arXiv subjects

M. Elitzur

Publications and source records attributed to M. Elitzur.

At least 37 records · Page 2Linked to original sources

arXiv2007

An Investigation into the Effects of Luminosity on the Mid-Infrared Spectral Energy Distributions of Radio-Quiet Quasars

We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (>99.99% confidence) correlation between the 1.8-8.0 micron spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for spectral curvature in more luminous quasars, we conclude this trend is likely a manifestation of the `near-infrared (3-5 micron) bump' noticed in earlier quasar SED surveys. The strength of this feature is indicative of the contribution of emission from the hottest (>1000 K) dust to the mid-infrared spectrum; higher luminosity quasars tend to show more hot dust emission. Finally, the comparable distribution of bolometric corrections from the monochromatic 3 micron luminosity as well as its lack of sensitivity to dust extinction as compared to the standard bolometric correction from nu*L_{5100A} suggest that the former may be a more robust indicator of bolometric quasar luminosity. The close link between the power in the mid-infrared and optical and the effect of luminosity on the shape of the mid-infrared continuum indicate that considering mid-infrared emission independent of the properties of the quasar itself is inadequate for understanding the parsec-scale quasar environment.

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arXiv2007

Dust and PAH emission in the star-forming active nucleus of NGC 1097

The nucleus of the nearby galaxy, NGC 1097, is known to host a young, compact (r < 9 parsec) nuclear star cluster as well as a low-luminosity active galactic nucleus (AGN). It has been suggested both that the nuclear stellar cluster is associated with a dusty torus, and that low-luminosity AGN like NGC 1097 do not have the torus predicted by the unified model of AGN. To investigate these contradictory possibilities we have acquired Gemini/T-ReCS 11.7 micron and 18.3 micron images of the central few hundred parsecs of this galaxy at < 45 parsec angular resolution, in which the nucleus and spectacular, kiloparsec-scale star-forming ring are detected in both bands. The small-scale mid-infrared (mid-IR) luminosity implies thermal emission from warm dust close to the central engine of this galaxy. Fitting of torus models shows that the observed mid-IR emission cannot be accounted for by dust heated by the central engine. Rather, the principal source heating the dust in this object is the nuclear star cluster itself, suggesting that the dust that we detect is not the torus of AGN unified schemes (although it is also possible that the dusty starburst itself could provide the obscuration invoked by the unified model). Comparison of Spitzer/IRS and Gemini/GNIRS spectra shows that, although polyaromatic hydrocarbon emission (PAH) bands are strong in the immediate circumnuclear region of the galaxy, PAH emission is weak or absent in the central 19 parsecs. The lack of PAH emission can probably be explained largely by destruction/ionization of PAH molecules by hard photons from the nuclear star cluster. If NGC 1097 is typical, PAH emission bands may not be a useful tool with which to find very compact nuclear starbursts even in low-luminosity AGN.

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arXiv2006

The distribution of silicate strength in Spitzer spectra of AGNs and ULIRGs

A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS) on Spitzer is analyzed to study the distribution of the strength of the 9.7 micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate features in emission and Seyfert 1s equally by emission or weak absorption. Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are characterized by strong silicate absorption (mean apparent optical depth about 1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron are similar for the most luminous quasars and ULIRGs and are almost 10^5 times more luminous than the least luminous AGN in the sample. The distributions of spectral characteristics and luminosities are compared to those of optically faint infrared sources at z~2 being discovered by the IRS, which are also characterized by strong silicate absorption. It is found that local ULIRGs are a similar population, although they have lower luminosities and somewhat stronger absorption compared to the high redshift sources.

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arXiv2006

Mid-IR Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 micron PAH emission feature and the strength of the 9.7 micron silicate feature. Based on the position in this diagram we classify galaxies into 9 classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus non-clumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

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arXiv2006

Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei

The silicate cross section peak near 10um produces emission and absorption features in the spectra of dusty galactic nuclei observed with the Spitzer Space Telescope. Especially in ultraluminous infrared galaxies, the observed absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A foreground screen of obscuration cannot reproduce this observed feature, even at large optical depth. Instead, the deep absorption requires a nuclear source to be deeply embedded in a smooth distribution of material that is both geometrically and optically thick. In contrast, a clumpy medium can produce only shallow absorption or emission, which are characteristic of optically-identified active galactic nuclei. In general, the geometry of the dusty region and the total optical depth, rather than the grain composition or heating spectrum, determine the silicate feature's observable properties. The apparent optical depth calculated from the ratio of line to continuum emission generally fails to accurately measure the true optical depth. The obscuring geometry, not the nature of the embedded source, also determines the far-IR spectral shape.

