C. Ramos Almeida

TESTING THE UNIFICATION MODEL FOR ACTIVE GALACTIC NUCLEI IN THE INFRARED: ARE THE OBSCURING TORI OF TYPE 1 AND 2 SEYFERTS DIFFERENT?

We present new mid-infrared (MIR) imaging data for three Type-1 Seyfert galaxies obtained with T-ReCS on the Gemini-South Telescope at subarcsecond resolution. Our aim is to enlarge the sample studied in a previous work to compare the properties of Type-1 and Type-2 Seyfert tori using clumpy torus models and a Bayesian approach to fit the infrared nuclear spectral energy distributions (SEDs). Thus, the sample considered here comprises 7 Type-1, 11 Type-2, and 3 intermediate-type Seyferts. The unresolved IR emission of the Seyfert 1 galaxies can be reproduced by a combination of dust heated by the central engine and direct AGN emission, while for the Seyfert 2 nuclei only dust emission is considered. These dusty tori have physical sizes smaller than 6 pc radius, as derived from our fits. Unification schemes of AGN account for a variety of observational differences in terms of viewing geometry. However, we find evidence that strong unification may not hold, and that the immediate dusty surroundings of Type-1 and Type-2 Seyfert nuclei are intrinsically different. The Type-2 tori studied here are broader, have more clumps, and these clumps have lower optical depths than those of Type-1 tori. The larger the covering factor of the torus, the smaller the probability of having direct view of the AGN, and vice-versa. In our sample, Seyfert 2 tori have larger covering factors (CT=0.95� 0.02) and smaller escape probabilities (Pesc=0.05� 0.08 0.03 %) than those of Seyfert 1 (CT=0.5� 0.1; Pesc=18� 3 %). All the previous differences are significant according to the KullbackLeibler divergence. Thus, on the basis of the results presented here, the classification of a Seyfert galaxy as a Type-1 or Type-2 depends more on the intrinsic properties of the torus rather than on its mere inclination towards us, in contradiction with the simplest unification model. Subject headings: galaxies: active – galaxies: nuclei – galaxies: Seyfert – infrared: galaxies

The Infrared Nuclear Emission of Seyfert Galaxies on Parsec Scales: Testing the Clumpy Torus Models

We present subarcsecond resolution mid-infrared (mid-IR) photometry in the wavelength range from 8 to 20 μm of 18 Seyfert galaxies, reporting high spatial resolution nuclear fluxes for the entire sample. We construct spectral energy distributions (SEDs) that the active galactic nucleus (AGN) dominates, relatively uncontaminated by starlight, adding near-IR measurements from the literature at similar angular resolution. We find that the IR SEDs of intermediate-type Seyferts are flatter and present higher 10 to 18 μm ratios than those of Seyfert 2 galaxies. We fit the individual SEDs with clumpy dusty torus models using the in-house-developed BayesClumpy tool. We find that the clumpy models reproduce the high spatial resolution measurements. Regardless of the Seyfert type, even with high spatial resolution data, near- to mid-IR SED fitting poorly constrains the radial extent of the torus. For the Seyfert 2 galaxies, we find that edge-on geometries are more probable than face-on views, with a number of clouds along equatorial rays of N 0 = 5-15. The 10 μm silicate feature is generally modeled in shallow absorption. For the intermediate-type Seyferts, N 0 and the inclination angle of the torus are lower than those of the Seyfert 2 nuclei, with the silicate feature appearing in weak emission or absent. The columns of material responsible for the X-ray absorption are larger than those inferred from the model fits for most of the galaxies, which is consistent with X-ray absorbing gas being located within the dust sublimation radius, whereas the mid-IR flux arises from an area farther from the accretion disk. The fits yield both the bolometric luminosity of the intrinsic AGN and the torus-integrated luminosity, from which we derive the reprocessing efficiency of the torus. In the models, the outer radial extent of the torus scales with the AGN luminosity, and we find the tori to be confined to scales less than 5 pc.

