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Search Results: 1 - 10 of 586364 matches for " N. A. Levenson "
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Shocking Clouds in the Cygnus Loop
N. A. Levenson,James R. Graham
Physics , 2001, DOI: 10.1086/322502
Abstract: With Hubble Space Telescope Wide-Field Planetary Camera 2 observations of the Cygnus Loop supernova remnant, we examine the interaction of an interstellar cloud with the blast wave on physical scales of 10^15 cm. The shock front is distorted, revealing both edge-on and face-on views of filaments and diffuse emission, similar to those observed on larger scales at lower resolution. We identify individual shocks in the cloud of density n~15 cm^-3 having velocity v_s~170 km/s. We also find the morphologically unusual diffuse Balmer-dominated emission of faster shocks in a lower-density region. The obstacle diffracts these shocks, so they propagate at oblique angles with respect to the primary blast wave. The intricate network of diffuse and filamentary H alpha emission arises during the early stage of interaction between the cloud and blast wave, demonstrating that complex shock propagation and emission morphology occur before the onset of instabilities that destroy clouds completely.
The Seyfert-Starburst Connection in X-rays. I. The Data
N. A. Levenson,K. A. Weaver,T. M. Heckman
Physics , 2000, DOI: 10.1086/320355
Abstract: We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. Although all sample members contain active galactic nuclei (AGNs), supermassive black holes or other related processes at the galactic centers alone cannot account for the total X-ray emission in all instances. Eleven of the twelve observed galaxies are significantly resolved with the ROSAT HRI, while six of the eight sources observed with the lower-resolution PSPC also appear extended on larger scales. The X-ray emission is extended on physical scales of 10 kpc and greater, which we attribute to starburst-driven outflows and supernova-heating of the interstellar medium. Spectrally, a physically-motivated composite model of the X-ray emission that includes a heavily absorbed (N_H > 10^{23} cm^{-2}) nuclear component (the AGN), power-law like scattered AGN flux, and a thermal starburst describes this sample well. Half the sample exhibit iron K alpha lines, which are typical of AGNs.
The Seyfert-Starburst Connection in X-rays. II. Results and Implications
N. A. Levenson,K. A. Weaver,T. M. Heckman
Physics , 2000, DOI: 10.1086/319726
Abstract: We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] 5007 luminosity distinguishes most of these composite galaxies from ``pure'' Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from ``pure'' starbursts. The hard nuclear X-ray source is generally heavily absorbed (N_H > 10^{23} cm^{-2}) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole, or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies, and starbursts and active galactic nuclei may be part of a common evolutionary sequence.
Mid-ir emission from the nucleus of centaurus a
J. T. Radomski,C. Packham,N. A. Levenson,E. Perlman
Revista mexicana de astronomía y astrofísica , 2007,
Abstract: We present high spatial resolution mid-IR images of the nuclear region of Centaurus A. Images were obtained at 8.8 um, N-band (10.5uem), and 18.3um using the mid-IR in- strument T-ReCS on Gemini South, a pro- genitor to CanariCam on the Gran Telescopio Canarias (GTC).
Dynamic Shocks in the Inhomogeneous Environment of the Cygnus Loop
N. A. Levenson,J. R. Graham,J. L. Walters
Revista mexicana de astronomía y astrofísica , 2003,
Shell Shock and Cloud Shock: Results from Spatially-Resolved X-ray Spectroscopy with Chandra in the Cygnus Loop
N. A. Levenson,J. R. Graham,J. L. Walters
Physics , 2002, DOI: 10.1086/341802
Abstract: We use the Chandra X-ray Observatory to analyze interactions of the blast wave and the inhomogeneous interstellar medium on the western limb of the Cygnus Loop supernova remnant. This field of view includes an initial interaction between the blast wave and a large cloud, as well as the encounter of the shock front and the shell that surrounds the cavity of the supernova progenitor. Uniquely, the X-rays directly trace the shock front in the dense cloud, where we measure temperature kT = 0.03 keV. We find kT~0.2 keV in regions where reflected shocks further heat previously-shocked material. Applying one-dimensional models to these interactions, we determine the original blast wave velocity v_bw~330 km/s in the ambient medium. We do not detect strong evidence for instabilities or non-equilibrium conditions on the arcsecond scales we resolve. These sensitive, high-resolution data indicate no exceptional abundance variations in this region of the Cygnus Loop.
A Chandra Survey of the Nearest ULIRGs: Obscured AGN or Super-Starbursts?
