CHANG-ES. XXXVIII. A Thin Radio Halo Shaped by Slow Cosmic-Ray Transport in the Quiescent Galaxy NGC 4565

Abstract We present the Very Large Array C-array S -band (2–4 GHz) radio continuum observations of the nearby edge-on spiral galaxy NGC 4565, a target from the Continuum Halos in Nearby Galaxies—an EVLA Survey. We conduct rotation measure (RM) synthesis to probe the magnetic field structure and analyze the vertical radio continuum intensity profiles using the 1D cosmic-ray (CR) transportation models. The radio continuum emission of NGC 4565 is vertically compact, with a vertical-to-radial extent ratio of ∼1/6. Its vertical profile is optimally described by a two-component Gaussian distribution, yielding a mean Gaussian halo scale height of ∼3.0 kpc. The magnetic field is weak, predominantly disk parallel, with an equipartition strength of ≲5 μ G and an RM profile indicative of an axisymmetric spiral structure. Nevertheless, we identify a localized, faint vertical magnetic field component in the northeastern region, hinting at an X-shaped structure that spatially coincides with extraplanar structures detected in H i and soft X-ray emission. The CR transport modeling favors a flux-tube advection scenario, with a slow initial velocity of v 0 ≈ 60 km s −1 , consistent with a limited energy input from star formation. Therefore, the absence of an extended radio halo can be explained by the low star formation rate, the weak magnetic field, and the inefficient CR transport. The localized X-shaped field may trace a weak, magnetically guided outflow or a tidal perturbation induced by the nearby companion. NGC 4565 is thus a key quiescent benchmark for understanding the physical conditions required to drive large-scale outflows and generate extended radio halos.