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arXiv2006

Detached shells as tracers of AGB-ISM bow shocks

New Spitzer imaging observations have revealed the structure around the Mira variable star R Hya to be a one-sided parabolic arc 100 arcsec to the West stretching from North to South. We successfully model R Hya and its surroundings in terms of an interaction of the stellar wind from an asymptotic giant branch (AGB) star with the interstellar medium (ISM) the star moves through. Our three-dimensional hydrodynamic simulation reproduces the structure as a bow shock into the oncoming ISM. We propose this as another explanation of detached shells around such stars which should be considered alongside current theories of internal origin. The simulation predicts the existence of a tail of ram-pressure-stripped AGB material stretching downstream. Indications for such a tail behind R Hya are seen in IRAS maps.

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arXiv2006

Detection of a Far-Infrared Bow-Shock Nebula Around R Hya: the First MIRIAD Results

We present the first results of the MIRIAD (MIPS [Multiband Imaging Photometer for Spitzer] Infra-Red Imaging of AGB [asymptotic giant branch] Dustshells) project using the Spitzer Space Telescope. The primary aim of the project is to probe the material distribution in the extended circumstellar envelopes (CSE) of evolved stars and recover the fossil record of their mass loss history. Hence, we must map the whole of the CSEs plus the surrounding sky for background subtraction, while avoiding the central star that is brighter than the detector saturation limit. With our unique mapping strategy, we have achieved better than one MJy/sr sensitivity in three hours of integration and successfully detected a faint (< 5 MJy/sr), extended (~400 arcsec) far-infrared nebula around the AGB star R Hya. Based on the parabolic structure of the nebula, the direction of the space motion of the star with respect to the nebula shape, and the presence of extended H alpha emission co-spatial to the nebula, we suggest that the detected far-IR nebula is due to a bow shock at the interface of the interstellar medium and the AGB wind of this moving star. This is the first detection of the stellar-wind bow-shock interaction for an AGB star and exemplifies the potential of Spitzer as a tool to examine the detailed structure of extended far-IR nebulae around bright central sources. \

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arXiv2006

Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry -- Signatures of flow precession

NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow, an associated UCHII region, and a linear methanol maser structure, which might trace a Keplerian-rotating circumstellar disk. The directions of the various associated axes are misaligned with each other. We investigate the near-infrared morphology of the source to clarify the relations among the various axes. K'-band bispectrum speckle interferometry was performed at two 6-meter-class telescopes -- the BTA 6m telescope and the 6.5m MMT. Complementary IRAC images from the Spitzer Space Telescope Archive were used to relate the structures detected with the outflow at larger scales. High-dynamic range images show fan-shaped outflow structure in which we detect 18 stars and several blobs of diffuse emission. We interpret the misalignment of various outflow axes in the context of a disk precession model, including numerical hydrodynamic simulations of the molecular emission. The precession period is approx. 280 years and its half-opening angle is 40 degrees. A possible triggering mechanism is non-coplanar tidal interaction of an (undiscovered) close companion with the circumbinary protostellar disk. Our observations resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with separation of 195 mas. We find indications for shock interaction between the outflow activities in IRS1 and IRS2. Indications of outflow precession have been discovered to date in a number of massive protostars, all with large precession angles 20--45 degrees. This might explain the difference between the outflow widths in low- and high-mass stars and add support to a common collimation mechanism.

