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Gas origin in the extended narrow line region of Nearby Seyfert galaxies
Full text linkThe Extended Narrow Line Region (ENLR) of Active Galactic Nuclei (AGN) is a conical or bi-conical region of highly ionized gas with a size of few up to 15–20 kpc and the apexes pointing towards the active nucleus. This region is also called ionization cones. These cones are an evidence of the Unified Model (Antonucci 1993) that predicts an anisotropic escape of ionizing photons from the nucleus confined in a conical flow by a dusty torus. Many details about the complex structure of the ENLR still remains unveiled. For example it is not clear which is the origin of the ionized gas. This is an important issue, being related to the fuelling of the active nucleus. The gas could be simply part of the interstellar medium of the host galaxy, ionized by the active nucleus or by shocks; or it could be intergalactic medium acquired by the surroundings of the host galaxy through gravitational interactions. Up to now only 25 Seyfert 2 galaxies with ionization cones are known and studied. The lack of cones in Seyfert 2 galaxies could be a selection effect, or it could be due to the absence of gas distributed over large scales in the host galaxy, but it can also indicate that some particular initial conditions are needed in order to form an ENLR. In this work we tackle the question of the origin of the gas, by combining the information obtained from IFU, longslit and echelle spectra. Being the IFU field of view typically small, in order to correctly interpret the spectroscopic results we analyse also broad band images. We reduced and analyzed the collected data from our observational campaign of nearby Seyfert galaxies. IFU spectra were taken with MPFS at SAO 6-mt telescope, longslit spectra with AFOSC at the 1.8-mt telescope
of Asiago Astrophysical Observatory and the echelle spectra with MagE at 6.5-mt Magellan telescope.
The broad band images were taken with SCORPIO at SAO 6-mt telescope and from the ING archive.
This work is based on: (i) analysis of gas physical properties (density, temperature and metallicity), (ii) analysis of emission lines ratios, (iii) study of gas and stars kinematics, (iv) morphological analysis. We applied this method to two Seyfert galaxies showing extended [O III] emission, NGC 7212, a Seyfert 2 galaxy and MRK 6 a Seyfert 1.5 galaxy. We obtained strong proof of an external origin for the ionized
gas in both cases. This is more likely to occur in NGC 7212 because it is in a strongly interacting system, however also in MRK 6 we found hints of a past merger. We took advantage of integral field data which allow to spatially sample the ENLR in 1"×1" emission line sub-regions and we selected 12
Seyfert 2 galaxies to perform a statistical study of spectroscopic properties of a large number of these regions. From the analysis of about 850 sub-regions we pointed out that most of them share the properties generally observed in the NLR of Seyfert galaxies. All these results strongly suggest that particular initial conditions are mandatory in order to generate a kpc-size NLR.La Extended Narrow Line Region (ENLR) dei Nuclei Galattici Attivi (AGN) e' una regione di gas ad alta ionizzazione, di forma conica o biconica con dimensioni fino a 15-20 kpc e gli apici in direzione del nucleo attivo. Questa struttura, detta anche coni di ionizzazione, e' una prova della validita' del Modello Unificato (Antonucci 1993), che predice che il flusso della radiazione ionizzante emessa dal nucleo sia collimato dal toro di polveri. La struttura della ENLR e' complicata e sono numerose le questioni irrisolte, per esempio non e' chiara l’origine del gas ionizzato presente in queste regioni. Il gas potrebbe essere parte del mezzo interstellare della galassia ospite, ed essere semplicemente ionizzato dal nucleo attivo, oppure potrebbe essere mezzo intergalattico acquisito dall’esterno in seguito a interazioni gravitazionali.
