arXiv subjects

C. Ramos Almeida

Publications and source records attributed to C. Ramos Almeida.

At least 109 records · Page 6Linked to original sources

arXiv2014

The dust masses of powerful radio galaxies: clues to the triggering of their activity

We use deep Herschel Space Observatory observations of a 90% complete sample of 32 intermediate-redshift 2Jy radio galaxies (0.05 < z < 0.7) 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 (7.2x10^5 < M_d < 2.6x10^8 M_sun) are intermediate between those of quiescent elliptical galaxies on the one hand, and ultra luminous infrared galaxies (ULIRGs) on the other. Consistent with simple models for the co-evolution of supermassive black holes and their host galaxies, these results suggest that most of the 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.

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arXiv2014

Polarized Mid-Infrared Synchrotron Emission in the Core of Cygnus A

We present high-angular (~0.4") resolution mid-infrared (MIR) polarimetric observations in the 8.7 $μ$m and 11.6 $μ$m filters of Cygnus A using CanariCam on the 10.4-m Gran Telescopio CANARIAS. A highly polarized nucleus is observed with a degree of polarization of 11${\pm}$3% and 12${\pm}$3% and position angle of polarization of 27${\pm}$8 degrees and 35${\pm}$8 degrees in a 0.38" (~380 pc) aperture for each filter. The observed rising of the polarized flux density with increasing wavelength is consistent with synchrotron radiation from the pc-scale jet close to the core of Cygnus A. Based on our polarization model, the synchrotron emission from the pc-scale jet is estimated to be 14% and 17% of the total flux density in the 8.7 $μ$m and 11.6 $μ$m filters, respectively. A blackbody component with a characteristic temperature of 220 K accounts for >75% of the observed MIR total flux density. The blackbody emission arises from a combination of (1) dust emission in the torus; and (2) diffuse dust emission around the nuclear region, but the contributions of the two components cannot be well constrained in these observations.

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arXiv2014

Nuclear 11.3$μ$m PAH emission in local active galactic nuclei

We present Gran Telescopio CANARIAS CanariCam 8.7$μ$m imaging and 7.5-13$μ$m spectroscopy of six local systems known to host an active galactic nucleus (AGN) and have nuclear star formation. Our main goal is to investigate whether the molecules responsible for the 11.3$μ$m polyclyclic aromatic hydrocarbon (PAH) feature are destroyed in the close vicinity of an AGN. We detect 11.3$μ$m PAH feature emission in the nuclear regions 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 increasing radial distances, reaching typical star-forming values a few hundred parsecs away from the nucleus. The reduced nuclear EW are interpreted as due to increased dilution from the AGN continuum rather than destruction of the PAH molecules. We conclude that at least those molecules responsible for the 11.3$μ$m PAH feature survive in the nuclear environments as close as 10pc from the AGN and for Seyfert-like AGN luminosities. We propose that material in the dusty tori, nuclear gas disks, and/or host galaxies of AGN is likely to provide the column densities necessary to protect the PAH molecules from the AGN radiation field.

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arXiv2014

Molecular line emission in NGC1068 imaged with ALMA. I An AGN-driven outflow in the dense molecular gas

We investigate the fueling and the feedback of star formation and nuclear activity in NGC1068, a nearby (D=14Mpc) Seyfert 2 barred galaxy, by analyzing the distribution and kinematics of the molecular gas in the disk. We have used ALMA to map the emission of a set of dense molecular gas tracers (CO(3-2), CO(6-5), HCN(4-3), HCO+(4-3) and CS(7-6)) and their underlying continuum emission in the central r ~ 2kpc of NGC1068 with spatial resolutions ~ 0.3"-0.5" (~ 20-35pc). Molecular line and dust continuum emissions are detected from a r ~ 200pc off-centered circumnuclear disk (CND), from the 2.6kpc-diameter bar region, and from the r ~ 1.3kpc starburst (SB) ring. Most of the emission in HCO+, HCN and CS stems from the CND. Molecular line ratios show dramatic order-of-magnitude changes inside the CND that are correlated with the UV/X-ray illumination by the AGN, betraying ongoing feedback. The gas kinematics from r ~ 50pc out to r ~ 400pc reveal a massive (M_mol ~ 2.7 (+0.9, -1.2) x 10^7 Msun) outflow in all molecular tracers. The tight correlation between the ionized gas outflow, the radio jet and the occurrence of outward motions in the disk suggests that the outflow is AGN-driven. The outflow rate estimated in the CND, dM/dt ~ 63 (+21, -37) Msun yr^-1, is an order of magnitude higher than the star formation rate at these radii, confirming that the outflow is AGN-driven. The power of the AGN is able to account for the estimated momentum and kinetic luminosity of the outflow. The CND mass load rate of the CND outflow implies a very short gas depletion time scale of <=1 Myr.

