The AGN fuelling/feedback cycle in LERGs: the molecular gas component and its interplay with radio jets

This Thesis has been carried out as part of an international collaboration overall aiming at a multi-component (stars, warm/cold gas, dust, radio jets) investigation of the AGN fuelling/feedback loop in the cores of a volume-limited (z<0.03) sample of eleven low excitation radio galaxies (LERGs). In this general context, this PhD Thesis mainly aimed at i) shedding light on the properties, kinematics and origin of the cold gas using ALMA CO(2-1) observations of nine sample members; ii) investigating the radio jets/cold ISM interplay on (sub-)kpc scales, coupling ALMA CO(2-1) and 230~GHz continuum observations with newly-acquired radio JVLA data at 10 GHz of five sample sources. Our work shows that rotating (sub-)kpc CO discs are very common in LERGs (six out of nine sources detected in CO). The 3D modelling of these discs demonstrates that the bulk of the gas is in ordered rotation (at least at the resolution of our ALMA observations). This may explain the relatively low accretion rate of LERGs. Nevertheless, low-level perturbations and/or non-circular motions are ubiquitous, indicating that the gas is not fully relaxed into the host galaxy potential. Whenever possible, we demonstrated that the observed CO discs are co-spatial with dust discs/lanes. In a couple of cases multi-wavelength constraints point towards an external origin of the gas. These evidence are difficult to reconcile with the chaotic cold accretion scenario, with implications for the powering mechanism of LERGs. The relative jet/CO disc inclination angles are found to span a wide range (27°-64°), with a marginal preference for angles around 45°. This is consistent with previous statistical 3D studies of the relative jet/dust disc orientation in radio galaxies. Hints of the presence of a jet/disc interaction are also observed in one case and tentatively found in the kinematics of other two sources

The role of molecular gas in the nuclear regions of IRAS 00183-7111. ALMA and X-ray investigations of an ultraluminous infrared galaxy

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 Cycle 0 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. We derived a molecular gas column density of $(8.0\pm0.9)\times10^{21}$ 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 $6.8^{+2.1}_{-1.5}\times10^{22}$ cm$^{-2}$. 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$\sim260\pm28$ M$_{\odot}$ 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.Comment: 15 pages, 7 figures, 5 tables. Accepted for publication in A&

Molecular gas kinematics in local early-type galaxies with ALMA

Local early-type galaxies (ETGs) are mostly populated by old stars, with little or no recent star formation activity. For this reason, they have historically been believed to be essentially devoid of cold gas, which is the fuel for the formation of new stars. Over the past two decades, however, increasingly-sensitive instrumentation observing the sky at (sub-)millimetre wavelengths has revealed the presence of significant amounts of cold molecular gas in the hearts of nearby ETGs. The unprecedented capabilities offered by the Atacama Large Millimeter/submillimeter Array (ALMA), in particular, have allowed us to obtain snapshots of the central regions of these ETGs with unprecedented detail, mapping this gas with higher sensitivity and resolution than ever before possible. Studies of the kinematics of the observed cold gas reservoirs are crucial for galaxy formation and evolution theories, providing, e.g., constraints on the fundamental properties and fuelling/feedback processes of super-massive black holes (SMBHs) at the centre of these galaxies. In this brief review, we summarise what the first 10 years of ALMA observations have taught us about the distribution and kinematics of the cold molecular gas component in nearby ellipticals and lenticulars

Kinematics Modelling of Molecular Gas in NGC 3100

SlidesA kinematics modelling of molecular gas in the centre of NGC 3100, a FRI radio galaxy hosted by a S0 galaxy at redshift z = 0.0088. The inner region of NGC 3100 has been observed with ALMA at Band 6 during Cycle 3. The CO(2-1) 230-GHz line (rest-frequency) was clearly detected. The analysis of the line through the integrated intensity map (moment 0) and the integrated velocity map (moment 1) revealed a ring-like rotating structure. A tilted-ring model was built through the software 3D-Barolo to better understand the complex kinematics of the gas. The modelling allowed us to explore purely rotational velocity fields as well as fields including non-circular motions

The AGN fuelling/feedback cycle in nearby radio galaxies - III. 3D relative orientations of radio jets and CO discs and their interaction

