Radio Galaxy B2 Research Articles

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in the Radio Galaxy B2 0902+34 at Redshift 3.4

Using the Karl G. Jansky Very Large Array, we have detected absorption lines due to carbon monoxide, CO(J = 0 → 1), and the cyano radical, CN(N = 0 → 1), associated with radio galaxy B2 0902+34 at redshift z = 3.4. The detection of millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates studying molecular clouds down to gas masses inaccessible to emission-line observations. The CO absorption in B2 0902+34 has a peak optical depth of τ ≥ 8.6% and consists of two components, one of which has the same redshift as previously detected 21 cm absorption of neutral hydrogen (H i) gas. Each CO component traces an integrated H2 column density of NH2 ≳ 3 × 1020 cm−2. CN absorption is detected for both CO components, as well as for a blueshifted component not detected in CO, with CO/CN line ratios ranging from ≲0.4 to 2.4. We discuss the scenario that the absorption components originate from collections of small and dense molecular clouds that are embedded in a region with more diffuse gas and high turbulence, possibly within the influence of the central active galactic nucleus or a starburst region. The degree of reddening in B2 0902+34, with rest-frame color B − K ∼ 4.2, is lower than the very red colors (B − K > 6) found among other known redshifted CO absorption systems at z < 1. Nevertheless, when including the many nondetections from the literature, a potential correlation between the absorption-line strength and B − K color is evident, giving weight to the argument that the red colors of CO absorbers are due to a high dust content.

The Multiwavelength Environment of Second Bologna Catalog Sources

We present the first results of the Chandra Cool Targets (CCT) survey of the Second Bologna Catalog (B2CAT) of powerful radio sources, aimed at investigating the extended X-ray emission surrounding these sources. For the first 33 sources observed in the B2CAT CCT survey, we performed both imaging and spectral X-ray analysis, producing multiband Chandra images, and compared these images with radio observations. To evaluate the presence of extended emission in the X-rays, we extracted surface flux profiles comparing them with simulated ACIS point-spread functions. We detected X-ray nuclear emission for 28 sources. In addition, we detected eight regions of increased X-ray flux originating from radio hot spots or jet knots, and a region of decreased flux, possibly associated with an X-ray cavity. We performed X-ray spectral analysis for 15 nuclei and found intrinsic absorption significantly larger than the Galactic values in four of them. We detected significant extended X-ray emission in five sources, and fitted their spectra with thermal models with gas temperatures ∼2 keV. In the case of B2.1 0742+31, the surrounding hot gas is compatible with the intracluster medium of low-luminosity clusters of galaxies, while the X-ray diffuse emission surrounding the highly disturbed wide-angle-tailed radio galaxy B2.3 2254+35 features a luminosity similar to those of relatively bright galaxy groups, although its temperature is similar to those of low-luminosity galaxy clusters. These results highlight the power of low-frequency radio selection, combined with short Chandra snapshot observations, for investigating the properties of X-ray emission from radio sources.

Duty cycle of the radio galaxy B2 0258+35

Context. Radio loud active galactic nuclei (AGN) are episodic in nature, cycling through periods of activity and quiescence. The study of this duty cycle has recently gained new relevance because of the importance of AGN feedback for galaxy evolution.Aims. In this work we investigate the duty cycle of the radio galaxy B2 0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio source consists of a pair of kpc-scale jets embedded in two large-scale lobes (∼240 kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity.Methods. We have combined new LOFAR data at 145 MHz and new Sardinia Radio Telescope data at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral properties of the outer lobes and derive their age.Results. Interestingly, the spectrum of both the outer northern and southern lobes is not ultra-steep as expected for an old ageing plasma with spectral index values equal to α1451400= 0.48 ± 0.11 and α14006600= 0.69 ± 0.20 in the outer northern lobe, and α1451400= 0.73 ± 0.07 in the outer southern lobe. Moreover, despite the wide frequency coverage available for the outer northern lobe (145–6600 MHz), we do not identify a significant spectral curvature (SPC ≃ 0.2 ± 0.2).Conclusions. While mechanisms such as in-situ particle reacceleration, mixing or compression can temporarily play a role in preventing the spectrum from steepening, in no case seem the outer lobes to be compatible with being very old remnants of past activity as previously suggested (with age ≳ 80 Myr). We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago, allowing us to observe both the inner and outer structure simultaneously. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.

