J0011+3217: A peculiar radio galaxy with a one-sided secondary lobe and misaligned giant primary lobes

The peculiar radio galaxy J0011+3217¹¹1This object was first catalogued as a bright radio source in the B2 survey, and so is best known as B2 0008+32 (Colla et al., 1970). In the LoTSS DR2 optical identification catalogue (Hardcastle et al., 2023), this object has the standard LOFAR identifier ILTJ001121.28+321638.8. Throughout the current article, we refer to the object by the shorter name J0011+3217 for convenience. was identified using the International LOw-Frequency ARray (LOFAR) Two-metre Sky Survey second data release (LoTSS DR2; Shimwell et al., 2022). LOFAR is currently the largest ground-based radio telescope operating in the low-frequency range of 120–168 MHz. The survey images were developed using a fully automated, direction-dependent calibration and imaging pipeline. The survey consists of mosaics of several 8-hour LOFAR observations. The survey has a median RMS sensitivity of 83 $\mu$Jy beam^-1 with 6″ resolution and operates at a central frequency of 144 MHz.

To study the nature of the source at a relatively high frequency, we used Giant Meterwave Radio Telescope (GMRT; Swarup et al., 1991) archival data²²2https://naps.ncra.tifr.res.in/goa/data/search (proposal code:29_058) with a central frequency of 607 MHz. The source was observed for 7h using multiple scans. 3C 48 was used as the flux calibrator and observed for 15 min, whereas 0029+349 was used as the phase calibrator and observed for 1.8 hours in various scans. The data were recorded in 256 frequency channels. We used the Common Astronomy Software Applications (CASA) package for data analysis (McMullin et al., 2007). Standard steps of flagging (removal of bad data), calibration of complex gains, and bandpass are carried out. The flux density scale of Perley & Butler (2017) is used for absolute flux calibration. Several rounds of phase-only self-calibration followed by amplitude and phase self-calibration are carried out to improve sensitivity. We imaged the final visibilities using the Briggs weighting scheme (Briggs, 1995), with robust = 0. To observe the diffuse emission of the source at 607 MHz, we used a UV distance cut of 5k$\lambda$. J0011+3217 is located 9.6 arcmin from the centre of the field. A primary beam correction was applied to the image during the analysis.

XMM-Newton (Jansen et al., 2001; den Herder et al., 2001) observation (obs id: 0827021201) was carried out centred on the nearby cluster Abell 7 (see Sect. 3.2) with field centre of $\alpha$:00h11m41.85s; $\delta$: +32^∘24^′33.3^′′ on 20-06-2018. The two Multi-Object Spectrometer cameras (MOS1 and MOS2) and the European Photon Imaging Camera (EPIC pn) on board XMM-Newton were operated in the nominal full-frame mode. The exposure time for MOS1, MOS2, and pn data were 29752s, 31192s, and 24190s, respectively. We used XMM-Newton Science Analysis System (SAS) v18.0.0 (Gabriel et al., 2004) for XMM-Newton European Photon Imaging Camera (EPIC) data reduction. All the obtained data were reprocessed based on the latest current calibration files. To obtain the photon event lists, we employed tasks EMPROC and EPPROC on the MOS and pn event files. The out-of-time pn event files are produced by the task EPPROC. To apply the standard filter to the event files and create the image, we used task EVSELECT to select single, double, triple, and quadruple events (PATTERN$\leq$12) for MOS and single and double events (PATTERN$\leq$4) for pn, with an energy range from 0.2 to 12 keV. FLAG was set to zero to reject events that were close to the charge-coupled device (CCD) gap and that contained bad pixels. In addition, we also imposed the criterion of #XMMEA_EM and #XMMEA_EP for MOS and PN, respectively, in order to filter out artefact events. We then filtered for background flares in the standard manner, which reduces the pn exposure time to 20.4 ks. The response matrix file (rmf) and the ancillary response file (arf) were created using the SAS tools rmfgen and arfgen, respectively. For spectral extraction, spectra were grouped to a minimum of 100 counts per bin before background subtraction using the task GRPPHA in FTOOLS in order to facilitate standard $\chi^{2}$ fitting techniques.