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arXiv2014

Investigating the sensitivity of observed spectral energy distributions to clumpy torus properties in Seyfert galaxies

We present nuclear spectral energy distributions (SEDs) from 1 to 18 micron of a small sample of nearby, nearly face-on and undisturbed Seyfert galaxies without prominent nuclear dust lanes. These nuclear SEDs probe the central ~35 pc of the galaxies, on average, and include photometric and spectroscopic infrared (IR) data. We use these SEDs, the clumpy torus models of Nenkova et al. and a Bayesian approach to study the sensitivity of different IR wavelengths to the torus parameters. We find that high angular resolution 8-13 micron spectroscopy alone reliably constrains the number of clumps and their optical depth (N0 and tau_V). On the other hand, we need a combination of mid- and near-IR subarcsecond resolution photometry to constrain torus width and inclination, as well as the radial distribution of the clouds (sigma, i and q). For flat radial profiles (q=0,1), it is possible to constrain the extent of the mid-IR-emitting dust within the torus (Y) when N-band spectroscopy is available, in addition to near-IR photometry. Finally, by fitting different combinations of average and individual Seyfert 1 and Seyfert 2 data, we find that, in general, for undisturbed, nearly face-on Seyferts without prominent nuclear dust lanes, the minimum combination of data necessary to reliably constrain all the torus parameters is J+K+M-band photometry + N-band spectroscopy.

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arXiv2013

Uncovering the deeply embedded AGN activity in the nuclear regions of the interacting galaxy Arp299

We present mid-infrared (MIR) 8-13micron spectroscopy of the nuclear regions of the interacting galaxy Arp299 (IC694+NGC3690) obtained with CanariCam (CC) on the 10.4m Gran Telescopio Canarias (GTC). The high angular resolution (~0.3-0.6arcsec) of the data allows us to probe nuclear physical scales between 60 and 120pc, which is a factor of 10 improvement over previous MIR spectroscopic observations of this system. The GTC/CC spectroscopy displays evidence of deeply embedded Active Galactic Nucleus (AGN) activity in both nuclei. The GTC/CC nuclear spectrum of NGC3690/Arp299-B1 can be explained as emission from AGN-heated dust in a clumpy torus with both a high covering factor and high extinction along the line of sight. The estimated bolometric luminosity of the AGN in NGC3690 is 3.2(+/-0.6)x10^44 erg/s. The nuclear GTC/CC spectrum of IC694/Arp299-A shows 11.3micron polycyclic aromatic hydrocarbon (PAH) emission stemming from a deeply embedded (A_V~24mag) region of less than 120pc in size. There is also a continuum-emitting dust component. If associated with th putative AGN in IC694, we estimate that it would be approximately 5 times less luminous than the AGN in NGC3690. The presence of dual AGN activity makes Arp299 a good example to study such phenomenon in the early coalescence phase of interacting galaxies.

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arXiv2013

The circumnuclear environment of NGC613: a nuclear starburst caught in the act?

We present near-infrared ($H$- and $K$-band) integral-field observations of the inner $\sim$700pc of the active spiral galaxy NGC613, obtained with SINFONI on the Very Large Telescope. We use emission-line ratios to determine the dominant excitation mechanisms in different regions within our field-of-view, in particular the active nucleus and the star-forming circum-nuclear ring. Diagnostic diagrams involving [FeII] and H$_2$ fluxes indicate that the gas is not only photoionized by the AGN in the nucleus of NGC613, but also shock-heated. On the other hand, the emission line ratios measured in the ``hot spots'' along the ring are fully consistent with them being young star forming regions. We find no sign of radial gas transport from the ring into the core region dominated by the AGN. The ring morphology appears disturbed by a radial outflow of material from the AGN, which is confirmed by the existence of a weak jet in archival radio maps. However, this jet does not seem to have any significant effect on the morphology of the large ($\sim$8$\times$10$^7$ solar masses) reservoir of molecular gas that has accumulated inside the central $\sim$100pc. Such a concentration of molecular gas around an AGN is unusual, and supports a scenario in which star formation is recurrent and episodic in spiral galaxies. In this context, NGC613 appears to be in final stages of the gas accumulation phase, and is likely to undergo a nuclear starburst in the near future.

