We present a high sensitivity, ground-based spectral line survey of low-J carbon monoxide (CO(Jup → Jup − 1) with Jup = 1, 2, 3) and neutral carbon [CI] 3P1−3P0 ([CI](1–0)) in 36 local ultra-luminous infrared galaxies (ULIRGs) and 4 additional LIRGs, all of which have previous Herschel OH 119 μm observations. The study is based on new single-dish observations conducted with the Atacama Pathfinder Experiment (APEX) and complemented with archival APEX and Atacama Large Millimeter Array (ALMA and ACA) data. Our methods are optimized for a multi-tracer study of the total molecular line emission from these ULIRGs, including any extended low-surface-brightness components. We find a tight correlation between the CO and [CI] line luminosities, which suggests that the emission from CO(1–0) (and CO(2–1)) arises from similar regions as the [CI](1–0), at least when averaged over galactic scales. By using [CI] to compute molecular gas masses, we estimate a median CO-to-H2 conversion factor of ⟨αCO⟩ = 1.7 ± 0.5 M⊙ (K km s−1pc2)−1 for ULIRGs. We derive median galaxy-integrated CO line ratios of 〈r21〉 = LCO(2-1)′/LCO(1-0)′ = 1.09, 〈r31〉 = LCO(3-2)′/LCO(1-0)′ = 0.76, and 〈r32〉 = LCO(3-2)′/LCO(2-1)′ = 0.76, significantly higher than normal star-forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that the r21 and r32 ratios are poor tracers of CO excitation in ULIRGs, while r31 shows a positive trend with LIR and star formation rates and a negative trend with the H2 gas depletion timescales (τdep). Our investigation of CO line ratios as a function of gas kinematics shows no clear trends, except for a positive relation between r21 and σv, which can be explained by CO opacity effects. These ULIRGs are also characterized by high L[CI](1-0)′/LCO(1-0)′ ratios, with a measured median value of ⟨rCICO⟩ = 0.18, higher than values from previous interferometric studies that were affected by missing [CI] line flux. The rCICO values do not show a significant correlation with any of the galaxy properties investigated, including OH outflow velocities and equivalent widths. We find that the widths of [CI](1–0) lines are ∼10% smaller than those of CO lines, and that this discrepancy becomes more significant in ULIRGs with broad lines (σv > 150 km s−1) and when considering the high-v wings of the lines. This suggests that the low optical depth of [CI] can challenge its detection in diffuse, low-surface-brightness outflows and, therefore, its use as a tracer of CO-dark H2 gas in these components. Finally, we find that higher LAGN are associated with longer τdep, consistent with the hypothesis that active galactic nucleus feedback may reduce the efficiency of star formation. Our study highlights the need for sensitive single-dish multi-tracer H2 surveys of ULIRGs that are able to recover the flux that is missed by interferometers, especially in the high-frequency lines such as [CI]. The Atacama Large Aperture Submillimeter Telescope (AtLAST) will be transformational for this field.

We present Atacama Large Millimetre/submillimetre Array (ALMA) CO(1–0) observations of the nearby infrared luminous (LIRG) galaxy pair IRAS 05054+1718 (also known as CGCG 468-002), as well as a new analysis of X-ray data of this source collected between 2012 and 2021 using the Nuclear Spectroscopic Telescope Array (NuSTAR), Swift, and the XMM-Newton satellites. The western component of the pair, NED01, hosts a Seyfert 1.9 nucleus that is responsible for launching a powerful X-ray ultra-fast outflow (UFO). Our X-ray spectral analysis suggests that the UFO could be variable or multi-component in velocity, ranging from v/c ∼ −0.12 (as seen in Swift) to v/c ∼ −0.23 (as seen in NuSTAR), and constrains its momentum flux to be ṗoutX−ray ∼ (4 ± 2) × 1034 g cm s−2. The ALMA CO(1–0) observations, obtained with an angular resolution of 2.2″, although targeting mainly NED01, also include the eastern component of the pair, NED02, a less-studied LIRG with no clear evidence of an active galactic nucleus (AGN). We study the CO(1–0) kinematics in the two galaxies using the 3D-BAROLO code. In both sources we can model the bulk of the CO(1–0) emission with rotating disks and, after subtracting the best-fit models, we detect compact residual emission at S/N = 15 within ∼3 kpc of the centre. A molecular outflow in NED01, if present, cannot be brighter than such residuals, implying an upper limit on its outflow rate of Ṁoutmol ≲ 19 ± 14 M⊙ yr−1 and on its momentum rate of ṗoutmol ≲ (2.7 ± 2.4) × 1034 g cm s−1. Combined with the revised energetics of the X-ray wind, we derive an upper limit on the momentum rate ratio of ṗoutmol/ṗoutX−ray < 0.67. We discuss these results in the context of the expectations of AGN feedback models, and we propose that the X-ray disk wind in NED01 has not significantly impacted the molecular gas reservoir (yet), and we can constrain its effect to be much smaller than expectations of AGN ‘energy-driven’ feedback models. We also consider and discuss the hypothesis of asymmetries of the molecular disk not properly captured by the 3D-BAROLO code. Our results highlight the challenges in testing the predictions of popular AGN disk-wind feedback theories, even in the presence of good-quality multi-wavelength observations.

We report on the changing-look nature of the active galactic nucleus (AGN) in the galaxy NGC 4156, as serendipitously discovered thanks to data acquired in 2019 at the Telescopio Nazionale Galileo (TNG) during a students' observing programme. Previous optical spectra had never shown any signatures of broad-line emission, and evidence of the AGN had come only from X-ray observations, being the optical narrow-line flux ratios unable to unambiguously denote this galaxy as a Seyfert. Our 2019 TNG data unexpectedly revealed the appearance of broad-line components in both the Hα and Hβ profiles, along with a rise of the continuum, thus implying a changing-look AGN transitioning from a type 2 (no broad-line emission) towards a (nearly) type 1. The broad-line emission has then been confirmed by our 2022 follow-up observations, whereas the rising continuum has no longer been detected, which hints at a further evolution backwards to a (nearly) type 2. The presence of broad-line components also allowed us to obtain the first singleepoch estimate of the black hole mass (log(M BH /M ) ∼ 8.1) in this source. The observed spectral variability might be the result of a change in the accretion activity of NGC 4156, although variable absorption cannot be completely excluded.

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