RAD@home Citizen Science Discovery of Two Spiral Galaxies Where the 30–220 kpc Radio Lobes Are Possibly Shaped by Ram Pressure Stripping

Abstract

We report the RAD@home citizen science discovery of two rare spiral-host radio galaxies (NGC 3898 and WISEA J221656.57-132042434.1 or RAD-“Thumbs up” galaxy), both exhibiting asymmetric radio lobes extending over 30 to 220 kiloparsec scales. We present a multi-wavelength image analysis of these two sources using radio, optical, and ultraviolet data. Both host galaxies are young, star-forming systems with asymmetric or distorted stellar disks. These disks show similarities to those in galaxies undergoing ram pressure stripping, and the radio morphologies resemble those of asymmetric or bent FR-II and wide-angle-tailed radio galaxies. We suggest that non-uniform gas density in the environment surrounding the ram pressure-stripped disks may contribute to the observed asymmetry in the size, shape, and brightness of bipolar radio lobes. Such environmental effects, when properly accounted for, could help explain many of the non-standard radio morphologies observed in Seyfert galaxies and in recently identified populations of galaxies with galaxy-scale radio jets, which are now being revealed through deep and sensitive radio surveys with uGMRT, MeerKAT, LOFAR, and, in the future, SKAO. These findings also underscore the potential of citizen science to complement professional research and data-driven approaches involving machine learning and artificial intelligence in the analysis of complex radio sources.

1. Introduction

Double-lobed radio galaxies were first discovered in 1953, and since then, they have been almost exclusively linked to elliptical hosts. Only in the past ∼15 years has a modest but growing sample demonstrated that spiral and disk galaxies can also launch episodic, megaparsec-scale radio jets [1,2,3]. While many nearby Seyferts in spiral galaxies are known to host radio jets and lobes, with a few of the kpc scale [4,5,6,7], only recently, due to the advancement of sensitive low-frequency surveys, has a sample of galaxy-scale radio-lobed galaxies been discovered [8]. Hence, for a unified picture of radio jets in the kpc-to-Mpc and bulge-less spirals-to-giant ellipticals scales, the study of jets at intermediate scales is important. Since these galaxy-scale jets in spiral hosts are naturally easier to observe for jet–interstellar medium (jet–ISM) and jet–circum-galactic medium (jet-CGM) interactions, their implications for understanding the detailed physics of AGN-feedback in galaxy evolution are naturally high. Large, 200 kpc-to-Mpc-scale radio lobes are likely too large to show signs of interaction with the CGM. Small kpc-scale Seyfert jets are easy to show interacting with the ISM. In some cases of jet–ISM interactions, jet-driven shock ionization is clearly observed [9,10]. As in the well-known case of Minkowski’s object, jet-triggered young star formations have also been observed in many galaxies. Radio jets are always seen orthogonal to the small-scale or nuclear dust lanes in elliptical galaxies, irrespective of cases where the large-scale dust ellipse is aligned with the major axis of the optical galaxy [11]. Such orthogonal correlations naturally suggest gas/dust feeding of the accretion disks. However, jets in spiral galaxies do not show any such correlation with the rotation axis [12]. This can be attributed to intense jet–ISM interaction and short duty cycles of accretion and jet episodes. In some cases, instead of well-defined jets, radio emissions from spirals are seen in the shape of bipolar bubbles. In cases where bubbles are seen to be orthogonal to the stellar disk, bubbles are proposed to have been shaped by the galactic wind outflow [4]. In a representative sample of ten radio bubbles, the mandatory presence of AGNs was observed [7]. The radio plasma in the bubble was probably supplied by the AGN jet. The current study reports the findings of two galaxies where the intermediate-scale (30–200 kpc) radio lobes from two spiral galaxies are seen asymmetrically located with respect to the host galaxies. Taking clues from multi-wavelength observations, we bring an analogy with ram pressure-stripped galaxies [13,14,15] and propose a novel idea that these intermediate-scale radio lobes seen here, and possibly in other cases, can be explained by the incorporation of ram pressure stripping by the intra-group or intra-cluster medium. The discovery and study of more such cases with deep H i and CO observations will grow our understanding of radio-jet feedback-driven galaxy evolution [16,17].

