The sub-arcsecond ILT view of the Boötes Deep Field: A link between low-frequency kiloparsec radio morphology and AGN driven ionised outflows

TL;DR

This study harnesses sub-arcsecond, low-frequency (144 MHz) ILT imaging of the Boötes Deep Field to resolve kiloparsec-scale radio morphologies for a luminosity- and redshift-matched sample of 47 AGN with [O III] outflow diagnostics. By combining wide-field VLBI-scale imaging with brightness-temperature analyses, the authors demonstrate a strong link between [O III] outflows and compact/extended radio emission on kpc scales, indicating AGN-driven mechanisms (jets, disk winds, shocks) as primary drivers in many cases, while non-outflow sources are largely dominated by diffuse, star-formation–related emission. Key findings include a 90±7% outflow detection rate when an AGN is detected at both 0.3'' and 6'', versus 63±9% for large-scale detections without small-scale confirmation; 17±6% of non-outflow sources show kpc-scale radio emission, compared to 51±12% of outflow hosts. The work also shows all extended sources host outflows, and that brightness-temperature–based AGN identifications align with the outflow detections, underscoring the AGN origin of the radio emission in these cases. The results illuminate the co-evolution of AGN activity and ionised gas dynamics and establish a framework for leveraging sub-arcsecond, low-frequency imaging in future wide-field surveys to study AGN feedback across cosmic time.

Abstract

Active Galactic Nuclei (AGN) outflows can regulate host galaxy evolution via AGN feedback. Ionised gas outflows have been linked to enhanced radio emission. In the first paper of this series, AGN detected with the International LOFAR Telescope (ILT) at 6" were more likely to host an [O III] 5007 outflow than AGN not detected, although only high-powered jets were ruled out as the origin of radio emission. New wide-field, sub-arcsecond resolution imaging at 144 MHz with the ILT now enables a resolved morphological study of this sample. We present the first wide-field, sub-arcsecond images of the Boötes Deep Field at 144 MHz, detecting 4074 sources in the $\sim$0.3" image with a central sensitivity of 33.8 uJy $\mathrm{beam^{-1}}$. For 47 AGN matched in AGN luminosity, we probe radio emission on kiloparsec-scales to investigate correlations with [O III] outflows. This sample spans $z<0.83$, $10^{40}<L_{\mathrm{[O III]}}<10^{43}~\mathrm{erg~s^{-1}}$, and $10^{21}<L_{\mathrm{144MHz}}<10^{24.5}~\mathrm{W~Hz^{-1}}$. We find that if we detect an AGN on both large-scales (6") and small-scales (0.3"), 90$\pm$7 per cent have an [O III] outflow, compared to 63$\pm$9 per cent of sources detected on large-scales, but undetected on small-scales. Furthermore, 17$\pm$6 per cent of sources without an [O III] outflow are detected on kiloparsec-scales, compared to 51$\pm$12 per cent of sources with an [O III] outflow. This implies a connection between [O III] outflows and kiloparsec-scale radio emission, which is likely AGN-driven. In contrast, AGN without an [O III] outflow are dominated by diffuse radio emission, likely to be associated with star formation.

Figures (13)

• Figure 1: $\textit{Left:}$ The $L_{\mathrm{6\muup\\ m}}$ and redshift relation of the AGN with and without an [O iii] outflow. The coloured markers show the matched populations in $L_{\mathrm{6\muup\\ m}}$ and redshift, with an [O iii] outflow (purple stars) and without an [O iii] outflow (pink circles). The grey points represent the AGN which are removed as these are unmatched, with diamonds portraying the AGN with [O iii] outflows, and circles for the AGN without an [O iii] outflow. We show uncertainties but these are minimal. The top histogram conveys the redshift distribution and the right histogram is the distribution of $L_{\mathrm{6\muup\\ m}}$. The hashed purple histograms are the AGN with [O iii] outflows and the pink solid histograms are the AGN which do not have an [O iii] outflow. $\textit{Right:}$$L_{\mathrm{144MHz}}$ as a function of $L_{\mathrm{[OIII]}}$, the coloured markers are from the $L_{\mathrm{6\muup\\ m}}$ and redshift matched population we use in this analysis, and the grey are unmatched. $L_{\mathrm{144MHz}}$ in this figure is calculated at 0.3$^{\prime\prime}$. Circles represent the detected AGN and downward triangles are the upper radio luminosity limits.
• Figure 2: The first $\sim$0.3$^{\prime\prime}$ resolution image of the Boötes Deep Field. This image consists of $\sim$8 billion pixels, contains over 4,000 sources, and has a central sensitivity of 33.8 $\muup$Jy $\mathrm{beam^{-1}}$. The image is 2.5 $\times$ 2.5 $\mathrm{deg^{2}}$ with a restoring beam of 0.50$^{\prime\prime}$$\times$ 0.34$^{\prime\prime}$. We highlight three interesting extended sources within this field beneath with the respective locations indicated by arrows. We can now probe radio emission of these sources to a new level thanks to the sub-arcsecond resolution.
• Figure 3: Demonstration of the morphology definitions in this work. Left: 6$^{\prime\prime}$ resolution, right: 0.3$^{\prime\prime}$ resolution. Top: Example of an "undetected" source, which does not have a $5\sigmaup$ detected at 0.3$^{\prime\prime}$ but is detected at 6$^{\prime\prime}$. Middle: A "compact" morphology. Bottom: "Extended" source. The black cross represents the LoTSS position, the white circle is the SDSS fibre and the contours are at a $5\sigmaup$ level.
• Figure 4: Montage showing the SDSS spectra and various morphologies of ILTJ143445.81+332818.1 ($z=0.197$) which lie within the Boötes field. Left: SDSS spectra data (black) with the MCMC fitting results overlaid as a dark purple solid line. We display both the $\lambdaup$4959 Å and $\lambdaup$5007 Å [O iii] emission lines. The pink Gaussians show the narrow component of [O iii], the Gaussians in magenta are the broad component of [O iii] which implies an [O iii] outflow is occurring. The yellow dotted line shows the continuum. The lower panel of this subplot displays the residuals between the fitted model and SDSS data and the pink shaded region corresponds to the 1$\sigmaup$ RMS region which we calculate over the full spectral range of the model. This AGN hosts an [O iii] fitted outflow. Right: Cutouts of ILTJ143445.81+332818.1 at multiple resolutions. Starting top left and proceeding clockwise: 6$^{\prime\prime}$, 1.2$^{\prime\prime}$, 0.3$^{\prime\prime}$, and 0.6$^{\prime\prime}$ resolution. The background and the pink 5$\sigmaup$ contours are from the respective resolution image. The scale bar in the bottom right corner demonstrates 6$^{\prime\prime}$ and the white circle near the centre of each cutout is the location of the SDSS fibre. At each resolution this source has been classified as extended, and we believe the radio core is located in the northern components due to its compact structure.
• Figure 5: Fraction of population in relation to sub-arcsecond low-frequency radio morphologies, where a radio source is classified as either undetected, compact, or extended. Left: Distribution of morphology with respect to [O iii] outflow population, i.e. for each [O iii] outflow population, the fractions of different morphology categories add up to unity. Right: Distribution of [O iii] outflow population with respect to the morphology category, i.e. for each morphology class, the fractions of the [O iii] outflow population adds to unity. The pink circle markers demonstrate the no [O iii] outflow population and the purple stars represent the outflowing population. In cases where the fraction of the population is either zero or one, the uncertainty is omitted. The grey dashed line separates the undetected AGN from the detected AGN.