The optical morphologies of the 2 Jy sample of radio galaxies : Evidence for galaxy interactions

We present deep Gemini Multi-Object Spectrograph South (GMOS-S)/Gemini optical broad-band images for a complete sample of 46 southern 2 Jy radio galaxies at intermediate redshifts (0.05 <z <0.7). Based on them, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). The high-quality observations presented here show for the first time that the overall majority of PRGs at intermediate redshifts (78-85 per cent) show peculiarities in their optical morphologies at relatively high levels of surface brightness (?; ?). The observed morphological peculiarities include tails, fans, bridges, shells, dust lanes, irregular features, amorphous haloes and multiple nuclei. While the results for many of the galaxies are consistent with them being observed at, or after, the time of coalescence of the nuclei in a galaxy merger, we find that more than one-third of the sample are observed in a pre-coalescence phase of the merger, or following a close encounter between galaxies that will not necessarily lead to a merger. By dividing the sample into Weak-Line Radio Galaxies (WLRGs; 11 objects) and Strong-Line Radio Galaxies (SLRGs; 35 objects) we find that only 27 per cent of the former show clear evidence for interactions in their optical morphologies, in contrast to the SLRGs, of which at least 94 per cent appear interacting. This is consistent with the idea that many WLRGs are fuelled/triggered by the Bondi accretion of hot gas. However, the evidence for interactions and dust features in a fraction of them indicates that the accretion of cold gas cannot always be ruled out. Of the 28 per cent of the sample that display evidence for significant starburst activity, we find that 92 per cent present disturbed morphologies, following the same general trend as the total and SLRG samples. By comparing our PRGs with various samples of quiescent ellipticals from the literature, we conclude that the percentage of morphological disturbance that we find here exceeds that found for quiescent ellipticals when similar surface brightnesses are considered. Overall, our study indicates that galaxy interactions are likely to play a key role in the triggering of active galactic nuclei (AGN)/jet activity.

Are luminous radio-loud active galactic nuclei triggered by galaxy interactions?

We present the results of a comparison between the optical morphologies of a complete sample of 46 southern 2 Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7) and those of two control samples of quiescent early-type galaxies: 55 ellipticals at redshifts z ≤ 0.01 from the Observations of Bright Ellipticals at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2 < z < 0.7 in the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We find that a significant fraction of quiescent ellipticals at low and intermediate redshifts show evidence for disturbed morphologies at relatively high surface brightness levels, which are likely the result of past or on-going galaxy interactions. However, the morphological features detected in the galaxy hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2 mag brighter than those present in their quiescent counterparts. Indeed, if we consider the same surface brightness limits, the fraction of disturbed morphologies is considerably smaller in the quiescent population (53 per cent at z < 0.2 and 48 per cent at 0.2 ≤ z < 0.7) than in the PRGs (93 per cent at z < 0.2 and 95 per cent at 0.2 ≤ z < 0.7 considering strong-line radio galaxies only). This supports a scenario in which PRGs represent a fleeting active phase of a subset of the elliptical galaxies that have recently undergone mergers/interactions. However, we demonstrate that only a small proportion (≲20 per cent) of disturbed early-type galaxies are capable of hosting powerful radio sources.

Starburst radio galaxies: general properties, evolutionary histories and triggering

In this paper we discuss the results of a programme of spectral synthesis modelling of a sample of starburst radio galaxies in the context of scenarios for the triggering of the activity and the evolution of the host galaxies. New optical spectra are also presented for a subset of the objects discussed. The starburst radio galaxies - comprising ˜15-25 per cent of all powerful extragalactic radio sources - frequently show disturbed morphologies at optical wavelengths, and unusual radio structures, although their stellar masses are typical of radio galaxies as a class. In terms of the characteristic ages of their young stellar populations (YSPs), the objects can be divided into two groups: those with YSP ages tYSP≤ 0.1 Gyr, in which the radio source has been triggered quasi-simultaneously with the main starburst episode, and those with older YSP in which the radio source has been triggered or re-triggered a significant period after the starburst episode. Most of the former group are associated with a large mid- to far-IR (MFIR) continuum and [O III] emission-line luminosities (LIR > 1011 L⊙, ? W), while most of the latter have lower luminosities. Combining the information on the YSP with that on the optical morphologies of the host galaxies, we deduce that the majority of the starburst radio galaxies have been triggered in galaxy mergers in which at least one of the galaxies is gas rich. However, the triggering (or re-triggering) of the radio jets can occur immediately before, around or a significant period after the final coalescence of the merging nuclei, reflecting the complex gas infall histories of the merger events. Although ˜25 per cent of starburst radio galaxies are sufficiently bright at MFIR wavelengths to be classified as ultraluminous infrared galaxies (ULIRGs), we show that only the most massive ULIRGs are capable of evolving into radio galaxies. Finally, for a small subset of starburst radio galaxies in rich clusters of galaxies, cooling flows associated with the hot X-ray haloes offer a viable alternative to mergers as a trigger for the radio jet activity. Overall, our results provide further evidence that a powerful radio jet activity can be triggered via a variety of mechanisms, including different evolutionary stages of major galaxy mergers; clearly, radio-loud AGN activity is not solely associated with a particular stage of a unique type of gas accretion event.