This is the third paper of a series exploring the multifrequency properties of a sample of eleven nearby low-excitation radio galaxies (LERGs) in the southern sky. We are conducting an extensive study of different galaxy components (stars, dust, warm and cold gas, radio jets) with the aim of better understanding the AGN fuelling/feedback cycle in LERGs. Here, we present new, deep, sub-kpc resolution Karl G. Jansky Very Large Array (JVLA) data for five sample sources at 10 GHz. Coupling these data with previously acquired Atacama Large Millimetre/submillimetre Array (ALMA) CO(2–1) observations and measurements of comparable quality from the literature, we carry out for the first time a full 3D analysis of the relative orientations of jet and disc rotation axes in six FR I LERGs. This analysis shows (albeit with significant uncertainties) that the relative orientation angles span a wide range (≈30○–60○). There is no case where both axes are accurately aligned and there is a marginally significant tendency for jets to avoid the disc plane. Our study also provides further evidence for the presence of a jet-CO disc interaction (already inferred from other observational indicators) in at least one source, NGC 3100. In this case, the limited extent of the radio jets, along with distortions in both the molecular gas and the jet components, suggest that the jets are young, interacting with the surrounding matter and rapidly decelerating

The WISDOM of power spectra: how the galactic gravitational potential impacts a galaxy’s central gas reservoir in simulations and observations

Observations indicate that the central gas discs are smoother in early-type galaxies than their late-type counterparts, while recent simulations predict that the dynamical suppression of star formation in spheroid-dominated galaxies is preceded by the suppression of fragmentation of their interstellar media. The mass surface density power spectrum is a powerful tool to constrain the degree of structure within a gas reservoir. Specifically here, we focus on the power spectrum slope and aim to constrain whether the shear induced by a dominant spheroidal potential can induce sufficient turbulence to suppress fragmentation, resulting in the smooth central gas discs observed. We compute surface density power spectra for the nuclear gas reservoirs of fourteen simulated isolated galaxies and twelve galaxies observed as part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project. Both simulated and observed galaxies range from disc-dominated galaxies to spheroids, with central stellar mass surface densities, a measure of bulge dominance, varying by more than an order of magnitude. For the simulations, the power spectra steepen with increasing central stellar mass surface density, thereby clearly linking the suppression of fragmentation to the shear-driven turbulence induced by the spheroid. The WISDOM observations show a different (but potentially consistent) picture: while there is no correlation between the power spectrum slopes and the central stellar mass surface densities, the slopes scatter around a value of 2.6. This is similar to the behaviour of the slopes of the simulated galaxies with high central stellar mass surface densities, and could indicate that high shear eventually drives incompressible turbulence

A plausible link between dynamically unsettled molecular gas and the radio jet in NGC 6328

We report the detection of outflowing molecular gas at the center of the nearby radio galaxy NGC6328 (z=0.014), which has a gigahertz-peaked spectrum radio core and a compact (2 pc) young double radio lobe tracing jet. Utilizing Atacama Large Millimeter/submillimeter Array (ALMA) CO(2-1) and CO(3-2) observations, as well as a novel code developed to fit the 3D gas distribution and kinematics, to study the molecular gas kinematics, we find that the bulk of the gas is situated within a highly warped disk structure, most likely the result of a past merger event. Our analysis further uncovers, within the inner regions of the gas distribution (R<300 pc) and at a position angle aligning with that of the radio jet (150 degrees), the existence of two anti-diametric molecular gas structures kinematically detached from the main disk. These structures most likely trace a jet-induced cold gas outflow with a total lower limit mass of $2\times 10^6\,\mathrm{M_\odot}$ mass, corresponding to an outflow rate of $2\,\mathrm{M_\odot\,yr^{-1}}$ and a kinetic power of $2.7\times 10^{40}\,\mathrm{erg\,s^{-1}}$. The energy required to maintain such a molecular outflow is aligned with the mechanical power of the jet.Comment: 19 pages, 13 figures, Accepted for publication in A

WISDOM Project -- XXIV. Cross-checking supermassive black hole mass estimates from ALMA CO gas kinematics and SINFONI stellar kinematics in the galaxy NGC 4751

