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Final Report Summary - FEEDING & HIDING AGN (Feeding and hiding the monster: triggering and obscuring the central engine of active galaxies)

Some of the most important open questions in astrophysics concern how and when active galactic nuclei (AGN) are triggered, and how important they are in the evolution of galaxies. In order to understand these processes and interpret the results found for AGN at cosmological distances, it is fundamental to account for the structure of their central engines in the local universe. This includes the unifying torus, whose properties are still uncertain. Thus, the first objective of this proposal was to characterize the AGN obscuring torus using an infrared survey of a homogeneous sample of nearby active galaxies. In addition, we aimed to use spectropolarimetric observations of a smaller sample of Seyfert galaxies to test the validity of the clumpy torus scheme, and ultimately, of the unified model for AGN. The second objective of this proposal was to confirm the hypothesis that AGN of different luminosities are triggered in different ways, with galaxy interactions triggering luminous AGN (quasar-like), and secular processes (bars or disc instabilities) triggering lower luminosity AGN (Seyfert-like). We also aimed to investigate the role of other factors such as the environment in the triggering of the nuclear and star formation activity, and the connection between them.

Research objective 1: Unveiling the properties of the AGN unifying torus.
Whilst critical to unified models of AGN, the torus itself remains poorly constrained. Since models agree that the torus peaks at mid-infrared (MIR) wavelengths, observations in this range offer the best opportunity for progress in constraining the torus properties. Dr. C. Packham, deputy-PI of the MIR instrument CanariCam on the 10.4 m Gran Telescopio Canarias (GTC), created the CanariCam AGN science team (Los Piratas) to maximize the scientific return from the instrument guaranteed time (GT): 100 hours of imaging, spectroscopic and polarimetry observations plus another 20 nights from an ESO/GTC large programme (PI: A. Alonso-Herrero). We have conducted a MIR homogeneous survey of ~100 nearby AGN covering a large range in luminosity and activity class (the sample includes ~50 Seyfert galaxies) and published 9 refereed papers based on individual sources and/or small subsets (Alonso-Herrero et al. 2013, 2014; López-Rodríguez et al. 2014, 2016; Mori et al. 2014; Ramos Almeida et al. 2014, García-Bernete et al. 2015; Martínez-Paredes et al. 2015; Pereira-Santaella et al. 2016) as well as the atlas describing the whole dataset (Alonso-Herrero et al. 2016). In Alonso-Herrero et al. (2014) we presented spectroscopic observations of six local Seyfert galaxies with nuclear star formation and extended MIR emission. We detected 11.3 micron PAH feature emission (a widely-used star formation tracer; hereafter PAH) in the nuclear region of the galaxies as well as extended PAH emission over a few hundred parsecs. The equivalent width (EW) of the feature shows a minimum at the nucleus but increases with radial distance, reaching typical star-forming values a few hundred parsecs away from the nucleus. The reduced nuclear EWs are interpreted as due to increased dilution from the AGN continuum rather than destruction of the PAH molecules. We concluded that the molecules responsible for the PAH feature survive in the nuclear environments as close as 10 pc from the AGN for Seyfert-like AGN luminosities. In Ramos Almeida et al. (2014) we presented a detailed study of the Seyfert 2 galaxy Mrk1066. Once we subtract the dominant AGN emission from the MIR image of the galaxy, four star-forming knots are revealed, including one in the nucleus (see Figure 1). We extracted MIR spectra of the knots, and subtracted the AGN unresolved component. We then measured the EWs of the PAH feature, and found values typical of pure starburst galaxies. In fact, the closer to the nucleus, the larger the EW, confirming that the AGN dilutes, rather than destroys, the molecules responsible for the PAH emission.
During this project, I have been the supervisor of the PhD of Ismael García Bernete at my host institution, and he has submitted his first publication, based on infrared observations of the Seyfert galaxy NGC 2992 and the interacting system Arp 245 (García-Bernete et al. 2015). We have now submitted his second paper, in which he used the new CanariCam MIR observations, as well as publicly available data in the same wavelength range obtained with other instruments on 8-10 m telescopes, and studied the extended MIR emission of Seyfert galaxies (García-Bernete et al. 2016, submitted). We find that the majority of the galaxies (75%) are extended in the MIR, and this extent is somehow related to the dominance of the AGN and/or the star formation in the inner kiloparsec of the galaxies. Star-formation-dominated systems show more extended MIR emission (~600 pc) than AGN-dominated systems (~300 pc).
I have also participated in a project aimed to exploit Cycle 0 and Cycle 2 ALMA observations of the Seyfert 2 galaxy NGC 1068 (PI: Santiago García Burillo). Our aim was to investigate nuclear inflows/outflows in this galaxy, and to unveil the unifying torus at sub-mm wavelengths. Although in principle the bulk of the torus emission is emitted in the infrared, sub-mm wavelengths allow to probe the coolest dust within the torus, and therefore it is possible to determine its properties by using high angular resolution ALMA observations. In 2014 we published the first results based on the Cycle 0 data (García-Burillo et al. 2014), where we could not resolve the torus because of the limited number of antennas available at the time, but we found evidence for an AGN-driven outflow in the dense molecular gas. More recently, and thanks to the angular resolution provided by ALMA in Cycle 2, we have finally imaged and spatially resolved the submillimeter counterpart of the putative torus of NGC 1068 (García-Burillo et al. 2016).
As part of research objective 1, I also proposed to perform a comparison between the polarized spectra and the torus properties of a sample of Seyfert galaxies, with the aim of revisiting the AGN unified model. I was awarded observing time at the 8 m Very Large Telescope (VLT) in visitor mode in April 2013. We obtained new optical spectropolarimetry data with the instrument FORS2, and the main result is the detection of a hidden broad-line region (BLR; where the broad lines are produced in AGN) in 73% of the galaxies observed with FORS2, which includes galaxies previously classified as non-hidden BLR objects and others without any previous spectropolarimetry data. Our result supports the AGN unification model, and highlights the importance of high-quality observations with 8-10 m telescopes to unveil hidden BLRs (see Figure 2; Ramos Almeida et al. 2016).