We also inspected an archival short Chandra Advanced CCD Imaging Spectrometer (ACIS-I) observation of Abell 7, observation ID 15157, but J0011+3217 lies outside the field of view of this observation.

We carry out an independent optical identification of the optical counterpart of J0011+3217 using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; Chambers et al., 2016). Figure 4 shows an optical r-band Pan-STARRS1 (Chambers et al., 2016) image of J0011+3217 overlaid with a LOFAR image of J0011+3217 at 144 MHz. To identify the radio core of J0011+3217, we used the Very Large Array (VLA) Sky Survey (VLASS) (Lacy et al., 2020) image of J0011+3217 at 3 GHz with 2.5″resolution, which we show in Fig. 5. The radio core of the VLASS image of J0011+3217 coincides with the optical counterpart (SDSS J001119.35+321713.8 or WISEA J001119.34+321713.8) denoted by G1 in the right panel of Fig. 5. We conclude that G1 is the optical and IR host galaxy of J0011+3217, as also reported in the LoTSS DR2 optical ID catalogue by Hardcastle et al. (2023). Due to the low resolution in the LOFAR image in Fig. 4, an offset between the radio and optical peak can be seen whereas the high-resolution VLASS image in Fig. 5 does not show any offset between the radio and optical peak. The optical galaxy G1 has a spectroscopic redshift of 0.107144 (Ahumada et al., 2020). This corresponds to a physical scale of 2.031 kpc arcsec^-1 for our adopted cosmology.

The optical or IR galaxy (SDSS J001119.75+321709.0 or WISEA J001119.75+321709.0), denoted by G2 in the zoomed right panel of Figs. 4 and 5 has a spectroscopic redshift of 0.105552. The projected linear distance between the optical galaxies G1 and G2 is 16 kpc (7 arcsec).

J0011+3217 resides at a distance of 9.9 arcmin (1.2 Mpc projected distance) away from the centre of the cluster Abell 7, also known as Planck cluster PSZ2 G113.29–29.69. The mass of the Abell 7 cluster (M₅₀₀) within the $r_{500}$ radius is $(3.71\pm 0.27)\times 10^{14}$ M_⊙, with $r_{500}=1060\pm 25$ kpc (Botteon et al., 2022). The cluster lies at a very similar mean redshift ($z=0.104$, Rozo et al. 2018) to the host galaxy G1 and its companion G2, so it seems very likely that the radio galaxy is physically associated with the cluster.

We used the NASA Extragalactic Database (NED) to search for known galaxies within 30 arcmin (3.6 Mpc) of the centre of Abell 7, and as expected, the distribution of their spectroscopic redshifts from the Sloan Digital Sky Survey (SDSS; Ahumada et al., 2020) is strongly peaked around $z\sim 0.105$, with 162 galaxies with redshifts having $0.095<z<0.115$, of which 62 lie within $r_{500}$ in projection. The bulk of the galaxies in this redshift slice are concentrated around the centre of Abell 7 but there is a clear overdensity (see Fig. 6) of 6-8 bright galaxies around the position of J011+3217 and its host galaxy G1. Taking the mean redshifts of the nearest 7 of these galaxies and comparing them to the 62 objects within $r_{\rm 500}$, there is a radial velocity offset of the order $(1000\pm 300)$ km s^-1, which suggests that the galaxies around G1 may be a group interacting with the cluster.

We searched for X-ray emission around the cluster and from the large structure of J0011+3217 and detected strong X-ray emission from the centre of Abell 7. The overlaid X-ray image at the energy range 2–10 keV with the LOFAR image (6″) at 144 MHz of J0011+3217 is presented in Fig. 7. There is no evidence of either extended or compact X-ray emission coincident with the radio galaxy itself or the group that may surround it. This could imply that interactions with the cluster have partially or wholly stripped away the hot-gas environment of the group.

