Radio-Near Infrared Imaging of Dual Active Galactic Nuclei Candidates

TL;DR

We investigate two local galaxies with double-peaked [OIII] emission for evidence of dual/binary AGNs by combining high-resolution HST NIR imaging with VLBA radio continuum imaging. In both targets, we detect two compact radio sources with $T_b > 10^{8}$ K and locate the NIR AGN via GALFIT modeling, finding no significant spatial offsets between radio and NIR AGN positions, with 3σ upper limits on projected separations of $<268$ pc and $<297$ pc. The results admit either a parsec-scale dual/binary AGN or a radio core–jet scenario, and the current single-frequency radio data cannot distinguish between them. Future matched-resolution, multi-band radio observations are needed to measure spectral indices and definitively identify dual cores versus jet components, advancing our understanding of the prevalence and nature of closely separated AGN pairs in merging galaxies.

Abstract

We report the results of a pilot study that searched for dual active galactic nuclei (AGN) in local ($z<$0.25) galaxies hosting double-peaked narrow emission lines in their optical spectra. We present high-resolution $L-$band (1.5 GHz or 18 cm) continuum images from the Very Long Baseline Array (VLBA) as well as WFC3/IR F160W images from the \textit{Hubble Space Telescope} of two candidate dual AGN systems: J0948+6848 and J1223+5409. In both targets, we detected compact non-thermal radio emission that is approximately co-spatial with the near-infrared AGN. Both systems host two high brightness temperature ($>10^{8}$ K) radio sources that indicate the presence of either a parsec-scale-separation dual AGN ($d_{\text{sep}} \sim 90$ pc and $\sim 56$ pc, respectively) or a radio jet. Matched-resolution multi-band radio observations are necessary to further characterize the AGN activity in these systems.

Figures (5)

• Figure 1: SDSS spectra showing the [OIII]$\lambda$5007 lines of J0948+6848 (left) and J1223+5409 (right). The [OIII] line spectra are fitted best with the double narrow line components (blue) and one broad line component (green). Kim2020 identified these systems as candidate dual/binary AGN based on these double-peaked [OIII] line profiles.
• Figure 2: Best-fit GALFIT model of J0948+6848. Top row shows (a) the original image, (b) the model, and (c) the residual image. The bottom row shows the model components: (d) the PSF, (e) the host galaxy (modeled by a single Sérsic component with index $n\approx1.66$), and (f) the neighboring galaxies. Images are on a logarithmic scale and are $0.77'\times0.54'$ in angular size. The $\chi^{2}_{\nu}$ value of the model fit is reported above the images. We observe a slight artifact of an imperfect PSF which resulted in a higher $\chi^{2}_{\nu}$, but the majority of the flux is well accounted for by the model.
• Figure 3: Best-fit GALFIT model of J1223+5409. Top row shows (a) the original image, (b) the model, and (c) the residual image. The bottom row shows the model components: (d) the PSF, (e) the host galaxy (modeled by four Sérsic components), and (f) the companion galaxies. Images are on a logarithmic scale and are $0.93'\times0.67'$ in angular size. The $\chi^{2}_{\nu}$ value of the model fit is reported above the images.
• Figure 4: VLBA continuum images at 1.5 GHz for J0948+6848 (left) and J1223+5409 (right) on a logarithmic color scale. Black contours show $5\sigma$, $10\sigma$, $20\sigma$, $40\sigma$, $80\sigma$, $100\sigma$ levels. Detections are labeled as components 1 and 2 in each system. The magenta stars indicate the GALFIT-determined position of the NIR-detected AGN, with the magenta circle representing the 2$\sigma$ relative astrometric uncertainty of the HST and VLBA images. The white star is the Gaia-defined position of the host galaxy's nucleus with an uncertainty approximately consistent with the magenta circle. The synthesized beam sizes are shown in the bottom left as white filled ellipses. We do not observe a significant ($>3\sigma$) spatial offset between the NIR-detected AGN and the radio-detected AGN.
• Figure 5: Greyscale WFC3/IR F160W images of J0948+6848 (left) and J1223+5409 (right) on a logarithmic scale. Images are oriented such that north is up and east is to the left and are $\sim$0.21$'\times0.21'$ in angular size. The red star is the position of the brightest VLBA-detected radio source, though the position of the secondary source also falls within the area of the star. VLASS 3 GHz radio contours are overlaid in blue, showing $100\sigma$ levels (100$\sigma$, 200$\sigma$, 300$\sigma$....) where the 1$\sigma$ RMS is $\sim120\mu$Jy beam$^{-1}$. The synthesized radio beam is shown as a white ellipse in the bottom left. For both systems, the VLBA-detected AGN cores and NIR center are relatively spatially coincident. The VLASS radio peak of J1223+5409, however, is offset by kiloparsec-scales.