Research objective 2: Investigating the triggering mechanisms of AGN and starbursts.
In Ramos Almeida et al. (2011, 2012) and Bessiere et al. (2012) we analysed the galaxy morphologies of a complete sample of radio galaxies and type-2 quasars, compared them with control samples and studied their environments. We concluded that galaxy interactions trigger nuclear activity in high luminosity AGN. As part of this research objective, I proposed to characterize the environments of radio galaxies to study its influence in the triggering of AGN. Using dedicated offset fields for the complete sample of 2Jy radio galaxies and the type-2 quasars studied in Bessiere et al. (2012), as well as Subaru images of a control sample of non-active galaxies of the same mass and redshift, I quantified the environments and published the results in Ramos Almeida et al. (2013). We concluded that the environment plays a key role in the triggering of the radio jets (see Figure 3).
We also used deep Herschel Space Observatory observations of a 90% complete sample of 32 intermediate-redshift 2Jy radio galaxies with strong emission lines to estimate the dust masses of their host galaxies and thereby investigate the triggering mechanisms for their quasar-like AGN. The dust masses derived for the radio galaxies are intermediate between those of quiescent elliptical galaxies on the one hand, and ultraluminous infrared galaxies (ULIRGs) on the other. These results suggest that most radio galaxies represent the late time re-triggering of AGN activity via mergers between the host giant elliptical galaxies and companion galaxies with relatively low gas masses. However, a minority of the radio galaxies in our sample (~20%) have high, ULIRG-like dust masses, along with evidence for prodigious star formation activity. The latter objects are more likely to have been triggered in major, gas-rich mergers that represent a rapid growth phase for both their host galaxies and their supermassive black holes.
Finally, in Bessiere et al. (2016, submitted) we performed a detailed study of the stellar populations of the host galaxies of 21 type-2 quasars, with the aim of understanding the sequence of events between the onset of the merger, the triggering of the associated starburst and the initiation of the quasar activity. We modelled high quality optical spectra of these quasars and found evidence for young stellar populations in 90% of the sample. Since the ages of the young stellar population in most type-2 quasar host galaxies are comparable with the expected lifetimes of the AGN activity, these results provide strong evidence that the quasars are triggered close to the peaks of the merger-induced starbursts.

The main results from this project can be summarized as follows:
-We do not find evidence for star formation suppression in the AGN vicinity for moderately-luminous AGN (Seyfert galaxies). Instead, the PAH features seem to be diluted by the intense AGN continuum.
-The extent of the MIR emission of Seyfert galaxies is somehow related to the dominance of the AGN and/or the star formation in the inner kiloparsec. Star-formation-dominated systems show more extended MIR morphologies (~600 pc) than AGN-dominated systems (~300 pc).
-In a major breakthrough, using ALMA Cycle 2 observations of the Seyfert 2 galaxy NGC 1068, we have imaged, for the first time, the AGN unifying torus at sub-mm wavelengths. This image provides fundamental support to the unified model of AGN.
-Using new spectropolarimetry data we found >4sigma detections of a hidden broad line region in 73% of a sample of Seyfert 2 galaxies. These results support the AGN unified model and confirm that at least some galaxies without polarized broad lines reported are misclassified, bringing previous publications into question.
-Using deep optical images of a complete sample of radio galaxies and type-2 quasars, we concluded that the environment plays a key role in the triggering of the radio jets.
-According to far-infrared observations of powerful radio galaxies, we found that most radio galaxies represent the late time re-triggering of AGN activity via mergers between the host giant elliptical galaxies and companion galaxies with relatively low gas masses.
-From the study of the stellar populations of the host galaxies of type-2 quasars, we find strong evidence that these quasars are triggered close to the peaks of the merger-induced starbursts.

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Record Number: 187661 / Last updated on: 2016-08-16
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