arXiv subjects

K. I. Kellermann

Publications and source records attributed to K. I. Kellermann.

At least 73 records · Page 4Linked to original sources

arXiv2004

Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei III. Kinematics of Parsec-Scale Radio Jets

We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity which may represent an underlying continuous jet flow, but we also see some stationary and even apparently inward moving features which co-exist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure. The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the time scale of flux density variations is consistent with a steep power law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows (abridged).

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arXiv2003

Variability and Velocity of Superluminal Sources

We investigate the relation between the Doppler factor determined from variations in total flux at 22 and 37 GHz, and the apparent transverse velocity determined from VLBA observations at 2 cm. The data are consistent with the relativistic beaming theory for compact radio sources, in that the distribution of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close to the "equipartition value" calculated by Readhead (1994). We deduce the distribution of Lorentz factors for a group of 48 sources; the values range up to about gamma=40.

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arXiv2003

Jet Collimation in Action: Re-alignment on Kiloparsec Scales in 3C279

We report a change in the trajectory of a well studied jet component of the quasar 3C279. The component changes in apparent projected speed and direction, and we find it to be moving with a Lorentz factor gamma >~ 15 at an initial angle of <~ 1 degree to the line of sight. The new trajectory of the component has approximately the same speed and direction as an earlier superluminal feature, originally seen in the early 1970s. The new direction for the component is also much better aligned with larger scale VLBA and VLA structure out to 0.1 arcseconds. We suggest that the trajectory change is a collimation event occurring at >~ 1 kiloparsec (deprojected) along the jet. While the change in trajectory on the sky appears to be 26 degrees the intrinsic change is <~ 1 degree. We estimate the Doppler factor prior to the change in direction to be D >~ 28 and after the change to be D >~ 23. Comparison to independent constraints on the Doppler factor suggest that the energy in the radiating particles cannot greatly exceed the energy in the magnetic field unless the volume filling factor is very much less than one.

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arXiv2003

Ionised, Atomic, and Molecular Gas around the Twin Radio Jets of NGC1052

The bright radio structure of the LINER elliptical galaxy NGC1052 is dominated by bi-symmetric jets on parsec scales. Features move outward on both sides of the core at v_{app}~0.26c. We have established the occurrence of free-free absorption, and suggest the presence of a patchy, geometrically region oriented roughly orthogonal to the jets; components have a wide range of spectral shapes and brightness temperatures. We distinguish three velocity systems in HI absorption. The ``high velocity'' system is the most prominent of these; it is redshifted from systemic by about 150 km/s. In HI VLBI it is seen towards both jets, but appears to be restricted to a shell 1 to 2 pc away from the core. The central hole might be largely ionised, and could be connected to the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH main line absorption over at least the full ~250 km/s velocity range seen in HI. In the ``high velocity'' system, the profiles of the OH main lines and HI are similar, which suggests co-location of molecular and atomic gas. The OH satellite lines are also detected in the ``high velocity'' system: 1612 MHz in absorption and 1720 MHz in emission, with complementary strength. But we have no satisfactory model to explain all properties; the connection to H_2O masing gas at the same velocity but apparently a different location is also unclear.

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arXiv2003

Variability, Brightness Temperature, Superluminal Motion, Doppler Boosting, and Related Issues

We review the observations of rapid flux density variations in compact radio sources, and discuss the inverse Compton limit to the maximum brightness temperature of incoherent synchrotron sources in comparison with recent VLBA observations. The apparent agreement of the theoretical brightness temperature limit due to inverse Compton cooling and the brightness temperatures observed by early VLBI observations appears to have been fortuitous. VLBA observations have greatly improved the quality of the data, but many of the early issues remain unresolved.

