Chemical abundances in active galaxies

ABSTRACT The Sloan Digital Sky Survey (SDSS) has proved to be a powerful resource for understanding the physical properties and chemical composition of star-forming galaxies in the local Universe. The SDSS population of active galactic nuclei (AGNs) remains as of yet less explored in this capacity. To extend the rigorous study of H ii regions in the SDSS to AGNs, we adapt methods for computing direct-method chemical abundances for application to the narrow-line regions (NLR) of AGNs. By accounting for triply ionized oxygen, we are able to more completely estimate the total oxygen abundance. We find a strong correlation between electron temperature and oxygen abundance due to collisional cooling by metals. Furthermore, we find that nitrogen and oxygen abundances in AGNs are strongly correlated. From the metal–temperature relation and the coupling of nitrogen and oxygen abundances, we develop a new, empirically and physically motivated method for determining chemical abundances from the strong emission lines commonly employed in flux-ratio diagnostic diagrams (BPT diagrams). Our approach, which for AGNs reduces to a single equation based on the BPT line ratios, consistently recovers direct-method abundances over a 1.5 dex range in oxygen abundance with an rms uncertainty of 0.18 dex. We have determined metallicities for thousands of AGNs in the SDSS, and in the process have discovered an ionization-related discriminator for Seyfert and LINER galaxies.

Optical spectroscopy of type-2 LINERs

Low-Ionisation Nuclear Emission-line Regions (LINERs) are the least luminous and the most numerous among the local population of Active Galactic Nuclei (AGN). They can be classified as type-1 or type-2 if their optical spectra show or do not show, respectively, a broad component. It is associated with the presence of a Broad Line Region (BLR) in these systems. However, recent studies have proven that the classification of type-1 LINERs may be controversial, since space- and ground-based spectroscopy provide contradicting results on the presence of very broad components (Cazzoli et al.2018). We have studied the nuclear spectra of 9 type-2 LINERs with intermediate spectral resolution HST/STIS data. We present the results on our analysis of the different spectral components, and discuss the eventual presence of BLR components in type-2 LINER galaxies, together with the possible presence of outflows, both in comparison with type-1 LINERs. We have found a BLR component in 7 out of the 9 analysed objects within the HST/STIS data.

Radio spectral index distribution of SDSS-FIRST sources across optical diagnostic diagrams

Context. The empirical relations between supermassive black holes and their host spheroids point towards the crucial role of galactic nuclei in affecting the properties of their hosts. A detailed understanding of how the activity of a galactic nucleus regulates the growth of its host is still missing. Aims. To understand the activity and the types of accretion of supermassive black holes in different hosts, it is essential to study the radio-optical properties of a large sample of extragalactic sources. In particular, we aim to study the radio spectral index trends across the optical emission line diagnostic diagrams to search for potential (anti)correlations. Methods. To this goal, we combined flux densities from the radio FIRST survey at 1.4 GHz (with the flux density range 10 mJy ≤ F1.4 ≤ 100 mJy) for 396 SDSS sources at intermediate redshift (0.04 ≤ z ≤ 0.4) with the Effelsberg radiotelescope measurements at 4.85 GHz and 10.45 GHz. The information about the optical emission-line ratios is obtained from the SDSS-DR7 catalogue. Results. Using the Effelsberg data, we were able to infer the two-point radio spectral index distributions for star-forming galaxies, composite galaxies (with a combined contribution to the line emission from the star formation and AGN activity), Seyferts, and low ionization narrow emission region (LINER) galaxies. Conclusions. While studying the distribution of steep, flat, and inverted sources across optical diagnostic diagrams, we found three distinct classes of radio emitters for our sample: (i) sources with a steep radio index, high ionization ratio, and high radio loudness, (ii) sources with a flat radio index, lower ionization ratio, and intermediate radio loudness, (iii) sources with an inverted radio index, low ionization ratio, and low radio loudness. The classes (i), (ii), and (iii) cluster mainly along the transition from Seyfert to LINER sources in the optical diagnostic (Baldwin, Phillips & Telervich; BPT) diagram. We interpret these groups as a result of the recurrent nuclear-jet activity.

