Mathematics – Logic
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50008i&link_type=abstract
Spitzer Proposal ID #50008
Mathematics
Logic
Scientific paper
Ultraluminous infrared galaxies (ULIRGs) have been used extensively to trace the history of star formation in the early universe. However, the major issue of whether ULIRGs are powered primarily by starbursts, or if AGNs hidden behind dust are energetically important, is poorly understood. Unlike optically detectable AGNs obscured by torus-shaped dust, most of putative AGNs in very dusty ULIRGs' nuclei may be deeply 'buried' in all direction. It is fundamental to quantitatively determine the energetic importance of such elusive buried AGNs in ULIRGs, if we are to understand the true nature of the ULIRG population. Based on infrared 3-4 micron (Subaru) and 5-35 micron (Spitzer) low-resolution spectra of optically non-AGN ULIRGs at z < 0.15, we have investigated the equivalent widths of PAH emission features and geometry between energy sources and dust. We have succeeded in detecting luminous buried AGN signatures in half of the observed ULIRGs. With the advent of AKARI's 2.5-5 micron spectroscopic capability, it is now possible to extend this successful approach to ULIRGs at z = 0.15-0.3. We propose Spitzer IRS 5-35 micron low-resolution spectroscopy of optically non-AGN ULIRGs at z = 0.15-0.3. By including these more distant ULIRGs, we can for the first time make a meaningful investigation of the fraction of luminous buried AGN as a function of infrared luminosity. If we find that luminous buried AGNs are more common in galaxies with higher infrared luminosities, as our preliminary analysis of archival data suggests, then we may be able to provide the first observational evidence that the well-known galaxy downsizing phenomena are indeed due to stronger AGN feedbacks in currently more massive (= previously more infrared luminous) galaxies. Additionally, the proposed observations will be an important bridge for the comprehensive understanding of the ULIRG population from nearby (z < 0.15) to cosmologically distant (z = 0.3-4) universe.
Dudley Chris
Imanishi Masatoshi
Maiolino Roberto
Nakagawa Takao
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