We extracted a spectrum from the central 5 arcmin of the XMM-Newton observation, which allowed us to measure³³3We fitted using xspec, assuming a Galactic $N_{H}$ of $4.40\times 10^{20}$ cm^-2 and taking an annular background subtraction region of 5–5.5 arcmin around the cluster. The metal abundance, a free parameter of the fit, is $0.31\pm 0.03$ solar. a cluster temperature of $4.56\pm 0.095$ keV for the central regions of the cluster, consistent with the temperature in the outer regions of $4.05\pm 0.07$ keV measured by Iqbal et al. (2023) as part of their temperature profile analysis. This temperature, which is also consistent with the temperature-$M_{500}$ relation of Arnaud et al. (2005), implies a sound speed for the central regions of the cluster around 1000 km s^-1, so on the rough velocity offset estimate derived above, the motion of the group hosting the radio galaxy is likely to be transonic or supersonic with respect to the cluster gas, particularly if the temperature is lower beyond $r_{500}$.

The measured total integrated flux densities of J0011+3217 at 144 MHz (including diffuse emission of LOFAR image at 6″$\times$6″resolution) and 607 MHz (GMRT image at UV distance cut of 5k$\lambda$ including diffuse emission) are 6.30$\pm 0.31$ Jy and 1.55$\pm 0.08$ Jy, respectively. The error in flux density measurements for 144 MHz LOFAR images (Shimwell et al., 2022) is $\sim$5%.

The error associated with our measured flux density at 607 MHz GMRT image is calculated using Eq. 1 (Hopkins et al., 2003):

where $\sigma_{I}$ is the total error on the integrated flux density, $\sigma$ is the RMS error in the image, and $I$ is the total integrated flux density of J0011+3217. We assume 0.05 (5%) as the calibration error of GMRT (Swarup et al., 1991).

Using the measured flux density, we calculated the two-point spectral index of this peculiar radio galaxy between 144 MHz and 607 MHz. We included the diffuse emission; the GMRT image at 607 MHz with UV distance cut of 5k$\lambda$ and measured the spectral index as –0.97$\pm 0.05$ (using $S_{\nu}\propto\nu^{\alpha}$). Here, $S_{\nu}$ is the radiative flux density at a given frequency $\nu$ and $\alpha$ is the spectral index.

The left panel of Fig. 8 presents the spectral index map of J0011+3217 between 144 and 607 MHz, and the right panel of Fig. 8 presents the corresponding error in the spectral index map of J0011+3217. The contour plot in Fig. 8 shows the LOFAR image of J0011+3217 at 144 MHz with 20″$\times$20″. The typical error in the spectral index map is $\sim$0.05 (see the right panel of Fig. 8). The error is uniform, except at the edge of the radio emission, where it is up to 0.3. For the spectral index map, we used the LOFAR image at 144 MHz with 20″$\times$20″resolution and the GMRT image at 607 MHz. The full resolution GMRT image (5.0″$\times$4.4″; position angle 46.6^∘) does not detect one-sided extended diffuse emission of J0011+3217 as observed in the LOFAR image. We therefore use a uv-taper of 22″in the GMRT 607 MHz image to detect the diffuse secondary wing. The tapered GMRT image captures the one-sided extended diffuse emission with a resolution of $\sim$21″. To create the spectral index map we compare this with the LOFAR image at 20″, as this has a very similar resolution and is expected not to miss any extended diffuse emission. We convolve both images (LOFAR and GMRT images) to a resolution of 21″$\times$21″. To create the sub-images of the same size (pixel to pixel), we used the aips task OHGEO. After that, we used the task COMB with OPCODE ‘SPIX’ to produce the spectral index map and spectral index error map of J0011+3217. Here we use 3$\sigma$ clip for both images. The spectral index at the hotspots of both sides of the structure is $\sim-0.60$. The spectral index in the right or western lobe varies from $-0.5$ to $-1.2$ whereas in the left or eastern lobe varies from $-0.62$ to $-0.89$ (see the left panel of Fig. 8). The western lobe is steeper than the eastern lobe. The eastern lobe is more bent and has a relatively flat spectral index compared to the western lobe. The relatively flat spectral index in the lobe is typically seen for the sources that have the effect of the cluster environment. So, it is possible that the eastern lobe of J0011+3217 is more affected by the cluster environment. Spectral steepening in the ranges of $-1.2$ to $-2.0$ (see the left panel of Fig. 8) can be seen in the one-sided diffuse wing structure of J0011+3217.

