Radio continuum properties of OH megamaser galaxies

We present a study of the radio continuum properties of two luminous/ultraluminous infrared galaxy samples: the OH megamaser (OHM) sample (74 objects) and the control sample (128 objects) without detected maser emission. We carried out pilot observations for 140 objects with the radio telescope RATAN-600 at 1.2, 2.3, 4.7, 8.2, 11.2, and 22.3 GHz in 2019–2021. The OHM sample has two times more flat-spectrum sources (32 per cent) than the control sample. Steep radio spectra prevail in both samples. The median spectral index at 4.7 GHz α4.7 = −0.59 for the OHM sample, and α4.7 = −0.71 for the non-OHM galaxies. We confirm a tight correlation of the far-infrared (FIR) and radio luminosities for the OHM sample. We found correlations between isotropic OH line luminosity LOH and the spectral index α4.7 (ρ=0.26, p-val.=0.04) and between LOH and radio luminosity P1.4 (ρ=0.35, p-val.=0.005). Reviewing subsamples of masers powered by active galactic nuclei and star formation revealed insignificant differences for their FIR and radio properties. Nonetheless, AGN-powered galaxies exhibit larger scatter in a range of parameters and their standard deviations. The similarities in the radio and FIR properties in the two samples are presumably caused by the presence of a significant amount of AGN sources in both samples (47 and 30 per cent in the OHM and control samples) and/or possibly by the presence of undetected OH emission sources in the control sample.

Comprehensive Broadband X-Ray and Multiwavelength Study of Active Galactic Nuclei in 57 Local Luminous and Ultraluminous Infrared Galaxies Observed with NuSTAR and/or Swift/BAT

We perform a systematic X-ray spectroscopic analysis of 57 local luminous and ultraluminous infrared galaxy systems (containing 84 individual galaxies) observed with the Nuclear Spectroscopic Telescope Array and/or Swift/BAT. Combining soft X-ray data obtained with Chandra, XMM-Newton, Suzaku, and/or Swift/XRT, we identify 40 hard (>10 keV) X-ray–detected active galactic nuclei (AGNs) and constrain their torus parameters with the X-ray clumpy torus model XCLUMPY. Among the AGNs at z < 0.03, for which sample biases are minimized, the fraction of Compton-thick (N H ≥ 1024 cm−2) AGNs reaches 64 − 15 + 14 % (6/9 sources) in late mergers, while it is 24 − 10 + 12 % (3/14 sources) in early mergers, consistent with the tendency reported by Ricci et al. We find that the bolometric AGN luminosities derived from the infrared data increase but the X-ray to bolometric luminosity ratios decrease with merger stage. The X-ray-weak AGNs in late mergers ubiquitously show massive outflows at subparsec to kiloparsec scales. Among them, the most luminous AGNs (L bol,AGN ∼ 1046 erg s−1) have relatively small column densities of ≲1023 cm−2 and almost super-Eddington ratios (λ Edd ∼ 1.0). Their torus covering factors (C T (22) ∼ 0.6) are larger than those of Swift/BAT-selected AGNs with similarly high Eddington ratios. These results suggest a scenario where, in the final stage of mergers, multiphase strong outflows are produced due to chaotic quasi-spherical inflows, and the AGN becomes extremely X-ray weak and deeply buried due to obscuration by inflowing and/or outflowing material.

ALMA detection of millimetre 183 GHz H2O maser emission in the Superantennae galaxy at z ∼ 0.06