ALMA observations of molecular tori around massive black holes

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO(3-2) emission in a sample of seven Seyfert/LINER galaxies at the unprecedented spatial resolution of 0.″1 = 4−8 pc. Our aim is to explore the close environment of active galactic nuclei (AGN), and the dynamical structures leading to their fueling, through the morphology and kinematics of the gas inside the sphere of influence of the black hole. The selected galaxies host low-luminosity AGN and have a wide range of activity types (Seyferts 1 to 2, LINERs), and barred or ringed morphologies. The observed maps reveal the existence of circumnuclear disk structures, defined by their morphology and decoupled kinematics, in most of the sample. We call these structures molecular tori, even though they often appear as disks without holes in the center. They have varying orientations along the line of sight, unaligned with the host galaxy orientation. The radius of the tori ranges from 6 to 27 pc, and their mass from 0.7 × 107 to 3.9 × 107 M⊙. The most edge-on orientations of the torus correspond to obscured Seyferts. In only one case (NGC 1365), the AGN is centered on the central gas hole of the torus. On a larger scale, the gas is always piled up in a few resonant rings 100 pc in scale that play the role of a reservoir to fuel the nucleus. In some cases, a trailing spiral is observed inside the ring, providing evidence for feeding processes. More frequently, the torus and the AGN are slightly off-centered with respect to the bar-resonant ring position, implying that the black hole is wandering by a few 10 pc amplitude around the center of mass of the galaxy. Our spatial resolution allows us to measure gas velocities inside the sphere of influence of the central black holes. By fitting the observations with different simulated cubes, varying the torus inclination and the black hole mass, it is possible to estimate the mass of the central black hole, which is in general difficult for such late-type galaxies, with only a pseudo-bulge. In some cases, AGN feedback is revealed through a molecular outflow, which will be studied in detail in a subsequent article.

Study of FHILs in low luminosity AGNs in the optical spectra

This work presents the main results from a selection of optical spectra of Seyfert and LINER galaxies taken from the 9th release of the SDSS with detectable coronal emission. A catalogue of 345 Seyfert 1 (Sy1) and Seyfert 2 (Sy2) galaxies with Forbidden High Ionization Lines (FHILs) emission is presented. By analysing their spectra as well as utilising data from the literature we found evidence of anisotropy in optical FHIL emissions between Sy1 and Sy2 galaxies for the following lines: [Ne v] λ3426, [Fe vii] λ6087, [Fe x] λ6374 and [Fe xi] λ7892. Results continue indicating that optical FHILs are not observed in LINER type galaxies.

The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes

Galaxies, which often contain ionised gas, sometimes also exhibit a so-called low-ionisation nuclear emission line region (LINER). For 30 years, this was attributed to a central mass-accreting supermassive black hole (more commonly known as active galactic nucleus or AGN) of low luminosity, making LINER galaxies the largest AGN sub-population, which dominate in numbers over higher AGN-luminosity Seyfert galaxies and quasars. This, however, poses a serious problem. While the inferred energy balance is plausible, many LINERs clearly do not contain any other independent signatures of an AGN. Using integral field spectroscopic data from the CALIFA survey, we compare the observed radial surface brightness profiles with what is expected from illumination by an AGN. For 48 galaxies with LINER-like emission we show, that the radial emission-line surface brightness profiles are inconsistent with ionisation by a central point-source and hence cannot be due to an AGN alone. The most probable explanation for the excess LINER-like emission is ionisation by evolved stars during the short but very hot and energetic phase known as post-AGB. This leads us to an entirely new interpretation. Post-AGB stars are ubiquitous and their ionising effect should be potentially observable in every galaxy with gas present and with stars older than ~1 Gyr, unless a stronger radiation field from young hot stars or an AGN outshines them. This means, that galaxies with LINER-like emission are not a class defined by a property but rather by the absence of a property. It also explains why LINER emission is observed mostly in massive galaxies with old stars and little star formation.

Optical Properties of the Host Galaxies of Extragalactic Nuclear H2O Masers

Although most nuclear 22GHz (λ=1.35 cm) H2O masers are in Seyfert 2 and LINER galaxies, only a small fraction of such galaxies host water masers. We systematically study the optical properties of the galaxies with and without nuclear H2O maser emission to better understand the relationship between H2O maser emission and properties of the central supermassive black hole and improve the detection rates in future surveys. To this end, we cross-matched the galaxies from H2O maser surveys, both detections and non-detections, with the Sloan Digital Sky Survey (SDSS) low-redshift galaxy sample. We find that maser detection rates are higher at higher optical luminosity (MB), larger velocity dispersion (σ), and higher ion [O III] λ5007 luminosity, with [O III] λ5007 being the dominant factor, and that the isotropic maser luminosity is correlated with these variables. These correlations are natural if maser emission depends on the host SMBH mass and AGN activity. We also find that the detection rate is higher for galaxies with higher extinction. These results indicate that, by pre-selecting galaxies with high extinction-corrected [O III] λ5007 flux, future maser surveys can increase detections efficiencies by a factor of ~3 to ~5.