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arXiv2002

The Parkes quarter-Jansky flat-spectrum sample 2. New Optical Spectra and Redshift Measurements

We present optical spectra and redshift measurements for 178 flat-spectrum objects from the Parkes quarter-Jansky flat-spectrum sample. These spectra were obtained in order to compile a complete sample of quasars for use in a study of quasar evolution. We present a composite optical spectrum made from the subset of 109 quasars that have flux densities in the range 0.25Jy < S(2.7GHz)< 0.5Jy, and make a comparison with a composite for radio-quiet QSOs from the Large Bright Quasar Survey. Our large sample of radio-loud quasars allows us to strengthen previous reports that the Ly-alpha and CIV emission lines have larger equivalent width in radio-loud quasars than radio-quiet QSOs to greater than the 3-sigma level. However we see no significant difference in the equivalent widths of CIII] or MgII. We also show that the flux decrements across the Lyman-alpha line (D_A) measured from these spectra show the same trend with redshift as for optically selected QSOs.

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arXiv2002

The Shroud Around the Twin Radio Jets in NGC 1052

(Abridged) We discuss multiple VLBI continuum and spectral line observations and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at approximately 0.26c in bi-symmetric jets, most likely oriented near the plane of the sky. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium. Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption in the inner parsec, probably together with synchrotron self-absorption. There is apparently a geometrically thick but patchy structure oriented roughly orthogonal to the jets. The western jet is receding: it is covered more deeply and extensively. HI spectral line VLBI reveals atomic gas in front of both jets. There appear to be three velocity systems. The deepest, at "high velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2 pc away from the core, within which this gas might be largely ionised. WSRT spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity system the OH profiles are similar to HI, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H2O masing gas seen elsewhere. We have also detected both 18cm OH satellite lines in the high velocity system. They have conjugate profiles: 1612 MHz is in absorption, and 1720 MHz in emission.

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arXiv2002

Superluminal Motion and Relativistic Beaming in Blazar Jets

High resolution radio observations remain the most direct way to study the formation and evolution of radio jets associated with the accretion onto massive black holes. We report preliminary results of our seven year VLBA observational program to understand the nature of relativistic beaming in blazars and the surrounding environment of massive black holes. Most blazars show an apparent outward flow away from an active core. However, in a few sources the motion appears inward, most likely the result of projection of a curved trajectory which bends back toward along the line of sight. The apparent motion of jet features is not always oriented along the direction separating the feature from the core, and in a few cases we have observed a clear change in the direction and velocity of a feature as it flows along the jet. In other sources, the motion appears to follow a simple ballistic trajectory. We find no simple relation between the time scales of flux density changes and apparent component velocities.

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arXiv2002

4C +12.50: A Superluminal Precessing Jet in the Recent Merger System IRAS 13451+1232

We present the results of a multi-epoch VLBA study of the powerful radio source 4C +12.50 (PKS 1345+125) at a wavelength of 2 cm. This compact radio source is associated with a hidden quasar whose host galaxy shows signs of a recent merger. It has been classified as a compact symmetric source (CSO) due to its small overall size (~220 pc) and twin-jet morphology, although it also has faint extended emission that may be a relic of previous activity. We report the detection of exceedingly high linear fractional polarization in isolated features of the southern jet (up to 60%), which is highly unusual for a CSO. Given the large amount of gas present in the host galaxy, we would expect significant Faraday depolarization across the whole source, unless the depolarizing gas is fairly clumpy. The southern jet also contains two features that are moving outward from the core at apparent speeds of v/c = 1.0 +/- 0.3 and 1.2 +/- 0.3. These represent the first positive detections of superluminal motion in a CSO, and taken together with the jet to counter-jet flux ratio, suggest an intrinsic flow speed of v/c = 0.84 +/- 0.12. The apparent ridgeline of the jet and counter-jet are consistent with a conical helix of wavelength 280 pc that is the result of Kelvin-Helmholtz instabilities driven by a slow precession of the jet nozzle. A fit to the data implies that the nozzle is precessing around a cone with half-angle 23 degrees, whose axis lies at an angle of 82 degrees to the line of sight. We suggest that the "S"-shaped jet morphologies commonly seen in recent AGN outflows such as 4C +12.50 may simply reflect the fact that their black hole spin axes are still precessing, and have not had sufficient time to align with their accretion disks.

