N. A. Levenson

Gemini Observatory, Hilo, Hawaii, United States

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Publications (145)

  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We present high-angular (0.17$-$0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 $\mu$m filter using MMT-Pol on the 6.5-m MMT, and in the 8.7 $\mu$m, 10.3 $\mu$m, and 11.6 $\mu$m filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1$-$12 $\mu$m total and polarized nuclear spectral energy distribution (SED). The total flux SED in the central 400 pc is explained as the combination of 1) a hot (731 $\pm$ 4 K) dusty structure, directly irradiated by the central engine, which is at 1.6 $\pm$ 0.1 pc away and attributed to be in the pc-scale polar region, 2) an optically-thick, smooth and disk-like dusty structure (`torus') with an inclination of 48 $\pm$ 23$^{\circ}$ surrounding the central engine, and 3) an extinguished (A$_{\mbox{V}} =$ 36 $\pm$ 5 mag) starburst component. The polarized SED decreases from 0.77 $\pm$ 0.14 per cent at 1.2 $\mu$m to 0.31 $\pm$ 0.15 per cent at 11.6 $\mu$m and follows a power-law function, $\lambda^{\sim0.57}$. The polarization angle remains constant ($\sim$108$^{\circ}$) in the 1$-$12 $\mu$m wavelength range. The dominant polarization mechanism is explained as scattering off hot dust grains in the pc-scale polar regions.
    Full-text available · Article · Sep 2016
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    [Show abstract] [Hide abstract] ABSTRACT: We investigate the evolutionary connection between local IR-bright galaxies ($\log L_{\rm IR}\ge 11.4\,L_\odot$) and quasars. We use high angular resolution ($\sim$ 0.3-0.4 arcsec $\sim$ few hundred parsecs) $8-13\,\mu$m ground-based spectroscopy to disentangle the AGN mid-IR properties from those of star formation. The comparison between the nuclear $11.3\,\mu$m PAH feature emission and that measured with Spitzer/IRS indicates that the star formation is extended over a few kpc in the IR-bright galaxies. The AGN contribution to the total IR luminosity of IR-bright galaxies is lower than in quasars. Although the dust distribution is predicted to change as IR-bright galaxies evolve to IR-bright quasars and then to optical quasars, we show that the AGN mid-IR emission of all the quasars in our sample is not significantly different. In contrast, the nuclear emission of IR-bright galaxies with low AGN contributions appears more heavily embedded in dust although there is no clear trend with the interaction stage or projected nuclear separation. This suggests that the changes in the distribution of the nuclear obscuring material may be taking place rapidly and at different interaction stages washing out the evidence of an evolutionary path. When compared to normal AGN, the nuclear star formation activity of quasars appears to be dimming whereas it is enhanced in some IR-bright nuclei, suggesting that the latter are in an earlier star-formation dominated phase.
    Full-text available · Article · Aug 2016 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract] ABSTRACT: We present subarcsecond resolution mid-infrared (MIR) images obtained with 8-10 m-class ground-based telescopes of a complete volume-limited (D$_L<$40 Mpc) sample of 24 Seyfert galaxies selected from the Swift/BAT nine month catalog. We use those MIR images to study the nuclear and circumnuclear emission of the galaxies. Using different methods to classify the MIR morphologies on scales of $\sim$400 pc, we find that the majority of the galaxies (75-83%) are extended or possibly extended and 17-25% are point-like. This extended emission is compact and it has low surface brightness compared with the nuclear emission, and it represents, on average, $\sim$30% of the total MIR emission of the galaxies in the sample. We find that the galaxies whose circumnuclear MIR emission is dominated by star formation show more extended emission (650$\pm$700 pc) than AGN-dominated systems (300$\pm$100 pc). In general, the galaxies with point-like MIR morphologies are face-on or moderately inclined (b/a$\sim$0.4-1.0), and we do not find significant differences between the morphologies of Sy1 and Sy2. We used the nuclear and circumnuclear fluxes to investigate their correlation with different AGN and SF activity indicators. We find that the nuclear MIR emission (the inner $\sim$70 pc) is strongly correlated with the X-ray emission (the harder the X-rays the better the correlation) and with the [O IV] $\lambda$25.89 $\mu$m emission line, indicating that it is AGN-dominated. We find the same results, although with more scatter, for the circumnuclear emission, which indicates that the AGN dominates the MIR emission in the inner $\sim$400 pc of the galaxies, with some contribution from star formation.
