A search for steep low-frequency radio spectra among quasars and clusters of galaxies

Using published flux densitiesS at low frequenciesv, radio spectra were constructed for 3C, 4C, and 4CT radio sources in Abell clusters of galaxies, radio galaxies outside Abell clusters, and quasars with known redshifts. About half the sources in rich Abell clusters (richness classesR>-2) have steep spectra between 38 and 178 MHz with spectral indices ₃₈ ¹⁷⁸ > whereSv ^–. However, radio galaxies outside clusters have values of ₃₈ ¹⁷⁸ 1.2, and no steep spectra were found among 170 quasars. The radio sources in rich clusters are probably confined by intergalactic gas, and the steep spectra develop over a period of 10⁹ yr as relativistic electrons lose energy. The absence of steep spectra among quasars does not necessarily mean that quasars never occur in rich clusters of galaxies, since quasars are probably being observed only in their early high-luminosity phases. The possibility that some quasar events occur in the nuclei of the dominant cD galaxies in clusters is discussed, but quasar events may occur in more than one type of galaxy.     

Dynamical evolution in clusters of galaxies with low-frequency radio emission

Clusters of galaxies in which radio emission at low frequencies (178 MHz) has been detected were classified on the Bautz-Morgan (BM) system according to the dominance of the brightest galaxy. Radio sources with steep low-frequency spectra occur in clusters of all BM types but more often in rich clusters; the distributions of BM types for clusters with high and low spectral indices between 38 and 178 MHz are similar. Glass copies of Mount Palomar Sky Survey plates were measured to determine the distribution of the ten brightest galaxies in clusters without dominant galaxies. Some clusters were found to have central cores of bright galaxies which may reflect mass segregation of galaxies due to dynamical friction. The bright galaxies in such cores may later merge to form dominant cD galaxies. The positions of the cD galaxies and cores of bright galaxies are often at projected distances <200 kpc from the low-frequency radio emission. The low-frequency spectrum of radio emission associated with a cD galaxy may be either steep or normal, but the low-frequency spectrum from a core of bright galaxies is usually steep. A steep spectrum may develop when a radio source is confined by hot gas in a cluster over a long period (10⁹ yr). Confinement would probably occur for radio sources associated with bright galaxies in the cores of clusters and cD galaxies in clusters. However, cD galaxies may have recurrent radio outbursts so that steep spectra are not always observed.     

A VLA 20 and 90 centimetre radio survey of distant A-bell clusters with central cD galaxies

A radio survey, using the Very Large Array at 20 and 90 cm λ has been carried out in the direction of 46 distant Abell clusters (0.1 ≲ z ≲ 0.3) dominated by a cD galaxy (clusters classified to be Bautz-Morgan I type). A radio source coincident with the cD galaxy was detected in 16 clusters. We find that the radio luminosity function of the cD galaxies at 20cm λ, and below the luminosityP _1.4ghz ≲ 10^24.5 W Hz^-1, is similar to that of brightest ellipticals in less clustered environments. Above this luminosity, the cDs seem to have a higher probability of becoming radio sources. The effect of optical brightness on radio emission is shown to be the same for the two classes. No significantly large population of very-steep-spectrum sources with spectral index α >1.2 (flux density ∝ frequency^-α) was found to be associated with cD galaxies. A significant negative correlation is found between the radio luminosity of the cD galaxy and the cooling-time of the intra cluster medium near the galaxy. We also present evidence that the probability of radio emission from first-ranked galaxies is dependent upon their location relative to the geometrical centres of clusters and thus related to the morphological class and the evolutionary state of the clusters. We argue that both these effects are primarily caused by the dynamical evolution of these distant clusters of galaxies.     

Stochastic simulation of fields of galaxies,III

Stochastic simulations of galaxy fields, using the cluster multiplicity function obtained for the Lick galaxy counts, reproduces satisfactorily the observed distribution of galaxies from COSMOS measures on a deep UKST and AAT plate limited atB22.0 andB23.2, respectively. The results imply that no strong evolutionary effect is present in the clustering of galaxies, at least out to redshiftsz ^*0.65.     