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arXiv2002

The shroud around the "compact, symmetric" radio jets in NGC 1052

This is a paper on young jet material in a frustratingly complex environment. NGC 1052 has a compact, flat- or GHz-peaked-spectrum radio nucleus consisting of bi-symmetric jets, oriented close to the plane of the sky. Many features on both sides move away at v_app~0.26c (H_0=65 km/s/Mpc). VLBI at seven frequencies shows a wide range of spectral shapes and brightness temperatures; there is clearly free-free absorption, probably together with synchrotron self-absorption, on both sides of the core. The absorbing structure is likely to be geometrically thick and oriented roughly orthogonal to the jets, but it is patchy. HI VLBI shows atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems, at least two of which are local to the AGN environment. The "high velocity system", 125 to 200 km/s redward of systemic, seems restricted to a shell 1 to 2 pc away from the core. Closer to the centre, this gas might be largely ionised; it could cause the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH absorption over a wide velocity range. OH and HI profile similarity suggests co-location of molecular and atomic ``high velocity'' gas; the connection to H2O masing gas is unclear. Further, at ``high velocity'' we detected the OH 1612 MHz satellite line in absorption and the 1720 MHz line in emission, with complementary strengths.

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arXiv2002

The Central Parsecs of the Bright Quasar PKS 1921 - 293

We report on a VLBA imaging study of the nearby bright southern blazar PKS 1921 - 293 (OV - 236). High resolution VLBA observations, made at four frequencies (5, 12, 15, and 43 GHz) over the period 1994 - 2000, have revealed a strongly curved jet extending out to about 50 parsecs from the presumed central engine. Two epoch VLBA observations, each simultaneously carried out at both 5 and 43 GHz, show a large position angle difference of 51 - 67 degrees between the jet emission at 5 and 43 GHz. Although the core of PKS 1921 - 293 has one of the highest brightness temperatures measured in any compact radio source, unlike other bright blazars it is not a source of gamma-ray emission. However, there is evidence in these images for superluminal motion within the central region (a few parsecs from the core) and within the north-east diffuse emission region. In all six-epoch 43 GHz images, two equally compact bright components within the central parsec are seen.

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arXiv2002

The Diverse Properties of GPS Sources

We discuss the morphology and kinematics of five gigahertz-peaked spectrum (GPS) sources that have been observed with the VLBA. We find a wide range of observed properties including core-jet structure, superluminal motion, variability, extended structure, and polarization, all of which appear to deviate from commonly-accepted GPS paradigms (e.g., O'Dea 1998). We suggest that the observed low frequency cutoff in GPS sources may be primarily due to free-free absorption rather than synchrotron self-absorption.

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arXiv2002

Kinematics of parsec-scale structures in AGN: the 2cm VLBA Survey

We are investigating the kinematics of jets in active galactic nuclei on parsec scales by studying a representative population of sources. This study is being carried out using the Very Long Baseline Array at 15 GHz, with more than 800 images taken since 1994. In this contribution we present an overview of the diversity of kinematics for a complete sample of sources.

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arXiv2002

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. II. Additional Sources

We report further results from our imaging survey at 15GHz (lambda=2 cm) with the Very Long Baseline Array. This paper presents single epoch images for 39 sources, bringing the total number of objects in the sample to 171. Our sample is representative of a complete unbiased sample and it will be used for statistical analysis of source properties. We compare the observed brightness temperatures derived from our VLBA observations to those derived from total intensity variations at 22 and 37 GHz. These are consistent with intrinsic brightness temperatures in the range 10^10 to 10^12 K. We also present three new spectroscopic redshift values: z=0.517+/-0.001 for 0026+346, z=1.591+/-0.003 for 0727-115, and z=0.2016+/-0.0004 for 1155+251. Images from this VLBA 2 cm survey are available on the Internet under http://www.cv.nrao.edu/2cmsurvey.

