Radio properties of the OH megamaser galaxy IIZw 096

Abstract

Based on the two-epoch European Very Long Baseline Interferometry (VLBI) Network (EVN) archive data from OH line observations of IIZw 096, we confirm that the high-resolution OH emission in this source mainly comes from two spots (OH1 and OH2) of component (comp) D1 of this merging system. We found no significant variations in the OH 1667 MHz line emission, including flux densities and peak positions. The OH 1665 MHz line emission is detected at about the 6σ level in the OH1 region by combining two epochs of EVN observations. By using archival data from the Very Long Baseline Array (VLBA), Very Large Array (VLA), and Atacama Large Millimeter Array (ALMA) observations, we investigated the properties of the environment of this component through H I, CO(3-2), and HCO+(4-3) lines and the multi-band radio continuum emission. We found that the comp D1 shows the brightest CO, HCO+ line emission, as well as multi-band radio continuum emission. The environment around D1 shows no clear velocity structure associated with circular motions, making it different from most other OH megamasers (OHMs) in the literature, which might have been caused by an effect during the merger stage. Meanwhile, we found that the CO emission shows three velocity structures around D1, including the central broad full width at half maximum (FWHM) region, the double-peak region where the CO line profile shows two separated peaks, and the region of the high-velocity clouds where the CO line peaks at a high velocity (∼11 000 km s−1). Similarly, H Iobservations in absorption also show high-velocity clouds around the D1 region, which might be due to inflows caused by the merging of two or more galaxy components. Based on the high-resolution K-band VLA and L-band VLBA observations of the radio continuum emission, we derived the brightness temperature in the range 105 K–106 K, which is consistent with other starburst dominant OHM sources in the literature. The multi-band VLA observations show that the radio continuum emission of comp D might also have contributions from free-free emission in addition to synchrotron emission. As a consequence, these results support a starburst origin for the OHMs, without the presence of an active galactic nucleus (AGN).

GOALS-JWST: Unveiling Dusty Compact Sources in the Merging Galaxy IIZw096

“…Although ID 7 (D0) is the brightest source at 1.6 μm, the JWST mid-IR data demonstrate that ID 8 (D1) generates the bulk of the total IR emission in IIZw096. The location of ID 8 also coincides with two OH megamasers (Migenes et al 2011;Wu et al 2022). 34 Megamasers are often found in merging (U) LIRGs, in close proximity to the nuclei (e.g., Roberts et al 2021), marking regions of extremely high gas density and strong far-IR radiation.…”

“…The diffuse, extended emission around ID 8 would then be the remnants of the third galaxy's disk. The estimated stellar mass of ID 8 is ∼10 9 M e and there is a similarly large mass of gas in this region (Inami et al 2010;Wu et al 2022). These estimates might indicate ID 8 is a partially stripped third nucleus.…”

“…These authors estimated a SFR surface density of 470 ± 60 M e yr −1 kpc −2 , corresponding to an L IR surface density of (3.9 ± 0.5) × 10 12 L e kpc −2 , which agrees with the value derived from the mid-IR. Given the L IR surface density limit from the mid-IR and the column density of ∼10 25 cm −2 obtained via the molecular gas mass (within an aperture of 0 2 × 0 16; Wu et al 2022), ID 8 appears to be below the Eddington limit if its size is 175 pc. (e.g., Pereira-Santaella et al 2021;Barcos-Munoz et al 2015).…”

“…The sources detected in the F770W image are used as priors for photometry in all three bands (Figure 1). We assign identification (ID) numbers for the detected sources in ascending order of R.A. For a subset of sources shown by Wu et al (2022), we also refer to their source names (Figure 1(c)). The same 12 clumps are detected in the F560W image, while in the F1500W image, four of them (IDs 4, 9, 10, and 11/C0) lie on the structure of the point-spread function (PSF) of the brightest source (ID 8/D1), making confident detections and flux measurements difficult.…”

Characterizing Compact 15–33 GHz Radio Continuum Sources in Local U/LIRGs

“…Here we tentatively classify this region as an extranuclear "SF" following Inami et al (2010). We note that while VLBI observations of the OH megamaser in this region have suggested that it may host an obscured AGN (Migenes et al 2011), recent multifrequency analysis of the radio spectrum shows that it is well described by pure star formation (Wu et al 2022). 60.…”