The produced spectral index map of J0011+3217 has a resolution of 21″. The radio core in the spectral index map of J0011+3217 at 21″ resolution is indistinguishable from the surroundings. We measured the spectral index of the radio core of J0011+3217 with the high-resolution images at 144 MHz, 607 MHz, and 3000 MHz in which the radio core is resolved. The measured spectral index of the core is $-0.33\pm 0.06,$ which is relatively flat, as expected for the core of a radio galaxy.

The nature of the spectral slope of a typical radio galaxy with jets, primary components (cores), and secondary components (hotspots) can be understood in terms of the spectral index as follows (Zajaček et al., 2019): (1) $\alpha<-0.7$ is described as a steep spectrum and is typical for optically thin synchrotron structures, where electrons have cooled off, such as radio lobes or faint diffuse emissions in the structure. The western lobe of J0011+3217 (in the left panel of Fig. 8) and the extended one-sided secondary wing of diffuse emission along the southern side of J0011+3217, all show a steep spectral index. (2) $-0.7\leq\alpha\leq-0.4$, denoted as flat, indicative of recent particle acceleration in optically thin synchrotron emission, typical for jet emission and hotspots. A similar spectral index $\sim$ –0.6 at the hotspots of both sides (eastern and western) of J0011+3217 is measured. (3) $\alpha>-0.4$, this range of spectral index characterises sources where synchrotron self-absorption or free-free absorption is involved such as in AGN core components (primary components) as observed (fitted spectral index $\sim$ –0.33) for the core of J0011+3217.

The spectral index of a typical radio galaxy including the whole structure lies between $-0.7$ to $-0.8$ whereas the total spectral index as calculated for J0011+3217 between 144 MHz and 607 MHz is $-0.97$. The steepening in the spectral index of J0011+3217 is due to the presence of extended one-sided diffuse emission in the structure, which is as expected very steep ($-1.2$ to $-2.0$) because of the very faint diffuse emission.

We measured the radio luminosity ($L_{\textrm{rad}}$) of this peculiar radio galaxy using the standard formula (Donoso et al., 2009):

where $\alpha$ is the spectral index, $z$ is the spectroscopic redshift of J0011+3217 ($z=0.107144$ for G1; see the right panel of Fig. 4), $D_{L}$ is the luminosity distance of J0011+3217 in metres (m), and $S_{\nu}$ denotes the flux density of J0011+3217 at a frequency $\nu$ in the unit W m^-2 Hz^-1. The total radio luminosity of J0011+3217 is calculated as 1.65$\times$ 10²⁶ W Hz^-1 at 144 MHz. This places it just above the nominal Fanaroff-Riley break at this frequency of $10^{26}$ W Hz^-1 and is similar to the average radio luminosity measured for 215 FRII giant radio galaxies at 144 MHz (Dabhade et al., 2020a).

