal free-free emission from an X-ray-heated corona or wind arising from the molecular disk. We demonstrate that the hidden active nucleus is sufficiently luminous, to within the current estimates, to provide the requisite heating.

Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NG C 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92± 0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 � upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is int rinsic to the jet-formation process or results from deceler ation of an initially relativistic jet by interaction with the pc or sub -pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not s een in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0 ◦ and 90 ◦ . The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

Evidence for Jet Domination of the Nuclear Radio Emission in Low-Luminosity Active Galactic Nuclei

We present simultaneous, subarcsecond (≤50 pc) resolution 5, 8.4, and 15 GHz Very Large Array observations of a well-defined sample of 16 low-luminosity active galactic nuclei. The radio emission in most of these nuclei does not show the rising spectrum (0.2 s 1.3, Lν νs) expected from thermal electrons in an advection-dominated accretion flow (ADAF) with or without weak to moderately strong outflows. Rather, the flat radio spectra are indicative of either synchrotron self-absorbed emission from jets, convection-dominated accretion flows (CDAFs) with L 10-5LEdd, or ADAFs with strong (p 0.6) outflows. The jet interpretation is favored by three factors: (1) the detection of parsec-scale radio extensions, morphologically reminiscent of jets, in the five nuclei with the highest peak radio flux density, (2) the domination of parsec-scale jet radio emission over unresolved core emission in the three best-studied nuclei, and (3) the lack of any clear correlation between radio spectral shape and black hole mass as would be expected from the dependence of the radio turnover frequency on black hole mass in ADAF and CDAF models. A jet domination of nuclear radio emission implies significantly lower accretion rates in ADAF-type models than earlier estimated from core radio luminosities.

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

We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-μm AGN sample of Rush et al. Our main results can be summarized 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, extranuclear emission regions; extranuclear 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 ‘ionization 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-μm flux densities have well-matched properties.

TheBeppoSAXView of the X‐Ray Active Nucleus of NGC 4258

BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 1041 ergs s-1. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5 ± 1.2) × 1022 cm-2; this component is detected at up to 70 keV with a signal-to-noise ratio of 3 and with a steep power-law energy spectral index of αE = 1.11 ± 0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E 2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6 ± 0.1 keV and 1.3 keV. The cooler (L0.1-2.4 keV~ 1040 ergs s-1) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with αE = 0.2, is L0.1-2.4 keV~ 7 × 1039 ergs s-1, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.

Radio Sources in Low‐Luminosity Active Galactic Nuclei. II. Very Long Baseline Interferometry Detections of Compact Radio Cores and Jets in a Sample of LINERs

We have used the VLBA at 5 GHz to observe all galaxies with nuclear radio flux densities above 3.5 mJy found in a VLA survey at 15 GHz of a sample of nearby LINER galaxies. All galaxies were detected revealing high brightness temperature (Tb 108 K) radio sources. Free-free emission is unlikely since it greatly overpredicts the soft X-ray luminosities. We infer the presence of active galactic nucleus (AGN)-like, nonthermal radio emission most likely powered by underfed black holes. Together with our VLA sample we estimate from our observations that at least one-half of LINER galaxies host genuine AGNs. We find no evidence for highly inverted radio cores as predicted in the advection-dominated accretion flow model: the (nonsimultaneous) spectral indices are on average around α = 0.0. In the two brightest sources we detect some extended emission, which appears to originate in jets in at least one of these galaxies. Together with the spectral indices this suggests that the nuclear emission at centimeter radio waves is largely dominated by emission from radio jets, very similar to the situation in more luminous AGNs. The energy released in these jets could be a significant fraction of the energy budget in the accretion flow.

COMPACT GROUPS OF GALAXIES

▪ Abstract Compact groups of galaxies have posed a number of challenging questions. Intensive observational and theoretical studies are now providing answers to many of these and, at the same time, are revealing unexpected new clues about the nature and role of these systems. Most compact groups contain a high fraction of galaxies having morphological or kinematical peculiarities, nuclear radio and infrared emission, and starburst or active galactic nuclei (AGN) activity. They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Compact groups are surprisingly numerous and may play a significant role in galaxy evolution.

Detection of a nuclear radio source in NGC 7331: A relatively 'radio-quiet' liner

view Abstract Citations (22) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of a Nuclear Radio Source in NGC 7331: A Relatively ``Radio-quiet'' Liner Cowan, John J. ; Romanishin, W. ; Branch, David Abstract We have used the Very Large Array to observe the nearby spiral galaxy NGC 7331. The observations detected an unresolved radio source in the nucleus of this galaxy, with flux densities of 234+/-25 microJy and 121+/-16 microJy at 20 and 6 cm, respectively. This is equivalent (at the distance of this galaxy) to luminosities of approximately 3 and 4 times that of the Galactic center source Sgr A at 20 and 6 cm. The source in NGC 7331, with a radio spectral index (Snu_ is proportional to νalpha^) of α = -0.6 +/- 0.1, sits in a region of very low gas density, surrounded by a ring of radio emission. Optical spectra show that NGC 7331 has a weak LINER nucleus. We compare the radio and optical properties of this nucleus with other LINER nuclei detected in the radio, and find that NGC 7331 has an unusually low radio to Hα flux ratio. The combination of an unresolved radio source and a LINER nucleus spectrum suggests that this galaxy harbors a massive black hole. The radio observations of NGC 7331 did not detect emission from the historical supernova 1959D with a 3 σ upper limit of 75 microJy at 20 cm. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/187652 Bibcode: 1994ApJ...436L.139C Keywords: Active Galactic Nuclei; Black Holes (Astronomy); Light (Visible Radiation); Optical Emission Spectroscopy; Radio Sources (Astronomy); Spiral Galaxies; Supernovae; Astronomical Coordinates; Radio Emission; Spectral Emission; Very Large Array (Vla); Wavelengths; Astrophysics full text sources ADS | data products SIMBAD (8) NED (1)