    Article · Aug 2016 · Monthly Notices of the Royal Astronomical Society
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    [Show abstract] [Hide abstract] ABSTRACT: We present 31.5 micron imaging photometry of 11 nearby Seyfert galaxies observed from the Stratospheric Observatory For Infrared Astronomy (SOFIA) using the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST). We tentatively detect extended 31 micron emission for the first time in our sample. In combination with this new data set, subarcsecond resolution 1-18 micron imaging and 7.5-13 micron spectroscopic observations were used to compute the nuclear spectral energy distribution (SED) of each galaxy. We found that the turnover of the torus emission does not occur at wavelengths <31.5 micron, which we interpret as a lower-limit for the wavelength of peak emission. We used CLUMPY torus models to fit the nuclear infrared (IR) SED and infer trends in the physical parameters of the AGN torus for the galaxies in the sample. Including the 31.5 micron nuclear flux in the SED 1) reduces the number of clumpy torus models compatible with the data, and 2) modifies the model output for the outer radial extent of the torus for 10 of the 11 objects. Specifically, six (60%) objects show a decrease in radial extent while four (40%) show an increase. We find torus outer radii ranging from <1pc to 8.4 pc
    Full-text available · Article · Jul 2016 · Monthly Notices of the Royal Astronomical Society
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    Mischa Schirmer · Sangeeta Malhotra · Nancy A. Levenson · [...] · James E. H. Turner
    [Show abstract] [Hide abstract] ABSTRACT: We report the discovery of 14 Lyman-alpha blobs (LABs) at z~0.3, existing at least 4-7 billion years later in the Universe than all other LABs known. Their optical diameters are 20-70 kpc, and GALEX data imply Ly-alpha luminosities of (0.4-6.3)x10^43 erg/s. Contrary to high-z LABs, they live in low-density areas. They are ionized by AGN, suggesting that cold accretion streams as a power source must deplete between z=2 and z=0.3. We also show that transient AGN naturally explain the ionization deficits observed in many LABs: Their Ly-alpha and X-ray fluxes decorrelate below 10^6 years because of the delayed escape of resonantly scattering Ly-alpha photons. High Ly-alpha luminosities do not require currently powerful AGN, independent of obscuration. Chandra X-ray data reveal intrinsically weak AGN, confirming the luminous optical nebulae as impressive ionization echoes. For the first time, we also report mid-infrared thermal echoes from the dusty tori. We conclude that the AGN have faded by 3-4 orders of magnitude within the last 10^(4-5) years, leaving fossil UV, optical and thermal radiation behind. The host galaxies belong to the group of previously discovered Green Bean galaxies (GBs). Gemini optical imaging reveals smooth spheres, mergers, spectacular outflows and ionization cones. Because of their proximity and high flux densities, GBs are perfect targets to study AGN feedback, mode switching and the Ly-alpha escape. The fully calibrated, coadded optical FITS images are publicly available.
    Full-text available · Article · Jul 2016 · Monthly Notices of the Royal Astronomical Society
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    A. Alonso-Herrero · P. Esquej · P. F. Roche · [...] · E. Perlman
    [Show abstract] [Hide abstract] ABSTRACT: We present an atlas of mid-infrared (mid-IR) ∼ 7.5-13 μm spectra of 45 local active galactic nuclei (AGN) obtained with CanariCam on the 10.4 m Gran Telescopio CANARIAS (GTC) as part of an ESO/GTC large programme. The sample includes Seyferts and other low-luminosity AGN (LLAGN) at a median distance of 35 Mpc and luminous AGN, namely PG quasars, (U)LIRGs, and radio galaxies (RG) at a median distance of 254 Mpc. To date, this is the largest mid-IR spectroscopic catalogue of local AGN at sub-arcsecond resolution (median 0.3 arcsec). The goal of this work is to give an overview of the spectroscopic properties of the sample. The nuclear 12 μm luminosities of the AGN span more than four orders of magnitude, νL12 μm ∼ 3 × 1041–1046 erg s−1. In a simple mid-IR spectral index versus strength of the 9.7 μm silicate feature diagram most LLAGN, Seyfert nuclei, PG quasars, and RGs lie in the region occupied by clumpy torus model tracks. However, the mid-IR spectra of some might include contributions from other mechanisms. Most (U)LIRG nuclei in our sample have deeper silicate features and flatter spectral indices than predicted by these models suggesting deeply embedded dust heating sources and/or contribution from star formation. The 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature is clearly detected in approximately half of the Seyfert nuclei, LLAGN, and (U)LIRGs. While the RG, PG quasars, and (U)LIRGs in our sample have similar nuclear νL12 μm, we do not detect nuclear PAH emission in the RGs and PG quasars.
