High-resolution radio imaging of TGSSJ1530+1049, a radio galaxy in a dense environment at z=4

TL;DR

This work presents high-resolution radio imaging of TGSSJ1530$+$1049 at $z=4.0$ using EVN ($1.7$ GHz) and e-MERLIN ($1.5$ and $5$ GHz) to resolve mas-scale jet features and compare them with JWST/NIRSpec IFU H$ extalpha$ maps. It reveals two compact mas-scale components (A and B) separated by about $\sim 400$ mas ($\sim 2.8$ kpc) with high brightness temperatures, indicating AGN-driven jets but no clear radio core is detected at cm wavelengths. Additional jet-like emission is found at $\sim 780$ mas scales (N and S) with steep spectra, yielding a total radio power at $1.4$ GHz of $\approx 3\times10^{27}$ W Hz$^{-1}$ and a linear size of $\sim 5.5$ kpc, placing the source in the category of medium-sized symmetric objects. The radio structures are closely aligned with JWST-detected ionized gas, which extends to $\sim 25$ kpc, signaling strong jet–gas interactions within a densely packed galaxy environment that is likely in the process of assembling a massive central galaxy.

Abstract

High-redshift radio galaxies can provide insights into the structure formation and galaxy evolution at earlier cosmological epochs. TGSSJ1530+1049 was selected as a candidate high-redshift radio galaxy. Subsequent observations with the James Webb Space Telescope (JWST) presented in a companion paper (Saxena et al., submitted) have shown that it is located at a redshift z=4.0. The JWST data furthermore showed that the radio source is part of one of the densest structures of galaxies and ionized gas known at these redshifts. The complex system qualitatively resembles a massive (cluster) galaxy forming early through a rapid succession of mergers. TGSSJ1530+1049 is an unresolved source down to ~0.6" scale in multiple radio surveys. To reveal its high-resolution radio structure and allow for a detailed comparison with JWST observations, we studied its morphology at various angular scales with different radio interferometric instruments. We observed TGSSJ1530+1049 at milliarcsecond (mas) scale angular resolution with the European VLBI Network (EVN), and at ~100-mas scale resolution with the enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN). We recovered a complex north--south oriented structure with steep-spectrum radio-emitting features, which are associated with lobes and hot spots of a jetted active galactic nucleus. The centre of the radio galaxy proved to be too faint at cm wavelengths to be unambiguously detected in our observations. Nevertheless, the linear size (~5.5 kpc) and the radio power place it among the so-called medium-sized symmetric objects, a smaller and/or confined version of larger radio galaxies. Comparison between the radio morphology and that of the ionized gas observed by JWST shows that the two are closely aligned. However, the optical emission line gas extends out to ~25 kpc, which is well beyond the detected radio structures. (Abridged)

Figures (6)

• Figure 1: EVN 1.7-GHz maps of J1530$+$1049. On the right-hand side, the two detected features are shown zoomed-in. The peak intensity of the whole image is $0.70\mathrm{\,mJy\,beam}^{-1}$. The contour levels are at $(\pm0.23, 0.33, 0.47, 0.66)\mathrm{\,mJy\,beam}^{-1}$. The lowest positive contour level corresponds to $5\sigma$ image noise. The restoring beam is shown in the lower left corner of the panels. Its FWHM size is $4.48\mathrm{\,mas} \times 3.61\mathrm{\,mas}$ at a position angle of $27\degr$.
• Figure 2: 1.5-GHz e-MERLIN image of J1530+1049. The peak intensity is $3.7\mathrm{\,mJy\,beam}^{-1}$. The lowest contours are drawn at $\pm0.24\mathrm{\,mJy\,beam}^{-1}$ corresponding to $6\sigma$ image noise level. Further positive contour levels increase by a factor of two. The restoring beam is shown in the upper left corner, its FWHM size is $284 \mathrm{\,mas} \times 120$ mas and its major axis is oriented at a position angle of $21\degr$.
• Figure 3: 5-GHz e-MERLIN image of J1530+1049. The peak intensity is $0.17\mathrm{\,mJy\,beam}^{-1}$. The lowest contours are drawn at $\pm0.046\mathrm{\,mJy\,beam}^{-1}$ corresponding to $3.5\sigma$ image noise level. Further positive contour levels increase by a factor of $\sqrt{2}$. The restoring beam is shown in the upper left corner, its FWHM size is $121 \mathrm{\,mas} \times 36$ mas and its major axis is oriented at a position angle of $23\degr$. The pink crosses indicate the positions of the EVN-detected components.
• Figure 4: The flux densities of the two Gaussian components that can be fitted to the e-MERLIN visibilities of J1530$+$1049. Circles and squares are for the components N and S, respectively. The solid and dashed lines are power-law fits to the corresponding data points. For comparison, the sum of the flux densities of the EVN-detected components is also shown with a yellow upward triangle. Empty downward brown triangles show lower-resolution flux density measurements from various surveys (see Sect. \ref{['sec:large_scale']} for details), and the brown line indicates the power-law fit to those points. The numbers show the corresponding spectral indices.
• Figure 5: The e-MERLIN images of the calibrator source, J1525$+$1107. Left: at $1.5$-GHz. The peak intensity is $420\mathrm{\,mJy\,beam}^{-1}$. The lowest positive contour is drawn at $1.1\mathrm{\,mJy\,beam}^{-1}$ corresponding to $7\sigma$ image noise level. The FWHM size of the restoring beam is $321 \mathrm{\,mas} \times 144$ mas and its major axis is oriented at a position angle of $21\degr$. Right:$5$-GHz. The peak intensity is $274\mathrm{\,mJy\,beam}^{-1}$. The lowest positive contour is drawn at $0.75\mathrm{\,mJy\,beam}^{-1}$ corresponding to $7\sigma$ image noise level. The restoring beam is shown in the lower left corner, its FWHM size is $91 \mathrm{\,mas} \times 44$ mas and its major axis is oriented at a position angle of $24\degr$. The two image cutouts have the same size. To illustrate that the $1.5$-GHz peak corresponds to the secondary rather than the primary brightness peak at $5$ GHz, the images are displaced by $110$ mas with respect to each other in the declination direction.