K.-H. Mack

Multi-frequency study of the B3-VLA sample III. Polarisation properties

This paper is the third of a series, and presents the results of the linear polarisation of the B3-VLA sample, at 1.4, 2.7, 4.8 and 10.5 GHz. We find that flat-spectrum sources are significantly less polarised than the steep ones at 10.5 GHz. A trend is seen for sources with larger linear size to be more strongly polarised. Compact steep-spectrum sources (CSSs) exhibit much stronger depolarisation than non-CSSs. Flat-spectrum sources are characterized by almost constant, and low, degrees of polarisation over the whole wavelength range studied here. We add 143 new Rotation Measures to the available database published by Wielebinski & Krause (1993). An important result is that the decrease of the degree of polarisation is almost linear with wavelength and does not drop to zero at lower frequencies. This behaviour cannot be fitted by any existing model of external depolarisation or intrinsic Faraday dispersion. We consider an intrinsic process producing the observed depolarisation, involving three source components with different emissivities and Faraday depths, while the Faraday rotation must arise from a foreground screen, most likely Galactic in origin.

Particle Accelerators in the Hot Spots of Radio Galaxy 3C 445, Imaged with the VLT

Hot spots (HSs) are regions of enhanced radio emission produced by supersonic jets at the tip of the radio lobes of powerful radio sources. Obtained with the Very Large Telescope (VLT), images of the HSs in the radio galaxy 3C 445 show bright knots embedded in diffuse optical emission distributed along the post-shock region created by the impact of the jet into the intergalactic medium. The observations reported here confirm that relativistic electrons are accelerated by Fermi-I acceleration processes in HSs. Furthermore, both the diffuse emission tracing the rims of the front shock and the multiple knots demonstrate the presence of additional continuous re-acceleration processes of electrons (Fermi-II).

Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies

Sensitive images of low-level, megaparsec-sized radio cocoons offer new opportunities to probe large-scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 reveal significant asymmetries and particularities in the morphology, radio spectrum, and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315s vicinity confirms its location to be near, or at, a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a weather station, probing the flow of the elusive and previously invisible intergalactic medium gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large-scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma rays and ultrahigh-energy cosmic rays is briefly discussed.

Ageing analysis of the giant radio galaxy J1343+3758

Deep 4860 and 8350 MHz observations with the VLA and 100-m Effelsberg telescopes, supplementing available radio survey maps at the frequencies of 327 MHz (WENSS survey) and 1400 MHz (NVSS survey), are used to study the syn- chrotron spectra and radiative ages of relativistic particles in opposite lobes of the giant radio galaxy J1343+3758 (Machalski & Jamrozy 2000). The classical spectral ageing analysis (e.g. Myers & Spangler 1985) with assumption of equipartition magnetic fields gives a mean separation velocity (� vsep� ) of about 0.16 c and 0.12 c measured with respect to the emitting plasma, and suggests a maximum particle age of about 48 and 50 Myr in the NE and SW lobes, respectively. On the contrary, a mean jet- head advance speed (� vadv� ) in the above lobes, derived from ram-pressure arguments, is about 0.016 c and 0.017 c, respectively. This would imply a substantial backflow of the lobe material from the hotspot regions towards the radio core, vbf, which is not supported by the available radio maps. A compromise is achieved by assuming an enhancement of 3 to 5 times the equipartition magnetic field strengths in the lobes which givesvsep �≈ 0.06 c andvadv �≈� vbf �≈ 0.03 c, hence a dynamical age of the source of 204 ± 40 Myr. A comparison of the radiative and dynamical ages of the investigated giant radio galaxy implies that the dynamical age is about 4 times the maximum synchrotron age of the emitting particles, which supports the conclusion of Blundell & Rawlings (2000) that the spectral and dynamical ages are comparable only when they are � 10 Myr, and suggests that for FRII-type sources the discrepancy between these ages increases with age.

DECLINE OF THE SPACE DENSITY OF QUASARS BETWEEN z = 2 AND z =4

We define a new complete sample of 13 optically luminous radio quasars [MAB(1450 A) 25.7] with redshift 3.8 < z < 4.5, obtained by cross-correlating the Faint Images of the Radio Sky at Twenty cm (FIRST) radio survey and the Automatic Plate Measuring Facility catalog of POSS I. We measure the space density to be 1.0 ± 0.3 Gpc-3, a factor of 1.9 ± 0.7 smaller than the space density of similar quasars at z ≈ 2 (FIRST Bright Quasar Survey). Using a new measurement of the radio-loud fraction of quasars, we find that at z = 4 the total space density of quasars with MAB(1450 A) < -26.9 mag is 7.4 ± 2.6 Gpc-3. This is a factor of 1.8 ± 0.8 less than the space density at z ≈ 2, found by the Two-Degree Field quasar survey. This z = 2/z = 4 ratio, consistent with that of the radio-loud quasars, is significantly different from the ratio of ~10 found for samples including lower luminosity quasars. This suggests that the decline of the space density beyond z ≈ 2 is slower for optically luminous quasars than for less luminous ones.

