arXiv subjects

Ralf Siebenmorgen

Publications and source records attributed to Ralf Siebenmorgen.

44 records · Page 3Linked to original sources

arXiv2004

SED models of AGN

Present AGN models aiming to account for the observed infrared SEDs consider a physical description of the dust and a solution of the radiative transfer problem. Mid infrared spectra obtained at different spatial scales are presented. They show that PAH bands are detected in starburst regions but significantly reduced near the centre of AGN. This may be explained by examining the heating mechanism of PAHs after hard photon interactions. On the radiative transfer side first a most economic model is presented where three parameters, luminosity, effective size and extinction of the nucleus are varied to obtain SED fits. A full grid of model spectra are made available at: "http://www.eso.org/~rsiebenm/agn_models". This model is sufficient to account for ISO broad band data of a sample of 68 radio galaxies and quasars of the 3CR catalogue. The hot dust component detected is mainly due to small grains and PAHs. In such models, type 1 AGNs are represented by a compact dust distribution with warm grains and weak PAH emission. In AGNs of type 2, the dust appears to be more extended, relatively colder and PAH bands are strong. Realistic AGN models which are consistent with the unification need to explain the overall absence of the 9.7mic. silicate emission feature. This can be done by considering various geometries (tapered discs). Models which combine AGN and starburst activity are presented for galaxies with hidden broad line region. It is found that the AGN torus dominate the mid IR continuum emission and that the starbursts dominate the PAH band as well as the far infrared and submillimeter emission.

astro-ph↗

arXiv2004

Mid-infrared selection of AGN

Since a large fraction of active galactic nuclei (AGN) is missed in common UV-excess surveys and is even hard to find in radio, near-IR and X-ray surveys, we have used a new AGN selection technique which is expected to be not affected by extinction. Within the scientific verification of the ISOCAM Parallel Survey at 6.7 micron we have discovered objects with exceptional mid-infrared (MIR) emission. They are essentially not detected on IRAS-ADDSCANs and only very few of them show up in the NVSS and FIRST radio surveys. Various colour criteria of the 6.7 micron data with 2MASS and optical wavebands show that the sources reach more extreme IR colours than the sources in the Hubble Deep Field-South and the ELAIS survey. The comparison with known object types suggests that we have found AGN with a pronounced MIR emission, probably due to circum-nuclear dust. First results from optical spectroscopy of ten candidates corroborate this interpretation showing four AGN, two reddened LINER and four extremely reddened emission-line galaxies with MIR/FIR flux ratios higher than for known pure starburst galaxies. The results will make a significant contribution to the debate on the entire AGN population.

astro-ph↗

arXiv2003

Hot Dust in Radio-Loud Active Galactic Nuclei

We have measured mid-infrared (MIR) fluxes of 3C sources on images taken with ISOCAM onboard the Infrared Space Observatory (ISO). The photometric data were combined with existing photometry at other wavelengths to assemble the spectral energy distribution (SED) for each galaxy from infrared to radio wavelengths. In addition, we used ISOPHOT spectra to compute average MIR spectra for different types of Active Galactic Nuclei (AGNs). The MIR emission of 53 of our sources shows evidence of dust. We find a clear correlation between the SEDs, emission from polycyclic aromatic hydrocarbons (PAHs), and the type of AGN. Specifically, we find that in broad emission line radio galaxies (BLRGs), the dust emission peaks at ~40 microns and PAH bands are weak, whereas emission in comparable narrow line radio galaxies (NLRGs) peaks at a longer wavelength of \~100microns and the PAH bands are much stronger. Although less pronounced, a similar trend is seen when comparing quasars (QSOs) with high-luminosity NLRGs. We used dust radiative transfer models with a central heating source to describe the SEDs. The difference in the dust emission appears to be the effect of heating by radiation from the central engine. In type~1 AGNs (BLRGs and QSOs), the broad line region (BLR) is directly visible and therefore hot (T>~300K) dust dominates the emission. In this region, PAHs are destroyed which explains the weak PAH emission. On the other hand, the BLR is hidden in type~2 AGNs (NLRGs). Their SEDs are therefore dominated by cooler dust including PAHs.

astro-ph↗

arXiv2002

Infrared Space Observatory Polarimetric Imaging of the Egg Nebula (RAFGL 2688)

