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arxiv: 2605.23750 · v1 · pith:TWEZQOH6new · submitted 2026-05-22 · 🌌 astro-ph.GA

Probing the environments of FRI and FRII radio galaxies in LoTSS DR2 with galaxy clusters

Tong Pan , Yuming Fu , H. J. A. Rottgering , J. M. G. H. J. de Jong , M. J. Hardcastle , B. Mingo , L. Clews , M. Magliocchetti
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J. W. Petley Bohan Yue
This is my paper

Pith reviewed 2026-05-25 03:23 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords FRI/FRII dichotomyradio galaxiesgalaxy clustersLoTSSintracluster mediumbrightest cluster galaxiesjet disruption
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The pith

FRII radio galaxies appear in galaxy clusters less often than FRI ones, especially at high luminosities, but occupy similar positions once inside clusters.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether galaxy cluster environments explain the morphological split between FRI and FRII radio galaxies. It matches a low-redshift LoTSS sample to DESI clusters using a fixed redshift and separation window, then compares association rates and internal properties. FRII sources show lower cluster membership overall, with the gap largest above 10^26 W/Hz. Inside clusters, however, the two classes follow matching trends in luminosity, stellar mass, cluster richness, and radial distance from the center, and both are mostly brightest cluster galaxies. The pattern implies cluster gas can suppress FRII structures but does not set the overall dichotomy.

Core claim

In the volume-limited sample, 48.6 percent of FRIs versus 30.6 percent of FRIIs meet the cluster association criteria of |Δz| < 0.01 and projected separation less than 2 R500; the fractions become 55.6 percent versus 19.0 percent at L_144 > 10^26 W Hz^{-1}. The luminosity-redshift paired sample yields similar differences. Cluster-associated FRIs and FRIIs nevertheless display comparable relations between radio luminosity, stellar mass, cluster richness, and M500, with both populations peaking near 0.5 R500 and declining beyond R500. Most sources in clusters are brightest cluster galaxies (74.8 percent FRIs, 61.9 percent FRIIs in the volume-limited sample).

What carries the argument

Cluster association defined by redshift difference |Δz| < 0.01 and projected separation < 2 R500, applied to compare occurrence rates, radial distributions, and scaling relations of FRI versus FRII radio galaxies.

If this is right

  • Dense intracluster gas can disrupt or slow jets, preventing stable FRII structures at high radio power.
  • Cluster-scale properties are unlikely to be the main cause of the FRI/FRII morphological difference.
  • Both FRI and FRII sources that reside in clusters are predominantly brightest cluster galaxies.
  • The radial distribution of cluster-associated sources of either type peaks near 0.5 R500 and drops beyond R500.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Smaller-scale factors such as jet power or the host galaxy's gas reservoir may dominate the morphological outcome.
  • Direct measurements of local intracluster medium density around individual sources could test the proposed jet-disruption picture.
  • Extending the same matching exercise to z > 0.4 would show whether the luminosity-dependent avoidance persists at earlier epochs.

Load-bearing premise

The redshift and separation thresholds correctly identify galaxies that are physically inside the same clusters rather than chance alignments.

What would settle it

Repeating the cross-match with spectroscopic redshifts accurate to |Δz| < 0.001 and finding that high-luminosity FRII association fractions equal those of FRIs would falsify the reported difference.

Figures

Figures reproduced from arXiv: 2605.23750 by B. Mingo, Bohan Yue, H. J. A. Rottgering, J. M. G. H. J. de Jong, J. W. Petley, L. Clews, M. J. Hardcastle, M. Magliocchetti, Tong Pan, Yuming Fu.

Figure 2
Figure 2. Figure 2: Distributions of radio luminosity at 144 MHz and redshift [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Distribution of radio luminosity at 144 MHz and redshift [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Cluster match fraction as a function of radio luminosity in [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Relationship between the cluster match fraction and stel [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 8
Figure 8. Figure 8: Relationship between the luminosity and distance to clus [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 10
Figure 10. Figure 10: Relationship between the radio luminosity and richness [PITH_FULL_IMAGE:figures/full_fig_p008_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: Relationship between the densities and distance to clus [PITH_FULL_IMAGE:figures/full_fig_p009_12.png] view at source ↗
read the original abstract

