Pair Of Radio Lobes Research Articles

Understanding the origin of radio outflows in Seyfert galaxies using radio polarimetry

AbstractThe role of starburst winds versus active galactic nuclei (AGN) jets/winds in the formation of the kiloparsec scale radio emission seen in Seyferts is not yet well understood. In order to be able to disentangle the role of various components, we have observed a sample of Seyfert galaxies exhibiting kpc-scale radio emission suggesting outflows, along with a comparison sample of starburst galaxies, with the EVLA B-array in polarimetric mode at 1.4 GHz and 5 GHz. The Seyfert galaxy NGC 2639, shows highly polarized secondary radio lobes, not observed before, which are aligned perpendicular to the known pair of radio lobes. The additional pair of lobes represent an older epoch of emission. A multi-epoch multi-frequency study of the starburst-Seyfert composite galaxy NGC 3079, reveals that the jet together with the starburst superwind and the galactic magnetic fields might be responsible for the well-known 8-shaped radio lobes observed in this galaxy. We find that many of the Seyfert galaxies in our sample show bubble-shaped lobes, which are absent in the starburst galaxies that do not host an AGN.

LoTSS DR1: Double-double radio galaxies in the HETDEX field

Context. Double-double radio galaxies (DDRGs) represent a short but unique phase in the life-cycle of some of the most powerful radio-loud active galactic nuclei (RLAGN). These galaxies display large-scale remnant radio plasma in the intergalactic medium left behind by a past episode of active galactic nuclei (AGN) activity, and meanwhile, the radio jets have restarted in a new episode. The knowledge of what causes the jets to switch off and restart is crucial to our understanding of galaxy evolution, while it is important to know if DDRGs form a host galaxy dichotomy relative to RLAGN. Aims. The sensitivity and field of view of LOFAR enables the observation of DDRGs on a population basis rather than single-source observations. Using statistical comparisons with a control sample of RLAGN, we may obtain insights into the nature of DDRGs in the context of their host galaxies, where physical differences in their hosts compared to RLAGN as a population may allow us to infer the conditions that drive restarting jets. Methods. We utilised the LOFAR Two-Metre Sky Survey (LoTSS) DR1, using a visual identification method to compile a sample of morphologically selected candidate DDRGs, showing two pairs of radio lobes. To confirm the restarted nature in each of the candidate sources, we obtained follow-up observations with the Karl. G. Jansky Very Large Array (VLA) at higher resolution to observe the inner lobes or restarted jets, the confirmation of which created a robust sample of 33 DDRGs. We created a comparison sample of 777 RLAGN, matching the luminosity distribution of the DDRG sample, and compared the optical and infrared magnitudes and colours of their host galaxies. Results. We find that there is no statistically significant difference in the brightness of the host galaxies between double-doubles and single-cycle RLAGN. The DDRG and RLAGN samples also have similar distributions in WISE mid-infrared colours, indicating similar ages of stellar populations and dust levels in the hosts of DDRGs. We conclude that DDRGs and “normal” RLAGN are hosted by galaxies of the same type, and that DDRG activity is simply a normal part of the life cycle of RLAGN. Restarted jets, particularly for the class of low-excitation radio galaxies, rather than being a product of a particular event in the life of a host galaxy, must instead be caused by smaller scale changes, such as in the accretion system surrounding the black hole.

Discovery of an unusual radio galaxy with a hybrid double-double and X-shaped morphology

AbstractWe report the discovery of the first ‘X’-shaped double-double radio galaxy (DDRG), MRC0929+164 (J0932+1611), from FIRST radio survey. The intersection angle of the lines of two pairs of radio lobes is around 20.6° for this object, much larger than other DDRGs. This unusual morphology may be a hint that the growth of its central super-massive black hole suffered a violent galaxy merger once.

CGCG 292-057: A Near-Distance Merger Galaxy with Double–Double Radio Lobes and X-shape Structure

J1159+5820 is an extended radio galaxy with a quite unusual morphology, featuring two pairs of radio lobes. Such sources, called double–double radio galaxies, constitute a very rare class of extragalactic radio sources. Furthermore, the extended radio structure of this source shows an X-shape form. According to a much likely scenario, such a morphology is due to interrupting nuclear activity in its central active galactic nucleus. Interestingly, the host of this source is a near-distance bright galaxy named CGCG 292-057, which is clearly disturbed, with tidal features and shells as plausible signs of a recent merger.

4C 02.27: a quasar with episodic activity?

Abstract Striking examples of episodic activity in active galactic nuclei are the double–double radio galaxies (DDRGs) with two pairs of oppositely directed radio lobes from two different cycles of activity. Although there are over about a dozen good examples of DDRGs, so far no case of one associated with a quasar has been reported. We present Giant Metrewave Radio Telescope observations of a candidate double–double radio quasar, J0935+0204 (4C 02.27), and suggest that radio jets in this source may also have been intrinsically asymmetric, contributing to the large observed asymmetries in the flux density and location of both pairs of radio lobes.

