RAD@home discovery of a bow-and-arrow radio galaxy tracing a ~560 kpc bow-shock structure in a multi-halo environment

Abstract: We report the RAD@home citizen science discovery of a unique bow-and-arrow-shaped radio galaxy (BAARG; RAD J104501.6+352852, z = 0.159) identified in LoTSS DR2. The source exhibits striking asymmetry: on the western side, a narrow jet feeds a sector-shaped emission region at ~115 kpc, extending backward to form a ~560 kpc arc-like structure; on the eastern side, the jet develops an S-shaped distortion extending to ~250 kpc, followed by a faint, offset tail reaching ~600 kpc. Our analysis shows that the host resides in a dynamically complex, multi-halo environment with nearby cluster-scale systems at similar redshifts. The observed morphology is consistent with interaction between the radio plasma and the surrounding medium, influenced by large-scale environmental gradients and bulk motions. The western structure is consistent with compression of radio plasma near a bow-shock-like feature, possibly linked to supersonic motion of the infalling host galaxy and its circumgalactic medium. This possibly represents one of the first instances in which morphology and environment together suggest signatures of infall- or shock-related processes; surveys such as LoTSS DR3 may reveal similar systems, offering new insights into the interplay between radio galaxies and their large-scale environments.

• The paper reports the RAD@home discovery of BAARG, a 715 kpc radio galaxy whose western arc extends about 560 kpc beyond the host and may trace a large-scale bow shock.
• The authors combine LoTSS radio imaging, optical redshifts, spectral-index data, and galaxy-environment analysis to identify three nearby cluster-scale halos and estimate a possible Mach number of about 2.4.
• The proposed infall-shock interpretation implies host-galaxy speeds of roughly 1,130–3,580 km s⁻¹, but deeper radio imaging and X-ray observations are needed to distinguish it from a wide-angle-tail or ram-pressure-bent galaxy.

Overview and discovery context

This paper reports the discovery, through the RAD@home citizen-science collaboratory, of a radio source with an unusual "bow-and-arrow" morphology, designated BAARG (RAD J104501.6+352852), at a spectroscopic redshift $z = 0.15939$. The source was identified during an online e-class on 24 May 2025 from LoTSS DR2 144 MHz data (2606.23106). Although previously catalogued as a giant radio galaxy (GRG) in the LoTSS GRG length-distribution study (Oei et al., 2022), its detailed morphology is presented here for the first time. The paper argues that the western arc-like structure traces a bow shock generated by the supersonic infall of the host galaxy into the cluster medium, making this possibly one of the first instances where radio morphology and environment jointly suggest infall- or shock-related processes.

The discovery pipeline is notable: RAD@home is a zero-funding, inter-university network in which trained citizen scientists combine multi-wavelength data via the RGB-maker tool. The authors emphasize that such rare, faint structures are often missed by automated classification; they cite their earlier ORC discovery that had been misclassified as a GRG by machine-learning-based studies (Mostert et al., 2024).

Radio morphology

At 144 MHz (6″ resolution, rms noise ~71 μJy beam⁻¹), the source shows a total flux density of ~320 mJy and a radio power of $2.2 \times 10^{25}$ W Hz⁻¹. Key morphological parameters are:

Feature	Extent	Notes
Western jet → sector-shaped region	~115 kpc	Conical flare brighter than the jet
Western bow/arc structure	~560 kpc total	NW half extends ~280 kpc
Eastern S-shaped jet distortion	~250 kpc	Brighter than initial plume
Eastern offset tail	~600 kpc	Undetected in NVSS/TGSS
Total projected size	~715 kpc	GRG class

The source is neither edge-brightened nor edge-darkened: the west resembles FR II-like features while the east resembles FR I plumes, giving it partial similarity to hybrid morphology radio sources (HyMoRS). However, the bow structure extends well beyond the lobe—unlike FR II backflow or bridge emission—which distinguishes it from typical HyMoRS and motivates the separate designation. Flux asymmetry is pronounced: 195 mJy on the west versus 125 mJy on the east despite comparable near-host jet brightness, indicating strong environmental influence.

Optical follow-up with BASS and HSCLA imaging identifies the host as a red ($u-r=3.2$) elliptical with $z_{\rm spec}=0.15939\pm0.00004$. An overlapping edge-on lenticular galaxy has $z_{\rm phot}=0.181$, inconsistent with interaction. Two galaxies (G1, G2) coincide with eastern S-structure radio peaks at photometric redshifts of $0.152\pm0.020$ and $0.196\pm0.046$ respectively; the authors note that a jet–galaxy interaction scenario for G1 cannot be ruled out but remains uncertain given photometric redshift uncertainties and the absence of deep high-resolution radio imaging.

