A multifaceted view of the interplay between gas and radio AGN

The supermassive blackholes inhabiting the centres of massive galaxies become active galactic nuclei (AGN) when the ambient matter gets accreted onto them. This results in the emission of enormous amount of energy which influences the evolution of their host galaxies. In radio-loud AGN, the energy is emitted in the form of collimated jets. These jets and the ambient interstellar medium (ISM) interact and affect each other: while the radio jets make the ISM turbulent and drive fast outflows, a dense ISM slows the growth of these radio jets. Understanding the nature of this jet-ISM interaction and its evolution with redshift is crucial to understand the role of radio AGN in galaxy evolution.In this thesis, we carried out a detailed study of the interaction between radio jets and the cold ISM: atomic (HI) and molecular, in a low-luminosity radio galaxy using the Very Large Array (VLA), European VLBI Network (EVN) and NOrthern Extended Millimeter Array (NOEMA). This resulted in the first discovery of a massive molecular-gas outflow in this AGN, driven single-handedly by radio jets. This showed that kpc-scale jet-ISM interaction in such sources is important for models of galaxy evolution. We further searched for HI in samples of radio AGN at higher redshifts and suggested that the incidence and kinematics of HI in radio AGN does not evolve up to z ~ 0.4 and provided the first evidence for the evolution of cold-gas content in radio AGN environments at z>0.7

Looking for other worlds out there

Are there worlds outside our Solar System? How do we look for them? What can we learn from them? In this article, the author shows how recent advances in technology have made the search for exoplanets more exciting than ever before

Disc galaxy resolved in HI absorption against the radio lobe of 3C 433: Case study for future surveys

The neutral atomic gas content of galaxies is usually studied in the HI 21cm emission line of hydrogen. However, at higher redshifts, we need very deep integrations to detect HI emission. The HI absorption does not suffer from this dependence on distance as long as there is a bright enough background radio source. However, resolved HI absorption studies of galaxies are rare. We report one such rare study of resolved HI absorption against the radio galaxy 3C 433 at $z = 0.101$ with the VLA. The resolved kinematics of the absorber, located against the southern lobe of the 3C 433, shows that it has regular kinematics with an HI mass $\lesssim 3.4 \times 10^{8} M_{\odot}$ for T$_{spin} =$ 100K. Our deep optical continuum and H$\alpha$ observations from the Gran Telescopio CANARIAS (GTC) show that the absorber is a faint disc galaxy in the same environment as 3C 433 with a stellar mass $\sim 10^{10} M_{\odot}$ and a star-formation rate of 0.15 $M_{\odot}~yr^{-1}$ or less. For its HI mass, HI column density, stellar mass, and star-formation rate, this galaxy lies well below the main sequence of star-forming galaxies. Its HI mass is lower than the galaxies studied in HI emission at $z \sim 0.1$. Our GTC imaging reveals interesting alignments between H$\alpha$ and radio emission in the HI companion and in the host galaxy of the AGN as well as in the circumgalactic medium in between. This suggests that the shock ionization of gas by the propagating radio source may happen across tens of kpc. Our work supports the potential of studying the HI content in galaxies via absorption in the case of a fortuitous alignment with an extended radio continuum. This allows us to trace galaxies with low HI masses which would otherwise be missed by deep HI emission surveys. In conjunction with the deep all-sky optical surveys, the blind HI surveys with the SKA pathfinders will be able to detect many such systems.Comment: 11 pages, 9 figures. Accepted for publication in Astronomy & Astrophysic