Recurrent radio emission and gas supply: the radio galaxy B2 0258+35

Outlined is the discovery of a very faint, diffuse, low surface-brightness (0.5 \mJybeam, 1.4 \mJyarcminsq on average) structure around the radio source B2 0258+35 hosted by an HI-rich early-type galaxy (NGC 1167). Since B2 0258+35 is a young Compact Steep Spectrum (CSS) source, the newly discovered structure could represent a remnant from an earlier stage of AGN activity. We go on by explaining in detail all the possibilities for triggering the radio activity in B2 0258+35 regarding gas accretion in a recurrent AGN activity framework. NGC 1167 hosts a very regular, extended and massive \HI\ disc that has been studied in great detail. Previous studies of the \HI\ closer to the core seem to go against the assumption of a circum-nuclear disc of \HI\ as the source of the accreting gas. We consider the cooling of gas from the hot, X-ray halo as a possible alternative option for the fueling of the AGN, as suggested in the case of other sources of similar radio power as B2 0258+35. Estimates are given for the age of the faint diffuse emission as well as for the current accretion rate, which are in good agreement with literature values. If our assumptions about the accretion mechanism are correct, similar large-scale, relic-like structures should be more commonly found around early-type galaxies and this will be hopefully confirmed by the next generation of sensitive, low-frequency radio surveys.

The magnetized medium around the radio galaxy B2 0755+37: an interaction with the intragroup gas

We explore the magneto-ionic environment of the isolated radio galaxy B2 0755+37 using detailed imaging of the distributions of Faraday rotation and depolarization over the radio source from Very Large Array observations at 1385,1465 and 4860 MHz and new X-ray data from XMM-Newton. The Rotation Measure (RM) distribution is complex, with evidence for anisotropic fluctuations in two regions. The approaching lobe shows low and uniform RM in an unusual `stripe' along an extension of the jet axis and a linear gradient transverse to this axis over its Northern half. The leading edge of the receding lobe shows arc-like RM structures with sign reversals. Elsewhere, the RM structures are reasonably isotropic. The RM power spectra are well described by cut-off power laws with slopes ranging from 2.1 to 3.2 in different sub-regions. The corresponding magnetic-field autocorrelation lengths, where well-determined, range from 0.25 to 1.4 kpc. It is likely that the fluctuations are mostly produced by compressed gas and field around the leading edges of the lobes. We identify areas of high depolarization around the jets and inner lobes. These could be produced by dense gas immediately surrounding the radio emission containing a magnetic field which is tangled on small scales. We also identify four ways in which the well known depolarization (Faraday depth) asymmetry between jetted and counter-jetted lobes of extended radio sources can be modified by interactions with the surrounding medium.

The disc-dominated host galaxy of FR-I radio source B2 0722+30

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper we use HI observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the Active Galactic Nucleus and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [OIII] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely HI-rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio-AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.

Shock heating in the group atmosphere of the radio galaxy B2 0838+32A

We present Chandra and radio observations, and analysis of Sloan Digital Sky Survey data, of the radio galaxy B2 0838+32A (4C 32.26) and its environment. The radio galaxy is at the centre of a nearby group that has often been identified wit h the cluster Abell 695, but we argue that the original Abell cluster is likely to be an unr elated and considerably more distant system. The radio source is a restarting radio galax y and, using our Chandra data, we argue that the currently active lobes are expanding supersonically, driving a shock with Mach number2.4 +1.0 0.5 into the inter-stellar medium. This would be only the third strong shock round a young radio source to be discovered, after Centaurus A and NGC 3801. However, in contrast to both these systems, the host galaxy of B2 0838+32A shows no evidence for a recent merger, while the AGN spectrum shows no evidence for the dusty torus that would imply a large reservoir of cold gas close to the central black hole. On the contrary, the AGN spectrum is of a type that has been associated with the presence of a radiatively inefficient accretion flow that could be controlled by AGN heating and sub sequent cooling of the hot, X-ray emitting gas. If correct, this means that B2 0838+32A is the first source in which we can directly see entropy-increasing processes (shocks) dr iven by accretion from the hot phase of the interstellar medium.

From major merger to radio galaxy: low surface-brightness stellar counterpart to the giant H I ring around B2 0648+27

We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale HI ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like HI structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation in the HI ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.

Timescales of merger, starburst and AGN activity in radio galaxy B2 0648+27

In this paper we use neutral hydrogen HI and optical spectroscopic observations to compare the timescales of a merger event, starburst episode and radio-AGN activity in the radio galaxy B2 0648+27. We detect a large ring-like structure of HI in emission around the early-type host galaxy of B2 0648+27 (M_HI = 8.5 x 10^9 Msun, diameter = 190 kpc). We interpret this as the result of a major merger that occurred > 1.5 Gyr ago. From modelling optical long-slit spectra we find that a young stellar population of 0.3 Gyr, indicative of a past starburst event, dominates the stellar light throughout the galaxy. The off-set in time between the merger event and the starburst activity in B2 0648+27 suggests that the starburst was triggered in an advanced stage of the merger, which can be explained if the gas-rich progenitor galaxies contained a bulge. Although the exact age of the radio source remains uncertain, there appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity. We also observe an outflow of emission-line gas in this system, which is likely related to superwinds driven by the stars that formed during the starburst event. We argue that the radio galaxy B2 0648+27 is a link in the evolutionary sequence between Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-type galaxies.