We present polarimetric imaging of the protoplanetary nebula RAFGL 2688 obtained at 4.5 microns with the Infrared Space Observatory (ISO). We have deconvolved the images to remove the signature of the point spread function of the ISO telescope, to the extent possible. The deconvolved 4.5 micron image and polarimetric map reveal a bright point source with faint, surrounding reflection nebulosity. The reflection nebula is brightest to the north-northeast, in agreement with previous ground- and space-based infrared imaging. Comparison with previous near-infrared polarimetric imaging suggests that the polarization of starlight induced by the dust grains in RAFGL 2688 is more or less independent of wavelength between 2 microns and 4.5 microns. This, in turn, indicates that scattering dominates over thermal emission at wavelengths as long as ~5 microns, and that the dust grains have characteristic radii < 1 micron.

astro-ph↗

arXiv2000

Galaxies in the infrared

The mid infrared spectra of the starbursts show the 9.7$μm$ silicate absorption feature and strong mid infrared emission bands centered at 6.2, 7.7, and 11.3 $μ$m. Illustrative models of the {\it active} galaxies are presented: As the starbursts are most likely confined to the central region of the galaxy the radiative transfer in the nucleus is solved under the assumption of spherical symmetry. The distribution of stars and dust are adjusted until the complete infrared spectrum of the galaxies are modeled. The dust is described as a mixture of large grains, very small grains and PAH, which are undergoing temperature fluctuations. Although the galactic nuclei are deeply hidden in the dust its global structure can be estimated by the simple calculations presented. ISOPHOT samples of eleven {\it active} galaxies and seven {\it inactive} spirals are presented. The far infrared and submillimeter spectrum of the {\it active} galaxies can be described by a single modified black--body at a color temperature of $31.5 \pm 2.8$ K. This leads to a ratio of infrared luminosity to gas mass, $L_{\rm IR}/M_{\rm gas}$, of $\sim 90$ L$_\odot$/ M$_\odot$. In contrast, the spectral energy distributions of {\it inactive} spirals require, apart from warm dust of $31.8 \pm 2.8$ K, an additional very cold component of at most $12.9\pm 1.7$ K. This implies a $L_{\rm IR}/M_{\rm gas}$ ratio of $\sim 3$ L$_\odot$/ M$_\odot$ for the {\it inactive} spirals, a factor $\sim 30$ lower than for the {\it active} galaxies. The detection of such cold dust can be predicted by radiative transfer models.

astro-ph↗

arXiv1999

ISO observations of obscured Asymptotic Giant Branch stars in the Large Magellanic Cloud

We present ISO photometric and spectroscopic observations of a sample of 57 bright Asymptotic Giant Branch stars and red supergiants in the Large Magellanic Cloud, selected on the basis of IRAS colours indicative of high mass-loss rates. PHOT-P and PHOT-C photometry at 12, 25 and 60 $μ$m and CAM photometry at 12 $μ$m are used in combination with quasi-simultaneous ground-based near-IR photometry to construct colour-colour diagrams for all stars in our sample. PHOT-S and CAM-CVF spectra in the 3 to 14 $μ$m region are presented for 23 stars. From the colour-colour diagrams and the spectra, we establish the chemical types of the dust around 49 stars in this sample. Many stars have carbon-rich dust. The most luminous carbon star in the Magellanic Clouds has also a (minor) oxygen-rich component. OH/IR stars have silicate absorption with emission wings. The unique dataset presented here allows a detailed study of a representative sample of thermal-pulsing AGB stars with well-determined luminosities.

astro-ph↗

arXiv1999

IRAS04496-6958: A luminous carbon star with silicate dust in the Large Magellanic Cloud

We describe ISO observations of the obscured Asymptotic Giant Branch (AGB) star IRAS04496-6958 in the Large Magellanic Cloud (LMC). This star has been classified as a carbon star. Our new ISOCAM CVF spectra show that it is the first carbon star with silicate dust known outside of the Milky Way. The existence of this object, and the fact that it is one of the highest luminosity AGB stars in the LMC, provide important information for theoretical models of AGB evolution and understanding the origin of silicate carbon stars.

astro-ph↗

arXiv1993

The Past and Present Infrared Spectrum of BD+30 3639

We present a radiative-transfer calculation which reproduces the infrared spectrum of the planetary nebula BD+30$^{\circ}$3639, fitting both the spectral energy distribution and the spatial extent at infrared wavelengths. We obtain an acceptable fit to most of the spectrum, including the infrared bands. The fit requires a distance of $\ge 2 \kpc$, which implies that BD+30$^\circ$3639 has evolved from a massive progenitor of several solar masses. Two surprising results are (1) a very low dust-to-gas ratio, (2) an absence of the smallest PAH molecules. Extrapolating back in time, we calculate the previous infrared evolution of BD+30$^\circ$3639.

astro-ph↗