Finora sono state scoperte solo 25 Seyfert 2 con coni di ionizzazione. Questo basso numero di oggetti potrebbe essere dovuto a un effetto di selezione, oppure all’assenza di gas distribuito su larga scala nella galassia ospite, ma potrebbe anche indicare che servono particolari condizioni per formare la ENLR. In questo lavoro abbiamo studiato l’orgine del gas utilizzando dati spettroscopici integral field (IFU), longslit e echelle. A causa del piccolo campo di vista dei dati IFU c’e' bisogno di un confronto con immagini broad band dell’intera galassia, per poter meglio interpretare l’informazione ottenuta dal dato spettroscopico. Abbiamo ridotto e analizzati i dati, che sono stati ottenuti nell’ambito della nostra campagna osservativa di galassie di Seyfert vicine. I dati IFU sono stati acquisiti con lo spettrografo MPFS al telescopio di 6-mt del SAO, i dati longslit con AFOSC al telescopio di 1.8-mt dell’Osservatorio Astrofisco di Asiago e i dati echelle con MagE al telescopio Magellan di 6.5-mt. Le immagini sono state in parte ottenute con SCORPIO al telescopio di 6-mt del SAO e in parte dall’archivio ING. Il lavoro e' basato sul seguente metodo: (i) analisi delle proprieta' fisiche del gas (densita', temperatura e metallicita'), (ii) analisi dei rapporti di righe, (iii) studio della cinematica del gas e delle stelle, (iv) analisi morfologica. Abbiamo applicato questo metodo a due galassie che mostrano emissione di [O III] estesa, NGC 7212 una Seyfert 2 e MRK 6 una Seyfert 1.5. Abbiamo ottenuto valide prove di un origine esterna del gas ionizzato per entrambi gli oggetti. L’origine esterna e' attesa nel caso di NGC 7212 che e' una galassia in un sistema fortemente interagente, comunque abbiamo trovato anche in MRK 6 indizi di un possibile merger. Abbiamo poi sfruttato il vantaggio offerto dai dati IFU che permettono di campionare spazialmente la ENLR in regioni da 1"×1" e abbiamo poi selezionato 12 Seyfert 2 per effettuare uno studio statistico delle proprieta' spettroscopiche di un elevato numero di queste regioni. Dall’analisi di 848 sub-regioni abbiamo notato che la maggior parte mostra proprieta' simili a quelle della NLR. Tutti questi risultati suggeriscono che siano necessarie particolari condizioni iniziali per formare delle NLR con dimensioni dell’ordine di alcuni kpc
The nature of gas and stars in the circumnuclear regions of AGN: a chemical approach
No full textAim of this communication is to describe the first results of a work-in-progress regarding the chemical properties of gas and stars in the circumnuclear regions of nearby galaxies. Different techniques have been employed to estimate the abundances of chemical elements in the gaseous and stellar components of nuclear surroundings in different classes of galaxies according to the level of activity of the nucleus (normal or passive, star forming galaxies and AGNs)
Morphology and photometry of isolated Seyfert galaxies
No full textWe present preliminary results about morphological and photometric analysis of a sample of 35 nearby (z<0.035) and isolated or partially isolated Seyfert galaxies, observed through broad-band BVR filters at the 1.8m Vatican Telescope and the 2.2m ESO-La Silla Telescope. We plotted radial profiles of surface brightness, ellipticity, position angles and deviations of axisymmetry for all galaxies. We also calculated magnitudes and colours and morphological parameters by means of 2D decomposition technique
Distribution of the heavy elements throughout the extended narrow-line region of the Seyfert galaxy NGC 7212
No full textThe latest observations of line and continuum spectra emitted from the extended narrow-line region (ENLR) of the Seyfert 2 galaxy NGC 7212 are analysed using models that account for photoionization from the active galactic nucleus (AGN) and shocks. The results show that relatively high (500–800 km s-1) shock velocities appear at the edge of the cone and beyond. The model-inferred AGN flux, which is lower than 10-11 photons cm-2 s-1 eV-1 at the Lyman limit, is more typical of low-luminosity AGN than Seyfert 2 galaxies. The preshock densities are characteristic of the ENLR and range between 80–150 cm-3. Nitrogen and sulphur are found to be depleted by a factor lower than 2, particularly at the eastern edge. Oxygen is depleted at several locations. The Fe/H ratio is approximately solar, whereas the Ne/H relative abundance is unusually high, 1.5–2 times the solar value. Modelling the continuum spectral energy distribution (SED), we have found radio synchrotron radiation generated by the Fermi mechanism at the shock front, whereas the X-rays are produced by the bremsstrahlung from a relatively high temperature plasma
The Starburst -- AGN Connection: a Critical Review
No full textNuclear activity and star formation processes represent two key elements in the evolution of galaxies across the cosmic ages. In spite of very different physical backgrounds, several arguments suggest that they should be closely connected. On the basis of simple theoretical considerations, the transport of appreciable amounts of fuel to the AGN scale is very likely to trigger star formation in the gas. Young stellar populations, in their turn, are expected to affect the properties of the interstellar medium, leading to a complex balance of interactions among nuclear activity and star formation. This scenario is also supported by the observation of super-massive black holes and host galaxy properties, which strongly suggest a common evolutionary track. However, despite several years of extensive investigation, the relationship among the two processes still has to be properly explained. Here we provide a review of some of the most important observations, which are relevant to the issue of the connection among AGN and starburst events. Based on a wide sample of observations, we present an analysis of the spectral signatures connected with AGN and star formation activity. Expanding the concept of the distinction among star forming galaxies and the true active nuclei, we provide systematic evidence for a role of recent starburst events in the circum-nuclear regions of active galaxies and discuss the possibility of its influence onto the AGN environment. We also analyze the age, mass and metallicity properties of star-forming and active galaxies, illustrating that they are arranged in a sequence that is consistent with the identified relation
The relation between nuclear activity and stellar mass in galaxies