Merger origin of radio galaxies investigated with HI observations

AbstractWe present results of an HI study of a complete sample of nearby radio galaxies. Our goal is to investigate whether merger or interaction events could be at the origin of the radio‐AGN activity. Around five of our radio galaxies, hosted mainly by early‐type galaxies, we detect extended HI in emission. In most cases this HI is distributed in large (up to 190 kpc) and massive (up to M HI ∼ 1010 M⊙) disk‐ or ring‐like structures, that show fairly regular rotation around the host galaxy. This suggests that in these systems a major merger likely occurred, but at least several Gyr ago. For the HI‐rich radio galaxy B2 0648+27 we confirm such a merger origin through the detection of a post‐starburst stellar population that dominates the visible light throughout this system. The timescale of the current episode of radio‐AGN activity in our HI‐rich radio galaxies is several orders of magnitude smaller than the merger timescales. Therefore the radio‐AGN activity either started late in the lifetime of the merger event, or is not directly related to the merger event at all. Another intriguing result is that the HI‐rich (&gt;109 M⊙) radio galaxies in our sample all have compact radio sources, while none of the extended radio sources contain these amounts of extended H I. This strongly suggests that there is a relation between the size of the radio jet and the presence of large amounts of neutral gas associated with the host galaxy. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The extremely asymmetric radio structure of the z = 3.1 radio galaxy B3 J2330 + 3927

Abstract We report on 1.7- and 5.0-GHz observations of the z= 3.087 radio galaxy B3 J2330 + 3927, using the Very Long Baseline Array (VLBA), and archival 1.4- and 8.4-GHz Very Large Array (VLA) data. Our VLBA data identify a compact, flat-spectrum (α5 GHz1.7 GHz=−0.2 ± 0.1; Sν∝να) radio component as the core. The VLA images show that the fraction of core emission is very large (fc≈ 0.5 at 8.4 GHz), and reveal a previously undetected, very faint counterjet, implying a radio lobe flux density ratio R≳ 11 and a radio lobe distance ratio Q≈ 1.9. Those values are much more common in quasars than in radio galaxies, but the optical/near-infrared spectra show a clear type II active galactic nucleus for B3 J2330 + 3927, confirming that it is indeed a radio galaxy. Unlike all other radio galaxies, the bright Lyα-emitting gas is located towards the furthest radio arm. We argue against environmental and relativistic beaming effects being the cause of the observed asymmetry, and suggest this source has intrinsically asymmetric radio jets. If this is the case, B3 J2330 + 3927 is the first example of such a source at high redshift, and seems to be difficult to reconcile with the unified model, which explains the differences between quasars and radio galaxies as being due to orientation effects.

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.

Relativistic models of two low-luminosity radio jets: B2 0326+39and B2 1553+24

We apply the intrinsically symmetrical, decelerating relativistic jet model developed by Laing & Bridle for 3C 31 to deep, full-synthesis 8.4-GHz VLA imaging of the two low-luminosity radio galaxies B2 0326+39 and B2 1553+24. After some modifications to the functional forms used to describe the geometry, velocity, emissivity and magnetic-field structure, these models can accurately fit our data in both total intensity and linear polarization. We conclude that the jets in B2 0326+39 and B2 1553+24 are at angles of 64° ± 5° and 7.°7 ± 1.°3 to the line of sight, respectively. In both objects, we find that the jets decelerate from 0.7-0.8c to < 0.2c over a distance of approximately 10 kpc, although in B2 1553+24 this transition occurs much further from the nucleus than in B2 0326+39 or 3C 31. The longitudinal emissivity profiles can be divided into sections, each fitted accurately by a power law; the indices of these power laws decrease with distance from the nucleus. B2 0326+39 also requires a discontinuity in emissivity to in order to fit a region with several bright knots of emission. In B2 1553+24, the sudden brightening of the jet can be explained by a combination of rapid expansion of the jet and a continuous variation of emissivity. The magnetic fields in both objects are dominated by the longitudinal component in the high-velocity regions close to the nucleus and by the toroidal component further out, but B2 0326+39 also has a significant radial component at large distances, whereas B2 1553+24 does not. Simple adiabatic models fail to fit the emissivity variations in the regions of high velocity but provide good descriptions of the emissivity after the jets have decelerated. Given the small angle to the line of sight inferred for B2 1553+24, there should be a significant population of similar sources at less extreme orientations. Such objects should have long (?200 kpc), straight, faint jets and we show that their true sizes are likely to have been underestimated in existing images.