2. RAD@home Citizen Science Discovery Method

The discovery and interpretation of the sources presented in this study were carried out through the RAD@home Citizen Science Research (CSR) Collaboratory [18,19]. Of over 4700 members, thousands were trained to interpret galaxy images through both online and in-person interactions. Undergraduate students across India were given the opportunity to explore and understand the fundamentals of multi-wavelength astronomy using red–green–blue-contour (RGB-C) images constructed from ultraviolet (UV), optical, infrared (IR), and radio data. To introduce the basic concepts of star formation, IR–optical–UV (IOU) RGB images were used, incorporating IR data from the Wide-field Infrared Survey Explorer [20] (WISE), UV data from the Galaxy Evolution Explorer [21] (GALEX), and optical images from the Digitized Sky Survey (DSS). Participants were encouraged to analyze RGB-C images of different galaxies and share their interpretations on various days throughout the year. These analyses were further discussed through text and image comments, as well as interactive online sessions. Regular e-classes were conducted via Google Meet on weekends, providing a platform for collaborative learning and feedback. A custom multi-wavelength RGB-C image maker tool (see Figure 1 for an example) ensured consistent and foundational training in galaxy image interpretation [22].

Upon successfully completing the initial RGB-C image analysis training, students were further guided to work with FITS image files from the TIFR GMRT Sky Survey (TGSS), conducted with the Giant Metrewave Radio Telescope (GMRT) at 150 MHz, offering a resolution of ∼${25}^{″}$ and an rms noise of ∼5 mJy ${\mathrm{beam}}^{-1}$ [23]. The primary goal of this exercise, performed using SAO ds9, was to identify faint, diffuse, and non-standard radio structures. Candidate sources identified by trained citizen scientists were discussed collaboratively through radio–optical–radio overlay images generated using the RAD@home RGB-maker tool. To protect the interests of the discoverer, the coordinates of the sources were withheld during the discussion. This process also helped ensure consistent multi-wavelength image analysis skill development for all participants. Faint diffuse structures may represent remnants of past AGN jet activity [18]. Non-standard morphologies, on the other hand, could indicate unusual AGN interactions or environmental distortions, such as in the case of RAD12 [19], where a radio jet from one galaxy impacts a companion galaxy and rebounds to form a mushroom-shaped radio bubble. Discoveries like these offer valuable insights into galaxy evolution, star formation, and AGN feedback via jet-driven outflows. Promising candidates identified through this process are submitted to the Collaboratory via a dedicated image submission Google form, after which they are reviewed by professional astronomers and selected for follow-up observations with GMRT or direct publication. In this study, we report on two such galaxies exhibiting non-standard and faint diffuse radio structures.

3. Results

3.1. Radio Lobes of NGC3898 or a Background Radio Galaxy?

Two radio blobs can be seen in the TGSS image. However, the possible host galaxy, a spiral galaxy, is not in the middle, but close to one of the blobs. This extreme case was posted on 21 April 2024 in the RAD@home online discussion group. The RGB-contour image constructed using radio–optical–radio data (Figure 1) shows the eastern lobe to be located close to the possible host galaxy, NGC3898, while the western lobe is nearly three times further away. Although the shorter eastern lobe is relatively bright, the western lobe appears faint and disconnected from the possible host. The optical disk of NGC3898 also exhibits faint radio emissions in the NRAO VLA Sky Survey (NVSS; 1400 MHz, beam size = ${45}^{″}$) data [24]. A comparison with the Faint Images of the Radio Sky at Twenty Centimeters (FIRST; 1400 MHz, beam size = $5^{″}$) survey [25] reveals a compact radio source coincident with the nucleus of NGC 3898 ($z_{spec}=0.003875$, $v=1162\pm 1$ km ${\mathrm{s}}^{-1}$, scale = 0.081 kpc ${\mathrm{arcsec}}^{-1}$ or $1^{\prime }$ = 4.86 kpc). We obtained the spectral index values (${\alpha }_{150\mathrm{MHz}}^{1400\mathrm{MHz}}$) for the target from the SPIDX database [26], which provides a spectral index derived from the NVSS and reprocessed TGSS data. In this dataset, UV tapering was applied to the original TGSS visibilities (original beam ∼${25}^{″}$) to match the coarser resolution of the NVSS (∼${45}^{″}$), ensuring consistent spatial sampling across the two frequencies. The eastern lobe, central component, and western lobe showed average spectral indices of −0.79, −1.0, and −0.58, respectively.