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arXiv2005

Spatially-resolved mid-infrared spectroscopy of NGC 1068: the nature and distribution of the nuclear material

We present spatially-resolved, near-diffraction-limited 10 micron spectra of the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle, the mid-IR imager and spectrometer on the 8.1 m Gemini North telescope. The spectra cover the nucleus and the central 6.0" x 0.4" of the ionization cones at a spatial resolution of approximately 0.4" (approx. 30 parsecs). The spectra extracted in 0.4" steps along the slit reveal striking variations in continuum slope, silicate feature profile and depth, and fine structure line fluxes on subarcsecond scales, illustrating in unprecedented detail the complexity of the circumnuclear regions of this galaxy at mid-IR wavelengths. A comparison of photometry in various apertures reveals two distinct components: a compact (radius <15 pc), bright source within the central 0.4" x 0.4" and extended, lower brightness emission. We identify the compact source with the AGN obscuring torus, and the diffuse component with the AGN-heated dust in the ionization cones. While the torus emission dominates the flux observed in the near-IR, the mid-IR flux measured with apertures larger than about 1" is dominated instead by the dust emission from the ionization cones. Many previous attempts to determine the torus spectral energy distribution are thus likely to be significantly affected by contamination from the extended emission. The observed spectrum of the compact source is compared with clumpy torus models, which require most of the mid-IR emitting clouds to be located within a few parsecs of the central engine. We also present a UKIRT/CGS4 5 micron spectrum covering the R(0) -- R(4) lines of the fundamental vibration-rotation band of 12CO. None of these lines was detected, and we discuss these non-detections in terms of the filling factor and composition of the nuclear clouds. (Abridged)

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arXiv2005

A compact starburst ring traced by clumpy OH megamaser emission

We model the OH megamaser emission from the luminous infrared galaxy IIIZw35 as arising from a narrow rotating starburst ring of radius 22 pc enclosing a mass of 7 x 10^6 Msun. We show how both the compact and apparently diffuse maser emission from this ring can arise from a single phase of unsaturated maser clouds amplifying background radio continuum. The masing clouds are estimated to have a diameter of <0.7 pc and internal velocity dispersion of about 20 km/s. We find that the clouds are neither self-gravitating nor pressure confined but are freely expanding. Their dispersal lifetimes may set the vertical thickness of the ring. For an estimated internal density of 3 x 10^3 cm^-3, cloud masses are of order 24 Msun. The observed spectral features and velocity gradients indicate that the clouds must be outflowing and escaping the nucleus. The cloud mass outflow rate is estimated to be 0.8 Msun/yr, while the star formation rate is about 19 Msun/yr. Associated ionised gas, possibly generated from dissipated clouds, provides free-free absorption along the source axis, explaining the observed East-West asymmetries. We show that the clumpiness of a maser medium can have a dramatic effect on what is observed even in a relatively low gain OH megamaser. Specifically, in IIIZw35 our clumpy maser model naturally explains the large line to continuum ratios, the large 1667MHz:1665MHz line ratios and the wide velocity dispersions seen in the compact maser spots. Other astrophysical masers showing both compact and apparently diffuse emission might be explained by similar clumpy structures.

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arXiv2005

W Hya through the eye of Odin Satellite observations of circumstellar submillimetre H$_2$O line emission

We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H2O, 1(10)-1(01), at 557GHz. The line is clearly of circumstellar origin. Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of (2.5 +/- 0.5)E-7 Msol/yr, and a circumstellar H2O abundance of (2.0 +/- 1.0)E-3. The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission, and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow. The very high water abundance, higher than the cosmic oxygen abundance, can be explained by invoking an injection of excess water from evaporating icy bodies in the system. The required extra mass of water is quite small, on the order of ~ 0.1 earth mass.

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arXiv2004

NG7538 IRS1 N: modeling a circumstellar maser disk

We present an edge-on Keplerian disk model to explain the main component of the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N. The brightness distribution and spectrum of the line of bright masers are successfully modeled with high amplification of background radio continuum emission along velocity coherent paths through a maser disk. The bend seen in the position-velocity diagram is a characteristic signature of differentially rotating disks. For a central mass of 30 solar masses, suggested by other observations, our model fixes the masing disk to have inner and outer radii of about 270 AU and 750 AU.

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arXiv2004

A thin ring model for the OH megamaser in IIIZw35

We present a model for the OH megamaser emission in the starburst galaxy IIIZw35. The observed diffuse and compact OH maser components in this source are explained by a single phase of unsaturated clumpy gas distributed in a thin ring structure and amplifying background continuum. We emphasize the importance of clumpiness in the OH masing medium, an effect that has not been fully appreciated previously. The model explains why multiple bright spots are seen only at the ring tangents while smoother emission is found elsewhere. Both the observed velocity gradients and the line to continuum ratios around the ring enquire a geometry where most of the seed photons come from a continuum emission which lies outside the OH ring. To explain both the OH and continuum brightness, free-free absorbing gas is required along the ring axis to partially absorb the far side of the ring. It is proposed that the required geometry arises from an inwardly propagating ring of starburst activity.