Spitzer Mid-IR Spectroscopy of Powerful 2?Jy and 3CRR Radio Galaxies. I. Evidence against a Strong Starburst-AGN Connection in Radio-loud AGN

We present deep Spitzer/Infrared Spectrograph (IRS) spectra for complete samples of 46 2 Jy radio galaxies (0.05 75%) than their more extended counterparts (≈15%-25%). We discuss this result in the context of a possible bias toward the selection of compact radio sources triggered in gas-rich environments.

The Nuclear Infrared Emission of Low-luminosity Active Galactic Nuclei

We present high-resolution mid-infrared (MIR) imaging, nuclear spectral energy distributions (SEDs), and archival Spitzer spectra for 22 low-luminosity active galactic nuclei (LLAGNs; L bol 1042 erg s–1). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGNs, the fate of the obscuring torus at low accretion rates, and, perhaps, the star formation histories of these objects. However, while comprehensively studied in higher-luminosity Seyferts and quasars, the nuclear IR properties of LLAGNs have not yet been well determined. We separate the present LLAGN sample into three categories depending on their Eddington ratio and radio emission, finding different IR characteristics for each class. (1) At the low-luminosity, low-Eddington-ratio (log L bol/L Edd < –4.6) end of the sample, we identify host-dominated galaxies with strong polycyclic aromatic hydrocarbon bands that may indicate active (circum-)nuclear star formation. (2) Some very radio-loud objects are also present at these low Eddington ratios. The IR emission in these nuclei is dominated by synchrotron radiation, and some are likely to be unobscured type 2 AGNs that genuinely lack a broad-line region. (3) At higher Eddington ratios, strong, compact nuclear sources are visible in the MIR images. The nuclear SEDs of these galaxies are diverse; some resemble typical Seyfert nuclei, while others lack a well-defined MIR dust bump. Strong silicate emission is present in many of these objects. We speculate that this, together with high ratios of silicate strength to hydrogen column density, could suggest optically thin dust and low dust-to-gas ratios, in accordance with model predictions that LLAGNs do not host a Seyfert-like obscuring torus. We anticipate that detailed modeling of the new data and SEDs in terms of accretion disk, jet, radiatively inefficient accretion flow, and torus components will provide further insights into the nuclear structures and processes of LLAGNs.

Dust in active galactic nuclei - Mid-infrared T-ReCS/Gemini spectra using the new RedCan pipeline

Context. The unified model of active galactic nuclei (AGN) claims that the properties of AGN depend on the viewing angle of the observer with respect to a toroidal distribution of dust surrounding the nucleus. Both the mid-infrared (MIR) attenuation and continuum luminosity are expected to be related to dust associated with the torus. Therefore, isolating the nuclear component is essential for studying the MIR emission of AGN. Aims. This work is aimed at studying the MIR emission of AGN with the highest spatial resolution available to date, isolating its contribution from extended emission. We would like to address three fundamental questions: (1) how important is the AGN contribution to the MIR spectrum; (2) where dust attenuation arises; and (3) how does spatial resolution affect these issues. Methods. We compiled all the T-ReCS spectra (Gemini Observatory) available in the N-band for 22 AGN: 5 Type-1 and 17 Type-2 AGN. The high angular resolution of the T-ReCS spectra allowed us to probe physical regions of 57 pc (median). We used a novel pipeline called RedCan capable of producing flux- and wavelength-calibrated spectra for the CanariCam (GTC) and T-ReCS (Gemini) instruments. We measured the fine-structure [S IV] at 10.5 μm and the PAH at 11.3 μm line strengths, together with the silicate absorption/emission features. We also compiled Spitzer/IRS spectra to understand how spatial resolution influences the results. We complemented our sample with the results of 19 VISIR/VLT spectra (Paranal Observatory) and 20 nearby, highly obscured AGN (N_H > 1.5 × 10^(24) cm^(-2)) Spitzer spectra. Results. The 11.3 μm PAH feature is only clearly detected in the nuclear spectra of two AGN, while it is more common in the Spitzer data. For those two objects, the AGN emission in NGC 7130 accounts for more than 80% of the MIR continuum at 12 μm, while in the case of NGC 1808 the AGN does not dominate the MIR emission. This is confirmed by the correlation between the MIR and X-ray continuum luminosities. The [S IV] emission line at 10.5 μm, which is believed to originate in the narrow line region, is detected in most AGN. We have found an enhancement of the optical depth at 9.7 μm (τ_(9.7)) in the high-angular resolution data for higher values of N_H. Clumpy torus models reproduce the observed values only if the host-galaxy properties are taken into account.

