Determine whether extended fossil radio lobes contribute to the SZ excess

Determine whether larger and fainter fossil radio lobes extending to tens of kiloparsecs around NGC 4594 contribute significantly to the measured Sunyaev–Zel'dovich signal.

Background

The observed SZ signal around NGC 4594 exceeds the signal predicted from the detected soft X-ray–emitting gas. One possible explanation is a nonthermal SZ contribution from relativistic electrons associated with fossil radio lobes. Although the known central radio source and approximately 10 kpc radio lobes lie within the excluded inner region, larger and fainter lobes extending farther into the halo have not been detected and could affect the SZ measurement.

Resolving this issue requires spatially resolved low-frequency radio observations capable of detecting or constraining extended fossil lobes and determining whether their cosmic-ray pressure and line-of-sight extent are sufficient to produce a significant nonthermal SZ distortion.

References

The known central radio source and the observed $10$~kpc radio lobes are contained within the excluded $r<10$ region \citep{Yang2024}, but larger and fainter fossil lobes extending to tens of kiloparsecs cannot presently be ruled out. Such lobes could, in principle, produce a nonthermal SZ distortion if their cosmic-ray pressure and line-of-sight extent are sufficiently large. However, no extended fossil lobes are currently detected around NGC~4594, and the conversion of a nonthermal spectral distortion into the thermal Compton-$y$ values reported by the Planck-based maps is model dependent, particularly on the low-energy cutoff and spectral index of the electron population \citep{Colafrancesco2008}. Deep spatially resolved low-frequency radio imaging will be required to determine whether fossil radio lobes contribute significantly to the measured signal (e.g., \citealt{Heald2022,Heesen2024}).

— A Possible Hidden Hot Halo Component Revealed by Joint X-ray and Sunyaev--Zel'dovich Observations  (2609.18006 - Li et al., 16 Sep 2026) in Section 5, Discussion and Prospects, paragraph beginning “A possible alternative contribution”

A smaller intrinsic discrepancy, a scenario in which the X-ray and SZ signals arise largely from the same gas, or a nonthermal contribution from an extended relativistic-electron population cannot currently be excluded.

— A Possible Hidden Hot Halo Component Revealed by Joint X-ray and Sunyaev--Zel'dovich Observations  (2609.18006 - Li et al., 16 Sep 2026) in Section 6, Summary and Prospects, paragraph beginning “We presented a joint X-ray”