A pilot VLBI study of the SQUAB quasar sample featuring multiple Gaia detections

TL;DR

This study tests the Gaia–VLBI approach on a radio-bright subset of the SQUAB quasar sample (SMGDs) by targeting three brightest GAIA-associated sources with VLBI at $1.6$ and $4.9$ GHz. High-resolution imaging at the Gaia positions reveals compact cores coincident with GAIA1 and no detectable radio emission at the secondary Gaia positions, with J1433+4842 additionally showing a jet-like feature between Gaia components. The results favor quasar–star pair interpretations over dual AGN or gravitationally lensed systems, validating the Gaia–VLBI method for disentangling ambiguous Gaia matches and removing contaminants from dual-AGN samples. The work demonstrates the capability and limitations of current VLBI sensitivity to test Gaia-based multiplicity, and motivates follow-up observations of the remaining radio-bright SMGDs and future, more sensitive surveys.

Abstract

Our previous work identified a class of SDSS quasars exhibiting multiple Gaia detections, classifying them as candidates for various astrophysical systems such as quasar-star pairs, dual quasars, and gravitationally lensed quasars. In this paper, we present a pilot VLBI study targeting a radio-bright subsample and report the first high-resolution imaging results. By leveraging the milliarcsecond-scale resolution of VLBI and its precise astrometric coordination incorporating with Gaia, we aim to refine the classification of these multiple matched sources, search for potential dual AGNs, and assess the efficacy of the combined Gaia-VLBI approach in resolving ambiguous quasar systems. We cross-matched the SQUAB quasar sample with the FIRST and NVSS catalogs, identifying 18 radio-emitting sources. The three brightest were selected for dual-frequency (1.6 and 4.9 GHz) VLBA observations. We performed VLBI imaging at both Gaia positions, constructed spectral index maps, and estimated brightness temperatures to characterize the radio morphology and physical properties. For the three target sources, our VLBI observations reveal compact radio structures consistent with single AGN at the primary Gaia positions. No significant emission is detected at the secondary Gaia locations. These results support the interpretation of the sources as quasar$-$star pairs, in line with earlier studies. This pilot study demonstrates the value of radio-VLBI high-resolution follow-ups on Gaia-selected quasar systems with multiple counterparts, showing how they can unambiguously reveal the true nature of these systems and help remove contaminants from dual AGN candidate samples.

Figures (9)

• Figure 1: Spectral index maps of the target sources. Contours denote the low-frequency (1.6 GHz) intensity map, with the corresponding synthesized beam shown in the bottom-left corners. The lowest contour corresponds to $4\sigma$, and subsequent levels increase by factors of two. The color scale represents the spectral index distribution between 1.6 and 4.9-GHz (see Sect. \ref{['sec:spx']}).
• Figure 2: Naturally weighted wide view VLBI image of J1433$+$4842 at 1.63 GHz. It includes the first Gaia position (GAIA1) the second Gaia position (GAIA2) and the extended emission that lies between the two positions (EXT1). To note that the shape of EXT1 in the picture is not exact the same with the zooming-in images showing in Fig. \ref{['fig:J1433_small']} due to the bandwidth smearing effect caused by the large distance between the source and the image phase center (I used the GAIA1 position as the big image's phase center). The lowest contours represent 4$\sigma$ ($\sigma$ the post-CLEAN background noise) and contour levels increase by a factor of 2.
• Figure 3: Naturally weighted wide view VLBI image of J1520$+$4211 at 1.63 GHz showing both GAIA1 and GAIA2 positions. The lowest contours represent 4$\sigma$ and contour levels increase by a factor of 2. The single-source zooming-in image of each position are shown in Fig. \ref{['fig:J1520_small']}.
• Figure 4: Naturally weighted wide view VLBI image of J0928$+$5707 at 1.63 GHz showing both GAIA1 and GAIA2 positions. The lowest contours represent 4$\sigma$ and contour levels increase by a factor of 2. The single-source zooming-in image of each position are shown in Fig. \ref{['fig:J0928_small']}.
• Figure 5: Naturally weighted VLBI single-source images of each target position for J1433$+$4842 at 1.7 and 4.9 GHz. The white crosses in the GAIA1 and GAIA2 images mark the corresponding Gaia positions in each source position. For each CLEANed image, the synthesized beam shape are shown at the bottom left corner. The lowest contours represent 4$\sigma$ and contour levels increase by a factor of 2. For the GAIA2 images, no significant signal can be detected above 5$\sigma$. For this source in the image, 1 mas represents 8.413 $\rm pc$.