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arXiv2002

The Micro-Jansky Sky at 8.4 GHz

We present the results from two radio integrations at 8.4 GHz using the VLA. One of the fields, at 13h,+43d (SA13 field), has an rms noise level of 1.49 microJy and is the deepest radio image yet made. Thirty-four sources in a complete sample were detected above 7.5 microJy and 25 are optically identified to a limit of I=25.8, using our deep HST and ground-based images. The radio sources are usually located within 0.5" (typically 5 kpc) of a galaxy nucleus, and generally have a diameter less than 2.5". The second field at 17h, +50d (Hercules Field) has an rms noise of 35 microJy and contains 10 sources. We have also analyzed a complete flux density-limited sample at 8.4 GHz of 89 sources from five deep radio surveys, including the Hubble deep field. Half of all the optical counterparts are with galaxies brighter than I=23 mag, but 20% are fainter than I=25.5 mag. We confirm the tendency for the micro-Jansky radio sources to prefer multi-galaxy systems. The distribution of the radio spectral index between 1.4 and 8.4 GHz peaks at alpha = -0.75~ with a median value of -0.6. The average spectral index becomes steeper (lower values) for sources below 35 microJy, and for sources identified with optical counterparts fainter than I=25.5 mag. The differential radio count between 7.5 and 1000 microJy has a slope of -2.11 +/-0.13 and a surface density of 0.64 sources per square-arcmin with flux density greater than $7.5 microJy.

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arXiv2000

VSOP observation of the quasar PKS 2215+020: a new laboratory for core-jet physics at z=3.572

We report results of a VSOP (VLBI Space Observatory Programme) observation of a high redshift quasar PKS 2215+020 (z=3.572). The ~1 milliarcsecond resolution image of the quasar reveals a prominent `core-jet' structure on linear scales from 5/h to 300/h pc ($H_0=100*h km/(s*Mpc). The brightness temperatures and sizes of bright features identified in the jet are consistent with emission from relativistic shocks dominated by adiabatic energy losses. The jet is powered by the central black hole with estimated mass of ~4*10^9 solar masses. Comparisons with VLA and ROSAT observations indicate a possible presence of an extended radio/X-ray halo surrounding 2215+020.

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arXiv1999

VLBA Absorption Imaging of Ionized Gas Associated with the Accretion Disk in NGC 1275

Nearly simultaneous VLBA observations of 3C 84, the radio source associated with NGC 1275, have been made at multiple frequencies to study the free-free absorption of the northern, or "counterjet", feature found by Walker, Romney, and Benson (1994, Ap.J. 430, L45) and by Vermeulen, Readhead, and Backer (1994, Ap.J. 430, L41). Our observations confirm that the spectra are consistent with free-free absorption and eliminate the possibility that the earlier result was an effect of variability. The northern feature is well resolved spatially, so images have been made showing the distribution of the absorption over a region of about 1.5 pc on a side, beginning about 1.5 pc from the presumed location of the central object. That distribution is dominated by a strong decrease with radial distance. The magnitude of the absoption near 2.5 pc projected distance from the central object is consistent with a 10^4 K gas with an emission measure of about 5 X 10^8 pc cm^-6. The geometry is consistent with absorption by ionized gas associated with an accretion disk. The data provide firm constraints for models of the outer regions of accretion disks and, perhaps, associated winds.

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arXiv1999

Optically Faint Microjansky Radio Sources

We report on the identifications of radio sources from our survey of the Hubble Deep Field and the SSA13 fields, both of which comprise the deepest radio surveys to date at 1.4 GHz and 8.5 GHz respectively. About 80% of the microjansky radio sources are associated with moderate redshift starburst galaxies or AGNs within the I magnitude range of 17 to 24 with a median of I = 22 mag. Thirty-one (20%) of the radio sources are: 1) fainter than $I>$25 mag, with two objects in the HDF $I_{AB}>$28.5, 2) often identified with very red objects $I-K>$4, and 3) not significantly different in radio properties than the brighter objects. We suggest that most of these objects are associated with heavily obscured starburst galaxies with redshifts between 1 and 3. However, other mechanisms are discussed and cannot be ruled out with the present observations.

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