Molecular Gas in the Nuclear Region of NGC 6240

NGC 6240 is a luminous infrared galaxy in the local universe in the midst of a major merger. We analyze high-resolution interferometric observations of warm molecular gas using CO J = 3–2 and 6–5 in the central few kpc of NGC 6240 taken by the Atacama Large Millimeter Array. Using these CO line observations, we model the density distribution and kinematics of the molecular gas between the nuclei of the galaxies. Our models suggest that a disk model represents the data poorly. Instead, we argue that the observations are consistent with a tidal bridge between the two nuclei. We also observe high-velocity redshifted gas that is not captured by the model. These findings shed light on small-scale processes that can affect galaxy evolution and the corresponding star formation.

A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar

Galaxies with nuclear bars are believed to efficiently drive gas inward, generating a nuclear starburst and possibly an active galactic nucleus. We confirm this scenario for the isolated, double-barred, luminous infrared galaxy ESO 320-G030 based on an analysis of Herschel and ALMA spectroscopic observations. Herschel/PACS and SPIRE observations of ESO 320-G030 show absorption or emission in 18 lines of H2O, which we combine with the ALMA H2O 423 − 330 448 GHz line (Eupper ∼ 400 K) and continuum images to study the physical properties of the nuclear region. Radiative transfer models indicate that three nuclear components are required to account for the multi-transition H2O and continuum data. An envelope, with radius R ∼ 130 − 150 pc, dust temperature Tdust ≈ 50 K, and NH2 ∼ 2 × 1023 cm−2, surrounds a nuclear disk with R ∼ 40 pc that is optically thick in the far-infrared (τ100 μm ∼ 1.5 − 3, NH2 ∼ 2 × 1024 cm−2). In addition, an extremely compact (R ∼ 12 pc), warm (≈100 K), and buried (τ100 μm > 5, NH2 ≳ 5 × 1024 cm−2) core component is required to account for the very high-lying H2O absorption lines. The three nuclear components account for 70% of the galaxy luminosity (SFR ∼ 16 − 18 M⊙ yr−1). The nucleus is fed by a molecular inflow observed in CO 2-1 with ALMA, which is associated with the nuclear bar. With decreasing radius (r = 450 − 225 pc), the mass inflow rate increases up to Ṁinf ∼ 20 Ṁ yr−1, which is similar to the nuclear star formation rate (SFR), indicating that the starburst is sustained by the inflow. At lower r, ∼100 − 150 pc, the inflow is best probed by the far-infrared OH ground-state doublets, with an estimated Ṁinf ∼ 30 Ṁ yr−1. The inferred short timescale of ∼20 Myr for nuclear gas replenishment indicates quick secular evolution, and indicates that we are witnessing an intermediate stage (< 100 Myr) proto-pseudobulge fed by a massive inflow that is driven by a strong nuclear bar. We also apply the H2O model to the Herschel far-infrared spectroscopic observations of H218O, OH, 18OH, OH+, H2O+, H3O+, NH, NH2, NH3, CH, CH+, 13CH+, HF, SH, and C3, and we estimate their abundances.

Highly turbulent gas on GMC scales in NGC 3256, the nearest luminous infrared galaxy

ABSTRACT We present the highest resolution CO (2–1) observations obtained to date (0.25 arcsec) of NGC 3256 and use them to determine the detailed properties of the molecular interstellar medium in the central 6 kpc of this merger. Distributions of physical quantities are reported from pixel-by-pixel measurements at 55 and 120 pc scales and compared to disc galaxies observed by PHANGS-ALMA (Physics at High Angular resolution in Nearby GalaxieS with Atacama Large Millimeter/Submillimeter Array). Mass surface densities range from 8 to 5500 M⊙ pc−2 and velocity dispersions from 10 to 200 km s−1. Peak brightness temperatures as large as 37 K are measured, indicating the gas in NGC 3256 may be hotter than all regions in nearby disc galaxies measured by PHANGS-ALMA. Brightness temperatures even surpass those in the overlap region of NGC 4038/9 at the same scales. The majority of the gas appears unbound with median virial parameters of 7–19, although external pressure may bind some of the gas. High internal turbulent pressures of 105–1010 K cm−3 are found. Given the lack of significant trends in surface density, brightness temperature, and velocity dispersion with physical scale we argue the molecular gas is made up of a smooth medium down to 55 pc scales, unlike the more structured medium found in the PHANGS-ALMA disc galaxies.

AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy

ABSTRACT We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multiwavelength follow-up spans ∼900 d, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad ∼2000 km s−1 hydrogen, He i, and O i emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 × 1050 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.

