Mid-Infrared Observations of Infrared Luminous Galaxies.

Astronomy and Astrophysics – Astronomy

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Scientific paper

I have undertaken a program to use SpectroCam -10, a new mid-infrared spectrometer/camera for the Palomar Observatory 200 inch telescope, to study a sample of infrared luminous galaxies (ILGs) discovered by the Infrared Astronomical Satellite (IRAS) mission in 1983. This category of galaxies includes the most luminous objects known in the universe, being bolometrically as luminous as quasars, and more numerous. I have looked for evidence to show whether the energy generation mechanism powering these galaxies comes from massive bursts of star formation, or from an active galactic nucleus (AGN). High resolution mid-infrared images trace the spatial extent of the infrared emitting dust, which must lie close to the heating source. The majority of the sample galaxies are indeed extended, implying a starburst as the source of at least part of the energy. However, most of the galaxies also contain an unresolved nucleus, and the presence of an AGN cannot be ruled out. The observations are consistent with an evolutionary model where the infrared luminous galaxies are seen as the dusty cocoons for the birth of quasars. Spatially resolved low resolution spectra of the polycyclic aromatic hydrocarbon (PAH) feature at 11.3 μm were obtained for NGC 7469, an ILG and a Seyfert 1 galaxy with a circumnuclear starburst. The results show that the PAHs are depleted in the nuclear environment of the galaxy, where the strong EUV and X-ray flux are expected to destroy them, and that they survive in the circumnuclear regions ~500 pc from the nucleus. Spatially resolved observations of the PAH feature can serve as test for the presence of an AGN in the midst of a dusty starburst, where the dust obscures the optical signatures. The design and development of SpectroCam-10 are also described in this thesis. SpectroCam-10 is a spectrometer/camera designed to operate from 8 to 13 mum. We have achieved diffraction limited imaging quality of 0.6 arcsec at 11.7 μm on the 200 inch telescope, providing the highest spatial resolution capability currently available shortward of the radio regime.

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