    Full-text available · Article · Oct 2015 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract] ABSTRACT: We present new mid-infrared (mid-IR) N-band spectroscopy and Q-band photometry of the local luminous IR galaxy NGC 1614, one of the most extreme nearby starbursts. We analyse the mid-IR properties of the nucleus (central 150 pc) and four regions of the bright circumnuclear (diameter∼600 pc) star-forming (SF) ring of this object. The nucleus differs from the circumnuclear SF ring by having a strong 8–12 μm continuum (low 11.3 μm PAH equivalent width). These characteristics, together with the nuclear X-ray and sub-mm properties, can be explained by an X-ray weak active galactic nucleus (AGN), or by peculiar SF with a short molecular gas depletion time and producing an enhanced radiation field density. In either case, the nuclear luminosity (LIR < 6 × 1043 erg s−1) is only <5 per cent of the total bolometric luminosity of NGC 1614. So this possible AGN does not dominate the energy output in this object. We also compare three star formation rate (SFR) tracers (Pa α, 11.3 μm PAH, and 24 μm emissions) at 150 pc scales in the circumnuclear ring. In general, we find that the SFR is underestimated (overestimated) by a factor of 2–4 (2–3) using the 11.3 μm PAH (24 μm) emission with respect to the extinction corrected Pa α SFR. The former can be explained because we do not include diffuse polycyclic aromatic hydrocarbon (PAH) emission in our measurements, while the latter might indicate that the dust temperature is particularly warmer in the central regions of NGC 1614.
    Article · Sep 2015 · Monthly Notices of the Royal Astronomical Society
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    [Show abstract] [Hide abstract] ABSTRACT: We present an analysis of the nuclear infrared (IR, 1.6–18 μm) emission of the ultraluminous IR galaxy UGC 5101 to derive the properties of its active galactic nucleus (AGN) and its obscuring material. We use new mid-IR high angular resolution (0.3–0.5 arcsec) imaging using the Si-2 filter (λC = 8.7 μm) and 7.5–13 μm spectroscopy taken with CanariCam (CC) on the 10.4 m Gran Telescopio CANARIAS. We also use archival Hubble Space Telescope/NICMOS and Subaru/COMICS imaging and Spitzer/IRS spectroscopy. We estimate the near- and mid-IR unresolved nuclear emission by modelling the imaging data with galfit. We decompose the Spitzer/IRS and CC spectra using a power-law component, which represents the emission due to dust heated by the AGN, and a starburst component, both affected by foreground extinction. We model the resulting unresolved near- and mid-IR, and the starburst subtracted CC spectrum with the CLUMPY torus models of Nenkova et al. The derived geometrical properties of the torus, including the large covering factor and the high foreground extinction needed to reproduce the deep 9.7 μm silicate feature, are consistent with the lack of strong AGN signatures in the optical. We derive an AGN bolometric luminosity Lbol ∼ 1.9 × 1045 erg s−1 that is in good agreement with other estimates in the literature.
    Full-text available · Article · Sep 2015 · Monthly Notices of the Royal Astronomical Society
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    E. Lopez-Rodriguez · C. Packham · T. J. Jones · [...] · E. Perlman
    [Show abstract] [Hide abstract] ABSTRACT: We present J′ and K′ imaging linear polarimetric adaptive optics observations of NGC 1068 using MMT-Pol on the 6.5-m MMT. These observations allow us to study the torus from a magnetohydrodynamical (MHD) framework. In a 0.5 arcsec (30 pc) aperture at K′, we find that polarization arising from the passage of radiation from the inner edge of the torus through magnetically aligned dust grains in the clumps is the dominant polarization mechanism, with an intrinsic polarization of 7.0 ± 2.2 per cent. This result yields a torus magnetic field strength in the range of 4–82 mG through paramagnetic alignment, and 139$^{+11}_{-20}$ mG through the Chandrasekhar–Fermi method. The measured position angle (P.A.) of polarization at K′ is found to be similar to the P.A. of the obscuring dusty component at few parsec scales using infrared interferometric techniques. We show that the constant component of the magnetic field is responsible for the alignment of the dust grains, and aligned with the torus axis on to the plane of the sky. Adopting this magnetic field configuration and the physical conditions of the clumps in the MHD outflow wind model, we estimate a mass outflow rate ≤0.17 M⊙ yr−1 at 0.4 pc from the central engine for those clumps showing near-infrared dichroism. The models used were able to create the torus in a time-scale of ≥105 yr with a rotational velocity of ≤1228 km s−1 at 0.4 pc. We conclude that the evolution, morphology and kinematics of the torus in NGC 1068 can be explained within a MHD framework.