A. Ptak,T. Heckman,N. A. Levenson,K. Weaver,D. Strickland
Physics , 2003, DOI: 10.1086/375766
Abstract: We present initial results from a Chandra survey of a complete sample of the 8 nearest (z <= 0.04) ultraluminous IR galaxies (ULIRGs), and also include the IR-luminous galaxy NGC 6240 for comparison. In this paper we use the hard X-rays (2-8 kev) to search for the possible presence of an obscured AGN. In every case, a hard X-ray source is detected in the nuclear region. If we divide the sample according to the optical/IR spectroscopic classification (starburst vs. AGN), we find that the 5 ``starburst'' ULIRGs have hard X-ray luminosities about an order-of-magnitude smaller than the 3 ``AGN'' ULIRGs. NGC 6240 has an anomalously high hard X-ray luminosity compared to the ``starburst'' ULIRGs. The Fe Kalpha line is convincingly detected in only two ULIRGs. The weakness of the Fe-K emission in these ULIRGs generally suggests that the hard X-ray spectrum is not dominated by reflection from high N_H neutral material. The hard X-ray continuum flux ranges from a few X 10^3 to a few X 10^-5 of the far-IR flux, similar to values in pure starbursts, and several orders-of-magnitude smaller than in Compton-thin AGN. The upper limits on the ratio of the Fe Kalpha to far-IR flux are below the values measured in Compton-thick type 2 Seyfert galaxies. While very large column densities of molecular gas are observed in the nuclei of these galaxies, we find no evidence that the observed X-ray sources are obscured by Compton-thick material. Thus, our new hard X-ray data do not provide direct evidence that powerful ``buried quasars'' dominate the overall energetics of most ultraluminous infrared galaxies.
The role of the accretion disk, dust, and jets in the IR emission of low-luminosity AGN
R. E. Mason,C. Ramos Almeida,N. A. Levenson,R. Nemmen,A. Alonso-Herrero
Physics , 2013, DOI: 10.1088/0004-637X/777/2/164
Abstract: 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.
Dust Emission from Unobscured Active Galactic Nuclei
G. D. Thompson,N. A. Levenson,S. A. Uddin,M. M. Sirocky
Physics , 2009, DOI: 10.1088/0004-637X/697/1/182
Abstract: We use mid-infrared spectroscopy of unobscured active galactic nuclei (AGNs) to reveal their native dusty environments. We concentrate on Seyfert 1 galaxies, observing a sample of 31 with the Infrared Spectrograph aboard the Spitzer Space Telescope, and compare them with 21 higher-luminosity quasar counterparts. Silicate dust reprocessing dominates the mid-infrared spectra, and we generally measure the 10 and 18 micron spectral features weakly in emission in these galaxies. The strengths of the two silicate features together are sensitive to the dust distribution. We present numerical radiative transfer calculations that distinguish between clumpy and smooth geometries, which are applicable to any central heating source, including stars as well as AGNs. In the observations, we detect the obscuring ``torus'' of unified AGN schemes, modeling it as compact and clumpy. We also determine that star formation increases with AGN luminosity, although the proportion of the galaxies' bolometric luminosity attributable to stars decreases with AGN luminosity.
Spatially-resolved mid-ir spectroscopy of ngc 1068
R. E. Mason,T. R. Geballe,C. Packham,N. A. Levenson
Revista mexicana de astronomía y astrofísica , 2006,
Abstract: Presentamos espectros espacialmente resueltos, cercanos al l mite de difracci on, de 10 m, del n ucleo de la galaxia Seyfert 2 NGC 1068. Estos revelan variaciones notables en la pendiente del cont nuo, el per l y la profundidad del rasgo de silicato, y ujos de l neas de estructura na en escalas de subsegundos de arco, que ilustran en detalles sin precedente la complejidad de las regiones circunnuclares de esta galaxia en longitudes de onda del IR medio. Las im agenes de adquisici on muestran dos componentes distintos: una fuente brillante compacta (radio < 15 pc) dentro de los 0.4 segarc centrales, identi cada con el toro obscurecedor de AGN, y una emisi on extendida, de brillo m as bajo proveniente del polvo de los conos de ionizaci on. El espectro observado de la fuente compacta se compara con modelos de toro grumoso, siendo esta la primera comparaci on detallada de estos modelos con observaciones. Los modelos requieren que la mayor parte de las nubes se localice dentro de unos cuantos parsecs de la m aquina central, lo cual concuerda muy bien con observaciones interferom etricas recientes del IR medio. Sin embargo, el ujo del IR medio medido con aperturas mayores de alrededor de 1 segarc est a dominado por la emisi on de polvo procedente de los conos de ionizaci on. Muchos de los intentos previos para determinar la distribuci on de la energ a espectral del toro estar an probablemente afectados por la contaminaci on procedente de la emisi on extendida, lo cual pone de relieve la importancia que la resoluci on espacial tiene para estudios IR de AGN cercanos.
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