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arXiv2013

Nuclear star formation activity and black hole accretion in nearby Seyfert galaxies

Recent theoretical and observational works indicate the presence of a correlation between the star formation rate (SFR) and the active galactic nuclei (AGN) luminosity (and, therefore, the black hole accretion rate) of Seyfert galaxies. This suggests a physical connection between the gas forming stars on kpc scales and the gas on sub-pc scales that is feeding the black hole. We compiled the largest sample of Seyfert galaxies to date with high angular resolution (0.4-0.8 arcsec) mid-infrared (8-13 micron) spectroscopy. The sample includes 29 Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalogue. At a median distance of 33 Mpc, our data allow us to probe nuclear regions on scales of 65 pc (median value). We found no general evidence of suppression of the 11.3 micron polycyclic aromatic hydrocarbon (PAH) emission in the vicinity of these AGN, and used this feature as a proxy for the SFR. We detected the 11.3 micron PAH feature in the nuclear spectra of 45% of our sample. The derived nuclear SFRs are, on average, five times lower than those measured in circumnuclear regions of 600 pc in size (median value). However, the projected nuclear SFR densities are a factor of 20 higher than those measured on circumnuclear scales. This indicates that the SF activity per unit area in the central 65 pc of Seyfert galaxies is much higher than at larger distances from their nuclei. We studied the connection between the nuclear SFR and the black hole accretion rate and showed that numerical simulations reproduce fairly well our observed relation.

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arXiv2013

The role of the accretion disk, dust, and jets in the IR emission of low-luminosity AGN

We use recent high-resolution infrared (IR; 1 - 20 micron) photometry to examine the origin of the IR emission in low-luminosity active galactic nuclei (LLAGN). The data are compared with published model fits that describe the spectral energy distribution of LLAGN in terms of an advection-dominated accretion flow (ADAF), truncated thin accretion disk, and jet. The truncated disk in these models is usually not luminous enough to explain the observed IR emission, and in all cases its spectral shape is much narrower than the broad IR peaks in the data. Synchrotron radiation from the jet appears to be important in very radio-loud nuclei, but the detection of strong silicate emission features in many objects indicates that dust must also contribute. We investigate this point by fitting the IR SED of NGC 3998 using dusty torus and optically thin (tau_MIR ~ 1) dust shell models. While more detailed modeling is necessary, these initial results suggest that dust may account for the nuclear mid-IR emission of many LLAGN.

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arXiv2013

Kinematics of Arp 270: gas flows, nuclear activity and two regimes of star formation

We have observed the Arp 270 system (NGC 3395 & NGC 3396) in Hα emission using the GHαFaS Fabry-Perot spectrometer on the 4.2m William Herschel Telescope (La Palma). In NGC 3396, which is edge-on to us, we detect gas inflow towards the centre, and also axially confined opposed outflows, characteristic of galactic superwinds, and we go on to examine the possibility that there is a shrouded AGN in the nucleus. The combination of surface brightness, velocity and velocity dispersion information enabled us to measure the radii, FWHM, and the masses of 108 HII regions in both galaxies. We find two distinct modes of physical behaviour, for high and lower luminosity regions. We note that the most luminous regions show especially high values for their velocity dispersions and hypothesize that these occur because the higher luminosity regions form from higher mass, gravitationally bound clouds while those at lower luminosity HII regions form within molecular clouds of lower mass, which are pressure confined.