Feedback from low-luminosity radio galaxies:B2 0258+35

Low-luminosity radio-loud active galactic nuclei (AGN) are of importance in studies concerning feedback from radio AGN since a dominant fraction of AGN belong to this class. We report high-resolution Very Large Array (VLA) and European VLBI Network (EVN) observations of HI-21cm absorption from a young, compact steep-spectrum radio source, B2 0258+35, nested in the early-type galaxy NGC 1167, which contains a 160 kpc HI disc. Our VLA and EVN HI absorption observations, modelling, and comparison with molecular gas data suggest that the cold gas in the centre of NGC 1167 is very turbulent (with a velocity dispersion of ~ 90 km/s) and that this turbulence is induced by the interaction of the jets with the interstellar medium (ISM). Furthermore, the ionised gas in the galaxy shows evidence of shock heating at a few kpc from the radio source. These findings support the results from numerical simulations of radio jets expanding into a clumpy gas disc, which predict that the radio jets in this case percolate through the gas disc and drive shocks into the ISM at distances much larger than their physical extent. These results expand the number of low-luminosity radio sources found to impact the surrounding medium, thereby highlighting the possible relevance of these AGN for feedback.Comment: 11 pages, 9 figures; Accepted for publication in Astronomy & Astrophysic

Polylysine-grafted Au144 nanoclusters: birth and growth of a healthy surface-plasmon-resonance-like band

Poly(amino acid)-coated gold nanoparticles hold promise in biomedical applications, particularly because they combine the unique physicochemical properties of the gold core, excellent biocompatibility, and easy functionalization of the poly(amino acid)-capping shell. Here we report a novel method for the preparation of robust hybrid core–shell nanosystems consisting of a Au144 cluster and a densely grafted polylysine layer. Linear polylysine chains were grown by direct N-carboxyanhydride (NCA) polymerization onto ligands capping the gold nanocluster. The density of the polylysine chains and the thickness of the polymer layer strongly depend on the amount and concentration of the NCA monomer and the initiator. The optical spectra of the so-obtained core–shell nanosystems show a strong surface plasmon resonance (SPR)-like band at 531 nm. In fact, despite maintenance of the gold cluster size and the absence of interparticle aggregation, the polylysine-capped clusters behave as if they have a diameter nearly 4 times larger. To the best of our knowledge, this is the first observation of the growth of a fully developed, very stable SPR-like band for a gold nanocluster of such dimensions. The robust polylysine protective shell makes the nanoparticles very stable under conditions of chemical etching, in the presence of glutathione, and at different pH values, without gold core deshielding or alteration of the SPR-like band. This polymerization method can conceivably be extended to prepare core–shell nanosystems based on other mono- or co-poly(amino acids)

Looking for other worlds out there

Are there worlds outside our Solar System? How do we look for them? What can we learn from them? In this article, the author shows how recent advances in technology have made the search for exoplanets more exciting than ever before

Redshift evolution of the H I detection rate in radio-loud active galactic nuclei

We present a search for associated H I 21 cm absorption in a sample of 29 radio-loud active galactic nuclei (AGNs) at 0.7 < z < 1, carried out with the upgraded Giant Metrewave Radio Telescope. We detect H I 21 cm absorption against none of our target AGNs, obtaining 3σ upper limits to the optical depth of ≲1% per 50 km s−1 channel. The radio luminosity of our sources is lower than that of most AGNs searched for H I 21 cm absorption at similar redshifts in the literature, and, for all targets except two, the UV luminosity is below the threshold 1023 W Hz−1, above which the H I in the AGN environment has been suggested to be completely ionised. We stacked the H I 21 cm spectra to obtain a more stringent limit of ≈0.17% per 50 km s−1 channel on the average H I 21 cm optical depth of the sample. The sample is dominated by extended radio sources, 24 of which are extended on scales of tens of kiloparsecs. Including similar extended sources at 0.7 < z < 1.0 from the literature, and comparing with a low-z sample of extended radio sources, we find statistically significant (≈3σ) evidence that the strength of H I 21 cm absorption towards extended radio sources is weaker at 0.7 < z < 1.0 than at z < 0.25, with a lower detection rate of H I 21 cm absorption at 0.7 < z < 1.0. Redshift evolution in the physical conditions of H I is the likely cause of the weaker associated H I 21 cm absorption at high redshifts, due to either a low H I column density or a high spin temperature in high-z AGN environments

Redshift evolution of the H

We present a search for associated H 

The H

We present an H