Thursday May 11th, 2012

Properties and Spatial Distribution of Dust Emission in the Crab Nebula

Authors: Tea Temim, George Sonneborn, Eli Dwek, Richard G. Arendt, Robert D. Gehrz, Patrick Slane, Thomas L. Roellig
(Submitted on 9 May 2012)

Abstract: Recent infrared (IR) observations of freshly-formed dust in supernova remnants (SNRs) have yielded significantly lower dust masses than predicted by theoretical models and measured from high redshift observations. The Crab Nebula’s pulsar wind is thought to be sweeping up freshly-formed supernova (SN) dust along with the ejected gas. The evidence for this dust was found in the form of an IR excess in the integrated spectrum of the Crab and in extinction against the synchrotron nebula that revealed the presence of dust in the filament cores. We present the first spatially resolved emission spectra of dust in the Crab Nebula acquired with the Infrared Spectrograph aboard the Spitzer Space Telescope. The IR spectra are dominated by synchrotron emission and show forbidden line emission from from S, Si, Ne, Ar, O, Fe, and Ni. We derived a synchrotron spectral map from the 3.6 and 4.5 microns images, and subtracted this contribution from our data to produce a map of the residual continuum emission from dust. The dust emission appears to be concentrated along the ejecta filaments and is well described by an amorphous carbon or silicate grain compositions. We find a dust temperature of 55+/- 4 K for silicates and 60 +/- 7 K for carbon grains. The total estimated dust mass is 0.0012-0.012 solar masses, well below the theoretical dust yield predicted for a core-collapse supernova. Our grain heating model implies that the dust grain radii are relatively small, unlike what is expected for dust grains formed in a Type IIP SN.

arXiv:1205.2062v1 [astro-ph.GA]

Deep Silicate absorption features in Compton-thick AGN predominantly arise due to dust in the host galaxy

Authors: A. D. Goulding (1), D. M. Alexander (2), F. E. Bauer (3), W. R. Forman (1), R. C. Hickox (4), C. Jones (1), J. R. Mullaney (2,5), M. Trichas (1) ((1) CfA, (2) Durham, UK, (3) UCC, Chile, (4) Dartmouth, (5) CEA-Saclay, France)
(Submitted on 8 May 2012)

Abstract: We explore the origin of mid-infrared (mid-IR) dust extinction in all 20 nearby (z < 0.05) bona-fide Compton-thick (N_H > 1.5 x 10^24 cm^-2) AGN with hard energy (E > 10 keV) X-ray spectral measurements. We accurately measure the silicate absorption features at lambda~9.7um in archival low-resolution (R~57-127) Spitzer Infrared Spectrograph (IRS) spectroscopy, and show that only a minority (~45%) of nearby Compton-thick AGN have strong Si-absorption features (S_9.7 = ln(f_{int}/f_{obs}) > 0.5) which would indicate significant dust attenuation. The majority (~60%) are star-formation dominated (AGN:SB<0.5) at mid-IR wavelengths and lack the spectral signatures of AGN activity at optical wavelengths, most likely because the AGN emission-lines are optically-extinguished. Those Compton-thick AGN hosted in low-inclination angle galaxies exhibit a narrow-range in Si-absorption (S_9.7 ~ 0-0.3), which is consistent with that predicted by clumpy-torus models. However, on the basis of the IR spectra and additional lines of evidence, we conclude that the dominant contribution to the observed mid-IR dust extinction is dust located in the host galaxy (i.e., due to disturbed morphologies; dust-lanes; galaxy inclination angles) and not necessarily a compact obscuring torus surrounding the central engine.

arXiv:1205.1800v1 [astro-ph.GA]

A Population of Dust-Rich Quasars at z ~ 1.5

Authors: Y. Sophia Dai, Jacqueline Bergeron, Martin Elvis, Alain Omont, Jia-Sheng Huang, Jamie Bock, Asantha Cooray, Giovanni Fazio, Evanthia Hatziminaoglou, Edo Ibar, Georgios E. Magdis, Seb J. Oliver, Mathew J. Page, Ismael Perez-Fournon, Dimitra Rigopoulou, Isaac G. Roseboom, Douglas Scott, Myrto Symeonidis, Markos Trichas, Joaquin D. Vieira, Christopher N. A. Willmer, Michael Zemcov
(Submitted on 8 May 2012)

Abstract: We report Herschel SPIRE (250, 350, and 500 micron) detections of 32 quasars with redshifts 0.5 < z < 3.6 from the Herschel Multi-tiered Extragalactic Survey. These sources are from a MIPS 24 micron flux-limited sample of 326 quasars in the Lockman Hole Field. The extensive multi-wavelength data available in the field permit construction of the rest-frame Spectral Energy Distributions (SEDs)from ultraviolet to the mid-infrared for all sources, and to the far-infrared (FIR) for the 32 objects. Most quasars with Herschel FIR detections show dust temperatures in the range of 25K to 60K, with a mean of 34K. The FIR luminosities range from 10^{11.3} to 10^{13.5} Lsun, qualifying most of their hosts as ultra- or hyper-luminous infrared galaxies. These FIR-detected quasars may represent a dust-rich population, but with lower redshifts and fainter luminosities than quasars observed at ~ 1 mm. However, their FIR properties cannot be predicted from shorter wavelengths (0.3–20 micron, rest-frame), and the bolometric luminosities derived using the 5100 A index may be underestimated for these FIR-detected quasars. Regardless of redshift, we observed a decline in the relative strength of FIR luminosities for quasars with higher near-infrared luminosities.

arXiv:1205.1808v1 [astro-ph.CO]
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