Mid-infrared interferometry of AGN cores

Active Galactic Nuclei (AGNs) are among the most luminous objects in the universe and are classified into a number of types and subtypes. Unified models of AGNs explain some of this variety as a result of different viewing angles towards their cores instead of intrinsic differences: from some viewing angles, our line of sight would be blocked by a dusty torus. The spatial resolution necessary to resolve these warm dust structures at 10 micron is currently only provided by the Very Large Telescope Interferometer (VLTI) in Chile. First, extensive observations of the radio galaxy Centaurus A are examined that show an extended structure, which is probably connected to the Northern nuclear radio jet, at a distance of about 40 milli arcseconds (0.7 parsec) in front of the nucleus. Secondly, a study of the brightest so-called type 1 Seyfert galaxy, NGC 4151, reveals for the first time a resolved nuclear emitter in such a source. Its properties (size, temperature profile, emissivity) are similar to those of the alternative type 2 galaxies, studied previ- ously. This is consistent with, though not necessarily confirmation of, unified models. Since the previous studies of type 2 source and modern torus models show a wide variety of nuclear dust structures, finally, the so-far most comprehensive study of resolved nuclear dust emission of AGNs is set up to study 13 AGNs of various luminosities L and distances reaching several hundred Mega Parsec. The first full analysis of this study shows in all but one galaxy that tori can be resolved and that their size on parsec scales is not simply proportional to L^0.5

Resolving the nucleus of Centaurus A at mid-IR wavelengths

We have observed Centaurus A with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 - 15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to earlier measurements. We are now able to describe the nuclear emission in terms of geometric components and derive their parameters by fitting models to the interferometric data. With simple geometrical models, the best fit is achieved for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x 35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also evidence that neither such a uniform nor a Gaussian disk are good fits to the data. This indicates that we are resolving more complicated small-scale structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees of the dust emission are compared with observations of gas and dust at larger scales.Comment: Accepted for the PASA special issue on Centaurus

A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies

The AGN-heated dust distribution (the "torus") is increasingly recognized not only as the absorber required in unifying models, but as a tracer for the reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most basic structural properties (such as its extent, geometry and elongation) are unknown for all but a few archetypal objects. Since most AGNs are unresolved in the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) that is sensitive to structures as small as a few milli-arcseconds (mas). We present here an extensive amount of new interferometric observations from the MIDI AGN Large Program (2009 - 2011) and add data from the archive to give a complete view of the existing MIDI observations of AGNs. Additionally, we have obtained high-quality mid-infrared spectra from VLT/VISIR. We present correlated and total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron using simple axisymmetric geometrical models. Perhaps the most surprising result is the relatively high level of unresolved flux and its large scatter: The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs meaning that a large part of the flux is concentrated on scales smaller than about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear components can be distinguished in the radial fits. While these models provide good fits to all but the brightest sources, significant elongations are detected in eight sources. The half-light radii of the fainter sources are smaller than expected from the size ~ L^0.5 scaling of the bright sources and show a large scatter, especially when compared to the relatively tight size--luminosity relation in the near-infrared.Comment: A&A in press; 93 pages, 63 figures, 39 tables; data available only via CD

The carbon footprint of large astronomy meetings

The annual meeting of the European Astronomical Society took place in Lyon, France, in 2019, but in 2020 it was held online only due the COVID-19 pandemic. The carbon footprint of the virtual meeting was roughly 3,000 times smaller than the face-to-face one, providing encouragement for more ecologically minded conferencing.Comment: Originated in a Twitter discussion (https://twitter.com/sarahkendrew/status/1144186571538739202) at EWASS 2019; followed up at the EAS 2020 conference sustainability session by https://astronomersforplanet.earth - published in Nature Astronomy, September 202

The Subarcsecond Mid-Infrared View of Local Active Galactic Nuclei. IV. The L- and M-band Imaging Atlas

