Size Of Radio Source Research Articles

Interactions of Jets with Interstellar and Intergalactic Media

Jets emanating from AGN propagate first through an isothermal, but roughly power-law in density, galactic halo and then into a hotter, less dense, and uniform intergalactic medium (IGM) or intracluster medium (ICM). We use a three-dimensional boundary-following code (Mitteldorf & Wiita 1988), altered to allow for a two-phase external medium. We vary the beam power, P, the redshift, z, the radius of the galactic halo/IGM interface, Rh, the steepness of the power-law fall-off within the halo, n, and the temperature ratio of the IGM (or ICM) to the halo, Tr to estimate the average linear sizes of extragalactic radio galaxies (RGs). Good agreement is obtained with regard to the relationships between the overall linear size of such radio sources and both the total radio power (at fixed redshift) and the cosmological redshift (at fixed power). These numerical models tend to support recent analytical models (Gopal–Krishna & Wiita [GW] 1987, 1988).

Interaction of the Beams of Active Galactic Nuclei with Their Environment at High Redshifts

Aiming at a physical interpretation of the cosmological evolution of radio galaxies, we extend to a high redshift our analytical model for the propagation of relativistic beams first through a hot gaseous halo of the parent elliptical galaxy and then through an even hotter, but less dense, diffuse IGM, after crossing a pressure-matched interface between the two media1–5. This model, verified by quasi-hydrodynamical numerical simulations5 has earlier explained: (1) the current mean size of classical double radio sources (D ∼350 kpc), (2) their steep linear-size evolution with redshift, z: D α (1+z)−3, (3). The correlation between size and radio luminosity (at fixed z): D α P0.3, (4) the number and <P> of giant radio galaxies, and (5) the break in the local radio luminosity function (LRLF), occurring near P ∼1024W.Hz−1 at 1 GHz (Ho = 50 kms−1 Mpc−1). Inputs to the model are observationally based average parameters of the halo1 {kTh∼1 keV, n(r) ∼10−2cm−3[1+(r/2kpc)2]−3/4}, IGM7 {kTIGM ∼18 keV (1+z)2, nIGM ∼7.10−7cm−3(1+z)3} and the beam4 {opening angle θ (radian) = 0.02 + 0.03 [29 - log P(t=0)]}. We assume a reasonable value of ∊ = 0.1 for the initial efficiency of conversion of the beam power, Lb (Watts) into (total) radio output Pt ∼1010. P(W.Hz−1). A gradual weakening of magnetic field within the expanding source raises the significance of inverse Compton losses against the Cosmic Microwave Background (CMB), leading to a reduced radio efficiency (RRE)3,2.

Beams crossing a galactic halo-intergalactic medium interface and the size of extragalactic radio sources

view Abstract Citations (16) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Beams Crossing a Galactic Halo--Intergalactic Medium Interface and the Size of Extragalactic Radio Sources Rosen, Alexander ; Wiita, Paul J. Abstract Numerical simulations of extragalactic jets which penetrate the boundary between a power-law galactic halo and a hotter intergalactic medium are carried out using a boundary-following code. Comparisons of these models are made with both simplified analytical models and observations of powerful radio sources. Good agreement is obtained with regard to the relationships between the overall linear size of such radio sources and both the cosmological redshift (at fixed power) and the total radio power (at fixed redshift). These computations support the basic idea that the interaction between jets and a cosmologically significant intergalactic medium is very important. Publication: The Astrophysical Journal Pub Date: July 1988 DOI: 10.1086/166451 Bibcode: 1988ApJ...330...16R Keywords: Computerized Simulation; Extragalactic Radio Sources; Galactic Structure; Intergalactic Media; Radio Jets (Astronomy); Red Shift; Galactic Mass; Galactic Nuclei; Halos; Morphology; Virgo Galactic Cluster; Astrophysics; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: JETS; GALAXIES: STRUCTURE; RADIO SOURCES: GALAXIES full text sources ADS | Related Materials (1) Erratum: 1991ApJ...372..349R

Cosmic evolution of the physical sizes of extragalactic radio sources and their luminosity-size correlation

L'examen d'un echantillon de 669 radiosources, pour lesquelles ont ete recoltees un maximum de donnees concernant les dimensions angulaires, les densites de flux et le redshift, a permis d'etablir des correlations entre luminosite et dimension physique en fonction du type de source (radiogalaxie ou quasar), du redshift et de la classe de luminosite radio

Linear size distribution of double radio sources in the constant speed symmetric expansion model

The distribution of projected linear sizes of double radio sources is derived in an intrinsically-symmetric model in which the hotspots move away from the parent object at speedv before fading at timeT after their simultaneous origin. This is applied to a sample of doubles, separating the two asymmetry types closer-hotspot-brighter (yes-type) and closer-hotspot-fainter (no-type). Along with the arm-ratio distributions for the two classes, no-doubles very roughly indicatev∼0.3c, but no consistent value can be found for yes-doubles. A possible generalization of the model is suggested to allow different expansion speeds for different sources (though still retaining the intrinsic bilateral symmetry). Other effects that could be included in the model are also indicated.