NGC 3898 (UGC 06787) is a well-studied nearby galaxy, with observations spanning nearly all wavelengths. This SA(s)ab-type spiral galaxy, classified as a LINER-H ii composite, is a member of the “12-3” galaxy group [27]. NGC 3898 is also identified as a galaxy (ID 461333) in the catalogue of merging groups and clusters [28]. It belongs to Group ID 52914, which comprises 30 member galaxies, with an $R_{200}$ of 580 kpc and a total mass $M_{200}=2.1\times {10}^{13}{M}_{\odot }$. Likely due to the presence of extremely asymmetric radio emission blobs seen in NVSS, the galaxy was overlooked in earlier samples of nearby galaxies hosting radio jets. In this brief study, we focus on the asymmetric radio lobe morphology and complement the analysis with newly available data.

Radio images are also available for this target from the LOFAR Two-Meter Sky Survey [29] (LoTSS). In Figure 2, we present both low- (${20}^{″}$) and high-resolution ($6^{″}$) 144 MHz images from LoTSS. In the high-resolution image (right panel), the eastern lobe reveals a clearly resolved structure, with a hotspot and backflow forming a standard edge-brightened FR-II morphology resembling a “fish-head” or bow shock-like structure. The hotspot is located at the farthest end from NGC3898. The projected angular separation between the hotspot and the radio core, i.e., the nucleus of NGC3898, is ${113}^{″}$, corresponding to a projected size of 9 kpc, assuming the radio lobes are associated with NGC3898. Faint radio emission from the backflow appears to reach NGC3898. In contrast, the western lobe is well separated from the spiral galaxy and appears distorted. The western hotspot is offset to the north relative to the remaining diffuse emission of the lobe. Extended radio emission is observed farther from the spiral galaxy than the hotspot. The projected distance between the western hotspot and the radio core of the spiral is ${349}^{″}$ or, assuming the radio lobe and spiral association, 28 kpc. The resulting arm-length ratio, greater than 3, makes this an extreme example of lobe asymmetry. The total flux density of the radio core region is 10 mJy, with a peak flux of 5.73 mJy ${\mathrm{beam}}^{-1}$. The eastern lobe, including the radio core of the spiral, has a flux density of 175 mJy, while the western lobe has 141 mJy. Since the eastern lobe extends into the spiral galaxy, the core flux must be subtracted when estimating the true flux density of the lobe. It remains uncertain whether ongoing star formation in the galactic disc contributes to the measured flux density of the eastern lobe.

The low-resolution LoTSS image is shown in Figure 2 (left panel), with optical image contours overlaid. No additional features are evident, except that the eastern lobe’s backflow is clearly seen crossing the nucleus of the spiral galaxy. The backflow also extends twice as far to the north of the optical nucleus compared to the south. This suggests that the eastern backflow is unlikely to be interacting directly with the spiral galaxy but appears to do so primarily due to projection effects. The origin of this pronounced asymmetry will be discussed in a subsequent section.

To investigate further if the asymmetric radio lobes are indeed associated with NGC 3898 and it is not a background radio galaxy, we examine the LoTSS $6^{″}$-resolution radio image alongside the optical data from the Beijing–Arizona Sky Survey [30] (BASS). Near the midpoint between the two radio lobes, we identify a faint, compact radio source (R.A. 11:49:03.105, Dec. +56:06:33.049) that could potentially be the radio core of the double-lobed radio source. Upon closer inspection, we find that the peak of this compact radio source is offset by more than $2^{″}$ from the nearest optical galaxy (R.A. 11:49:02.958, Dec. +56:06:31.398), which has a photometric redshift of $z_{phot}=0.469\pm 0.091$ (see Figure 3 left panel). The positional accuracy of LoTSS $6^{″}$-resolution radio images is within ${0.2}^{″}$ [31]. At $z=$ 0.469, the $2^{″}$ separation corresponds to nearly 12 kpc between the radio point source and the optical galaxy, assuming that the radio source is at the distance of the galaxy. Furthermore, we also verify the astrometric accuracy of the LoTSS image by cross-matching point sources across the field. The right panel of the same figure shows a collage of four such point radio sources whose LoTSS contours (green) are overlaid on the BASS optical image. The compact radio source (left panel) has a flux density of 0.8 mJy at 144 MHz but is not detected in higher-frequency radio surveys, including FIRST (3$\sigma$ upper limit: 0.5 mJy) and the Very Large Array Sky Survey (VLASS; 3$\sigma$ upper limit: 0.4 mJy). This leads to an upper limit in the spectral index of ${\alpha }_{150\mathrm{MHz}}^{1400\mathrm{MHz}}<-0.2$. This value is consistent with it being a flat-spectrum radio core, as well as with a steep-spectrum radio galaxy at an even higher redshift whose optical host is undetected in BASS. If the double-lobe structure is a background radio galaxy with its host galaxy at $z_{phot}=0.469\pm 0.091$, the total size (${113}^{″}$ + ${349}^{″}$ = ${462}^{″}$; scale = 5.95 kpc${/}^{″}$) is ∼2.75 Mpc. Radio galaxies with sizes greater than 700 kpc are called giant radio galaxies (GRGs), which are a relatively rare population [32]. In summary, the association of the LoTSS point source with the double radio lobes is not proven due to the lack of detection of a radio jet aligned with the lobes and the compact radio source being significantly offset from the centre of the nearest optical galaxy. Based on this analysis, we interpret that the asymmetric radio lobes are associated with NGC 3898 (hereafter, the host galaxy) rather than with a background galaxy.