Variability of the Nuclear Radio Source in M31

The unresolved nuclear radio source in the galaxy M31 has been detected at a second epoch, with a luminosity one-quarter that of Sgr A * . Between the two epochs of observation (1990 June 8 and 9 and 1992 November 4 and 5), the flux density at λ3.6 cm increased from 28±5 to 39±5 μJy. The value of χ 2 is consistent, at the 70% level, with variability about the mean flux density. The observed variability is similar to that of Sgr A * and strengthens the identification of the source in M31 as an analog of Sgr A *

Detection of an Unresolved Nuclear Radio Source in M31: Erratum

view Abstract Citations (2) References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of an Unresolved Nuclear Radio Source in M31: Erratum Crane, Patrick C. ; Dickel, John R. ; Cowan, John J. Abstract There were two typographical errors in the Letter "Detection of an Unresolved Nuclear Radio Source in M31" by Patrick C. Crane, John R. Dickel, & John J. Cowan (ApJ, 390, L9 [1992]). A comparison of the radio source's brightness included in the Abstract and the radio position for (J2000) listed in Table 1 were incorrect. 1. The corrected Abstract is shown below: An unresolved radio source with a flux density of 28 +/- 5 microJy has been detected at the center of the galaxy M 31. This source coincides with the optical stellar nucleus and with a variable Einstein X-ray source but is only 1/300 as luminous as the Galactic center source Sgr A and one fifth as luminous as Sgr A^*^. Consequently, this object is the weakest known nuclear radio source. No evidence is found for radio emission from the remnant of supernova 1885A in M31. An upper limit of 15 microJy (3 σ has been determined for the flux density of this source, which is 1/15 the value for Tycho's SNR if it were at the same distance. 2. The right ascension (J2000) given in Table 1 for the radio nucleus is incorrect. The correct right ascension (J2000) is 00^h^42^m^44.329^s^ +/- 0.15". Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/186924 Bibcode: 1993ApJ...411L.107C Keywords: ERRATA; ADDENDA full text sources ADS | data products NED (1) Related Materials (1) Original Paper: 1992ApJ...390L...9C

Detection of an unresolved nuclear radio source in M31

view Abstract Citations (65) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of an Unresolved Nuclear Radio Source in M31 Crane, Patrick C. ; Dickel, John R. ; Cowan, John J. Abstract An unresolved radio source with a flux density of 28 +/- 5 microJy has been detected at the center of the galaxy M 31. This source coincides with the optical stellar nucleus and with a variable Einstein X-ray source but is only 1/300 as luminous as the Galactic center source Sgr A and one fifth as luminous as Sgr A^*^. Consequently, this object is the weakest known nuclear radio source. No evidence is found for radio emission from the remnant of supernova 1885A in M31. An upper limit of 15 microJy (3 σ has been determined for the flux density of this source, which is 1/15 the value for Tycho's SNR if it were at the same distance. 2. The right ascension (J2000) given in Table 1 for the radio nucleus is incorrect. The correct right ascension (J2000) is 00^h^42^m^44.329^s^ +/- 0.15". Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/186359 Bibcode: 1992ApJ...390L...9C Keywords: Andromeda Galaxy; Galactic Nuclei; Radio Sources (Astronomy); Supernova Remnants; Supernovae; Synchrotron Radiation; X Ray Sources; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M31; GALAXIES: NUCLEI; ISM: INDIVIDUAL ALPHANUMERIC: SN 1985A; ISM: SUPERNOVA REMNANTS; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products SIMBAD (5) NED (4) Related Materials (1) Erratum: 1993ApJ...411L.107C

Radio galaxy jets as probes of galactic structure

It has been noted that the central source of an asymmetric nuclear galactic radio jet may experience considerable net thrust and consequently behave like a massive rocket. In this paper, simple models for the motion of a rocket through a galaxy are examined. It is found that the density distribution of the galaxy is important, and determines whether a given source can escape. Thus, observations of the location and velocity of a source relative to its galactic center may provide new constraints on models of the density distribution in galaxies. 35 references.

0III]-line emission associated with nuclear radio components in Seyfert galaxies

Close correlations between [0III] emission features and radio components have been found in a project of high resolution long-slit spectroscopy of Seyfert galaxies with linear radio structures in their nuclei. Substructures in the [0III] line profiles can, in several cases, be associated with individual radio components, within the Narrow Line Region. The observed properties of these [0III] features suggest that gas behind the bow shock of an outmoving radio plasmon has cooled and then is photoionized by the central continuum source. About half of the objects studied possess an Extended Narrow Line Region which could be interstellar gas, photoionized by energetic radiation from the nucleus, beamed along the axis of the (jet-like) radio structure.

Paschen Alpha as a Probe of Relativistic Beaming

Is the theme of core-brightening by relativistic beaming, so useful for compact sources, helpful in explaining the nuclear properties of classical doubles? Certainly one can construct an attractive model - involving the usual beaming together with an anisotropic dust distribution at r ≲ 1 pc - to explain the differences in nuclear radio and optical properties of radiogalaxies as due differences in orientation with respect to us. There are clear predictions about the strength of broad Pα. Preliminary results indicate that this kind of model is not correct.

Cores in Extended Quasars

The Nuclear Radio cores of several nearby extended radio galaxies (e.g. M87, 3C236) consist not only of optically thick (< 1 pc) components, but also of emission on somewhat larger scale. As extended radio sources associated with quasars have on average stronger and more luminous radio cores (see e.g. Miley, 1980, Ann. Rev. Astron. Astrophys. 18, 165), we have started a project to study the properties of these quasar cores.