Supermassive black hole (SMBH) masses can be measured by observing the impacts of the SMBHs on dynamical tracers around them. We present high angular resolution ($0.19$ arcsec or $\approx24$ pc) Atacama Large Millimeter/submillimeter Array observations of the $^{12}$CO(3-2) line emission of the early-type galaxy NGC 4751, which reveal a highly-inclined regularly-rotating molecular gas disc with clear central Keplerian motions. Using a Hubble Space Telescope image to constrain the stellar mass distribution, we forward model the molecular gas kinematics and data cube in a Bayesian framework using the Kinematic Molecular Simulation code. Assuming a constant mass-to-light ratio ($M/L$), we infer a SMBH mass $M_\text{BH}=3.43^{+0.45}_{-0.44}\times10^9$ $\text{M}_\odot$ and a F160W filter stellar $M/L$ $M/L_\text{F160W}=(2.68\pm0.11)$ $\text{M}_\odot/\text{L}_{\odot,\text{F160W}}$ (all quoted uncertainties are at $3\sigma$ confidence). Assuming a linearly spatially-varying $M/L$, we infer $M_\text{BH}=2.79_{-0.57}^{+0.75}\times10^9$ $\text{M}_\odot$ and $\left(M/L_\text{F160W}\right)/\left(\text{M}_\odot/\text{L}_{\odot,\text{F160W}}\right)=3.07^{+0.27}_{-0.35}-0.09^{+0.08}_{-0.06}\,\left(R/\text{arcsec}\right)$, where $R$ is the galactocentric radius. We also present alternative SMBH mass estimates using the Jeans Anisotropic Modelling (JAM) method and SINFONI stellar kinematics. Assuming a cylindrically-aligned velocity ellipsoid (JAM$_\text{cyl}$) we infer $M_\text{BH}=(2.52\pm 0.36)\times10^9$ $\text{M}_\odot$, while assuming a spherically-aligned velocity ellipsoid (JAM$_\text{sph}$) we infer $M_\text{BH}=(3.24\pm0.87)\times10^9$ $\text{M}_\odot$. Our derived masses are all consistent with one another, but they are larger than (and inconsistent with) one previous stellar dynamical measurement using Schwarzschil's method and the same SINFONI kinematics.Comment: 15 pages, 13 figures, submitted to MNRAS. arXiv admin note: text overlap with arXiv:2401.1637

The AGN fuelling/feedback cycle in nearby radio galaxies - I. ALMA observations and early results

This is the first paper of a series exploring the multi-frequency properties of a sample of eleven nearby low excitation radio galaxies (LERGs) in the southern sky. We are conducting an extensive study of different galaxy components (stars, warm and cold gas, radio jets) with the aim of improving our understanding of the AGN fuelling/feedback cycle in LERGs. We present ALMA Band 6 $^{12}$CO(2-1) and continuum observations of nine sources. Continuum emission from the radio cores was detected in all objects. Six sources also show mm emission from jets on kpc/sub-kpc scales. The jet structures are very similar at mm and cm wavelengths. We conclude that synchrotron emission associated with the radio jets dominates the continuum spectra up to 230 GHz. The $^{12}$CO(2-1) line was detected in emission in six out of nine objects, with molecular gas masses ranging from $2 \times 10^{7}$ to $2 \times 10^{10}$ M$_{\rm \odot}$. The CO detections show disc-like structures on scales from $\approx$0.2 to $\approx$10 kpc. In one case (NGC 3100) the CO disc presents some asymmetries and is disrupted in the direction of the northern radio jet, indicating a possible jet/disc interaction. In IC 4296, CO is detected in absorption against the radio core, as well as in emission. In four of the six galaxies with CO detections, the gas rotation axes are roughly parallel to the radio jets in projection; the remaining two cases show large misalignments. In those objects where optical imaging is available, dust and CO appear to be co-spatial.Comment: Accepted for publication in MNRAS. The Appendix is provided as on-line only supplementary materia

WISDOM Project - XVI. The link between circumnuclear molecular gas reservoirs and active galactic nucleus fuelling

We use high-resolution data from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) project to investigate the connection between circumnuclear gas reservoirs and nuclear activity in a sample of nearby galaxies. Our sample spans a wide range of nuclear activity types including radio galaxies, Seyfert galaxies, low-luminosity active galactic nuclei (AGN) and inactive galaxies. We use measurements of nuclear millimetre continuum emission along with other archival tracers of AGN accretion/activity to investigate previous claims that at, circumnuclear scales (<100 pc), these should correlate with the mass of the cold molecular gas. We find that the molecular gas mass does not correlate with any tracer of nuclear activity. This suggests the level of nuclear activity cannot solely be regulated by the amount of cold gas around the supermassive black hole (SMBH). This indicates that AGN fuelling, that drives gas from the large-scale galaxy to the nuclear regions, is not a ubiquitous process and may vary between AGN type, with time-scale variations likely to be very important. By studying the structure of the central molecular gas reservoirs, we find our galaxies have a range of nuclear molecular gas concentrations. This could indicate that some of our galaxies may have had their circumnuclear regions impacted by AGN feedback, even though they currently have low nuclear activity. Alternatively, the nuclear molecular gas concentrations in our galaxies could instead be set by secular processes