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arXiv2013

Estimations of the magnetic field strength in the torus of AGN using near-infrared polarimetry

An optically and geometrically thick torus obscures the central engine of Active Galactic Nuclei (AGN) from some lines of sight. From a magnetohydrodynamical framework, the torus can be considered to be a particular region of clouds surrounding the central engine where the clouds are dusty and optically thick. In this framework, the magnetic field plays an important role in the creation, morphology and evolution of the torus. If the dust grains within the clouds are assumed to be aligned by paramagnetic alignment, then the ratio of the intrinsic polarisation and visual extinction, P(%)/Av, is a function of the magnetic field strength. To estimate the visual extinction through the torus and constrain the polarisation mechanisms in the nucleus of AGN, we developed a polarisation model to fit both the total and polarised flux in a 1.2" (~263pc) aperture of the type 2 AGN, IC5063. The polarisation model is consistent with the nuclear polarisation observed at K being produced by dichroic absorption from aligned dust grains with a visual extinction through the torus of 48$\pm$2 mag. We estimated the intrinsic polarisation arising from dichroic absorption to be P$_{K}^{dic}$=12.5$\pm$2.7%. We consider the physical conditions and environment of the gas and dust for the torus of IC5063. Then, through paramagnetic alignment, we estimate a magnetic field strength in the range of 12-128mG in the NIR emitting regions of the torus of IC5063. Alternatively, we estimate the magnetic field strength in the plane of the sky using the Chandrasekhar-Fermi method. The minimum magnetic field strength in the plane of the sky is estimated to be 13 and 41 mG depending of the conditions within the torus of IC5063. These techniques afford the chance to make a survey of AGN, to investigate the effects of magnetic field strength on the torus, accretion, and interaction to the host galaxy.

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arXiv2012

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

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 to study the MIR emission of AGN. We have 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 allows us to probe physical regions of 57 pc (median). We have used a novel pipeline called RedCan capable of producing flux- and wavelength-calibrated spectra for the CanariCam (GTC) and T-ReCS (Gemini) instruments. We have measured the fine-structure [SIV] at 10.5 microns and the PAH at 11.3 microns line strengths together with the silicate absorption/emission features. We have also compiled Spitzer/IRS spectra to understand how spatial resolution influences the results. The 11.3 microns 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 NGC7130 accounts for more than 80% of the MIR continuum at 12 microns while in the case of NGC1808 the AGN is not dominating the MIR emission. This is confirmed by the correlation between the MIR and X-ray continuum luminosities. The [SIV] emission line at 10.5 microns, 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 microns in the high-angular resolution data for higher values of NH. Clumpy torus models reproduce the observed values only if the host-galaxy properties are taken into account.

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arXiv2012

SAFIR: testing the coexistence of AGN and star formation activity and the nature of the dusty torus in the local universe

We present the Seyfert and star formation Activity in the Far-InfraRed (SAFIR) project, a small (15.1h) Herschel guaranteed time proposal performing PACS and SPIRE imaging of a small sample of nearby Seyfert galaxies. This project is aimed at studying the physical nature of the nuclear IR emission by means of multi-component spectral energy distribution (SED) fitting and the star formation properties of AGN hosts, as traced by cold dust. We summarize the results achieved so far and outline the on-going work.

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arXiv2012

Constraining clumpy dusty torus models using optimized filter sets

Recent success in explaining several properties of the dusty torus around the central engine of active galactic nuclei has been gathered with the assumption of clumpiness. The properties of such clumpy dusty tori can be inferred by analyzing spectral energy distributions (SEDs), sometimes with scarce sampling given that large aperture telescopes and long integration times are needed to get good spatial resolution and signal. We aim at using the information already present in the data and the assumption of clumpy dusty torus, in particular, the CLUMPY models of Nenkova et al., to evaluate the optimum next observation such that we maximize the constraining power of the new observed photometric point. To this end, we use the existing and barely applied idea of Bayesian adaptive exploration, a mixture of Bayesian inference, prediction and decision theories. The result is that the new photometric filter to use is the one that maximizes the expected utility, which we approximate with the entropy of the predictive distribution. In other words, we have to sample where there is larger variability in the SEDs compatible with the data with what we know of the model parameters. We show that Bayesian adaptive exploration can be used to suggest new observations, and ultimately optimal filter sets, to better constrain the parameters of the clumpy dusty torus models. In general, we find that the region between 10 and 200 um produces the largest increase in the expected utility, although sub-mm data from ALMA also prove to be useful. It is important to note that here we are not considering the angular resolution of the data, which is key when constraining torus parameters. Therefore, the expected utilities derived from this methodology must be weighted with the spatial resolution of the data.

