LeMMINGs VII: 5 GHz, 50 mas e-MERLIN observations of a statistically complete sample of nearby AGN

Abstract

We present 5 GHz e-MERLIN radio images at 50 mas resolution of the nuclear regions of the Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies survey (LeMMINGs), the deepest statistically complete radio-band survey of the local Universe (<120 Mpc), consisting of 280 galaxies spanning all morphological and nuclear types. We detect nuclear radio emission above a median 5 sigma threshold of 0.33 mJy beam^-1 in 68 of 280 sources (24 percent), with core luminosities in the range 10^35 to 10^41.9 erg s^-1. The radio emission is attributed to active galactic nuclei, circumnuclear star formation, or, in the case of NGC 3690, a tidal disruption event. The brightest radio nuclei, with brightness temperatures >=10^6 K, reside in optically active galaxies such as LINERs and Seyferts. The detection rate for inactive systems (H II and absorption-line galaxies), which may host low-luminosity active galactic nuclei, is 8 percent. Most detections (78 percent) are compact (<10 pc), while the remaining 22 percent show extended jet-like features up to 380 pc. Compared to the 1.5 GHz LeMMINGs data, the 5 GHz observations provide superior resolution and spatial filtering, resolving out large-scale structures and isolating genuine nuclear emission. Our results suggest that low-luminosity active galactic nuclei are the primary manifestation of black hole activity in the local Universe in the form of compact jets and cores, with a preference for early-type hosts. The two LeMMINGs campaigns indicate that up to 30 percent of the local galaxy population hosts a radio-active nucleus, highlighting the necessity of high-resolution, high-sensitivity imaging for uncovering nuclear emission at the lowest luminosities.

Figures (11)

• Figure 1: The distribution of optical position offsets relative to the e-MERLIN observation pointing positions for all 280 sources in the sample. Four sources are further than 1-arminute away from the central pointing, with only NGC 5055 ($>$3-arcmin away) being re-processed in a specific way to ameliorate the problems of smearing (see Section \ref{['sec:selfcal']}). The vertical dashed line is the median offset of the sample which is 7.5 arcsec.
• Figure 2: Central 2.5$\times$2.5 arcsec$^{2}$ of the 5 GHz radio image of NGC 4203 as an example of the LeMMINGs 5 GHz data, made using APLpyAPLpy. See online supplementary material for all the images. The false colour background image ranges from 0 to 5 mJy beam$^{-1}$, with the colour bar shown at the top. The contour levels are set at the image r.m.s. (in this case 77 $\upmu$Jy beam$^{-1}$) multiplied by $-$3, 3, 5 (black contours) and 10, 50, 100, 250 (light blue contours). Negative contours are dashed. The light-blue plus represents the Gaia DR3 optical position, where available (Section \ref{['sec:Gaia']}). The pink circle denotes the average 1.5 GHz synthesized beam positioned at the 1.5 GHz 'core' position. The synthesized beam is shown in the bottom left corner of the image.
• Figure 3: Histograms of the image r.m.s. distribution of the noise in the LeMMINGs C-Band (5 GHz, grey) and L-Band (1.5 GHz, blue) samples. The median of the distributions are marked with dashed lines, corresponding to 66 $\upmu$Jy beam$^{-1}$ at 5 GHz and 83 $\upmu$Jy beam$^{-1}$ at 1.5 GHz.
• Figure 4: Radio core peak intensity (panel a) and luminosity (panel b), using the peak of the brightest component plotted as a function of distance (Mpc) for the full sample. The dashed lines represent the 5$\sigma$ flux density limit of the 5 GHz survey (0.33 mJy beam$^{\rm -1}$).
• Figure 5: The 5 GHz radio luminosity distribution (erg s$^{-1}$) from the observations presented in this work per panel a: optical nuclear class and panel b: host morphological type. The radio core luminosity and the total radio luminosity distributions are outlined by the solid-line and the dashed-line histograms, respectively. The "Undetected" plot in panel a shows the 5$\sigma$ upper-limit radio luminosity distribution for the undetected sources. No irregular galaxies are detected and core-identified.