arXiv subjects

D. M. Alexander

Publications and source records attributed to D. M. Alexander.

310 records · Page 18Linked to original sources

arXiv2000

On the Seyfert 2 warm and cool infrared dichotomy

An optical spectropolarimetric study has shown that the detectability of polarised broad H-alpha in Seyfert 2 galaxies is correlated to the IRAS f60/f25 flux ratio where only those Seyfert 2s with "warm" IRAS colours show polarised broad line emission. It was suggested that those Seyfert 2s with "cool" IRAS colours have highly inclined tori which obscure the broad line scattering screen. I present here hard X-ray observations inconsistent with this picture showing that the derived column densities of warm and cool Seyfert 2 galaxies are statistically the same. I suggest that the IRAS f60/f25 flux ratio is more consistent with implying the relative strength of galactic to Seyfert emission and provide supporting evidence for this view.

astro-ph↗

arXiv2000

Modelling the Infrared continuum of Centaurus A

We present ISOPHOT_S, ISOSWS and 8 to 13 micron ground based observations of Centaurus A that show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that in a nuclear sized aperture (~4 arcsec, ~60 pc) the SED is characteristic of emission from a starburst and AGN torus; in larger apertures an additional component of cirrus emission is required. Based on our model, the torus diameter is estimated to be 3.6 pc and the best fitting inclination angle of the torus is 45 degrees. This result has implications for the detectability of tori in low power AGN and in particular for the use of the IRAS 60/25 micron flux ratio as an indicator of the torus inclination.

astro-ph↗

arXiv2000

A Near Infrared Polarized Bipolar Cone in the CIRCINUS Galaxy

We present near-infrared broad-band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south-eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J-K and H-K colour maps reveal a complex colour gradient toward the south-east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the star-formation ring. We model the polarized flux images with the scattering and torus model of Young etal, with the same basic input parameters as used by Alexander etal in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of ~16pc, and a visual extinction Av, through the torus, to the near-infrared emission regions of >66 mags.

astro-ph↗

arXiv2000

A local infrared perspective to deeper ISO surveys

We present new techniques to produce IRAS 12 micron samples of galaxies and stars. We show that previous IRAS 12 micron samples are incompatible for detailed comparison with ISO surveys and review their problems. We provide a stellar infrared diagnostic diagram to distinguish galaxies from stars without using longer wavelength IRAS colour criteria and produce complete 12 micron samples of galaxies and stars. This new technique allows us to estimate the contribution of non-dusty galaxies to the IRAS 12 micron counts and produce a true local mid-infrared extragalactic sample compatible with ISO surveys. We present our initial analysis and results.

astro-ph↗