The central black hole masses of the pair of interacting galaxies (G1 and G2; see the right panel of Fig. 4) of J0011+3217 are estimated using the $M_{\text{BH}}-{\sigma}$ relation (Ferrarese & Merritt, 2000; Gebhardt et al., 2000):

where $\sigma_{\ast}$ is the velocity dispersion, and $M_{\odot}$ is the stellar mass. Here, we use the central velocity dispersion ($\sigma_{\ast}$) for the optical hosts taken from SDSS (Ahumada et al., 2020). We found that the black hole masses of J001119.35+321713.8 (G1) and J001119.75+321709.0 (G2) are 7.37$\pm$1.08 $\times$10⁸ M_⊙ (with a velocity dispersion of 248.34$\pm$9.13 km s^-1) and 2.68$\pm$0.51 $\times$10⁸ M_⊙ (with a velocity dispersion of 192.86$\pm$9.19 km s^-1), respectively. The optical host J001119.35+321713.8 (G1) is a luminous red galaxy and more massive than J001119.75+321709.0 (G2).

The comparison of black hole mass for both optical galaxies (G1 and G2) with statistically measured average black hole mass for radio galaxies (with the spectroscopic data collected from Best & Heckman (2012) and giant radio galaxies by Dabhade et al. (2020b) (see Fig. 11a of Dabhade et al. (2020b)) shows that J0011+3217 is relatively less massive.

We measure the projected linear size of this peculiar source along with that of the secondary wing using the following formula:

where $\theta$ is the largest angular size (LAS) of the source (in arcsec), $z$ is the redshift of J0011+3217, $D_{co}$ is the comoving distance of J0011+3217 in Mpc, and $D_{p}$ is the projected linear size (in Mpc). We measure the projected linear size of the primary lobe as 0.99 Mpc with the largest angular size of 407 arcsec and a comoving distance of 463.7 Mpc. The measured projected linear size of the secondary wing is 0.85 Mpc (approximately 85% of the primary lobe), with the largest angular size of 476 arcsec.

The cutoff projected linear size for a giant radio galaxy is usually taken as $\geq 0.7$ Mpc (Kuźmicz et al., 2018; Dabhade et al., 2020a, b; Bhukta et al., 2024). Therefore, based on the projected linear size of the primary lobe (0.99 Mpc) and the diffuse secondary wing (0.85 Mpc), this peculiar source is a giant radio galaxy with a giant one-sided diffuse secondary wing. This source is also catalogued as a giant radio galaxy in the list of 162 giant radio galaxies identified by Dabhade et al. (2020b) using the National Radio Astronomy Observatory (NRAO) VLA Sky Survey (NVSS) survey data. Because of the coarser resolution (45″) and lower sensitivity (0.45 mJy beam^-1), the NVSS survey did not detect the southward extended wing of J0011+3217 in Dabhade et al. (2020b). The low-frequency LOFAR survey has much better sensitivity (83 $\mu$Jy beam^-1) and high resolution (6″) in comparison to NVSS, which helped us to detect the diffuse one-sided southward extended wing of J0011+3217 as presented in the current paper.

The peculiar radio galaxy J0011+3217 has a large diffuse secondary wing elongated out to 0.85 Mpc from the centre towards the southern part of the galaxy, which is 85% of the size of the primary lobe. Although extremely rare, one-sided secondary lobes have previously been discovered in some XRGs. Cheung (2007) discovered small, one-sided wings for a few X-shaped sources; however, Roberts et al. (2018) examined these sources using VLA at 1.4 GHz and discovered a very faint wing in the other direction for some of the sources. The source presented in the current paper also seems to have a very faint wing to the north of the eastern lobe. The faintness and small size of the wings in other directions could be a consequence of the energy losses sustained by the relativistic plasma produced in these older or fossil radio components. Bhukta et al. (2022) also catalogued 13 sources as single-wing XRGs. The sizes of the one-sided secondary lobes in all these sources were very small (¡25% of the primary lobes). Thus, although secondary wings are also observed in X-shaped sources, giant one-sided secondary wings or plumes have never been observed in any other XRGs, as seen in the peculiar source, J0011+3217, which makes this source special.