Probing the Obscuring Medium Around Active Nuclei Using Masers: The Case of 3C 403

We report the first detection of a water megamaser in a radio-loud galaxy, 3C 403, and present a follow-up study using the VLA. 3C 403 has been observed as a part of a small sample of FR II galaxies with evidence of nuclear obscuration. The isotropic luminosity of the maser is 1200 L. With a recessional velocity of cz 17680 km s^–1 it is the most distant water maser so far reported. The line arises from the densest (> 10⁸ cm^–3) interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are identified, likely bracketing the systemic velocity of the galaxy. Our interferometric data clearly indicate that these arise from a location within 0.1 (110 pc) from the active galactic nucleus. We conclude that the maser spots are most likely associated with the tangentially seen parts of a nuclear accretion disk, while an association with dense warm gas interacting with the radio jets cannot yet be ruled out entirely.     

Number-magnitude count for galaxies at the South Galactic Pole

COSMOS measures on a deep UK Schmidt Telescope Plate have been used to obtain the number-magnitude count for galaxies in a field of 14.6 square degrees near the South Galactic Pole. The results are in excellent agreement with data for the North Galactic Pole for galaxies fainter thanB=18.0, indicating no large-scale differences between north and south. A deficiency in numbers is observed for galaxies withB16.0. This is comparable to the deficiency atB17.5 for counts at the North Galactic Pole and supports the suggested asymmetry of the bright galaxy distribution between north and south galactic poles.     

Continuum radiation from active galactic nuclei

Summary Active galactic nuclei (AGN) can be divided into two broad classes, where the emitted continuum power is dominated either by thermal emission (radio-quiet AGN), or by nonthermal emission (blazars). Emission in the 0.01–1 m range is the primary contributor to the bolometric luminosity and is probably produced through thermal emission from an accretion disk, modified by electron scattering and general relativistic effects. The 1–1000 m continuum, the second most important contributor to the power, is generally dominated by thermal emission from dust with a range of temperatures from 40 K to 1000–2000 K. The dust is probably reemitting 0.01–0.3 m continuum emission, previously absorbed in an obscuring cone (or torus) or an extended disk. The 1–10 keV X-ray emission is rapidly variable and originates in a small region. This emission may be produced through Compton scattering by hot thermal electrons surrounding an accretion disk, although the observations are far from being definitive. The weak radio emission, which is due to the nonthermal synchrotron process, is usually elongated in the shape of jets and lobes (a core may be present too), and is morphologically distinct from the radio emission of starburst galaxies.In the blazar class, the radio through ultraviolet emission is decidedly non-thermal, and apparently is produced through the synchrotron process in an inhomogeneous plasma. The plasma probably is moving outward at relativistic velocities within a jet in which the Lorentz factor of bulk motion (typically 2–6) increases outward. This is inferred from observations indicating that the opening angle becomes progressively larger from the radio to the optical to the X-ray emitting regions. Shocks propagating along the jet may be responsible for much of the flux variability. In sources where the X-ray continuum is not a continuation of the optical-ultraviolet synchrotron emission, some objects show variability consistent with Compton scattering by relativistic electron in a large region (in BL Lacertae), while other objects produce their X-ray emission in a compact region, possibly suggesting pair production.When orientation effects are included, all AGN may be decomposed into a radio-quiet AGN, a blazar, or a combination of the two. Radio-quiet AGN appear to have an obscuring cone or torus containing the broad emission line clouds and an ionizing source. Most likely, the (non-relativistic) directional effects of this obscuring region give rise to the difference between Seyfert 1 and 2 galaxies or narrow and broad line radio galaxies. For different orientations of the nonthermal jet, relativistic Doppler boosting can produce BL Lacertae objects or FR I radio galaxies, or at higher jet luminosities, flat-spectrum high-polarization quasars or FR II radio galaxies.     