We present the largest currently existing subarcsecond 3-5 $\mu$m atlas of 119 local ($z < 0.3$) active galactic nuclei (AGN). This atlas includes AGN of 5 subtypes: 22 are Seyfert 1; 5 are intermediate Seyferts; 46 are Seyfert 2; 26 are LINERs; and 20 are composites/starbursts. Each AGN was observed with VLT ISAAC in the $L$- and/or $M$-bands between 2000 and 2013. We detect at 3$\sigma$ confidence 92 sources in the $L$-band and 83 sources in the $M$-band. We separate the flux into unresolved nuclear flux and resolved flux through two-Gaussian fitting. We report the nuclear flux, extended flux, apparent size, and position angle of each source, giving $3\sigma$ upper-limits for sources which are undetected. Using WISE W1- and W2-band photometry we derive relations predicting the nuclear $L$ and $M$ fluxes for Sy1 and Sy2 AGN based on their W1-W2 color and WISE fluxes. Lastly, we compare the measured mid-infrared colors to those predicted by dusty torus models SKIRTOR, CLUMPY, CAT3D, and CAT3D-WIND, finding best agreement with the latter. We find that models including polar winds best reproduce the 3-5$\mu$m colors, indicating that winds are an important component of dusty torus models. We find that several AGN are bluer than models predict. We discuss several explanations for this and find that it is most plausibly stellar light contamination within the ISAAC $L$-band nuclear fluxes.Comment: Main Text: 22 pages, 9 figures, 3 tables. Accepted by Ap

Sculpting the disk around T Cha: an interferometric view

(Abridged) Circumstellar disks are believed to be the birthplace of planets and are expected to dissipate on a timescale of a few Myr. The processes responsible for the removal of the dust and gas will strongly modify the radial distribution of the dust and consequently the SED. In particular, a young planet will open a gap, resulting in an inner disk dominating the near-IR emission and an outer disk emitting mostly in the far-IR. We analyze a full set of data (including VLTI/Pionier, VLTI/Midi, and VLT/NaCo/Sam) to constrain the structure of the transition disk around TCha. We used the Mcfost radiative transfer code to simultaneously model the SED and the interferometric observations. We find that the dust responsible for the emission in excess in the near-IR must have a narrow temperature distribution with a maximum close to the silicate sublimation temperature. This translates into a narrow inner dusty disk (0.07-0.11 AU). We find that the outer disk starts at about 12 AU and is partially resolved by the Pionier, Sam, and Midi instruments. We show that the Sam closure phases, interpreted as the signature of a candidate companion, may actually trace the asymmetry generated by forward scattering by dust grains in the upper layers of the outer disk. These observations help constrain the inclination and position angle of the outer disk. The presence of matter inside the gap is difficult to assess with present-day observations. Our model suggests the outer disk contaminates the interferometric signature of any potential companion that could be responsible for the gap opening, and such a companion still has to be unambiguously detected. We stress the difficulty to observe point sources in bright massive disks, and the consequent need to account for disk asymmetries (e.g. anisotropic scattering) in model-dependent search for companions.Comment: Removed the word "first" in the abstract of the paper: "obtained with the first 4-telescope combiner (VLTI/Pionier)

Surprisingly Strong K-band Emission Found in Low Luminosity Active Galactic Nuclei