To investigate the regions where the radio lobe seems to touch the spiral-host galaxy for possible signs of jet–ISM interaction, we checked available images in GALEX (UV band) and deep optical images in BASS [30]. Although we did not see any sign of jet–ISM interaction or signs of jet-triggered young star formation, we noticed that the western side of the galaxy showed extended young star-forming regions (Figure 4). This was similar to the extended UV disks found in some galaxies from GALEX UV in earlier studies [33]. These regions are also clearly seen in BASS images as arms/arcs in blue optical light. Note that they are still too far from the western lobe to interact.

We also looked for signatures of asymmetry in the gaseous medium. Previous H i and H$\alpha$ studies found the galaxy to have an inclination angle of ∼${54}^{°}$ with a smooth gaseous velocity field [35,36]. The H i 21cm line observation data were obtained from the Westerbork Synthesis Radio Telescope (WSRT). The total intensity H i-emission contours (cyan) were overlaid on the optical image from the Sloan Digital Sky Survey (SDSS) (Figure 5a). The nuclear region shows a dip, the surrounding region shows two peaks, and the regions surrounding them show clear east–west asymmetry. While the eastern region shows a truncated/shrunken H i, the western emission is wider. In the same image, we overlaid LoTSS high-resolution contours (thin white). While the short eastern lobe is seen, in projection, inside the H i disk on the truncated side, the longer western lobe is seen outside the H i disk on the wider disk side. We also inspected the H$\alpha$ line emission images to check for similar asymmetry. A false-color H$\alpha$ image of the central region of the galaxy is presented in Figure 5b. Ionized gas distribution is also seen as asymmetric, with an extended gas tail seen to the south-west of the nuclear region. In summary, the radio lobe asymmetry, asymmetry in the atomic and ionized gas distribution, and asymmetry in the star-forming regions in the extended disk of the host galaxy may have some causal connections. We shall return to this discussion in later sections.

3.2. RAD-Thumbs up Galaxy: Speca or Radio Phoenix?

In December 2018, towards the end of a citizen science research training programme (also known as the RAD@home Discovery Camp), one participant spotted a radio source that seemed to be two radio lobes associated with a disturbed spiral galaxy but peculiarly showing emission on only one side of the host. As seen in Figure 6, there is no radio emission on the eastern side of the central optical galaxy. The radio features, seen in the NVSS image, look like a “Thumbs up” sign, and, thus, were discussed as the “Thumbs up” galaxy. The disturbed spiral galaxy, hereafter the host galaxy, can be identified with WISEA J221656.57-042434.1 ($z_{spec}=$ 0.095963 (scale = 1.79 kpc${/}^{″}$ or $1^{\prime }$ = 107 kpc). There are three radio peaks that can be seen in both the NVSS and TGSS images. The brightest peak is away from the host galaxy on the west side, the secondary peak roughly coincides (shifted slightly to the west) with the host galaxy, and the faintest peak, the ‘thumb’ feature, is seen north-west of the host galaxy.