No full textThe existence of correlations between the nuclear properties of galaxies, such as the mass of their central black holes, and larger scale features, such as the bulge mass and luminosity, represents a fundamental constraint on galaxy evolution. Although the actual reasons for these relations have not yet been identified, it is widely believed that they could stem from a connection between the processes that lead to black hole growth and stellar mass assembly. The problem of understanding how the processes of nuclear activity and star formation can affect each other became known to the literature as the starburst-active galactic nucleus (AGN) connection. Despite years of investigation, the physical mechanisms which lie at the basis of this relation are known only in part. In this work, we analyse the problem of star formation and nuclear activity in a large sample of galaxies. We study the relations between the properties of the nuclear environments and their host galaxies. We find that the mass of the stellar component within the galaxies of our sample is a critical parameter, which we have to consider in an evolutionary sequence, which provides further insight into the connection between AGN and star formation processes
Supersoft X-Ray Sources Identified with Be Binaries in the Magellanic Clouds
No full textWe investigated four luminous supersoft X-ray sources (SSSs) in the Magellanic Clouds suspected to have optical counterparts of Be spectral type. If the origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in accreted envelopes of white dwarfs (WDs), like in the majority of SSSs, these objects are close binaries with very massive WD primaries. Using the South African Large Telescope, we obtained the first optical spectra of the proposed optical counterparts of two candidate Be stars associated with SUZAKU J0105-72 and XMMU J010147.5-715550, respectively, a transient and a recurrent SSS, and confirmed the proposed Be classification and Small Magellanic Cloud membership. We also obtained new optical spectra of two other Be stars proposed as optical counterparts of the transient SSS XMMU J052016.0-692505 and MAXI J0158-744. The optical spectra with double-peaked emission-line profiles are typical of Be stars and present characteristics similar to many high-mass X-ray binaries with excretion disks truncated by the tidal interaction with a compact object. The presence of a massive WD that sporadically ignites nuclear burning, accreting only at certain orbital or evolutionary phases, explains the supersoft X-ray flares. We measured equivalent widths and distances between line peaks and investigated the variability of the prominent emission-line profiles. The excretion disks seem to be small in size and are likely to be differentially rotating. We discuss possible future observations and the relevance of these objects as a new class of SN Ia progenitors
V4743 Sgr, a magnetic nova?
No full textTwo XMM-Newton observations of Nova V4743 Sgr (Nova Sgr 2002) were performed shortly after it returned to quiescence, 2 and 3.5 yr after the explosion. The X-ray light curves revealed a modulation with a frequency of ≃0.75 mHz, indicating that V4743 Sgr is most probably an intermediate polar (IP). The X-ray spectra have characteristics in common with known IPs, with a hard thermal plasma component that can be fitted only assuming a partially covering absorber. In 2004, the X-ray spectrum had also a supersoft blackbody-like component, whose temperature was close to that of the white dwarf (WD) in the supersoft X-ray phase following the outburst, but with flux by at least two orders of magnitude lower. In quiescent IPs, a soft X-ray flux component originates at times in the polar regions irradiated by an accretion column, but the supersoft component of V4743 Sgr disappeared in 2006, indicating a possible origin different from accretion. We suggest that it may have been due to an atmospheric temperature gradient on the WD surface, or to continuing localized thermonuclear burning at the bottom of the envelope, before complete turn-off. An optical spectrum obtained with Southern African Large Telescope (SALT) 11.5 yr after the outburst showed a prominent He II λ4686 line and the Bowen blend, which reveal a very hot region, but with peak temperature shifted to the ultraviolet range. V4743 Sgr is the third post-outburst nova and IP candidate showing a low-luminosity supersoft component in the X-ray flux a few years after the outburst
The scientific use of the 1.2-m Galileo telescope of the Asiago Astrophysical Observatory after its recent refurbishment
No full textDuring the last 10 years the Galileo 1.2-m telescope has been completely refurbished in its optical, mechanical and electronic parts. This successful result opened suddenly the way to new scientific programs involving observations of Galactic and extragalactic sources. It gave the chance to participate efficiently in international campaigns of spectroscopic monitoring of variable sources like AGNs, supernovae, novae, dwarf novae, etc
Nuclear activity and star formation properties of Seyfert 2 galaxies.
No full textIn order to characterize the amount of recent or ongoing stellar formation in the circumnuclear region of active galaxies on a statistically sound basis, we have studied the stellar component of the nuclear spectra in three different samples of galaxies, namely Seyfert 2 galaxies (hereafter S2G), star-forming galaxies (SFG) and passive normal galaxies (NG), i.e., no emission lines observed, using Sloan Digital Sky Survey data (SDSS) (Adelman-McCarthy, 2008). The stellar component of the observed spectra has been extracted using STARLIGHT (Cid Fernandes et al., 2004), which fits an observed spectrum with a model (template) spectrum obtained by combining a library of pre-defined simple stellar populations spectra, with distinct ages and metallicities. The resulting template spectra for the different samples of galaxies have been compared to determine the features of the stellar emission component and to evaluate the presence and intensity of the star formation in the nuclear regions of different families of galaxies. From a first qualitative analysis it results that the shape of the Spectral Energy Distribution (SED) of S2G and NG is very similar, while that of SFG is characterized by a strong blue excess. The presence of the 4000 Å break in the spectra of S2G and NG together with the lack of a strong blue continuum clearly indicate the absence of ongoing star formation in the circumnuclear regions of S2G and obviously of NG. Anyway traces of a recent star formation history are evident in the spectra of S2G galaxies, which show a 4000 Å break systematically shallower than in NG
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