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Register a new userThe AGN fuelling/feedback cycle in nearby radio galaxies III. 3D relative orientations of radio jets and CO discs and their interaction
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This is the third 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, 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 Millimeter/submillimeter 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 ($approx$30$^{circ}-60^{circ}$). 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.
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This is the second paper of a series exploring the multi-component (stars, warm and cold gas and radio jets) properties of a sample of eleven nearby low excitation radio galaxies (LERGs), with the aim of better understanding the AGN fuelling/feedback cycle in these objects. Here we present a study of the molecular gas kinematics of six sample galaxies detected in $^{12}$CO(2-1) with ALMA. In all cases, our modelling suggests that the bulk of the gas in the observed (sub-)kpc CO discs is in ordered rotation. Nevertheless, low-level distortions are ubiquitous, indicating that the molecular gas is not fully relaxed into the host galaxy potential. The majority of the discs, however, are only marginally resolved, preventing us from drawing strong conclusions. NGC 3557 and NGC 3100 are special cases. The features observed in the CO velocity curve of NGC 3557 allow us to estimate a super-massive black hole (SMBH) mass of $(7.10pm0.02)times10^{8}$ M$_{odot}$, in agreement with expectations from the M$_{rm SMBH}- sigma_{*}$ relation. The rotation pattern of NGC 3100 shows distortions that appear to be consistent with the presence of both a position angle and inclination warp. Non-negligible radial motions are also found in the plane of the CO disc, likely consistent with streaming motions associated with the spiral pattern found in the inner regions of the disc. The dominant radial motions are likely to be inflows, supporting a scenario in which the cold gas is contributing to the fuelling of the AGN.
We investigate the kinematic properties of a large (N=998) sample of COSMOS spectroscopic galaxy members distributed among 79 groups. We identify the Brightest Group Galaxies (BGGs) and cross-match our data with the VLA-COSMOS Deep survey at 1.4 GHz, classifying our parent sample into radio/non-radio BGGs and radio/non-radio satellites. The radio luminosity distribution spans from $L_Rsim2times10^{21}$ W Hz$^{-1}$ to $L_Rsim3times$10$^{25}$ W Hz$^{-1}$. A phase-space analysis, performed by comparing the velocity ratio (line-of-sight velocity divided by the group velocity dispersion) with the galaxy-group centre offset, reveals that BGGs (radio and non-radio) are mostly ($sim$80%) ancient infallers. Furthermore, the strongest ($L_R>10^{23}$ W Hz$^{-1}$) radio galaxies are always found within 0.2$R_{rm vir}$ from the group centre. Comparing our samples with HORIZON-AGN, we find that the velocities and offsets of simulated galaxies are more similar to radio BGGs than to non-radio BGGs, albeit statistical tests still highlight significant differences between simulated and real objects. We find that radio BGGs are more likely to be hosted in high-mass groups. Finally, we observe correlations between the powers of BGG radio galaxies and the X-ray temperatures, $T_{rm x}$, and X-ray luminosities, $L_{rm x}$, of the host groups. This supports the existence of a link between the intragroup medium and the central radio source. The occurrence of powerful radio galaxies at group centres can be explained by Chaotic Cold Accretion, as the AGN can feed from both the galactic and intragroup condensation, leading to the observed positive $L_{rm R}-T_{rm x}$ correlation.
We investigate the correlations between optical and radio isophotal position angles for 14302 SDSS galaxies with $r$ magnitudes brighter than 18 and which have been associated with extended FIRST radio sources. We identify two separate populations of galaxies using the colour, concentration and their principal components. Surprisingly strong statistical alignments are found: late-type galaxies are overwhelmingly biased towards a position angle differences of $0^{circ}$ and early-type galaxies to $90^{circ}$. The late-type alignment can be easily understood in terms of the standard picture in which the radio emission is intimately related to areas of recent star-formation. In early-type galaxies the radio emission is expected to be driven by accretion on to a nuclear black hole. We argue that the observed correlation of the radio axis with the minor axis of the large-scale stellar distribution gives a fundamental insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled form the rest of the galaxy. Our results imply that the galaxies are oblate spheroids with their radio emission aligned with the minor axis. Remarkably the strength of the correlation of the radio major axis with the optical minor axis depends on radio loudness. Those objects with a low ratio of FIRST radio flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This may reflect different formation histories for the different objects and we suggest we may be seeing the different behaviour of rationally supported and non-rotationally supported ellipticals.
We investigate radio-mode AGN activity among post-starburst galaxies from the Sloan Digital Sky Survey to determine whether AGN feedback may be responsible for the cessation of star formation. Based on radio morphology and radio-loudness from the FIRST and NVSS data, we separate objects with radio activity due to an AGN from ongoing residual star formation. Of 513 SDSS galaxies with strong A-star spectra, 12 objects have 21-cm flux density above 1 mJy. These galaxies do not show optical AGN emission lines. Considering that the lifetime of radio emission is much shorter than the typical time-scale of the spectroscopic features of post-starburst galaxies, we conclude that the radio-emitting AGN activity in these objects was triggered after the end of the recent starburst, and thus cannot be an important feedback process to explain the post-starburst phase. The radio luminosities show a positive correlation with total galaxy stellar mass, but not with the mass of recently formed stars. Thus the mechanical power of AGN feedback derived from the radio luminosity is related to old stellar populations dominating the stellar mass, which in turn are related to the masses of central supermassive black holes.
Dwarf galaxies are thought to host the remnants of the early Universe seed black holes (BHs) and to be dominated by supernova feedback. However, recent studies suggest that BH feedback could also strongly impact their growth. We report the discovery of 35 dwarf galaxies hosting radio AGN out to redshift $sim$3.4, which constitutes the highest-redshift sample of AGN in dwarf galaxies. The galaxies are drawn from the VLA-COSMOS 3 GHz Large Project and all are star-forming. After removing the contribution from star formation to the radio emission, we find a range of AGN radio luminosities of $L^mathrm{AGN}_mathrm{1.4 GHz} sim 10^{37}$-$10^{40}$ erg s$^{-1}$. The bolometric luminosities derived from the fit of their spectral energy distribution are $gtrsim 10^{42}$ erg s$^{-1}$, in agreement with the presence of AGN in these dwarf galaxies. The 3 GHz radio emission of most of the sources is compact and the jet powers range from $Q_mathrm{jet} sim 10^{42}$ to 10$^{44}$ erg s$^{-1}$. These values, as well as the finding of jet efficiencies $geq 10$ % in more than 50% of the sample, indicate that dwarf galaxies can host radio jets as powerful as those of massive radio galaxies whose jet mechanical feedback can strongly affect the formation of stars in the host galaxy. We conclude that AGN feedback can also have a very strong impact on dwarf galaxies, either triggering or hampering star formation and possibly the material available for BH growth. This implies that those low-mass AGN hosted in dwarf galaxies might not be the untouched relics of the early seed BHs, which has important implications for seed BH formation models.
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