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

Abstract

We present the VLA 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 (CHANG-ES) Survey. We conduct rotation measure synthesis to probe the magnetic field structure and analyze the vertical radio continuum intensity profiles using the 1-D cosmic ray transportation models. The radio continuum emission of NGC 4565 is vertically compact, with a vertical-to-radial extent ratio of $\sim 1/6$. Its vertical profile is optimally described by a two-component Gaussian distribution, yielding a mean Gaussian halo scale height of $\sim 3.0$ kpc. The magnetic field is weak, predominantly disk-parallel, with an equipartition strength of $\lesssim 5\ μ$G and a rotation measure 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 $v_0 \approx 60$ 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.

Figures (10)

• Figure 1: $S$-band total intensity images of NGC 4565. (a) Contours from the robust zero weighting image are overlaid on a three-color SDSS optical image constructed from the $i$, $r$, and $g$ filters. The contour levels start at $3\sigma = 10.5 \,\mu{\rm Jy\,beam}^{-1}$ and increase by factors of 2. The beam size of this panel is $4\farcs99 \times 4\farcs62$. The white box indicates the region shown in panel (c). (b) Contours from the $10\arcsec$ uv-tapered image, overlaid on the same SDSS optical image. The contours begin at $3\sigma = 11.2 \,\mu{\rm Jy\,beam}^{-1}$ and increase by factors of 2. The beam size of this panel is $10\farcs32 \times 10\farcs26$. (c) A detailed grayscale view of the central ring-shaped structure from the robust zero-weighting image. In each panel, the synthesized beam is shown as a black ellipse in the bottom-right corner.
• Figure 2: Polarization properties of NGC 4565 derived from RM synthesis. Left panel: Map of the polarized intensity. Right panel: RM map, with the Galactic foreground contribution subtracted. The intrinsic magnetic field orientations, corrected for Faraday rotation, are shown as black lines. The RM values and field lines are blanked for polarized intensities below $5\sigma$, where $\sigma = 3.12 \,\mu{\rm Jy\,beam}^{-1}$. In both panels, total intensity contours from Figure \ref{['fig:total_intensity']}b at levels of 3, 24, and 192$\sigma$ are overlaid to outline the galaxy's structure. The $15\arcsec$ synthesized beam is displayed in the bottom-right corner.
• Figure 3: RM distribution along the major axis of NGC 4565. (a) The RM map overlaid with $S$-band total intensity contours (black lines, levels at 30, 40, 60, and $120\,\sigma$; see Figure \ref{['fig:total_intensity']}c) to highlight the inner radio ring structure. The rectangular boxes indicate the $15\arcsec\times15\arcsec$ bins used to extract the RM profile. (b) The RM profile along the major axis. Each data point represents the inverse-variance weighted mean RM within the bin. The $x$-axis represents the distance from the galactic center, where negative values correspond to the southeast side. In each panel, the red arrows mark the positions of the local maximum ($\sim 4.5$ kpc, NW) and local minimum ($\sim -7$ kpc, SE).
• Figure 4: Distribution of magnetic field orientations in four galactic quadrants. (Top) Map of the magnetic field orientations, where the data from Figure \ref{['fig:magnetic_field']} has been divided into four quadrants (labeled I, II, III, and IV). Regions around the galactic major and minor axes have been masked out. (Bottom) Rose diagrams illustrating the magnetic field orientation distribution for each quadrant. In each plot, individual data points are scattered around the circle, and the angular distribution is shown as a gray histogram. The solid red line indicates the circular mean of the distribution.
• Figure 5: Vertical profiles of the non-thermal radio continuum emission at $S$-band. The top-center panel shows the locations of the five analysis strips (E2, E1, C, W1, and W2) overlaid on the non-thermal intensity map. The other five panels display the corresponding vertical profile for each strip, arranged symmetrically around the setup panel. In each profile panel, the black points represent the measured flux density, averaged within $6\arcsec\times120\arcsec$ ($0.35\times6.92$ kpc) boxes. Positive distances are to the north of the major axis, and negative distances are to the south. The red curve is the best-fit two-component Gaussian profile.