    Full-text available · Article · Jun 2015 · Monthly Notices of the Royal Astronomical Society
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    I. García-Bernete · C. Ramos Almeida · J. A. Acosta-Pulido · [...] · N. A. Levenson
    [Show abstract] [Hide abstract] ABSTRACT: We present subarcsecond resolution infrared (IR) imaging and mid-IR (MIR) spectroscopic observations of the Seyfert 1.9 galaxy NGC 2992, obtained with the Gemini North Telescope and the Gran Telescopio CANARIAS (GTC). The N-band image reveals faint extended emission out to ∼3 kpc, and the polycyclic aromatic hydrocarbon features detected in the GTC/CanariCam 7.5–13 μm spectrum indicate that the bulk of this extended emission is dust heated by star formation. We also report arcsecond resolution MIR and far-IR imaging of the interacting system Arp 245, taken with the Spitzer Space Telescope and the Herschel Space Observatory. Using these data, we obtain nuclear fluxes using different methods and find that we can only recover the nuclear fluxes obtained from the subarcsecond data at 20–25 μm, where the active galactic nuclei (AGN) emission dominates. We fitted the nuclear IR spectral energy distribution of NGC 2992, including the GTC/CanariCam nuclear spectrum (∼50 pc), with clumpy torus models. We then used the best-fitting torus model to decompose the Spitzer/IRS 5–30 μm spectrum (∼630 pc) in AGN and starburst components, using different starburst templates. We find that, whereas at shorter MIR wavelengths the starburst component dominates (64 per cent at 6 μm), the AGN component reaches 90 per cent at 20 μm. We finally obtained dust masses, temperatures and star formation rates for the different components of the Arp 245 system and find similar values for NGC 2992 and NGC 2993. These measurements are within those reported for other interacting systems in the first stages of the interaction.
    Full-text available · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society
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    [Show abstract] [Hide abstract] ABSTRACT: We present results from the fitting of infrared (IR) spectral energy distributions of 21 active galactic nuclei (AGN) with clumpy torus models. We compiled high spatial resolution ($\sim 0.3$--$0.7$ arcsec) mid-IR $N$-band spectroscopy, $Q$-band imaging and nuclear near- and mid-IR photometry from the literature. Combining these nuclear near- and mid-IR observations, far-IR photometry and clumpy torus models, enables us to put constraints on the torus properties and geometry. We divide the sample into three types according to the broad line region (BLR) properties; type-1s, type-2s with scattered or hidden broad line region (HBLR) previously observed, and type-2s without any published HBLR signature (NHBLR). Comparing the torus model parameters gives us the first quantitative torus geometrical view for each subgroup. We find that NHBLR AGN have smaller torus opening angles and larger covering factors than those of HBLR AGN. This suggests that the chance to observe scattered (polarized) flux from the BLR in NHBLR could be reduced by the dual effects of (a) less scattering medium due to the reduced scattering volume given the small torus opening angle and (b) the increased torus obscuration between the observer and the scattering region. These effects give a reasonable explanation for the lack of observed HBLR in some type-2 AGN.
    Full-text available · Article · Jan 2015 · The Astrophysical Journal
  • R. E. Mason · C. Ramos Almeida · N. A. Levenson · [...] · A. Alonso Herrero
    [Show abstract] [Hide abstract] ABSTRACT: To test recent suggestions that the infrared emission of low-luminosity AGN arises in a truncated thin accretion disk, we compare recent, high-resolution IR data with published SED model fits that include emission from the truncated disk. We also fit the data with clumpy torus and optically thin dust shell models. These comparisons suggest that dust can better account for the IR emission of the objects in question than can the truncated disk. That optically thin models give a good fit to the data may support a scenario in which the torus of the AGN unified model does not persist in low accretion rate AGN.