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arXiv2004

Spectral line and continuum radiation propagation in a clumpy medium

We discuss the propagation of spectral line and continuum radiation in a clumpy medium and give general expressions for the observed absorption or emission from a cloud population. We show that the affect of the medium clumpiness can usually be characterised by a single number multiplying the mean column opacity. Our result provides a simpler proof and generalisation of the result of Martin et al (1984). The formalism provides a simple way to understand the effects of clumping on molecular line profiles and ratios, for example how clumping effects the interpretation of 13CO(1-0) to 12CO(1-0) line ratios. It can also be used as a propagation operator in physical models of clumpy media where the incident radiation effects the spectral line emissivity. We are working to extend the formalism to the propagation of masers in a clumpy medium, but in this case there are special difficulties because formal expectation values are not characteristic of observations because they are biased by rare events.

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arXiv2004

Bipolar outflow on the Asymptotic Giant Branch - the case of IRC+10011

Near-IR imaging of the AGB star IRC+10011 (= CIT3) reveals the presence of a bipolar structure within the central ~0.1 arcsec of a spherical dusty wind. We show that the image asymmetries originate from ~1E-4 Msun of swept-up wind material in an elongated cocoon whose expansion is driven by bipolar jets. We perform detailed 2D radiative transfer calculations with the cocoon modeled as two cones extending to ~1,100 AU within an opening angle of ~30deg, imbedded in a wind with the standard r^{-2} density profile. The cocoon expansion started <~200 years ago, while the total lifetime of the circumstellar shell is ~5,500 years. Similar bipolar expansion, at various stages of evolution, has been recently observed in a number of other AGB stars, culminating in jet breakout from the confining spherical wind. The bipolar outflow is triggered at a late stage in the evolution of AGB winds, and IRC+10011 provides its earliest example thus far. These new developments enable us to identify the first instance of symmetry breaking in the evolution from AGB to planetary nebula.

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arXiv2003

Evidence for bipolar jets in late stages of AGB winds

Bipolar expansion at various stages of evolution has been recently observed in a number of AGB stars. The expansion is driven by bipolar jets that emerge late in the evolution of AGB winds. The wind traps the jets, resulting in an expanding, elongated cocoon. Eventually the jets break-out from the confining spherical wind, as recently observed in W43A. This source displays the most advanced evolutionary stage of jets in AGB winds. The earliest example is IRC+10011, where the asymmetry is revealed in high-resolution near-IR imaging. In this source the jets turned on only ~200 years ago, while the spherical wind is ~4000 years old.

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arXiv2003

The Most Detailed Picture Yet of an Embedded High-mass YSO

High-mass star formation is not well understood chiefly because examples are deeply embedded, relatively distant, and crowded with sources of emission. Using VLA and VLBA observations of water and SiO maser emission, we have mapped in detail the structure and proper motion of material 20-500 AU from the closest high-mass YSO, radio source-I in the Orion KL region. We observe streams of material driven in a rotating, wide angle, bipolar wind from the surface of an edge-on accretion disk. The example of source-I provides strong evidence that high-mass star formation proceeds via accretion

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arXiv2003

The Flaring H2O Megamaser and Compact Radio Source in Mrk 348

We report on single-dish monitoring and extremely high angular resolution observations of the flaring H2O megamaser in the Seyfert 2 galaxy Mrk 348. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is very broad, with a FWHM of 130 km/s, and has no detectable high velocity components within 1500 km/s on either side of the strong line. Monitoring observations made with the Effelsberg 100m telescope show that the maser varies significantly on timescales as short as one day and that the integrated line flux is loosely correlated with the continuum flux. VLBA observations indicate that the maser emission arises entirely from a region less than 0.25 pc in extent, located toward a continuum component thought to be associated with the receding jet. We also report on integrated flux monitoring with the VLA between 1.4 and 43 GHz, and VLBA continuum observations of the milliarcsecond scale jets at 1.7, 8, 15, and 22 GHz. These observations have allowed us to tentatively pinpoint the location of the core, and also show the ejection of a new jet component during the current radio "flare."

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