Dressing a Naked Quasar: Star Formation and Active Galactic Nucleus Feedback in HE 0450−2958

We present Australia Telescope Compact Array radio continuum observations of the quasar/galaxy system HE 0450-2958. An asymmetric triple linear morphology is observed, with the central radio component coincident with the quasar core and a second radio component associated with a companion galaxy at a projected distance of 7 kpc from the quasar. The system obeys the far-infrared to radio continuum correlation, implying that the radio emission is energetically dominated by star formation activity. However, there is undoubtedly some contribution to the overall radio emission from a low-luminosity active galactic nucleus (AGN) core and a pair of radio lobes. Long-baseline radio interferometric observations of the quasar core place a 3 ? upper limit of 0.6 mJy at 1400 MHz on the AGN contribution to the quasar's radio emission: less than 30% of the total. The remaining 70% of the radio emission from the quasar is associated with star formation activity and provides the first direct evidence of the quasar's host galaxy. A reanlaysis of the Very Large Telescope (VLT) spectroscopic data shows extended emission-line regions aligned with the radio axis and extended on scales of ~20 kpc. This is interpreted as evidence for jet-cloud interactions, similar to those observed in radio galaxies and Seyferts. The emission lines in the companion galaxy are consistent with radiative shocks, and its spatial association with the eastern radio lobe implies that large-scale jet-induced star formation has played a role in this galaxy's evolution.

The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes

Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes that cannot be naturally explained by either the galactic merger or radio-lobe backflow scenarios, the two dominant models for these X-shaped radio sources. We show that the merger picture can explain these morphologies provided one takes into account that, prior to the coalescence of their supermassive black holes, the smaller galaxy releases significant amounts of gas into the interstellar medium of the dominant active galaxy. This rotating gas, whose angular momentum axis will typically not be aligned with the original jets, is likely to provide sufficient ram pressure at a distance ~10 kpc from the nucleus to bend the extant jets emerging from the central engine, thus producing a Z-symmetry in the pair of radio lobes. Once the two black holes have coalesced some 107 yr later, a rapid reorientation of the jets along a direction close to that of the orbital angular momentum of the swallowed galaxy relative to the primary galaxy would create the younger primary lobes of the X-shaped radio galaxy. This picture naturally explains why such sources typically have powers close to the FR I/II break. We suggest that purely Z-symmetric radio sources are often en route to coalescence and the concomitant emission of substantial gravitational radiation, while X-shaped ones have already merged and radiated.

Radio Galaxies Return From the Dead

ASTRONOMYEven for deep-space objects, radio galaxies are odd beasts--so odd that scientists have trouble explaining why they exist at all. Now astronomers in the Netherlands have deepened the mystery by discovering that some radio galaxies live twice. In the 21 June issue of the Monthly Notices of the Royal Astronomical Society , they report finding what they call double-double radio galaxies--classic double-lobed galaxies sporting a second, older pair of radio lobes much farther out than the first.

Radio galaxies with a 'double-double morphology'- I. Analysis of the radio properties and evidence for interrupted activity in active galactic nuclei

We present four Mpc-sized radio galaxies which consist of a pair of double-lobed radio sources, aligned along the same axis, and with a coinciding radio core. We call these peculiar radio sources 'double-double' radio galaxies (DDRGs) and propose a general definition of such sources: a 'double-double' radio galaxy consists of a pair of double radio sources with a common centre. Furthermore, the two lobes of the inner radio source must have a clearly extended, edge-brightened radio morphology. Adopting this definition, we find several other candidate DDRGs in the literature. We find that in all sources the smaller (inner) pair of radio lobes is less luminous than the larger (outer) pair, and that the ratio of 1.4-GHz flux density of these two pairs appears to be anticorrelated with the projected linear size of the inner source. Also, the outer radio structures are large, exceeding 700 kpc. We discuss possible formation scenarios of the DDRGs, and we conclude that an interruption of the jet-forming central activity is the most likely mechanism. For one of our sources (B 1834+620) we have been able observationally to constrain the length of time of the interruption to a few Myr. We discuss several scenarios for the cause of the interruption, and suggest multiple encounters between interacting galaxies as a possibility. Finally, we discuss whether such interruptions help the formation of extremely large radio sources.

High frequency radio observations of the nucleus of Cen A

Cen A (NGC5128) is a giant radio galaxy that in several ways behaves like a weak active galactic nucleus. It presents three pairs of radio lobes and a compact core with a one sided jet (Jauncey et al. 1995). The compact core is highly variable in wavelengths ranging from the radio region to X and γ rays (Kinzer et al. 1995). In the radio domain, the emission from the core dominates at high frequencies and a considerable effort was made to study variability at 22 and 43 GHz for more than ten years (Botti & Abraham 1995).