Peering into the heart of darkness with VLBA : Radio Quiet AGN in the JWST North Ecliptic Pole Time-Domain Field

Payaswini Saikia, Ramon Wrzosek, Joseph Gelfand, Walter Brisken, William Cotton, S. P. Willner, Hansung B. Gim, Rogier A. Windhorst, Vicente Estrada-Carpenter, Ivan Yu. Katkov, Ingyin Zaw, Michael Rosenthal, Hanaan Shafi, Kenneth Kellermann, James Condon, Anton M. Koekemoer, Christopher J. Conselice, Rafael Ortiz, Christopher N. A. Willmer, Brenda Frye, Norman A. Grogin, Heidi B. Hammel, Seth H. Cohen, Rolf A. Jansen, Jake Summers, Jordan C. J. D'Silva, Simon P. Driver, Nor Pirzkal, Haojing Yan, Min S. Yun

First listed 2025-06-22 | Last updated 2025-06-22

Abstract

We present initial results from the 4.8 GHz Very Long Baseline Array (VLBA) survey of the JWST North Ecliptic Pole Time-Domain Field (TDF). From 106 radio sources found in the Karl G. Jansky Very Large Array observations in the TDF, we detected 12 sources (11% detection rate) at 3.3 $μ$Jy rms sensitivity and 4 mas resolution. Most detections exhibit pc-scale emission (less than 40 pc) with high VLBA/VLA flux density ratios and brightness temperatures exceeding 10$^5$ K, confirming non-thermal AGN activity. Spectral indices ($>$ -0.5) correlate with higher VLBA/VLA flux ratios, consistent with synchrotron emission from AGN coronae or jets. In the majority of our sources star formation contributes less than 50% of the total VLBA radio emission, with a few cases where the emission is almost entirely AGN-driven. Although the radio emission from radio quiet AGN is thought to be primarily driven by star formation, our VLBA observations confirm that there is also often a contribution at various levels from black hole driven AGN. Eight VLBA detections have JWST/NIRCam counterparts, predominantly early-type, bulge-dominated galaxies, which we use to get an estimate of the redshift and star formation rate (SFR). WISE colors indicate that VLBA detections are either AGN or intermediate-disk-dominated systems, while VLBA non-detections correspond to extended, star-forming galaxies. We compare SFRs derived from previous SCUBA-2 850 $μ$m observations with new JWST-based estimates, and discuss the observed discrepancies, highlighting JWST's improved capability to disentangle AGN activity from star formation.

Short digest

A 4.8 GHz VLBA snapshot survey of the JWST NEP Time-Domain Field targeted 106 VLA-selected radio sources and detected 12 compact emitters at 3.3 μJy rms and 4 mas resolution. The detections show pc-scale (<40 pc) structure with high VLBA/VLA flux ratios and Tb >10^5 K, and flatter spectra (α > −0.5) correlate with higher compactness—consistent with synchrotron from AGN coronae or low-power jets. Star formation contributes <50% of the VLBA flux in most cases, with several cores essentially AGN-driven; eight have JWST/NIRCam counterparts in predominantly early-type, bulge-dominated hosts, and WISE colors separate compact AGN/intermediate disks from extended SF galaxies. Comparing SCUBA-2 850 μm SFRs to new JWST-based estimates reveals discrepancies, underscoring JWST’s leverage to disentangle black-hole–driven radio cores from host-galaxy star formation.

Key figures to inspect

• Figure 1: Use the point vs. Gaussian peak comparison to gauge source compactness; note PC 64 is resolved and PC 67 marginally so—key for distinguishing true pc-scale cores from slightly extended structures.
• Figure 2: Inspect each 4.8 GHz VLBA panel for mas-scale morphology and elongation; confirm which sources are point-like versus resolved and relate this to the inferred emission mechanism (coronal vs. jet).
• Figure 3: Cross-check the eight VLBA detections with JWST/NIRCam cutouts to see bulge-dominated hosts and the precise VLBA–NIR positional alignment within the VLA beam, informing counterpart reliability and host type.
• Figure 4: Examine Legacy Survey RGB cutouts for PC 24, PC 41, and PC 64 to verify optical morphology and environment, complementing the NIR classifications and compactness seen with VLBA.