A multi-wavelength approach of AGN feedback in LINERs: The case of NGC 4438

TL;DR

This study investigates jet-mode feedback in the LINER NGC 4438 by combining high-resolution radio data from e-MERLIN and the VLA (L, C, X bands) with Chandra X-ray observations and optical MEGARA IFS. A spatially resolved minimum-energy model is applied to the radio maps to derive $B_{ ext{min}}$, $u_{ ext{min}}^{ ext{CR}}$, $t_{ ext{syn}}$, diffusion length $D_L$, and jet power, revealing a dominant NW-lobe/bubble structure connected to the nucleus. The results show a jet power of approximately $5\times10^{44}$ erg s$^{-1}$, about three orders of magnitude larger than the ionised outflow power, consistent with jet-driven feedback that shock-ionises gas and yields thermal X-ray emission coincident with the radio/Hα cavity. The multi-wavelength morphology and energetics indicate that NGC 4438 hosts a low-luminosity, weakly collimated jet impacting the dense northern ISM, making this system a valuable pilot for a larger program targeting jet-ISM interactions across LINERs.

Abstract

The presence of multi-phase outflows in low ionisation nuclear emission-line regions (LINERs) has been confirmed to be frequent, but the mechanisms that launch them are still under study. We aim to explore the connections between the ionised gas outflow, radio continuum structures and X-ray emission detected in the LINER NGC4438. We analyse L, C and X-band images (from 1.4 to 12 GHz) of the LINER NGC4438, combining high-resolution data from enhanced Multi Element Radio Linked Interferometer Network (e-MERLIN) and Karl G Jansky Very Large Array (VLA). We produce radio flux, spectral index maps, and an energetic model that allows us to characterise the source. We incorporate optical integral field spectroscopy (IFS) data (GTC/MEGARA) and Chandra X-ray data, with comparable resolution, to better trace the outflow, the AGN and their potential connection. We present new L, C, and X-band high-resolution, high-sensitivity radio images and spectral-index maps that probe $\sim$ 25 pc scales in NGC 4438. These data reveal a close morphological correspondence between the radio structures and the ionised gas bubble. Using a spatially resolved energetic model based on radio flux and spectral index, we disentangle the compact AGN emission from the extended bubble for the first time, establishing their distinct physical origins. We measure a kinetic power of $\sim 5\times 10^{44}$ erg s$^{-1}$ for the radio bubble, exceeding the power of the ionised outflow by more than three orders of magnitude. Our multi-wavelength analysis indicates that NGC 4438 is undergoing jet-mode feedback, where a low-luminosity, weakly collimated jet impacts the dense northern interstellar medium. This interaction drives shock-ionised gas, produces a moderate velocity outflow that removes material from the region, and generates thermal X-ray emission coincident with the radio and H$α$ cavity.

Figures (7)

• Figure 1: NGC 4438 radio structures. Top: C-band from VLA data. Center: L-band image from blending e-MERLIN and VLA data. Bottom: X-band image with VLA data. Contours: We represent in green -3$\sigma$ regions, the two black contours represent 3$\sigma$ and 0.05 mJy, and the three white ones 0.075 mJy, 0.15 mJy and 0.35 mJy. The ellipse in the bottom left indicates the restoring beam. Despite the similar resolution in the C and X-band images, the diffuse emission captured in the C-band makes the image appear smoother. This is an intrinsic effect limited by wavelength.
• Figure 2: Scheme presenting the substructures of NGC 4438. It is based on the C-band image (see Fig. \ref{['fig:1']} and Sect. \ref{['Results_C']}). The contours are at 3$\sigma$, 0.03 mJy, 0.08 mJy, 0.2 mJy and 0.35 mJy. The orange line shows the hourglass-like structure mentioned in the Sect. \ref{['Results_C']}. The NW side of the orange figure encloses the region referred to as the 'base of the cone'. We name the circular cavity in the main bubble (blue circle) the 'hole'.
• Figure 3: Top: C-band spectral index map created pixel by pixel with a S/N threshold of 5. Bottom: Error in the spectral index value for each pixel.
• Figure 4: Maps of the minimum energy parameters assuming k = 40. (a) Magnetic field strength, $B_{min}\,$, in $\mu$G. (b) Cosmic-ray energy density, $u_{min}^{CR}\,$, in eV cm$^{-3}$. (c) Lifetime of the particles, $t_{syn}\,$, in Myr. The contour is 1.3 Myr. (d) Diffusion length, $D_{L}\,$, in pc. (e) Velocity in km s$^{-1}$. (f) Cosmic-ray power, P, in erg s$^{-1}$.
• Figure 5: X-ray Chandra images with radio C-band contours. Top: Soft X-rays (0.5-2 keV). Bottom: Hard X-rays (2-10 keV) (see Sects. \ref{['Data_reduction3']} and \ref{['Results_xray']} for details). Contours: 3$\sigma$ (black), 0.05 mJy (white), 0.075 mJy (black), 0.15 mJy (white) and 0.35 mJy (black). The 0.35 mJy contour locates the nucleus. In the bottom panel, we indicate the five regions in which we have fitted spectra: (1) Nuclear, (2) North, (3) South, (4) West and (5) External.