    Article · Oct 2014 · Proceedings of the International Astronomical Union
  • N. A. Levenson
    [Show abstract] [Hide abstract] ABSTRACT: Compton thick active galactic nuclei (AGN), which are obscured by column density NH > 1.5 × 104 cm−2, can be difficult to identify. They are certainly cosmically significant, both in producing the observed cosmic X-ray background, and in providing a location where black hole growth is hidden from view. Here I review some recent results from surveys that provide indications of Compton thick AGN, considering X-ray, radio, and infrared selection techniques. I also offer a caution against using mid-infrared silicate features to measure line-of-sight obscuration to active galactic nuclei. Instead, these features better indicate the geometric distribution of dust that the central engine heats. I conclude that the outstanding problem of Compton thick AGN is not the cases where the obscuration is directly associated with the environment of the active nucleus itself, even in the most obscured examples. Instead, we still risk missing the completely buried AGN, which are obscured by large amounts of gas and dust over large solid angles. The solution to finding Compton thick AGN may be to begin the search based on infrared emission and star formation, and then select for nuclear activity.
    Article · Oct 2014 · Proceedings of the International Astronomical Union
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    [Show abstract] [Hide abstract] ABSTRACT: We present a time series of 8 - 13 $\mu$m spectra and photometry for SN 2014J obtained 57, 81, 108, and 137 days after the explosion using CanariCam on the Gran Telescopio Canarias. These mid-IR spectra and their evolution can be understood within the framework of the delayed detonation model and the production of $\sim$0.6 M$_{\odot}$ of $^{56}$Ni, a value also consistent with the observed brightness, the brightness decline relation, and the $\gamma$-ray fluxes. The [Co III] line at 11.888 $\mu$m, which must be emitted predominantly in regions where the density is below the critical value for collisional de-excitation, is particularly useful for evaluating the time evolution of the photosphere and measuring the amount of $^{56}$Ni and, thus, the mass of the ejecta. Late-time line profiles of SN 2014J are rather symmetric and not shifted in the rest frame. We see Argon emission, which provides a unique probe of mixing in the transition layer between incomplete burning and nuclear statistical equilibrium. In the early spectra we may see [Fe III] emission and, in all spectra, [Ni IV] emission, both of which are observed to be substantially stronger than indicated by our models. If the latter identification is correct, then we are likely observing stable Ni, which might imply central mixing. In addition, electron capture, also required for stable Ni, requires densities larger than $\sim 1 \times 10^9$ g cm$^{-3}$, which are expected to be present only in white dwarfs close to the Chandrasekhar limit. This study demonstrates that mid-IR studies of Type Ia supernovae are feasible from the ground and provide unique information, but it also indicates the need for better atomic data.
    Full-text available · Article · Sep 2014 · The Astrophysical Journal
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    [Show abstract] [Hide abstract] ABSTRACT: We present mid-infrared (MIR) imaging and spectroscopic data of the Seyfert 2 galaxy Mrk 1066 obtained with CanariCam (CC) on the 10.4 m Gran Telescopio CANARIAS (GTC). The galaxy was observed in imaging mode with an angular resolution of 0.24 arcsec (54 pc) in the Si-2 filter (8.7 micron). The image reveals a series of star-forming knots within the central ~400 pc, after subtracting the dominant active galactic nucleus (AGN) component. We also subtracted this AGN unresolved component from the 8-13 micron spectra of the knots and the nucleus, and measured equivalent widths (EWs) of the 11.3 micron Polycyclic Aromatic Hydrocarbon (PAH) feature which are typical of pure starburst galaxies. This EW is larger in the nucleus than in the knots, confirming that, at least in the case of Mrk 1066, the AGN dilutes, rather than destroys, the molecules responsible for the 11.3 micron PAH emission. By comparing the nuclear GTC/CC spectrum with the Spitzer/IRS spectrum of the galaxy, we find that the AGN component that dominates the continuum emission at wavelengths below 15 micron on scales of ~60 pc (90-100%) decreases to 35-50% when the emission of the central ~830 pc is considered. On the other hand, the AGN contribution dominates the 15-25 micron emission (75%) on the scales probed by Spitzer/IRS. We find a good match between the MIR morphology of Mrk 1066 and the extended Pab, Brg and [O III] emission. This coincidence implies that the 8.7 micron emission is probing star formation, dust in the narrow-line region, and the oval structure previously detected in the near-infrared. On the other hand, the Chandra soft X-ray morphology does not match any of the previous, contrary to what it is generally assumed for Seyfert galaxies.
    Full-text available · Article · Sep 2014 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract] ABSTRACT: Gemini's Fast Turnaround program is intended to greatly decrease the time from having an idea to acquiring the supporting data. The scheme will offer monthly proposal submission opportunities, and proposals will be reviewed by the principal investigators or co-investigators of other proposals submitted during the same round. Here, we set out the design of the system and outline the plan for its implementation, leading to the launch of a pilot program at Gemini North in January 2015.