On the relation between radius,luminosity, surface brightness,and surface density in elliptical galaxies

We have analyzed luminosity profiles of E galaxies studied by Strom and Strom in six clusters of galaxies. We have found a relationship between radius, luminosity, and surface brightness for galaxies in each of the clusters. A dependence of the zero point of the relation with the local projected density of galaxies is likewise found:r _{e proj} ^–0.14 L ^0.445 I _e ^–0.413 . This relationship implies (i) that there is not a universal luminosity profile for elliptical galaxies, (ii) the environmental variation of radius is larger than that produced by mergers of galaxies, (iii) distance to a galaxy can be estimated from apparent magnitude, surface brightness, angular size, and apparent local projected density of galaxies.     

Cooling flows in clusters of galaxies

Summary X-ray images and spectra of clusters of galaxies show strong evidence for cooling flows. In many clusters, the hot gas in the core is cooling at rates of 100Myr^–1 and greater. Few traces of the cooled gas have been observed, but it probably forms into low-mass stars (perhaps brown dwarf or even Jupiter-mass objects). X-ray surface-brightness profiles show that the cooling gas is highly inhomogeneous. Overdense gas cools rapidly to form cooled clumps distributed throughout the flow, with little of the gas ever reaching the cluster centre. Cooled and cooling clumps are disrupted because of their motion relative to the remainder of the gas, tending to produce small cooled fragments and, ultimately, low-mass stars. Large molecular clouds, which are the sites of massive star formation in our galaxy, do not occur in the outer parts of cooling flows. There is evidence of larger gas clumps and the formation of more massive stars in the central few kpc of some cooling flows. It is argued that cooling flows efficiently form dark matter. This has wider implications for the formation of dark matter in massive galaxies.     

Multiple moving magnetic structures in the solar corona

We report the study of moving magnetic structures inferred from the observations of a moving type IV event with multiple sources. The ejection contains at least two moving radio emitting loops with different relative inclinations. The radio loops are located above multiple H flare loops in an active region near the limb. We investigate the relationship between the two systems of loops. The spatial, temporal and geometrical associations between the radio emission and near surface activities suggest a scenario similar to coronal mass ejection (CME) events, although no CME observations exist for the present event. From the observed characteristics, we find that the radio emission can be interpreted as Razin suppressed optically thin gyrosynchrotron emission from nonthermal particles of energy 100, keV and density 10²–10⁵ cm^–3 in a magnetic field 2 G.     

Production of a short-lived filament by a surge

A large surge was observed on September 17, 1971, part of which, after travelling 200 000 km through the corona, returned to the surface to form a filament. The filament lasted about 30 min, then rose up and returned to the source of the surge. We interpret this as the filling of a semi-stable magnetic trap.The energetics of radio, X-ray, and surge expulsion are estimated. The radio spectrum and flux correspond to a thermal source of area 4 (arcmin)², T 190 000 K, N _e ² V 7 × 10⁴⁸, which is optically deep at 8800 MHz.The soft X-ray source has T 12 × 10⁶ K, N _e ² V 3 × 10⁴⁸; and if an equal mass is expelled in the surge, the kinetic energy of the surge is similar to the thermal energy of the X-ray source.     

Infrared emission of dust grains in the clusters of galaxies

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 10⁷–10⁸ yr at a gas concentrationn _g 10^–3 cm^–3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300 are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near =100 and the same for the Perseus cluster near 50–70. The total fluxes of IR emission from these clusters are about 10⁵–10⁶ Jy and can be detected by modern observational methods.     

On the evolution of low mas binary systems

The evolution of parameters of close binary systems containing a red dwarf (with mass loss) and a condensed star is investigated. The mass loss from the system and asynchronism of the red dwarf rotation are taken into consideration. The calculations show that if the initial mass ratioq ₀2 then during 10⁷ yr the instability of mass loss process arises, and a bright X-ray source forms with luminosity close to the Eddington one. Ifq ₀>2 then during the mass loss phase the bright X-ray source arises twice. The models explain the existence of the forbidden interval of orbital periods and the absence of systems with periods less than 80 min.     