On the interpretation of the observed angular-size-flux-density relation for extragalactic radio sources

The interpretation of the observed relation between median angular sizes (θm) of extragalactic radio sources and flux density at 408 MHz has been examined. The predictedθm-S relations based on well-observed strong sources in parent samples selected at 178 and 1400 MHz, and existing models of the evolving radio luminosity function can be made to fit the observed relation only by invoking cosmological evolution in linear sizes even for theq0 = 0 universe. Predictions based on a parent sample at 2.7 GHz are shown to overestimate the contribution of steep-spectrum, compact (SSC) sources in low-frequency samples unless the downward curvature in the spectra of such sources is taken into account. When approximate corrections are made for this effect, predictions based on the 2.7 GHz parent sample cannot obviate the need for linear size evolution as claimed in the literature.

Compact radio sources associated with molecular outflows

The results of a search for radio emission associated with molecular outflows in regions with a compact object are reported. The data were collected with the VLA in 1981 and 1983 and covered 12 fields and 15 outflows. The frequencies used were 1.4 GHz, 5 GHz and 15 GHz aimed at S187, NGC 1333, L1551, NGC 2024, M78, NGC 2071, S255, CFK 961, S68, AS 353A, S106, and NGC 7129. The positions, flux densities, spectra and sizes of the compact radio sources scanned are provided for each region and discussed. Radio continuum emissions were recorded whenever the bolometric luminosity exceeded 100 solar luminosities, and from three sources with luminosities below that magnitude. The sources S106, CRL 961 and LkH-alpha 234 all had spectra and angular breadths typical of stars undergoing spherical mass outflow. Smooth surface brightness distributions for S254, S255 and S257 support an interpretation as uniform density spherical Stromgren spheres. High resolution radio maps of NGC 2071 and S255 reveal that both have three compact objects distributed as the objects in Trapezium. The NGC 2071 objects are confined to a space the size of the solar system, with electron densities greater than 10,000/cu cm. 85 references

A sample of 6C sources selected at the peak of the source counts - III. An investigation of the variation of the sizes of radio sources with cosmic epoch

The radio (Paper I) and optical data (Paper II) for a sample of 67 6C radio sources with flux density ≍2 Jy at 151 MHz are compared with data for a sample of bright sources. The 6C sources are found to be at higher redshifts but have similar radio luminosities, except that there are fewer 6C sources with very low luminosities. There is little evidence in samples with this level of flux density of the strong correlation between spectral index and radio luminosity or redshift present in brighter samples. An investigation has been made into whether the number density evolution of radio galaxies is different from that of quasars of the same radio luminosity. This was inconclusive; more spectroscopic redshifts will allow a much more stringent test of this possibility. A new method is presented for investigating possible cosmological evolution in the physical sizes of sources, and it is shown that such evolution is present, in the sense that the median size D of sources was smaller at earlier epochs, both for Ω0=0 and for Ω0=1. Because of the parameters chosen for the faint sample this result should not be due to selection effects. If a function D ∝(1+z)-1 is fitted to the data, n = 1.45 ± 0.4 for Ω0= 1, and n = 1.1 ± 0.5 for Ω0=0. No evidence is found for a correlation between luminosity and physical size which is not merely due to both being correlated with redshift. It does not seem that this evolution can be explained either by the increase with redshift of the energy density in the microwave background or by invoking special evolutionary tracks through the P-D diagram.