The origin of the Fanaroff--Riley Class I/II (FRI/FRII) morphological dichotomy remains uncertain. We investigate whether cluster-scale environment contributes to this distinction using a morphologically classified LoTSS DR2 catalogue at \(z<0.4\). We construct a volume-limited sample with \(L_{144}>4\times10^{24}\,\mathrm{W\,Hz^{-1}}\) and a luminosity--redshift paired sample, and cross-match them with DESI Legacy Imaging Survey galaxy clusters. A radio galaxy is associated with a cluster if \(|\Delta z|<0.01\), projected separation \(<2R_{500}\). In the volume-limited sample, \(48.6\%\) of FRIs and \(30.6\%\) of FRIIs are cluster-associated; in the paired sample, the corresponding fractions are \(45.6\%\) and \(32.6\%\). The difference is stronger at \(L_{144}>10^{26}\,\mathrm{W\,Hz^{-1}}\), where the fractions are \(55.6\%\) versus \(19.0\%\) in the volume-limited sample and \(50.0\%\) versus \(6.7\%\) in the paired sample. However, cluster-associated FRIs and FRIIs occupy similar environments: their radio luminosities and stellar masses show similar trends with cluster richness and \(M_{500}\), and their radial distributions both peak near \(0.5R_{500}\) and decline beyond \(R_{500}\). Most cluster-associated sources are brightest cluster galaxies (BCGs), with fractions of \(74.8\%\) for FRIs and \(61.9\%\) for FRIIs in the volume-limited sample, and \(78.1\%\) and \(65.9\%\) in the paired sample. These results show that FRIIs are less frequently found in clusters, especially at high radio luminosity, consistent with dense intracluster gas disrupting or decelerating jets and suppressing stable FRII structures. Nevertheless, once inside clusters, FRIs and FRIIs inhabit similar large-scale environments, implying that cluster-scale properties alone are unlikely to be the primary driver of the FRI/FRII dichotomy.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The paper claims that FRII radio galaxies from a morphologically classified LoTSS DR2 sample at z<0.4 are less frequently associated with galaxy clusters than FRIs (30.6% vs 48.6% in the volume-limited sample with L_144>4e24 W Hz^-1; 32.6% vs 45.6% in the luminosity-redshift paired sample), with the deficit stronger at L_144>1e26 W Hz^-1. Cluster-associated FRIs and FRIIs occupy similar large-scale environments (similar trends in radio luminosity and stellar mass with cluster richness and M_500; radial distributions peaking near 0.5 R_500), and most are BCGs. This is interpreted as evidence that dense intracluster gas can suppress stable FRII structures, but cluster-scale properties are unlikely to be the primary driver of the FRI/FRII dichotomy.

Significance. If the associations hold, the explicit percentages from defined volume-limited and paired samples provide a controlled observational constraint on environment's role in the FRI/FRII dichotomy, showing a morphology-dependent cluster occupancy that weakens at fixed luminosity while associated sources share similar properties. This adds to the literature on jet-environment interactions without relying on fitted parameters or self-referential derivations.

major comments (1)
  1. [Association criteria] Association criteria (as described in the abstract and sample construction): the adopted matching (|Δz|<0.01 and projected separation <2R500) is load-bearing for the headline cluster-fraction results (e.g., 48.6% vs 30.6% and the high-L split). At z<0.4 the redshift window spans ~3000 km/s (several times a typical cluster velocity dispersion) and the aperture reaches ~2-3 Mpc, raising the risk of line-of-sight interlopers. If contamination differs systematically between morphological classes, the reported FRII deficit could be partly spurious; the manuscript should quantify expected contamination (e.g., via random offsets or mock catalogs) or demonstrate robustness with stricter cuts.
minor comments (1)
  1. [Abstract and results] The abstract and results sections report fractions to one decimal place but do not state the absolute numbers of sources in each subsample; adding these (or a table of sample sizes) would improve transparency.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive comments on our manuscript. We address the single major comment below and will revise the paper accordingly to strengthen the robustness of the reported results.

read point-by-point responses

  1. Referee: [Association criteria] Association criteria (as described in the abstract and sample construction): the adopted matching (|Δz|<0.01 and projected separation <2R500) is load-bearing for the headline cluster-fraction results (e.g., 48.6% vs 30.6% and the high-L split). At z<0.4 the redshift window spans ~3000 km/s (several times a typical cluster velocity dispersion) and the aperture reaches ~2-3 Mpc, raising the risk of line-of-sight interlopers. If contamination differs systematically between morphological classes, the reported FRII deficit could be partly spurious; the manuscript should quantify expected contamination (e.g., via random offsets or mock catalogs) or demonstrate robustness with stricter cuts.

    Authors: We agree that a quantitative assessment of line-of-sight contamination is warranted given the adopted matching criteria. These criteria are standard in the literature for cluster associations at low redshift and were chosen to encompass typical cluster velocity dispersions and projected extents. Because the FRI and FRII samples are drawn from the same parent catalogue and classified morphologically (independent of environment), any differential contamination is expected to be modest; however, this remains an assumption. In the revised manuscript we will add an explicit contamination estimate using randomized offset catalogues (both positional and redshift shifts) applied separately to the FRI and FRII subsamples. The resulting false-association rates will be reported for the volume-limited and paired samples, including the high-luminosity subsets, together with a brief discussion of their impact on the observed FRII deficit. We will also present results for a stricter redshift cut (|Δz|<0.005) as a robustness check. These additions will directly address the referee's concern without altering the headline conclusions. revision: yes

Circularity Check

0 steps flagged

No circularity; purely observational catalog cross-matching and counting

full rationale

The paper's central results consist of empirical fractions of radio galaxies associated with clusters, obtained by applying fixed, externally motivated matching criteria (|Δz|<0.01 and projected separation <2R500) to existing catalogs and then counting. No model equations, parameter fits, predictions, or self-citations are invoked to derive these fractions; the association step is a direct data operation, not a derivation that reduces to its own inputs. Secondary statements about similar environments inside clusters are likewise direct comparisons of the matched subsamples. No load-bearing step matches any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The claim rests on the accuracy of the input morphological catalogue and the physical meaning of the adopted spatial-redshift matching window; no free parameters or new entities are introduced.

axioms (2)
  • domain assumption The morphological classification of FRI/FRII sources in the LoTSS DR2 catalogue is reliable and unbiased.
    The paper uses a pre-classified catalogue without re-deriving morphologies.
  • domain assumption The redshift and projected-separation cuts identify true physical associations rather than line-of-sight projections.
    These cuts define the cluster-associated subsample central to all reported fractions.

pith-pipeline@v0.9.0 · 6005 in / 1341 out tokens · 32308 ms · 2026-05-25T03:23:57.245863+00:00 · methodology

discussion (0)

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