FIRST survey data are available on the target. In Figure 7, we present the RAD@home composite contour image of the target, where the optical image is superposed with the same NVSS (cyan), TGSS (magenta), and FIRST (yellow) radio contours. FIRST, being a higher resolution ($5^{″}$) image compared to NVSS (${45}^{″}$), has resolved the compact peak in the host galaxy. However, the brightest NVSS/TGSS peak, the ’fist’ part to the west of the host, has been removed, which is evidence that it is unlikely to be a background radio source but a diffuse/remnant lobe possibly from the host galaxy. Similarly, the north-western peak, the ‘Thumb’ part, has also been removed in the FIRST image.

The right panel of the same figure shows a zoomed-in view of the same multi-contour image. The resolved nuclear peak shows a “<”-like structure. Its orientation and similarity with the larger-scale Thumbs-up structure are quite intriguing. The radio emission probably has a smaller, bent, and double-lobed structure. The western peak is comparatively brighter than the north-western peak. This radio structure is similar in size to that of the host galaxy. Such sub-galaxy-scale radio lobes/jets are typical of jetted Seyfert galaxies. From the center to the western peak of the mini-“Thumbs up”, the projected distance is ${12}^{″}$ (21.5 kpc), while that to the north-western peak is ${10}^{″}$ (17.9 kpc). In comparison, the nucleus-to-western radio peak of the large-scale “Thumbs up”, or the fist, is ${123}^{″}$ (220 kpc). Similarly, the nucleus to the peak at the Thumb is ${101}^{″}$ (181 kpc). Hence, in projection, large radio lobes are at least ten times larger than small-scale radio lobes. Since radio jet ejection must occur in the opposite direction from the nucleus and both the small and large radio lobes are on the western side of the nucleus, we are likely seeing the source whose jet axis is either parallel to the line of sight or in the plane of the sky. Thus, assuming an intermediate inclination, the real linear size of the radio source could be bigger than 400 kpc (${224}^{″}$).

We obtained the spectral index map (${\alpha }_{150\mathrm{MHz}}^{1400\mathrm{MHz}}$) of the “Thumbs Up” radio source from the SPIDX database [26]. This is presented in Figure 8, which clearly shows that the host galaxy region is typical of synchrotron radiation ($\alpha \sim -0.75$), and the diffuse region, which is further away from the host, is $\alpha \sim -1.5$, i.e., significantly steeper. This suggests very little contribution of star formation to the mini-“Thumbs up” and the presence of a small unresolved jetted AGN. This pattern is also consistent with expectations from the composite contour map, where NVSS peaks were removed in FIRST, suggesting the remnant or diffuse nature of large-scale radio lobes.

Owing to the limited resolution and sensitivity of TGSS and NVSS, the thumb-like feature could not be reliably discerned or its morphology clearly characterized. To reassess the morphology with improved sensitivity and resolution, we examined recent data releases from the Rapid ASKAP Continuum Survey (RACS-low and RACS-mid; [37,38]) and the Evolutionary Map of the Universe (EMU; [39]). RACS-mid (Epoch 1, Stokes I, 1367.5 MHz), RACS-low (Epoch 1, Stokes I, 887.5 MHz), and EMU (943.491 MHz) images are shown in Figure 9. The RACS-mid image reveals the radio core and a westward plume that extends toward the “fist” component of the “Thumbs up” structure. However, the “thumb” component is not detected, probably due to the reduced sensitivity to surface brightness at higher frequencies. In contrast, the RACS-low image captures the full extent of the “Thumbs up” morphology. While the bright “fist” remains connected to the radio core, the diffuse “thumb” appears detached, which is consistent with its low surface brightness and extended nature.

Figure 9 also presents the EMU map of the “Thumbs up” galaxy with significantly higher sensitivity and clarity compared to other radio images. The EMU image has an rms noise level of 46 $\mathsf{\mu }$Jy ${\mathrm{beam}}^{-1}$ and a spatial resolution of ${15}^{″}$. In addition to the previously identified core/wrist, fist, and thumb components, the improved sensitivity reveals filamentary connections between the thumb and fist, as well as a faint one between the core and thumb. The thumb component is now clearly resolved as an elongated structure oriented in the north–south direction. Its peak flux density is 1.8 mJy ${\mathrm{beam}}^{-1}$, corresponding to a signal-to-noise ratio of ∼39. The detection of filamentary bridges and the extended plume from the core to the fist argues against a chance alignment of unrelated diffuse radio emission with the core component associated with the disturbed spiral host galaxy.