Spectral evolution of bursts of star formation superimposed on old populations

We simulate the occurrence of star formation bursts, during the past 3×10⁹ yr, superimposed on old populations, by combining visible and near-infrared integrated spectra of star clusters with those of red galaxy nuclei. We track the resultant spectral evolution of the composite object for burst to old population mass ratios of 10, 1, and 0.1%. If a star formation burst uses 10% of the galaxy mass, the underlying old population will be undetectable, at least by means of integrated spectra, during 5×10⁷ yr. At intermediate ages, 2×10⁹ yr, the burst will still contribute around 20% of the total optical flux. For a 1% mass, the burst becomes barely visible at intermediate ages; and, finally, for an 0.1% mass, the burst will affect the galaxy spectrum during 2×10⁷ yr only.     

On the lifetime of bright X-ray sources

The lifetime of massive X-ray binaries is (2–5)×10⁵ yr, this time close to the nuclear one. The lifetime of nonmassive X-ray binaries close to thermal one, (0.5–1)×10⁷ yr. Massive systems may be conserved at supernova explosion, the probability of the conservation of nonmassive system is (1–3)×10^–3.     

Radio AGNs among Luminous Infrared Galaxies

An analysis of radio and FIR emission in over 1500IRAS selected galaxies produces a good linear correlationbetween radio and FIR luminosity, indicating that star formationin normal field galaxies dominates the infrared luminosityin the local volume. Galaxies with clear radio-excess (definedas having at least5 times larger radio flux over expected from FIR) are identified as hosting a radio AGN, and they account for onlyabout 1% of the whole sample. This fraction increases to 10% among themore luminous galaxies with L _1.4GHz 10²³ W Hz^-1 (equivalently L _60m 10¹¹ L), however. The characteristic mid-IR excess of a Seyfert nucleus is ubiquitously present amongthe radio-excess objects, suggesting that mid-IR excess isa robust tracer of an AGN despite the high mid-IR opacity.We conclude that about 30% of the luminous infrared galaxies(L _60m 10¹¹ L) host an AGN based on themid-IR excess, and about 40% of the mid-IR excess AGNs alsohost a radio AGN. A VLA imaging survey of a distance limited sample of IR luminousgalaxies has revealed the presence of 100 kpc scale giant radioplumes in 3 out of 9 cases (Mrk 231, Mrk 273, NGC 6240). Theirlarge spatial extent, energetics, and presence of a powerful AGN in each case suggests that an AGN is the power source. Such plumesare not detected in other ultraluminous infrared galaxies which lack clear evidence for an AGN, such as Arp 220.     

Positrons and low energy cosmic rays from supernovae

Burgeret al. (1970) calculated the positron flux from the decay of⁵⁶Co⁵⁶Fe from cosmic rays injected from supernovae. The plasma properties of the ejected matter are determined in the present calculation in order to include the ionization loss of the positrons as the matter expands. It is found that using the matter velocity distribution of previous supernova model calculations that roughly 10% of the positrons escape. The average lifetime in the galaxy due to ionization loss is found to be relatively small, 1.5×10⁵ yr, and with the above injection results in ×3, the observed flux. The same matter velocity distribution is subjected to ionization loss in the galaxy and a steady state low energy, 10E200 MeV, differential flux spectrum is found,J(E)E ^–1.2. This removes the difficulty of the high galactic energy density resulting from a steeper spectrum.     