Associated HI absorption in thez = 3.4 radio galaxy B2 0902 + 343 observed with the GMRT

Associated HI absorption in thez = 3.4 radio galaxy B2 0902 + 343 observed with the GMRT

Two new optical jets in B2 radio galaxies

We present HST observations of previously undetected optical jets in the low-luminosity radio galaxies B2 0755+37 and B2 1553+24. We show that there is accurate spatial coincidence between optical and radio emission, implying that the former is likely to be synchrotron radiation. The physical properties of the jets are similar to those known previously: their radio-optical spectral indices are ≈0.7, and in B2 0755+37 the spectrum steepens between optical and X-ray wavelengths. Our results support the hypothesis that optical emission is detectable from jets orientated within ≈20° of the line of sight for the B2 sample.

CO emission and associated H I absorption from a massive gas reservoir surrounding thez= 3 radio galaxy B3 J2330+3927

We present results of a comprehensive multi-frequency study of the radio galaxy B3 J2330+3927. The 1. �� 9 wide

B2 0648+27: A radio galaxy in a major merger

We present WSRT observations of the neutral hydrogen in the nearby radio galaxy B2 0648+27. In emission, we detect a very large amount of $\ion{H}{I}$ ($M_{\ion{H}{I}} = 1.1 \times 10^{10}\,M_\odot$) that is distributed in a very extended disk, or ring-like structure, of about 160 kpc in size. We also detect $\ion{H}{I}$ absorption against the central radio continuum component. The detection of the $\ion{H}{I}$, its structure and kinematics, give us key information for building a possible evolutionary scenario. The characteristics of the detected $\ion{H}{I}$ are explained as the result of a major merger event that is likely to have occurred ${\la} 10^9$ yr ago. Interestingly, we find that, when observed in radio continuum at higher resolution, this galaxy has a double lobed, steep spectrum structure of about 1 kpc in size. Thus, despite its low radio power, B2 0648+27 bears striking similarities with Compact Symmetric Objects, i.e. objects believed to represent the early phase of radio galaxies ($\la$few thousand yrs old). B2 0648+27 is one of the few nearby radio galaxies where extended neutral hydrogen has been detected so far. This, and other recent results, appear however to indicate that nearby radio galaxies are more often gas rich than commonly assumed. The phenomena described are likely to be much more common at high redshift and galaxies like B2 0648+27 may provide valuable information on the evolution of high redshift radio sources.

The HST survey of the B2 sample of radio galaxies: Detection of two optical jets

We present HST observations of previously undetected optical jets in the low-luminosity radio galaxies B2 0755+37 and B2 1553+24. We show that there is accurate spatial coincidence between optical and radio emission, implying that the former is likely to be synchrotron radiation. The physical properties of the jets are similar to those known previously: their radio-optical spectral indices are ~0.7 and in B2 0755+37, the spectrum steepens between optical and X-ray wavelengths. Our results support the hypothesis that optical emission is detectable from jets orientated within ~20 degrees of the line of sight for the B2 sample.

A Chandra observation of the distant radio galaxy B2 0902+343: a powerful obscured active galaxy

We report a Chandra observation of the z = 3.395 radio galaxy B2 0902+343. The unresolved X-ray source is centred on the active nucleus. The spectrum is well fitted by a flat power law of photon index of Γ ∼ 1.1 with intrinsic absorption of 8 X 10 2 2 cm - 2 , and an intrinsic 2-10 keV luminosity of 3.3 X 10 4 5 erg s - 1 . More complex models that allow for a steeper spectral index cause the column density and intrinsic luminosity to increase. The data limit any thermal luminosity of the hot magnetized medium, assumed responsible for high Faraday rotation measures seen in the radio source, to less than ∼10 4 5 erg s - 1 1 .

B3 0003+387: AGN-Marked Large-Scale Structure at Redshift 1.47?

We present evidence for a significant overdensity of red galaxies, as much as a factor of 14 over comparable field samples, in the field of the z = 1.47 radio galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies are consistent with their being at z > 0.8. The radio galaxy and one of the red galaxies are separated by 5'' and show some evidence of a possible interaction. However, the red galaxies do not show any strong clustering around the radio galaxy or around any of the brighter red galaxies. The data suggest that we are looking at a wall or sheet of galaxies, possibly associated with the radio galaxy at z = 1.47. Spectroscopic redshifts of these red galaxies will be necessary to confirm this large-scale structure.