The bottom-right panel of Figure 9 shows the EMU radio contours overlaid onto the g-band optical image from the Legacy Survey [40]. As seen in earlier images, the radio core is co-spatial with the host galaxy. The fist has no radio peak corresponding to any optical galaxy, suggesting diffuse emission. The elongated thumb component exhibits a sharp eastern edge and does not show any associated radio emission from the nearby elliptical galaxy WISEA J221654.97−042302.0 (also identified as WHL J221654.9−042302 BCG, $z=0.0869$), which is the brightest cluster galaxy (BCG) of the galaxy cluster WHL J221654.9−042302 ($z_{sepc}=0.098102$). This elliptical galaxy was also undetected in the FIRST radio survey image (Figure 7). In addition to this cluster at $z=0.0869$ [41], a supercluster was also reported in the background at $z\sim 0.38$ [42]. However, we detected no diffuse radio emission in the field other than that associated with the “Thumbs up” galaxy. The close redshift match suggests that the host galaxy (WISEA J221656.57−042434.1 ($z=0.095963$)) may be a member of the galaxy cluster WHL J221654.9−042302, and as suggested by the asymmetric WAT-like radio lobes, it is likely circling the cluster BCG located to the east of the thumb. Hence, the large-scale diffuse thumb and fist components can possibly be part of the cluster diffuse emission [43]. The fist–thumb diffuse radio emissions are unlike cluster radio relics, which are typically seen in pairs at the periphery of clusters. Instead they are steep in their spectral index and are associated with cluster mergers. This is also unlike cluster radio halos, which are steep-spectrum diffuse sources and are seen around cluster BCGs without links to any individual galaxy. Hence, the thumb and fist seem similar to cluster radio phoenixes. A radio phoenix is a remnant radio lobe from past AGN-jet activity, but has recently been revived due to cluster mergers or similar cluster dynamical processes [44,45]. Since the disturbed spiral host is the only possible host galaxy seen in the middle to have created these double remnant radio lobes, it is simply a remnant or older pair of lobes of the same host galaxy WISEA J221656.57−042434.1. Hence, it is likely a case of a spiral-host episodic radio galaxy or Speca [1], where the remnant lobes are misaligned and the host is a WAT. Recent observations suggest that radio phoenixes are even possible in densities lower than clusters, i.e., in group environments [46].

Ultraviolet images from GALEX and deeper optical images from the Legacy survey are available. Figure 10a shows a combined near and far UV image of the host galaxy. The arc-shaped or bowl-shaped young star-forming region of the host galaxy naturally draws similarities to the ram-pressure-stripped galaxies in nearby clusters like Virgo and Coma. This UV image can be compared with GALEX images of Virgo cluster galaxies, showing H i evidence of ram pressure stripping [47] (e.g., NGC 4522 and NGC 4330).

Similarly, the Legacy optical image shows a disturbed spiral galaxy with bright blue star-forming clumps on its southern arm, as well as disturbed regions to the north. The distribution of the star-forming regions of the galaxy has no correspondence with the sub-galactic mini-“Thumbs up” radio structure seen in the FIRST image. This, along with the spectral index value, is a clear sign that the mini-“Thumbs up” radio emission is due to AGN activity and not from star formation. The overall optical structure is also similar to that of the Virgo cluster galaxy NGC 4438. Interestingly, NGC 4438 also has a mini-radio double along with large-scale diffuse radio emission to the west of the galaxy, which is attributed to ram pressure stripping. A multi-wavelength study of NGC 4438 can be referred to for further comparisons [48]. Although the arms are disturbed, the central region of the galaxy is not. The host galaxy has a spectroscopic redshift $z=0.095963\pm 0.000150$ or (v (heliocentric) = 28,769 ± 45 km ${\mathrm{s}}^{-1}$). The early type galaxy seen to the north is WISEA J221656.42-042405.6, which has a heliocentric recession velocity of 20,870 km ${\mathrm{s}}^{-1}$ (measurement uncertainty not available in the NASA Extragalactic Database). Hence, these two galaxies, though they seem to be connected in projection, are widely separated in velocity space and are clearly not interacting. The late-type edge-on galaxy seen to the south-east of the host galaxy is undisturbed; hence, it is not part of any possible interaction.

In summary, plume and filamentary links indicate that the compact “mini-thumbs up” component is likely feeding the more diffuse fist and thumb features. Although the diffuse emission could, in principle, be a radio-phoenix associated with the surrounding cluster, the simpler explanation is that both episodes of the radio lobes originate from the same spiral host and have possibly been pushed westward by the ram pressure of the intra-cluster medium acting east to west. Similar environments have been reported for other spiral-host radio galaxies [49,50], and our results align with those studies.