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arXiv2012

PKS0347+05: a radio-loud/radio-quiet double AGN system triggered in a major galaxy merger

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double AGN system, comprising a weak line radio galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at the same redshift. Our deep Gemini optical images show a highly disturbed morphology, with a warped dust lane crossing through the halo and nuclear regions of the radio galaxy host, tidal tails, and a bridge connecting the radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our Gemini optical spectrum of the radio galaxy shows evidence for a reddened young stellar population of age <100 Myr. Further evidence for recent star formation activity in this source is provided by the detection of strong PAH features in mid-IR Spitzer/IRS spectra. Together, these observations support a model in which both AGN have been triggered simultaneously in a major galaxy merger. However, despite the presence of a powerful FRII radio source, and the apparently plentiful supply of fuel provided by the merger, the nucleus of the radio galaxy shows only weak, low ionization emission line activity. We speculate that the fuel supply to nuclear regions of the radio galaxy has recently switched off (within the last ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of t\sim 5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.

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arXiv2012

Probing the Nuclear and Circumnuclear Activity of NGC1365 in the Infrared

We present new far-infrared (70-500micron) Herschel PACS and SPIRE imaging observations as well as new mid-IR Gemini/T-ReCS imaging (8.7 and 18.3micron) and spectroscopy of the inner Lindblad resonance (ILR) region (R<2.5kpc) of the spiral galaxy NGC1365. We complemented these observations with archival Spitzer imaging and spectral mapping observations. The ILR region of NGC1365 contains a Seyfert 1.5 nucleus and a ring of star formation with an approximate diameter of 2kpc. The strong star formation activity in the ring is resolved by the Herschel/PACS imaging data, as well as by the Spitzer 24micron continuum emission, [NeII]12.81micron line emission, and 6.2 and 11.3micron PAH emission. The AGN is the brightest source in the central regions up to lambda~24micron, but it becomes increasingly fainter in the far-infrared when compared to the emission originating in the infrared clusters (or groups of them) located in the ring. We modeled the AGN unresolved infrared emission with the CLUMPY torus models and estimated that the AGN contributes only to a small fraction (~5%) of the infrared emission produced in the inner ~5kpc. We fitted the non-AGN 24-500micron spectral energy distribution of the ILR region and found that the dust temperatures and mass are similar to those of other nuclear and circumnuclear starburst regions. Finally we showed that within the ILR region of NGC1365 most of the on-going star formation activity is taking place in dusty regions as probed by the 24micron emission.

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arXiv2012

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 (LLAGN; Lbol \lesssim 10^42 erg/sec). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGN, 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 LLAGN 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. (I) At the low-luminosity, low-Eddington ratio (log Lbol/LEdd < -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. (II) 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 AGN that genuinely lack a broad line region. (III) 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 LLAGN do not host a Seyfert-like obscuring torus.

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arXiv2012

The starburst-AGN connection in the merger galaxy Mrk 938: an infrared and X-ray view

Mrk938 is a luminous infrared galaxy in the local Universe believed to be the remnant of a galaxy merger. It shows a Seyfert 2 nucleus and intense star formation according to optical spectroscopic observations. We have studied this galaxy using new Herschel far-IR imaging data in addition to archival X-ray, UV, optical, near-IR and mid-IR data. Mid- and far-IR data are crucial to characterise the starburst contribution, allowing us to shed new light on its nature and to study the coexistence of AGN and starburst activity in the local Universe. The decomposition of the mid-IR Spitzer spectrum shows that the AGN bolometric contribution to the mid-IR and total infrared luminosity is small (Lbol(AGN)/LIR~0.02), which agrees with previous estimations. We have characterised the physical nature of its strong infrared emission and constrained it to a relatively compact emitting region of <2kpc. It is in this obscured region where most of the current star formation activity is taking place as expected for LIRGs. We have used Herschel imaging data for the first time to constrain the cold dust emission with unprecedented accuracy. We have fitted the integrated far-IR spectral energy distribution and derived the properties of the dust, obtaining a dust mass of 3x10^7Msun. The far-IR is dominated by emission at 35K, consistent with dust heated by the on-going star formation activity.

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