On the size of the galactic centre compact radio source: diameter <20 AU

The existence of a compact nonthermal radio source at the galactic centre was first suggested by Lynden-Bell and Rees1 as the possible signature of a black hole. Such a source was detected unambiguously by Balick and Brown2.Very long baseline interferometry (VLBI) observations of the structure of this source, hereafter referred to as Sgr A*, have been made for several years3,4. In all previous experiments, the visibility amplitudes, measured over a restricted (U, V) range, could be fitted only to the simplest model of brightness distribution, that is, to a circular gaussian. We have re-observed Sgr A* at λ3.6 cm and λ 1.35 cm with many baselines, using the more sensitive receivers and VLBI recording terminals now available. These observations set an upper limit of 20 AU (3 × l014 cm) to the diameter of Sgr A* at λ 1.35 cm and reveal for the first time an elongated structure at λ 3.6 cm, with the position angle of the long axis almost parallel to the rotation axis of the Galaxy. Sgr A* is unique in our Galaxy, but resembles most closely the compact radio sources at the centre of external galaxies. Observations of the central 4 arc s (0.2 pc) at radio and other wavelengths are best explained by a single massive collapsed object at the galactic centre.

Rapid fluctuations in the position, size, and brightness of intense solar metre-wave radio sources

A comparison of quiescent type I solar radio sources with concurrent intense impulsive type III, V, and type II sources shows that whereas the type I sources are usually small and stable the type III, V, and II sources are usually large and unstable. We conclude that the large size and variability of the type III, V, and II radio sources cannot be attributed to instrumental error or ionospheric refraction but must instead reflect the size and variability of the coronal structures on which they arise.

Variations of the linear sizes of extragalactic radio sources with radio luminosity and redshift

The variations with radio luminosity and redshift of the characteristic linear size of extragalactic radio sources are investigated in two ways: (1) by examining bright flux-density-limited samples in order to determine the dependence explicitly, and (2) by comparing angular size–flux density data from faint samples (which generally have no redshift information) with numerical predictions, using individual sources in bright samples to define the local population. These predictions, using multi-frequency space-density evolution models, divide the total population into flat- and steep-spectrum components, thereby allowing populations found at different frequencies to be related. These investigations suggest that the apparent local distribution of linear sizes is determined by instrumental and physical selection effects which result in a weak inverse correlation of linear size with radio luminosity. For populations at earlier cosmological epochs, these effects cause an apparent decrease in linear size with redshift, although not by enough to account for the observational results. These may thus point to a variation in the relative proportions of physically distinct types of source with redshift.

Proposed Very Long Baseline Interferometry at 103 MHz in India

Three radio telescopes operating at 103 MHz are being installed at Ahmedabad, Rajkot and Surat separated by about 200 km from each other for observing interplanetary scintillations (IPS) of compact radio sources for study of solar wind plasma dynamics as well as radio source size measurements for cosmological studies. Of these, two radio telescopes at Ahmedabad and Rajkot have been commissioned and started synchronous daily observations of IPS of a few compact radio sources with relative time accuracy of about ± 1 millisec. The third telescope at Surat is expected to go in operation by the end of 1983. As soon as all the three telescopes go in for simultaneous operation, it is proposed to (1) augment the telescope sensitivity so as to detect sources with flux density ~ 1 Jansky (2) incorporate better time and frequency standards at each station which can be synchronised to better than μs relative time accuracy (3) develop suitable receivers and data acquisition system for generating interference fringes using a general purpose computer and (4) take advantage of the availability of three telescopes to incorporate ‘closure phase and amplitude’ techniques which eliminate undesirable atmospheric and ionospheric phase distortions.

Long time delay between the peaks of intense solar hard X-ray and microwave bursts

It is shown that the long time delays more than five seconds between the peaks of intense hard X-ray and microwave bursts are concerned with two independent phenomena. One is the energy dependent time delays in X-rays and the other is the frequency dependent time delays in microwaves. The time delays of 5 s to 10 s between the peaks of solar hard X-ray burst (≲100 keV) obtained with Hinotori spacecraft and microwave burst at 17 GHz were observed exceptionally in three intense events with a spectral maximum at about 17 GHz. It is found that the peak of harder X-rays (≳300 keV) also delays in these events by about the same amount with respect to the softer X-rays (≲100 keV), so that the peak at 17 GHz nearly coincides (≲4s) with that of the harder X-rays. This is quite reasonable because the gyro-synchrotron emissions from the electrons below about 100 keV in the solar flares are generally negligible at high microwave frequencies (≳10 GHz). The optical thickness of the radio source decreases with frequency and is unity generally at about 10–20 GHz in intense bursts as inferred from the radio spectrum. Further delay of the peaks at the lower microwave frequencies is attributed to the temporal increase in the effective size of radio source which is optically thick at the lower frequencies.