4. Discussion

Identifying the host galaxy of an extragalactic radio source can sometimes become challenging, particularly when the jet linking the radio core to the hotspots is absent. It is also complicated when the radio lobes are of a non-standard shape or remnant radio lobes with diffuse emission blobs. In such cases, multiple potential host galaxies may lie near the radio structure, making the association ambiguous. In both systems presented in this study, the presence of a surrounding galaxy cluster/group further complicates host identification. In the case of NGC3898, the radio lobes, which are asymmetric to NGC3898, can be a symmetric background radio galaxy at a redshift near 0.469 or even at a higher redshift. Similarly, in the case of the “Thumbs up” galaxy, the core radio component is clearly associated with the disturbed spiral (WISEA J221656.57-197042434.1), but the ’fist’ and ’thumb’ components can be the diffuse radio phoenix emission from some other host galaxy that is associated with the same cluster (WHL J221654.9-042302).

Several mechanisms can contribute to radio lobe asymmetries, including (1) light travel time effects, where the nearer lobe is observed at a later evolutionary stage compared to the lobe farther away, (2) Doppler boosting, wherein the approaching lobe appears brighter than the receding one, and (3) intrinsic asymmetries in the ambient medium on scales of several hundred kiloparsecs, leading to differential resistance to lobe expansion [51,52]. Previous studies have tried to explain the correlated radio lobe asymmetries and optical asymmetries with motion of the host galaxy through the cluster medium, where the ram pressure-stripped gas and dust causes the radio optical correlations [53]. Multi-wavelength images of the host galaxy and the asymmetric radio lobes present observational evidence in support of such models involving ram pressure stripping. In the absence of X-ray data, we do not attempt to investigate the role of hot intracluster gas in detail. However, the UV and optical signatures in the host galaxies suggest the possible influence of ram pressure stripping in shaping the radio morphology. There may not be strong signs of ram pressure stripping like H i/H$\alpha$ tails, but far from the stellar disk, where gravity is weak compared to ram pressure, the lobes may be bent/distorted relatively easily. Such effects are commonly observed in bent-double radio sources within group environments [54].

4.1. NGC3898: Not Bending but Stripping

Based on the available evidence (i.e., based on current data), we consider the asymmetric radio lobes to be most plausibly associated with NGC3898, which is a member of a group of galaxies. If the host galaxy moves roughly in the direction of eastern radio jets, the hotspot will naturally be ahead of the bridge/back flow, and the core-to-hotspot distance will be small compared to that on the other side. On the other hand, the western side hotspot may be seen closer to the host than some of the diffuse emission in the lobe. While the jets/hotspots may be moving at a few percent of the speed of light, the host galaxies may be moving only at a few hundred km ${\mathrm{s}}^{-1}$ in groups and to a couple of thousands km ${\mathrm{s}}^{-1}$ in clusters. Such a group/cluster environment also creates head–tail and wide-angle-tailed (WAT) radio galaxies. By analogy with WAT radio galaxies, if the observed arm-length asymmetry in NGC 3898 were attributed to bending, one would expect the shorter (9 kpc) eastern lobe to be aligned closer to the line of sight, and the longer (28 kpc) western lobe to lie closer to the plane of the sky. However, the observed morphology contradicts this interpretation: the hotspot of the eastern (shorter) lobe lies at the farthest extent from the host galaxy, while in the western (longer) lobe, the backflow or diffuse emission is located beyond the hotspot. These structural details indicate that bending alone cannot account for asymmetry, suggesting the influence of another force, such as ram pressure stripping. In optical and UV images, the eastern half of the host galaxy appears relatively clear, while the western half exhibits multiple star-forming clumps and extends further from the galactic center. This asymmetry in the stellar disc supports an east-to-west ram pressure stripping scenario. Additional supporting evidence includes the observed depletion of H i gas on the eastern side and the presence of an H$\alpha$ tail extending westward, both of which are consistent with ram pressure acting from east to west. The stripping of the eastern radio lobe may also enable the backflow to reach or overtake the host galaxy. Similarly, on the western lobe, ram pressure stripping can push the diffuse emission beyond the hotspot. Notably, evidence of ram pressure stripping affecting both H i and radio continuum emission has been reported in multiple group environments [54,55], reinforcing the plausibility of such a mechanism in NGC 3898. As illustrated in the schematic (Figure 11a,b), ram pressure stripping acting approximately along the jet axis can lead to two distinct effects: backflow from the eastern lobe may reach or overlap with the host galaxy, while the lobe itself remains short; conversely, backflow from the western lobe may be displaced beyond its hotspot, resulting in an elongated morphology. Gas stripping becomes effective only where the gravitational potential of the stellar disc is sufficiently weak enough to be overcome by the external ram pressure [13]. Consequently, the host galaxy often appears morphologically undisturbed, with regular gas kinematics. Only in deep H i or H$\alpha$ imaging are long (∼100–200 kpc) ram pressure-stripped tails typically revealed. Future deep H$\alpha$ observations may help confirm or rule out the proposed mechanism. Additionally, a spatially resolved radio spectral index map could serve as an independent diagnostic of ram pressure effects on radio lobes.