The presented source (J0011+3217) in this current paper possesses minor distortions or off-axis diversions (or bending of the primary lobe in a common direction) at both edges of the structure (see Fig. 3). Such bending in the jet is more common for FRI radio sources, that is, for wide-angle tailed sources (WATs; e.g. Owen & Rudnick, 1976; Komissarov, 1988; Burns, 1998; Mao et al., 2010; Mingo et al., 2019; Missaglia et al., 2019; Bhukta et al., 2022; Sasmal et al., 2022; Pal & Kumari, 2023). WATs are typically found in dense cluster regions and form when the lobes or jets of the WAT are swept back by the relative motion of the surrounding medium that bends the jets in a common direction (e.g. Owen & Rudnick, 1976; Komissarov, 1988; Burns, 1998; Mao et al., 2010; Mingo et al., 2019; Missaglia et al., 2019; Patra et al., 2019; Bhukta et al., 2022; Sasmal et al., 2022; Pal & Kumari, 2023).

X-ray emission can originate from radio galaxies in several ways (Birkinshaw & Worrall, 1996). Radiation identifiable with a component of a radio galaxy usually appears to come from jets, hot spots, or lobes and is consistent with either synchrotron or inverse-Compton emission (Harris et al., 1994; Hardcastle et al., 2004; Mingo et al., 2017). As shown in Fig. 7, no X-ray emission is detected from the primary lobe of J0011+3217. The strong and extended thermal X-ray emission in the Abell 7 cluster may restrict the detection of faint or weak non-thermal X-ray emission from J0011+3217 near the core and the primary lobe. As noted above, there is no evidence of thermal emission from the presumed host group of the radio galaxy either.

As discussed above, J0011+3217 is hosted by a group of galaxies that is offset from, but physically associated with, the cluster Abell 7, and it seems very likely that its unusual appearance is associated with its large-scale environment. The fact that the ‘missing’ northern wing would point back towards the centre of the cluster is significant in this regard. We suggest that the host group is currently moving radially inwards at a speed comparable to our rough estimate of the velocity offset (i.e. around 1000 km s^-1), which means that the motion is at least mildly supersonic with respect to the cluster atmosphere, and so by definition ram pressure dominates over the static thermal pressure of the cluster. We can use the universal pressure profile of Arnaud et al. (2010) to estimate that the thermal pressure of Abell 7 at $r_{500}$ is $\sim 5\times 10^{-14}$ Pa, while the ram pressure should be of order twice this value. Using pysynch (Hardcastle et al., 1998) we estimate an equipartition pressure in the active lobes around $3\times 10^{-14}$ Pa, and we know from inverse-Compton observations that the true pressure is likely to be a factor of a few higher. The pressure in the wings must be comparable to that in the lobes since the internal sound speed of the radio galaxy is very high. The approximate equality between the ram pressure and the internal pressure of the wings means that it is highly plausible that the asymmetrical interaction with the cluster material will tend to push both the primary lobes and the secondary wings away from the cluster centre.

We propose that as a result of this interaction, the northern wing of J001+3217 has been ‘folded’ behind the W lobe; due to projection effects, this wing is not visible for most of its length (perhaps appearing as steep-spectrum material to the N of the E lobe). It is thus possible that intrinsically J0011+3217 has both secondary wings, and that they would be visible as separate structures from a line of sight, for example through the cluster centre. Meanwhile, both buoyant effects and the large-scale motion would tend to stretch out the southern secondary wing, contributing to its very large projected size. Over the $10^{8}$ years needed to generate such a large radio source, the projected motion on the sky would be $\sim 100$–$200$ kpc (on the assumption of projected speeds between 1000 and 2000 km s${}^{-1})$.

This interpretation leaves open the question of what generates the wings in the first place. The close pair of interacting galaxies that form the host of J0011+3217 is certainly an environment that might be conducive to jet precession or reorientation as well as to the formation of an asymmetric small-scale hot gas environment. The multiple, offset hotspots in J0011+3217 are consistent with a jet precession model (Horton et al., 2023) but could also be related to the ram pressure bending of the primary lobes. Precession models for this source will be discussed in more detail in a separate paper.