Abstract
We present radio observations of two well-known double-double radio galaxies, J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using both the Giant Metrewave Radio ...Telescope and the Very Large Array. These observations, over a large radio frequency range, enable us to determine the spectra of the inner and outer lobes. Our detailed spectral ageing analysis of their inner and outer lobes demonstrates that the outer doubles of double-double radio galaxies are created by the previous cycle of activity, while the inner doubles are due to the present cycle of activity. The (core subtracted) spectra of the inner doubles of both sources are power laws over a large frequency range. We found that the duration of the quiescent phase of J0041+3224 is between 4 and 28 per cent of the active phase of the previous activity. The outer north-western lobe of J1835+6204 has a compact hotspot and the regions of both the outer hotspots have close to power-law (rather than curved) spectra, which indicates that the outer lobes are still fed by jet material ejected in the previous episode just before the central engine stopped powering the jet. We estimate that the duration of the quiescent phase of J1835+6204 is ≲5 per cent of the duration of the active phase of the previous activity. Therefore, we conclude that the duration of the quiescent phase can be as short as a few per cent of the active phase in radio galaxies of this type.
In this paper, we test the possibility that the structure of the largest radio galaxy J1420−0545 may have been formed by restarted rather than primary jet activity. This hypothesis was motivated by ...the unusual morphological properties of the source consisting of two edge-brightened, narrow, highly collinear, and symmetric lobes, thus suggesting an almost ballistic propagation of powerful jets into a particularly low-density external medium. New observations made with the VLA together with the currently available gleam and TGSS ADR1 data releases allow the detection of an excess emission at low frequencies. An extracted part (88 MHz-200 MHz) of the spectrum of the emission is fitted with the dynage model, giving a forecast for the environmental conditions and the energetic requirements for the presumed old cocoon related to a preceding epoch of jet activity.
We present a multiwavelength radio study of the nearby galaxy pair Arp 269 (NGC 4490/85). High sensitivity to extended structures gained by using the merged interferometric and single-dish maps ...allowed us to reveal a previously undiscovered extension of the radio continuum emission. Its direction is significantly different from that of the neutral gas tail, suggesting that different physical processes might be involved in their creation. The population of radio-emitting electrons is generally young, signifying an ongoing, vigorous star formation – this claim is supported by strong magnetic fields (over 20 μG), similar to the ones found in much larger spiral galaxies. From the study of the spectral energy distribution, we conclude that the electron population in the intergalactic bridge between member galaxies originates from the disc areas, and therefore its age (approximately 3.7–16.9 Myr, depending on the model used) reflects the time-scale of the interaction. We have also discovered an angularly near compact steep source – which is a member of a different galaxy pair – at a redshift of approximately 0.125.
The dynamical ages of the opposite lobes of selected giant radio sources are estimated using the DYNAGE algorithm of Machalski et al., and compared with their spectral ages estimated and studied by ...Jamrozy et al. in Paper II. As expected, the DYNAGE fits give slightly different dynamical ages and other model parameters for the opposite lobes modelled independently of each other, e.g. the age ratios are found to be between ∼1.1 and ∼1.4. Demanding similar values of the jet power and the radio core density for the same source, we look for a self-consistent solution for the opposite lobes, which results in different density profiles along them. We also show that a departure from the equipartition conditions assumed in the model, justified by X-ray observations of the lobes of some nearby radio galaxies, and a relevant variation of the magnetic field strengths may provide an equalization of the ages of the lobes. A comparison of the dynamical and spectral ages shows that a ratio of the dynamical age to the spectral age of the lobes of investigated giant radio galaxies is between ∼1 and ∼5, i.e. similar to that found for smaller radio galaxies. To supplement possible causes for this effect already discussed in the literature, such as uncertainty of the assumed parameters of the model, or influence of a possible departure from the energy equipartition assumption, a further two causes are identified and discussed: (i) a difference between the injection spectral indices describing the initial energy distributions of the emitting relativistic particles determined using the DYNAGE algorithm in the dynamical analysis and in the classical spectral-ageing analysis, and (ii) a different influence of the axial ratio of the lobes in the estimation of the dynamical age and the spectral (synchrotron) age. Arguments are given to suggest that DYNAGE can better take account of radiative effects at lower frequencies than the spectral-ageing analysis. The DYNAGE algorithm is especially effective for sources at high redshifts, for which an intrinsic spectral curvature is shifted to low frequencies.
Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) are used to determine the spectral breaks in consecutive strips along the lobes of a sample ...of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sources range from ∼6 to 46 Myr with a median value of ∼23 Myr using the classical equipartition fields. Using the magnetic field estimates from the Beck & Krause formalism the spectral ages range from ∼5 to 58 Myr with a median value of ∼24 Myr. These ages are significantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, confirming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots, as is the case in earlier studies of smaller sources. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from ∼0.55 to 0.88 with a median value of ∼0.6. We discuss these values in the light of theoretical expectations, and show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.
Context. Determining ages of the outer and the inner lobes of so-called double-double radio galaxies (DDRGs) is crucial for understanding the active cycles of galactic nuclei, the phases of ...interruption of the jet flow, and physical conditions in the surrounding galactic and intergalactic medium governing the jets' propagation. A recognition and understanding of these conditions during the restarted jet activity is of special interest. Aims. We determine the ages and other physical characteristics of the outer and the inner lobes of the DDRG J1548-3216, as well as the properties of the surrounding environment during the original and the restarted phase of the jets' activity. Methods. Using the new low-frequency and high-frequency radio images of this galaxy, we determined the shape of the spectrum along its lobes and performed the classical spectral-ageing analysis. On the other hand, we applied the analytical model of the jet's dynamics, which allowed us to derive the physical conditions for the source's evolution during the original jet propagation through the unperturbed IGM, as well as those when the restarted new jet propagates inside the outer cocoon formed by the old jet material that passed through the jet terminal shock. Results. The dynamical age estimate of the outer and the inner lobes is $132\pm28$ Myr and ${\sim}9\pm4$ Myr, respectively. The synchrotron age in the outer lobes systematically rises from ~25 Myr in the vicinity of the lobes' edges to about 65–75 Myr in the centre of the old cocoon. These ages imply an average expansion speed along the jets' axis: ($0.012\pm0.003$)c in the outer lobes and ($0.058\pm 0.025$)c in the inner lobes, but the latter speed would be ~0.25 c when they were of age less than 1 Myr. We find that the jet power during the restarted activity is about ten-fold fainter than that of the original jet. Similar disproportion is found for the internal pressures and the magnetic field strengths in the old cocoon and those in the inner lobes. This disproportion can be effectively reduced by assuming the same equations of state for the emitting particles and the magnetic fields within the old and the new lobes. However, we think that our assumption of the non-relativistic equation of state for the old cocoon and the relativistic one for the new lobes is more justified.
Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large ...scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~ 4″), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~ 30″), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α< − 1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated fluxdensity spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 kms-1), similar to what is found in other clusters. The derived column density is NHI ~ 4 × 1020 cm-2 for a Tspin = 100 K. This detection supports the connection – already suggested for other restarted radio sources – between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.
We use the LOFAR Two-metre Sky Survey (LoTSS) Data Release I to identify the groups of galaxies (and individual galaxies) from the Hickson compact groups (HCG) and magnitude-limited compact groups ...(MLCG) samples that emit at the frequency of 150 MHz, characterise their radio emission (extended or limited to the galaxies), and compare new results to earlier observations and theoretical predictions. The detection of 73 systems (and 7 more – probably) out of 120, of which as many as 17 show the presence of extended radio structures, confirms the previous hypothesis of the common character of the magnetic field inside galaxy groups and its detectability. In order to investigate the future potential of low-frequency radio studies of galaxy groups, we also present a more detailed insight into four radio-emitting systems, for which the strength of the magnetic field inside their intergalactic medium (IGM) is calculated. The estimated values are comparable to that found inside star-forming galaxies, suggesting a dynamical and evolutionary importance of the magnetic field in galaxy groups.
We present low-frequency observations with the Giant Metrewave Radio Telescope of a sample of giant radio sources, and high-frequency observations of three of these sources with the Very Large Array. ...From multifrequency observations of the lobes, we estimate the magnetic field strengths using three different approaches, and show that these differ at most by a factor of ∼3. For these large radio sources, the inverse-Compton losses usually dominate over synchrotron losses when estimates of the classical minimum energy magnetic field are used, consistent with earlier studies. However, this is often not true if the magnetic fields are close to the values estimated using the formalism of Beck & Krause. We also examine the spectral indices of the cores and any evidence of recurrent activity in these sources. We probe the environment using the symmetry parameters of these sources and suggest that their environments are often asymmetric on scales of ∼1 Mpc, consistent with earlier studies.
PDF