ABSTRACT We present the results of Atacama Large Millimeter/submillimeter Array (ALMA) band-5 (∼170 GHz) observations of the merging ultraluminous infrared galaxy, the ‘Superantennae’ (IRAS 19254−7245), at z = 0.0617, which has been diagnosed as containing a luminous obscured active galactic nucleus (AGN). In addition to dense molecular line emission (HCNJ = 2–1, HCO+J = 2–1, and HNC J = 2–1), we detect a highly luminous (∼6 × 10$^{4}\, \mathrm{L}_{\odot }$) 183 GHz H2O 31,3–22,0 emission line. We interpret the strong H2O emission as largely originating in maser amplification in AGN-illuminated dense and warm molecular gas, based on (1) the spatially compact (≲220 pc) nature of the H2O emission, unlike spatially resolved (≳500 pc) dense molecular emission, and (2) a strikingly different velocity profile from, and (3) significantly elevated flux ratio relative to, dense molecular emission lines. H2O maser emission, other than the widely studied 22 GHz 61,6–52,3 line, has been expected to provide important information on the physical properties of gas in the vicinity of a central mass-accreting supermassive black hole (SMBH), because of different excitation energy. We here demonstrate that with highly sensitive ALMA, millimetre 183 GHz H2O maser detection is feasible out to &gt;270 Mpc, opening a new window to scrutinize molecular gas properties around a mass-accreting SMBH far beyond the immediately local Universe.

Tracing circumnuclear dense gas in H2O maser galaxies

ABSTRACT A sample of 30 H$_{2}$O extra-galactic maser galaxies with their published HCN(J = 1 − 0) and HCO + (J = 1 − 0) observations has been compiled to investigate the dense gas correlation with H2O maser emission. Our sample number exceeds the size of the previous HCN samples studied so far by a factor of 3, and it is the first study on the possible relation with the dense gas tracer HCO + . We find a strong correlation between normalized H2O maser emission luminosity (LH2O/LCO) and normalized HCO + line luminosity (LHCO + /LCO). Moreover, a weak correlation has been found between LH2O/LCO and normalized HCN line luminosity (LHCN/LCO). The sample is also studied after excluding Luminous and ultraluminous infrared galaxy (U)LIRG sources, and the mentioned correlations are noticeably stronger. We show that ‘dense gas’ fractions as obtained from HCN and HCO + molecules tightly correlate with maser emission, especially for galaxies with normal IR luminosity (LIR &lt; 1011 L⊙) and we show that HCO + is a better ‘dense gas’ tracer than HCN. Further systematic studies of these dense gas tracers with higher transition level lines are vital to probe megamaser physical conditions and to accurately determining how maser emission interrelates with the dense gas.

The role of molecular gas in the nuclear regions of IRAS 00183-7111

Aims. We present a multi-frequency study of the ultraluminous infrared galaxy (ULIRG) IRAS 00183-7111 (z = 0.327), selected from the Spoon diagnostic diagram as a highly obscured active galactic nucleus (AGN) candidate. ALMA millimetre and X-ray observations are used; the main aim is to verify at what level the molecular gas, traced by the CO, may be responsible for the obscuration observed at X-ray energies. Theory and observations both suggest that galaxy-scale absorption may play a role in the AGN obscuration at intermediate (i.e. Compton-thin) column densities. Methods. We calibrated and analysed ALMA archival Cycle 0 data in two bands (Bands 3 and 6). The X-ray properties of IRAS 00183-7111 were studied by reducing and analysing separately archival Chandra and XMM-Newton data; recently acquired NuSTAR spectra were first examined individually and then added to the Chandra and XMM spectra for the broad-band (0.5 − 24 keV, observed frame) analysis. Results. We derived a molecular gas column density of (8.0 ± 0.9) × 1021 cm−2 from the ALMA CO(1−0) detection, while the best-fit column density of cold gas obtained from X-ray spectral fitting is 6.8−1.5+2.1×1022 cm−. The two quantities suggest that the molecular gas may contribute only a fraction of the AGN obscuration; however, the link between them is not straightforward. The nuclear regions of IRAS 00183-7111 are likely stratified into different layers of matter: one inner and highly ionized by the strong radiation field of the AGN (as inferred from the high-ionization iron line found in the X-ray spectra), and one outer and colder, extending more than 5 kpc from the nucleus (as traced by the molecular gas observed with ALMA). The molecular gas regions also give rise to a vigorous starburst with SFR ~260 ± 28 M⊙ yr−1. The complexity of this nuclear environment makes it difficult to identify the origin of the AGN obscuration given the quality of the data currently available. Higher resolution observations in the millimetre regime are needed to deeply investigate this issue.