Multi-halo environment

BAARG resides in a dynamically complex environment with three cluster-scale systems within ~12′, all spectroscopically confirmed and spanning a narrow redshift interval:

System	Redshift	Projected separation
Abell 1081 (≥83 members)	0.1588	5.9′
WHL J104449.3+352139	0.15759	7.6′
WHL J104454.9+354055	0.16294	12.1′

BAARG itself belongs to a Tempel et al. group (ID 325) with 12 spectroscopic members over $0.15480 \leq z \leq 0.16332$. The total redshift spread ($\Delta z \approx 0.0057$) corresponds to a line-of-sight velocity interval of ~1700–2000 km s⁻¹, exceeding typical cluster velocity dispersions (~10³ km s⁻¹) and even those of extremely massive systems (~1400 km s⁻¹). The authors therefore interpret the system as superposed cluster-scale halos rather than a single relaxed halo—an environment expected to host large-scale accretion and virial shocks associated with structure formation.

Bow-shock interpretation and velocity estimate

The authors propose that the host galaxy, together with its circumgalactic medium, moves supersonically relative to the ambient ICM, producing a large-scale bow shock highlighted by the jet-supplied plasma. Supporting evidence includes: (i) the bow extends to ~280 kpc, far beyond the ~115 kpc western lobe, ruling out backflow as the sole explanation; (ii) spectral index images (144–1400 MHz) show a tentative flattening toward the extreme western bow region rather than at the sector emission peak, consistent with compression-related re-acceleration—though the authors characterize this trend as tentative only.

Under the assumption that the arc is a bow shock, the Mach angle $\theta \approx 25°$ yields $2.2 \times 10^{25}$0. Adopting an ICM temperature range of $2.2 \times 10^{25}$1–$2.2 \times 10^{25}$2 K gives an infall speed of ~1130–3580 km s⁻¹, consistent with typical supersonic infall velocities [astro-ph/0408150-era estimates; Machacek et al. 2005]. This is an order-of-magnitude estimate resting on the simplified assumption that the arc geometry directly reflects a Mach cone.

The detection requires a temporal coincidence: simulations of NGC 1404 infall into Fornax (Sheardown et al., 2018) show the shock becomes indistinguishable roughly one billion years after first pericentric passage, whereas radio-jet activity lasts only tens of Myr. BAARG likely represents a rare alignment of active jet phase, favorable orientation (both jet and motion near the plane of the sky, supported by weak jet-sidedness), and first-passage timing.

An alternative interpretation is acknowledged explicitly: BAARG could be a wide-angle-tail or bent-tail galaxy shaped by ram pressure, buoyancy, precession, or bulk motions in the ambient medium. The complex multi-halo environment could account for the differing orientations of the tails. The paper does not decisively discriminate between the bow-shock and WAT scenarios.

Implications for related populations

Three consequences follow from the analysis. First, deeper observations of HyMoRS samples may reveal analogous bow features, since BAARG is partially HyMoRS-like. Second, if the host motion were aligned with the line of sight and jets faint, the hemispherical shock shell would appear as an edge-brightened ring—suggesting that some odd radio circles (ORCs) could be manifestations of the same phenomenon viewed differently; the authors propose investigating ORC host-galaxy motions. Third, deep low-frequency surveys (LoTSS DR3, SKAO) should reveal further systems with similar large-scale, low-surface-brightness features.

Limitations and open questions

Several caveats bear directly on the central claim. The bow-shock identification rests on morphology alone; no X-ray confirmation of a shock front exists, and X-ray evidence for galaxy-infall shocks remains sparse even in dedicated studies [Irwin & Sarazin 1996]. The Mach number and infall velocity derive from a single measured angle under idealized assumptions. The spectral-index flattening at the bow is described as tentative and lacks the resolution needed for a robust aging analysis. The possible jet–galaxy interactions with G1 and G2 require spectroscopic redshifts to establish physical association. Finally, the WAT/bent-tail alternative is not excluded by the present data; distinguishing between environmental ram-pressure bending and a genuine bow shock will require deeper multi-frequency radio imaging and X-ray observations of the field.

Conclusion

This paper presents a citizen-science discovery of a ~715 kpc radio galaxy whose western arc, extending to ~560 kpc, is interpreted as a bow-shock-like structure produced by supersonic infall of the host through a multi-halo environment containing Abell 1081 and two WHL clusters. With an estimated Mach number of ~2.4 and infall speeds of ~1130–3580 km s⁻¹, the system offers a candidate radio-band tracer of galaxy-infall shocks previously seen mainly in simulations and limited X-ray work. The interpretation remains provisional pending X-ray confirmation, spectroscopy of companion galaxies, and higher-resolution spectral aging measurements, but the source establishes a concrete observational template for future searches in LoTSS DR3 and SKAO-era surveys.