BASS XXXII: Studying the Nuclear Millimeter-wave Continuum Emission of AGNs with ALMA at Scales ≲100–200 pc

Abstract

To understand the origin of nuclear (≲100 pc) millimeter-wave (mm-wave) continuum emission in active galactic nuclei (AGNs), we systematically analyzed subarcsecond resolution Band-6 (211–275 GHz) Atacama Large Millimeter/submillimeter Array data of 98 nearby AGNs (z < 0.05) from the 70 month Swift/BAT catalog. The sample, almost unbiased for obscured systems, provides the largest number of AGNs to date with high mm-wave spatial resolution sampling (∼1–200 pc), and spans broad ranges of 14–150 keV luminosity { 40 < log [ L 14 − 150 / ( erg s − 1 ) ] < 45 }, black hole mass [ 5 < log ( M BH / M ⊙ ) < 10 ], and Eddington ratio ( − 4 < log λ Edd < 2 ). We find a significant correlation between 1.3 mm (230 GHz) and 14–150 keV luminosities. Its scatter is ≈0.36 dex, and the mm-wave emission may serve as a good proxy of the AGN luminosity, free of dust extinction up to N H ∼ 1026 cm−2. While the mm-wave emission could be self-absorbed synchrotron radiation around the X-ray corona according to past works, we also discuss different possible origins of the mm-wave emission: AGN-related dust emission, outflow-driven shocks, and a small-scale (<200 pc) jet. The dust emission is unlikely to be dominant, as the mm-wave slope is generally flatter than expected. Also, due to no increase in the mm-wave luminosity with the Eddington ratio, a radiation-driven outflow model is possibly not the common mechanism. Furthermore, we find independence of the mm-wave luminosity on indicators of the inclination angle from the polar axis of the nuclear structure, which is inconsistent with a jet model whose luminosity depends only on the angle.

UGC 4211: A Confirmed Dual Active Galactic Nucleus in the Local Universe at 230 pc Nuclear Separation

“…The copious unresolved nuclear millimeter emission at the location of the two nuclei, coincident with the unresolved emission-line emitters. While this could be due to star formation, the fact that they are spatially extremely compact, <30 pc, and (at least the south nucleus) consistent with a flat, nonthermal, spectrum, strongly suggests that these signals arise from millimeter wave emission, as seen in X-ray selected AGN (see the discussion in Kawamuro et al 2022). Furthermore, the millimeter continuum luminosity is consistent with the expected value for bright nearby X-ray selected AGN emission following the Kawamuro et al (2022) correlation, and is significantly higher than the values expected for star formation processes in such a small size.…”

“…While this could be due to star formation, the fact that they are spatially extremely compact, <30 pc, and (at least the south nucleus) consistent with a flat, nonthermal, spectrum, strongly suggests that these signals arise from millimeter wave emission, as seen in X-ray selected AGN (see the discussion in Kawamuro et al 2022). Furthermore, the millimeter continuum luminosity is consistent with the expected value for bright nearby X-ray selected AGN emission following the Kawamuro et al (2022) correlation, and is significantly higher than the values expected for star formation processes in such a small size. Furthermore, a non-AGN origin would require an extremely compact and dense star-forming region, which at the same time shows strong AGN-ionized BPT emission line ratios, and is in the center of an NIR nucleus tracing old stellar populations.…”

“…Our analysis and findings clearly demonstrate the benefits of multiwavelength, high-spatial resolution observations (<0 1). The ability of ALMA to identify other subkiloparsec dual AGN candidates is also promising, given the large number of high-resolution archival observations of BAT AGN (e.g., N > 100; Kawamuro et al 2022), though requiring further investigation.…”

“…The nuclear ALMA 231 GHz continuum emission (<200 pc), has been found to be a good proxy of the AGN luminosity, with a tight correlation with the 14-150 keV(0.36 dex; Kawamuro et al 2022). This emission is shown in the bottom right panel of Figure 1 from a high resolution ∼0 07 (49 pc) map, with a brighter southern source detected at an S/N of 116 and fainter northern source at an S/N of 6.2 based on the peak flux.…”

“…We estimate the AGN bolometric luminosity, using the tight correlation (∼0.5 dex scatter) between the nuclear peak millimeterwave luminosity and the hard X-ray emission (Kawamuro et al 2022), giving an absorption-corrected 2-10 keV luminosity of ). In a study of nearby dual AGN detected in the X-ray band (Koss et al 2012), the average ratio of X-ray luminosities was ≈11, while some dual pairs have X-ray ratios greater than ≈1000 (e.g., IRAS 05589+2828 and UGC 8327), so the predicted ratio of 32 suggested by their millimeter emission is not extreme.…”