    Article · Aug 2014 · Proceedings of SPIE - The International Society for Optical Engineering
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    [Show abstract] [Hide abstract] ABSTRACT: We present high-angular (~0.4") resolution mid-infrared (MIR) polarimetric observations in the 8.7 ${\mu}$m and 11.6 ${\mu}$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 ${\mu}$m and 11.6 ${\mu}$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.
    Full-text available · Article · Jul 2014 · The Astrophysical Journal
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    [Show abstract] [Hide abstract] ABSTRACT: We present Gran Telescopio CANARIAS CanariCam 8.7 μm imaging and 7.5–13 μm 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 μm polycyclic aromatic hydrocarbon (PAH) feature are destroyed in the close vicinity of an AGN. We detect 11.3 μm 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 EWs 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 μm PAH feature survive in the nuclear environments as close as 10 pc from the AGN and for Seyfert-like AGN luminosities. We propose that material in the dusty tori, nuclear gas discs, and/or host galaxies of AGN is likely to provide the column densities necessary to protect the PAH molecules from the AGN radiation field.
    Full-text available · Article · Jul 2014 · Monthly Notices of the Royal Astronomical Society
  • Tamami I. Mori · Masatoshi Imanishi · Almudena Alonso-Herrero · [...] · Nancy A. Levenson
    [Show abstract] [Hide abstract] ABSTRACT: We report near-infrared K ′, L′, and M ′ band imaging observations of the nearby merging galaxy NGC 6240 with the Infrared Camera and Spectrograph on the Subaru telescope. The observations were performed with the assistance of the Subaru Adaptive Optics System, and the achieved spatial resolutions were around 0${^{\prime\prime}_{.}}$10–0${^{\prime\prime}_{.}}$20. We also obtained new mid-infrared imaging in the Si-2 filter band (8.7 μm) and N-band (7.5–13 μm) spectroscopy of this galaxy with the CanariCam on the Gran Telescopio Canarias with a spatial resolution of 0${^{\prime\prime}_{.}}$4–0${^{\prime\prime}_{.}}$5. In the K ′ band image the two nuclei of the galaxy each show a double peak suggesting the complex geometry of the source, while the L′, M ′, and Si-2 band images show single compact structures in each of the two nuclei. Assuming that the center core observed at wavelengths longer than 3.8 μm is associated with dust heated by the buried AGN, we re-evaluated the spectral energy distributions (SEDs) of the southern nucleus from 2 to 30 μm with additional literature values, and performed the SED+spectroscopy fit using the clumpy torus models of Nenkova et al. (2008a, ApJ, 685, 145; 2008b, ApJ, 685, 160) and a Bayesian fitting approach. The model fit suggests that the high covering factor torus emission in the southern nucleus is also obscured by foreground dust in the host galaxy. The estimated AGN bolometric luminosity of the southern nucleus, Lbol(AGN) ∼ 1 × 1045 erg s−1, accounts for approximately 40% of the whole luminosity of the system.
    Article · Jun 2014 · Publications- Astronomical Society of Japan
  • Matthew J. Merlo · Eric S. Perlman · Robert Nikutta · [...] · Itziar Aretxaga
    [Show abstract] [Hide abstract] ABSTRACT: We present high angular resolution ($\sim$0.5$^\prime$$^\prime$) MIR spectra of the powerful radio galaxy, Cygnus A, obtained with the Subaru telescope. The overall shape of the spectra agree with previous high angular resolution MIR observations, as well as previous Spitzer spectra. Our spectra, both on and off nucleus, show a deep silicate absorption feature. The absorption feature can be modeled with a blackbody obscured by cold dust or a clumpy torus. The deep silicate feature is best fit by a simple model of a screened blackbody, suggesting foreground absorption plays a significant, if not dominant role, in shaping the spectrum of Cygnus A. This foreground absorption prevents a clear view of the central engine and surrounding torus, making it difficult to quantify the extent the torus attributes to the obscuration of the central engine, but does not eliminate the need for a torus in Cygnus A.
    Article · Apr 2014 · The Astrophysical Journal

Publication Stats

2k Citations


  • 2013-2015
    • Gemini Observatory
      Hilo, Hawaii, United States
  • 2002-2010
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, KY, United States
  • 1995-2009
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, CA, United States
  • 2006
    • Cornell University
      • Department of Astronomy
      Ithaca, New York, United States
  • 2001
    • Universidade Federal do Rio Grande do Sul
      • Institute of Physics
      Pôrto de São Francisco dos Casaes, Rio Grande do Sul, Brazil