Unusual high-redshift radio broad absorption-line quasar 1624+3758

We present observations of the most radio-luminous broad absorption-line (BAL) quasar known, 1624+3758, at redshift z = 3.377. The quasar has several unusual properties. (1) The Fe II UV191 1787-A emission line is very prominent. (2) The BAL trough (BALnicity index 2990 km s −1 )i sdetached by 21 000 km s −1 and extends to velocity v =− 29 000 km s −1 . There are additional intrinsic absorbers at −1900 and −2800 km s −1 . (3) The radio rotation measure of the quasar, 18 350 rad m −2 ,i sthe second highest known. The radio luminosity is P 1.4 GHz = 4.3 × 10 27 WH z −1 (H 0 = 50 km s −1 Mpc −1 , q 0 = 0.5) and the radio loudness is R ∗ = 260. The radio source is compact ( ∼ 2.8 kpc) and the radio spectrum is GHz-peaked, consistent with it being relatively young. The width of the C IV emission line, in conjunction with the total optical luminosity, implies a black hole mass M BH ∼ 10 9 M� , L/L Eddington ≈ 2. The high Eddington ratio and the radio-loudness place this quasar in one corner of Boroson’s two-component scheme for the classification of active galactic nuclei, implying a very high accretion rate, and this may account for some of the unusual observed properties. The v = −1900 km s −1 absorber is a possible Lyman-limit system, with N (H I) = 4 × 10 18 cm −2 , and a covering factor of 0.7. A complex mini-BAL absorber at v =− 2200 to −3400 km s −1 is detected in each of C IV ,N V and O VI. The blue and red components of the C IV doublet happen to be unblended, allowing both the covering factor and optical depth to be determined as a function of velocity. Variation of the covering factor with velocity dominates the form of the mini-BAL, with the absorption being saturated (e −τ ≈ 0) over most of the velocity range. The velocity dependence of the covering factor and the large velocity width imply that the mini-BAL is intrinsic to the quasar. There is some evidence of line-locking between velocity components in the C IV mini-BAL, suggesting that radiation pressure plays a role in accelerating the outflow. Ke yw ords: galaxies: high-redshift ‐ intergalactic medium ‐ quasars: absorption lines ‐ quasars: emission lines ‐ quasars: general ‐ early Universe.

Synchrotron spectra and ages of compact steep spectrum radio sources

The high-frequency integrated spectra of Compact Steep Spectrum (CSS) sources show breaks with a moderate spectral steepening well fitted by continuous injection syn- chrotron spectra. In lobe-dominated CSS sources the radiative ages deduced by the synchrotron theory are in the range of up to 10 5 years, if equipartition magnetic fields are assumed. These radiative ages are well correlated with the source size indicat- ing that the CSS sources are young . In order to maintain the frustration scenario, in which the sources lifetimes are 10 7 years, their equipartition magnetic field would be systemati- cally decreased by a factor > 20. To complete the sample used in this work, we conducted observations at 230 GHz with the IRAM 30-m telescope of those sources which did not have such high-frequency observations up to now.

A sample of radio-loud QSOs at redshift ∼ 4

We obtained spectra of 60 red, star-like objects (E 1 mJy. Eight are quasi-stellar objects (QSOs) with redshift z > 3.6. Combined with our earlier pilot search, our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8 per cent of candidates with S1.4 GHz 10 mJy are QSOs with z > 3.6. The surface density of E 1 mJy, z > 4 QSOs is 0.003 deg −2 . This is currently the only well-defined sample of radio-loud QSOs at z ≈ 4 selected independently of radio spectral index. The QSOs are highly luminous in the optical (eight have MB < −28, q0 = 0.5, H 0 = 50 km s −1 Mpc −1 ). The SEDs are as varied as those seen in optical searches for high-redshift QSOs, but the fraction of objects with weak (strongly self-absorbed) Lyα emission is marginally higher (3 out of 18) than for high-redshift QSOs from SDSS (5 out of 96).

Spectral ageing in the relic radio galaxy B2 0924+30

The radio spectrum of the relic radio galaxy B2 0924+30 is investigated using integrated flux densities between 0.151 and 10.6 GHz, as well as images at 0.325, 0.609, 1.400 and 4.750 GHz. Allowing for synchrotron and inverse Compton losses in this source, the observed spectral steepening yields a break frequency of ∼7 GHz, implying a mean particle age of about 50 Myrs. The corresponding particle advance speed is of the order of 2000 km s −1 . This relatively small particle lifetime following the exhaustion of the central source can explain the paucity of such sources and implies that such objects would easily escape detection if searched for in the cm wavelength regime.

Multi-frequency study of the B3 VLA sample. IV. 74-MHz flux densities from VLA A-array data

We present 74-MHz data of 365 B3 VLA sources, which were obtained as a by-product of the observations of Tschager et al. (2003, A&A, 402, 171) who observed a large area of sky with the VLA in A-array. Apart from the lowest- frequency observations of B3 VLA sources performed so far contributing to our on-going multi-frequency study of this survey, these A-array data also provide the first morphological information at this low frequency. The most intriguing result is the discovery of two new Giant Radio Galaxy candidates, B3 1232+397B at a redshift of 3.22 and B3 1419+419 at a redshift of 0.367. This means that B3 1232+397B would be the most distant GRG known to date. At the same time, these two objects are two new examples of sources with recurrent activity. Our results give a foretaste of the impact which extremely low-frequency radio continuum observations will have for the study of source evolution.