The Evolution of Linear Sizes

Possible evidence that the linear sizes of extragalactic radio sources were smaller at earlier epochs was first provided by the angular size redshift (θ-z) relation for double radio quasars (Legg 1970, Miley 1971, Wardle and Miley 1974). But because of the strong correlation between redshift (z) and radio luminosity (P) in flux limited radio samples, it is hard to decide if the observed decrease in sizes with z is caused by an epoch dependence of linear sizes (ℓ) or by an inverse correlation between P and ℓ. Several authors (eg. Stannard and Neal 1977, Wardle and Potash 1977, Hooley ET AL. 1978, Wills 1979, Masson 1980) have attempted to separate the two effects by comparing the properties of quasars from the 3CR survey with those from the 4C and Parkes samples. Although most of these studies appear to marginally favour a P-ℓ correlation, none of them can rule out even a fairly strong evolution in ℓ with z. Apart from the small numbers involved, the difficulty is that 3C and 4C quasars do not differ a great deal in their redshifts or luminosities. A complete sample of quasars at much weaker flux levels would be quite valuable in this regard.

A Study of Small Angular Size Ooty Sources

Recent theoretical models of radio radiation from central cores of radio sources invoke axial relativistic beaming of the radiation and predict significant enhancement of the radiation from the central cores when viewed at small angles to the direction of the beam. Investigations of the angular diameter distributions of radio sources by Swarup (1975) has shown that the median angular size of radio sources below 4 Jy at 327 MHz is about 10 arc seconds. Hence a sample of sources, with angular sizes substantially less than the above median, chosen from the Ooty occultation survey may be expected to contain a large fraction of double sources viewed along the axis of the double structure.

The heating of gas in clusters of galaxies by relativistic electrons - Collective effects

We show that the rate at which gas is heated in X-ray clusters of galaxies by streaming relativistic electrons can be much greater than the Coulomb heating rate because of the stimulated growth of a high level of electrostatic turbulence and its subsequent collapse to shorter wavelengths. This enhanced heating (and, hence, energy loss) rate allows the X-ray emitting gas to be heated by those particles which are observable through their synchrotron emission at low radio frequencies and yields a radio source size consistent with the observed radio halo sizes in the Coma cluster. The heating of gas in clusters of galaxies by relativistic electrons will significantly affect the cluster gas dynamics.

VLBI observations of galactic nuclei at 18 centimeters - NGC 1052, NGC 4278, M82, and M104

Compact radio sources about a light year in size have been detected in the nuclei of the galaxies NGC 1052, NGC 3034 (M82), NGC 4278, and NGC 4594 (M104) at a wavelength of 18 cm. The compact nucleus detected in M81 at 6 cm was not seen at 18 cm. The compact source in M82 is unique among extragalactic sources in its size-spectrum relationship. It is either broadened by scattering within M82 or it lies behind, and is absorbed by, an H II region. In these galaxies, the size of the nuclear radio source at 18 cm is larger than it is at higher frequencies. The nucleus of the giant radio galaxy DA 240 was not detected.

Statistical properties of double radio sources

We study a model of radio galaxies, where the galaxy is surrounded by an isothermal sphere of gas and the radio components move through this gas subsonically. Each radio component is assumed to be powered by either a supermassive particle accelerator, a head of a relativistic particle beam, or a train of plasmons. We find that the statistical properties involving the total size, radio luminosity, depolarization and symmetry of double radio sources can be explained independent of details of the radio source model.

Bent beams and the overall size of extragalactic radio sources

In a small sample of extragalactic radio sources, curvature is found to be a common feature in bright radio components coincident with quasi-stellar objects. In 3C273 and 3C345 the curvature is smooth between the nuclear ( 1″) regions. This contrasts sharply with objects having comparatively faint central radio components, which are often well aligned with the extended features.

Applications of statistical techniques to the angular size-Flux density relation for extragalactic radio sources

The data on the angular sizes and flux densities of extragalactic radio sources from the 3CR and the Ooty surveys is subjected to two independent statistical tests. The purpose of these tests is to investigate whether the data necessitates the conclusion that the properties of the radio sources evolve with the epoch of the Universe. The first test uses a minimum χ2 technique to compare the observed (θ,S) plot with a theroretical one. In the second, the so-called median test, the two distributions are compared in a narrow range of flux densities. It is shown that the present scatter in this data is such that within the usual confidence limits it is not possible to distinguish between a number of evolutionary and non-evolutionary models. In fact, a non-evolutionary model in which there is a reasonable correlation between the linear size and power of radio sources passes both the tests. It is shown how future improved data on (θ,S) may be able to distinguish between the evolutionary and non-evolutionary models.