We examine the near-infrared (NIR) emission from low-luminosity AGNs (LLAGNs). Our galaxy sample includes 15 objects with detected 2-10 keV X-ray emission, dynamical black hole mass estimates from the literature, and available Gemini/NIFS integral field spectroscopy (IFU) data. We find evidence for red continuum components at the center of most galaxies, consistent with the hot dust emission seen in higher luminosity AGN. We decompose the spectral data cubes into a stellar and continuum component, assuming the continuum component comes from thermal emission from hot dust. We detect nuclear thermal emission in 14 out of 15 objects. This emission causes weaker CO absorption lines and redder continuum ($2.05-2.28\:\mu$m) in our $K$-band data, as expected from hot dust around an AGN. The NIR emission is clearly correlated with the 2-10 keV X-ray flux, with a Spearman coefficient of $r_{spearman}=0.69$ suggesting a $>99\%$ significance of correlation, providing further evidence of an AGN origin. Our sample has typical X-ray and NIR fluxes $3-4$ orders of magnitude less luminous than previous work studying the NIR emission from AGN. We find that the ratio of NIR to X-ray emission increases towards lower Eddington ratios. The NIR emission in our sample is often brighter than the X-ray emission, with our $K$-band AGN luminosities comparable to or greater than the 2-10 keV X-ray luminosities in all objects with Eddington ratios below $0.01\%$. The nature of this LLAGN NIR emission remains unclear, with one possibility being an increased contribution from jet emission at these low luminosities. These observations suggest JWST will be a useful tool for detecting the lowest luminosity AGN.Comment: Submitted to ApJ, manuscript after resubmissio

Gemini NIFS survey of feeding and feedback processes in nearby active galaxies : VI. Stellar populations

We use Gemini Near-Infrared Integral Field Spectrograph (NIFS) adaptive optics assisted data cubes to map the stellar population of the inner few hundred parsec of a sample of 18 nearby Seyfert galaxies. The near-infrared light is dominated by the contribution of young to intermediate-age stellar populations, with light-weighted mean ages ‹t›L ≲ 1.5 Gyr. Hot dust (HD) emission is centrally peaked (in the unresolved nucleus), but it is also needed to reproduce the continuum beyond the nucleus in nearly half of the sample. We have analysed the stellar population properties of the nuclear region and their relation with more global properties of the galaxies. We find a correlation between the X-ray luminosity and the contributions from the HD, featureless continuum (FC), and reddening AV. We attribute these correlations to the fact that all these properties are linked to the mass accretion rate to the active galactic nuclei (AGNs). We also find a correlation of the bolometric luminosity log(LBolobs) with the mass-weighted mean age of the stellar population, interpreted as due a delay between the formation of new stars and the triggering/feeding of the AGN. The gas reaching the supermassive black hole is probably originated from mass loss from the already somewhat evolved intermediate-age stellar population (‹t›L ≲ 1.5 Gyr). In summary, our results show that there is a significant fraction of young to intermediate-age stellar populations in the inner few 100 pc of active galaxies, suggesting that this region is facing a rejuvenation process in which the AGN, once triggered, precludes further star formation, in the sense that it can be associated with the lack of new star formation in the nuclear region

Stellar populations in local AGNs : evidence for enhanced star formation in the inner 100 pc

In modern models and simulations of galactic evolution, the star formation in massive galaxies is regulated by an ad hoc active galactic nuclei (AGN) feedback process. However, the physics and the extension of such effects on the star formation history of galaxies is matter of vivid debate. In order to shed some light in the AGN effects over the star formation, we analysed the inner 500 × 500 pc of a sample of 14 Seyfert galaxies using GMOS and MUSE integral field spectroscopy. We fitted the continuum spectra in order to derive stellar age, metallicity, velocity, and velocity dispersion maps in each source. After stacking our sample and averaging their properties, we found that the contribution of young SP, as well as that of AGN featureless continuum both peak at the nucleus. The fraction of intermediate-age SPs is smaller in the nucleus if compared to outer regions, and the contribution of old SPs vary very little within our field of view (FoV). We also found no variation of velocity dispersion or metallicity within our FoV. Lastly, we detected an increase in the dust reddening towards the center of the galaxies. These results lead us to conclude that AGN phenomenon is usually related to a recent star formation episode in the circumnuclear region

The GRAVITY instrument software / High-level software

GRAVITY is the four-beam, near- infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.Comment: 8 pages, 7 figures, published in Proc. SPIE 9146, Optical and Infrared Interferometry IV, 91462