A Compton-thick nucleus in the dual active galactic nuclei of Mrk 266

We present the results from our analysis of NuSTAR data of the luminous infrared galaxy Mrk 266, which contains two nuclei, south-western (SW) and north-eastern (NE), which were resolved in previous Chandra imaging. Combining this with the Chandra data, we intepret the hard X-ray spectrum obtained from a NuSTAR observation to result from a steeply rising flux from a Compton-thick active galactic nuclei (AGN) in the SW nucleus which is very faint in the Chandra band, confirming the previous claim. This hard X-ray component is dominated by reflection, and its intrinsic 2–10 keV luminosity is likely to be ∼1 × 1043 erg s−1. Although it is bright in soft X-ray, only a moderately absorbed NE nucleus has a 2–10 keV luminosity of 4 × 1041 erg s−1, placing it in the low-luminosity AGN class. These results have implications for understanding the detectability and duty cycles of emission from dual AGN in heavily obscured mergers.

The 300-pc scale ALMA view of [C i] 3P1–3P0, CO J = 1–0, and 609-μm dust continuum in a luminous infrared galaxy

ABSTRACT We present high-quality Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 observations of the [C i] 3P1–3P0 line and 609-μm dust continuum emission towards the nearby luminous infrared galaxy (LIRG) IRAS F18293-3413, as well as matched resolution (300-pc scale) Band 3 CO J = 1–0 data, which allow us to assess the use of the [C i] 3P1–3P0 line as a total gas mass estimator. We find that the [C i] line basically traces structures detected in CO (and dust) and a mean (median) [C i]/CO luminosity ($L^{\prime }_{\rm [C\, {\small I}]}$/$L^{\prime }_{\rm CO}$) ratio of 0.17 (0.16) with a scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a radial $L^{\prime }_{\rm [C\, {\small I}]}$/$L^{\prime }_{\rm CO}$ gradient and a superlinear $L^{\prime }_{\rm CO}$ versus $L^{\prime }_{\rm [C\, {\small I}]}$ relation (slope = 1.54 ± 0.02) at this spatial scale, which can be explained by radial excitation and/or line opacity gradients. Based on the molecular gas masses converted from the dust continuum emission, we found that the CO-to-H2 and [C i]-to-H2 conversion factors are relatively flat across the molecular gas disc with a median value of 3.5$^{+1.9}_{-1.3}$ and 20.7$^{+9.2}_{-4.9}$ M⊙ (K km s−1 pc2)−1, respectively. A non-LTE calculation yields that typical molecular gas properties seen in nearby (U)LIRGs ($n_{\rm H_2}$ = 103−4 cm−3, Tkin ∼ 50 K, and $X_{\rm C\, {\small I}}$ = (0.8–2.3) × 10−5) can naturally reproduce the derived [C i]-to-H2 conversion factor. However, we caution that a careful treatment of the physical gas properties is required in order to measure H2 gas mass distributions in galaxies using a single [C i] line. Otherwise, a single [C i] line is not a good molecular gas estimator in a spatially resolved manner.

First detection of the 448 GHz ortho-H2O line at high redshift: probing the structure of a starburst nucleus at z = 3.63

Submillimeter rotational lines of H2O are a powerful probe in warm gas regions of the interstellar medium (ISM), tracing scales and structures ranging from kiloparsec disks to the most compact and dust-obscured regions of galactic nuclei. The ortho-H2O(423 − 330 line at 448 GHz, which has recently been detected in a local luminous infrared galaxy, offers a unique constraint on the excitation conditions and ISM properties in deeply buried galaxy nuclei because the line requires high far-infrared optical depths to be excited. In this letter, we report the first high-redshift detection of the 448 GHz H2O(423–330) line using ALMA in a strongly lensed submillimeter galaxy (SMG) at z = 3.63. After correcting for magnification, the luminosity of the 448 GHz H2O line is ∼106 L⊙. In combination with three other previously detected H2O lines, we build a model that resolves the dusty ISM structure of the SMG, and find that it is composed of a ∼1 kpc optically thin (optical depth at 100 μm τ100 ∼ 0.3) disk component with a dust temperature Tdust ≈ 50 K that emits a total infrared power of 5 × 1012 L⊙ with a surface density ΣIR = 4 × 1011 L⊙ kpc−2, and a very compact (0.1 kpc) heavily dust-obscured (τ100 ≳ 1) nuclear core with very warm dust (100 K) and ΣIR = 8 × 1012 L⊙ kpc−2. The H2O abundance in the core component, XH2O ∼ (0.3–5) × 10−5, is at least one order of magnitude higher than in the disk component. The optically thick core has the characteristic properties of an Eddington-limited starburst, providing evidence that radiation pressure on dust is capable of supporting the ISM in buried nuclei at high redshifts. The multicomponent ISM structure revealed by our models illustrates that dust and molecules such as H2O are present in regions that are characterized by highly differing conditions and scales, extending from the nucleus to more extended regions of SMGs.