We have investigated the properties of 142 galaxies both detected in X-ray and 70µm in the COSMOS field. X-ray data are obtained from both XMM and Chandra point source catalogs, and 70µm photometry is drawn from Spitzer-MIPS 70µm point source catalog.
We classified our sample into three distinct subsamples according to their respective total infrared luminosity (LIR): star-forming galaxies (LIR < 1011L⊙), luminous infrared galaxies (LIRGs, 1011L⊙ ≤ LIR< 1012L⊙), and ultra-luminous infrared galaxies (ULIRGs, LIR≥ 1012L⊙), with median redshifts of z∼ 0.168, 0.518 and 1.268, respectively. The major conclusions for this study are as follows:
1. We applied two methods to determine which mechanism dominates the SED, star formation or AGN:
i) Using Spitzer-IRAC colors, we have shown that the majority of our sample is dominated by AGN. Although the higher AGN fraction accompanies with higher
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total infrared luminosity (LIR), the presence of PAH emission lines (e.g. 3.3µm, 6.2µm, 7.7µm, 8.6µm, and 11.2µm) related to star formation could disturb AGN identification.
ii) Using the relation between intrinsic rest-frame hard X-ray (2-10keV) and total infrared luminosity LIR (8-1000µm), we found that our samples are dominated by AGN, with property comparable to AGN detected SMGs.
2. We provided evidences for additional X-ray obscuration in X-ray detected 70µm galaxies, in agreement with current AGN/starburst co-evolution model (Hopkins et al. 2008). A K-S test on the modified HR between 70µm/X-ray galaxies and whole X-ray samples rejected the assumption of them to be originated from the same population within 99% confidence level, supporting the idea that the excess of X-ray absorption does not come from the AGN dusty torus obscuration, but probably from additional diffuse obscuration generated by the star formation in the galaxy.
3. We estimated the dust temperature of our 70µm galaxy samples by fitting their far-infrared photometries with fixed emissivity models. Despite the presence of a warm dust component, the cold dust shows a similar temperature in host galaxy with and without AGN, indicating that the longer wavelengths are still dominated by star formation. This evidence conflicts with a scenario where radiative feedback from the AGN truncates the star formation in the host galaxy.
Acknowledgements
I would like to express my deepest appreciation to all people who help me the possibility to finish this master thesis. I would like to dedicate this thesis to my parents, Grace Chang and Ching-Yuan Lin, for their unconditional supports and allowing me to be
as ambitious as I wanted. Thanks to my dog - Pocky, he brings a lot of fun for me through my research process. A great gratitude I give to my supervisors, Yasuhiro Hashimoto and S´ebastien Foucaud, who provide plentiful advices and help me to coordinate my project especially in writing this report. A special thanks to Lin-Wen Chen, with his solid training for astronomical courses that inspired me to put more efforts on my master project.
We would like to acknowledge the wonderful work made by Jeyhan S. Kartaltepe on the Spitzer-COSMOS 70µm point source catalog on which our analyses are based. We would like to thank the following people for very useful discussions, suggestions, and comments:
Jeyhan S. Kartaltepe, Wei-Hao Wang, Albert Kong, Matthew A. Malkan, Nick Scoville, and David B. Sanders.
The work presented here is supported by the National Science Council of Taiwan under the grants NSCM-003-002-MY2, NSC99-2119-M-003-005, and NSC 99-2112-M-003-001-MY2. This research used extensively the COSMOS survey archive data of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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