The importance of galaxy interactions in triggering type II quasar activity

We present deep Gemini Multi-Object Spectrograph-South optical broad-band images for a complete sample of 20 Sloan Digital Sky Survey selected type II quasars taken from Zakamska et al., with redshifts in the range 0.3 < z < 0.41 and [O iii]λ5007 emission-line luminosities . The images were taken with the aim of investigating the interaction status of the quasar host galaxies, in order to determine the significance of galaxy interactions in triggering nuclear activity. We find that 15 of our sample of 20 (75 per cent) show evidence for interaction in the form of tails, shells, fans, irregular features, amorphous haloes and double nuclei. The median surface brightness of the features is and the range is . We find a similar rate of interaction signatures in the type II quasars as in a comparison sample of quiescent early-type galaxies at similar redshift (67 per cent) taken from Ramos Almeida et al. (RA11). However the surface brightness of the detected features is up to 2 mag brighter for the type II quasars than for the quiescent early types, which have surface brightnesses in the range and a median surface brightness . Despite the relatively small sample size, this may indicate that the mergers witnessed in the comparison sample galaxies could have different progenitors, or we may be viewing the interactions at different stages. We also compare our results with those of Ramos Almeida et al. (RA12) who made a similar analysis using a complete sample of radio-loud active galactic nuclei (AGN). They find a higher rate of interaction signatures in the radio-loud AGN (95 per cent) than the type II quasars, but a very similar range of surface brightnesses for the morphological features , possibly indicating a similarity in the types of triggering interactions. The wide range of features detected in the type II quasar sample suggests that AGN activity can be triggered before, during or after the coalescence of the black holes, with six of the 20 objects (30 per cent) having double nuclei. Overall, the results presented here are consistent with the idea that galaxy interaction plays an important role in the triggering of quasar activity. We also use time-scale arguments to show that it is unlikely that most radio-quiet quasars cycle through a radio-loud phase as part of a single quasar triggering event.

NEAR-INFRARED SPECTROSCOPY OF SEYFERT GALAXIES. NUCLEAR ACTIVITY AND STELLAR POPULATION

Near-infrared (near-IR) spectroscopic data for the five Seyfert galaxies with jet-gas interaction Mrk 348, Mrk 573, Mrk 1066, NGC 7212, and NGC 7465, taken with the Long-slit Intermediate Resolution Infrared Spectrograph near-IR camera/spectrometer at the William Herschel Telescope are reported. The long-slit spectra reveal the characteristic strong emission lines of this type of objects. Many forbidden transitions and hydrogen recombination lines are employed here to study the excitation and ionization mechanisms that are dominating the narrow-line region emission of these objects, that is affected by the radio-jet interaction. Several absorption features are also detected in the H and K bands of these galaxies, allowing us to identify the spectral types that are producing them. We find that the continuum can be reproduced by a combination of late-type stellar templates plus a blackbody component associated with hot dust, mainly contributing to the K-band emission. The detection of the permitted O I and Fe II lines and broad components of the hydrogen recombination lines in the spectra of Mrk 573 and NGC 7465 allows the reclassification of these two galaxies that are not canonical Type-2 Seyferts: Mrk 573 is confirmed to be an obscured narrow-line Seyfert 1, and NGC 7465 is revealed for the first time as a Type-1 LINER through its near-IR spectrum.