4.2. RAD-Thumbs up Galaxy: Bent, Episodic, and Stripped

When an FR I or FR II deviates from the standard structure and has diffuse emission due to remnant radio lobes, it becomes difficult to associate it with a particular host galaxy. This becomes even more complicated in groups/clusters of galaxies. The diffuse thumb and fist component can be debatable if it is associated with the disturbed spiral (WISEA J221656.57-197042434.1) or unrelated radio phoenix. Due to the plume connecting the core with the fist, a faint filament connecting the core to the thumb, and an east-to-west gradient of the spectral index, we consider it as evidence that the disturbed spiral is the supplier of the diffuse plasma that is seen in the fist and thumb, which has aged with time. In some groups/clusters where jetted AGN is seen near diffuse emission, it has been suggested that the remnant radio lobes have probably been re-ignited by shocks and turbulence [44,45,46]. “Thumbs up” may represent such a case of revived remnant radio lobes, known as a radio phoenix. The spectral index map and structure of the emission (compact vs. diffuse emission) clearly suggest that the “Thumbs up” galaxy has two episodes of AGN-driven radio emission in the form of bent double lobes. Figure 11c,d presents a schematic of the “Thumbs up” galaxy. Due to the ram pressure, both young and old pairs of lobes are seen to the west of the host galaxy. Remnant lobes (large orange ellipses) are located to the west, nearly 10 times further away from the inner/younger lobes (small orange ellipses), and extend up to nearly 200 kpc from the center of the galaxy. As both of these lobes are on the same side of the host, the jet axis is expected to be inclined between parallel to the line of sight and in the plane of the sky. Hence, the real linear size of this spiral-host episodic radio galaxy (Speca) may be larger than 400 kpc or larger than typical 200–300 kpc radio galaxies hosted in ellipticals. These lobes are unlike typical episodic FR-II or double-double radio galaxies [56,57]. The spectral index map of typical FR-II radio galaxies shows the backflow as a steep spectral index and the end of the lobe or hotspot regions as flat. If the lobe plasma was stripped away from the host, the plasma seen at the farthest point would be the oldest or steepest. This is what is seen in the present case, suggesting that ram pressure stripping has shaped large-scale radio emission.

4.3. Unique Context

The effects of ram pressure on WAT radio galaxies are commonly observed, but these systems are almost always hosted by ellipticals, where direct evidence of ram pressure stripping is difficult to trace due to the lack of gas and dust. In contrast, spiral galaxies often show clear signs of ram pressure stripping, yet they rarely host double-lobed radio structures that extend beyond the optical size of the galaxy. Some known examples of ram pressure-stripped spiral galaxies with double radio lobes, though small in extent, include NGC 4438, NGC 4388 [58], and NGC 4569 [59].

Unless the radio lobes extend significantly beyond the host, it is not possible to directly study the effect of ram pressure from the ambient thermal gas on the nonthermal plasma of AGN-driven lobes. The two sources reported in this study provide rare examples where such interactions may be examined, offering unique targets for future observations with upcoming mega-telescopes. The interaction between thermal gas and nonthermal outflows has direct implications for the modeling of AGN feedback [60], especially in understanding how the outflow energy couples with the surrounding gas in the interstellar medium (ISM) and circum-galactic medium (CGM), a process that remains poorly constrained. Moreover, galaxies undergoing ram pressure stripping by intrafilament gas flows around clusters can also serve as useful tracers to constrain the properties of diffuse gas in the cosmic web.