Decimetric type III radio bursts and associated hard X-ray spikes

A detailed comparison is made between hard X-ray spikes and decimetric type III radio bursts for a relatively weak solar flare on 1981 August 6 at 10: 32 UT. The hard X-ray observations were made at energies above 30 keV with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission and with a balloon-born coarse-imaging spectrometer from Frascati, Italy. The radio data were obtained in the frequency range from 100 to 1000 MHz with the analog and digital instruments from Zürich, Switzerland. All the data sets have a time resolution of 0.1 s or better. The dynamic radio spectrum shows many fast drift type III radio bursts with both normal and reverse slope, while the X-ray time profile contains many well resolved short spikes with durations of 1 s. Some of the X-ray spikes appear to be associated in time with reverse-slop bursts suggesting either that the electron beams producing the radio bursts contain two or three orders of magnitude more fast electrons than has previously been assumed or that the electron beams can trigger or occur in coincidence with the acceleration of additional electrons. One case is presented in which a normal slope radio burst at 600 MHz occurs in coincidence with the peak of an X-ray spike to within 0.1 s. If the coincidence is not merely accidental and if it is meaningful to compare peak times, then the short delay would indicate that the radio signal was at the harmonic and that the electrons producing the radio burst were accelerated at an altitude of 4 × 10⁹ cm. Such a short delay is inconsistent with models invoking cross-field drifts to produce the electron beams that generate type III bursts but it supports the model incorporating a MASER proposed by Sprangle and Vlahos (1983).     

A synchrotron radiation model for extragalactic infrared sources

For a number of extragalactic sources of infrared radiation, for example some quasars and nuclei of Class I Seyfert galaxies that are also radio sources, there is a characteristic maximum in the radiation spectrum atv10¹¹ Hz. We discuss a model in which this feature is regarded as intrinsic to the process of electron production, and in which the radiation is produced by the synchrotron mechanism. A two-component version of the model can be employed in the interpretation of the composite radiation spectra of several sources — 3C 84 (NGC 1275), 3C 120 and 3C 273.     

10–100 keV electron acceleration and emission from solar flares

We present an analysis of spacecraft observations of non-thermal X-rays and escaping electrons for 5 selected small solar flares in 1967. OSO-3 multi-channel energetic X-ray measurements during the non-thermal component of the solar flare X-ray bursts are used to derive the parent electron spectrum and emission measure. IMP-4 and Explorer-35 observations of > 22 keV and > 45 keV electrons in the interplanetary medium after the flares provide a measure of the total number and spectrum of the escaping particles. The ratio of electron energy loss due to collisions with the ambient solar flare gas to the energy loss due to bremsstrahlung is derived. The total energy loss due to collisions is then computed from the integrated bremsstrahlung energy loss during the non-thermal X-ray burst. For > 22 keV flare electrons the total energy loss due to collisions is found to be 10⁴ times greater than the bremsstrahlung energy loss and 10² times greater than the energy loss due to escaping electrons. Therefore the escape of electrons into the interplanetary medium is a negligible energetic electron loss mechanism and cannot be a substantial factor in the observed decay of the non-thermal X-ray burst for these solar flares.We present a picture of electron acceleration, energy loss and escape consistent with previous observations of an inverse relationship between rise and decay times of the non-thermal X-ray burst and X-ray energy. In this picture the acceleration of electrons occurs throughout the 10–100 sec duration of the non-thermal X-ray burst and determines the time profile of the burst. The average energy of the accelerated electrons first rises and then falls through the burst. Collisions with the ambient gas provide the dominant energetic electron loss mechanism with a loss time of 1 sec. This picture is consistent with the ratio of the total number of energetic electrons accelerated in the flare to the maximum instantaneous number of electrons in the flare region. Typical values for the parameters derived from the X-ray and electron observations are: total energy in > 22 keV electrons total energy lost by collisions = 10^28–29 erg, total number of electrons accelerated above 22 keV = 10³⁶, total energy lost by non-thermal bremsstrahlung = 10²⁴erg, total energy lost in escaping > 22 keV electrons = 10²⁶erg, total number of > 22 keV electrons escaping = 10^33–34.The total energy in electrons accelerated above 22 keV is comparable to the energy in the optical or quasi-thermal flare, implying a flare mechanism with particle acceleration as one of the dominant modes of energy dissipation.The overall efficiency for electron escape into the interplanetary medium is 0.1–1% for these flares, and the spectrum of escaping electrons is found to be substantially harder than the X-ray producing electrons.Currently at Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.