Study of local infrared bright galaxies with HERSHCEL-PACS

Infrared bright galaxies (LIRGs and ULIRGs) represent the bulk of the cosmic infrared background and play a major role in the cosmic star formation and accretion histories. For this reason they have been subject of intensive studies at all wavelengths. However, being very dusty galaxies, one of the key wavelength range to understand their evolutionary stages and the physic involved, is the Mid-Far-Infrared and sub-millimeter window. Previous (IRAS and ISO) and current (SPITZER) infrared missions, already shed light on the nature and the evolution of these galaxies, but still many phenomena lack of a complete understanding. For example, the processes triggering the starburst and AGN activities as well as trends with the interaction stage, are not well established yet, partially because at FIR wavelengths it has not been possible so far to spatially resolve these different components even in nearby objects. With its passively cooled 3.5 meter telescope, HERSCHEL will offer this opportunity for the first time. In particular, the PACS instrument, is unique for tackling some important open issues thanks to its spectro imaging capability at FIR wavelengths. We will illustrate some of these exciting new opportunities using examples from the Guaranteed Time program on infrared bright galaxies, that is currently being developed. ISO has undoubtedly shown that the use of Mid-Far-infrared spectroscopy is a powerful tool for establishing the physical conditions of the ISM and separating the starburst and AGN activity contributions which often coexist in (U)LIRGs. However, such a coexistence makes the central regions of (U)LIRG very peculiar such that we expect the ISM in their central regions to be very different than in normal star-forming galaxies. One known example is the fact that ultra luminous infrared galaxies have less [CII] emission w.r.t. the total FIR emission ([CII] deficiency) than what is found in normal galaxies. This result, found in several studies based on ISO spectroscopy, points towards a different heating/cooling balance of the ISM in infrared bright galaxies, but the causes and the related physic remain unknown. With PACS it will be possible to take full resolution complete PACS scans of representative nearby sources such that we can probe the ISM physics in central starbursts, the circum-nuclear molecular rings, disks and winds, separately. Moreover, a survey of the most important FIR structure lines will become feasable for large samples spanning from starburst, AGNs and obscured objects, at local and intermediate redshift. Observations with the PACS photometric camera in its three bands (70,110 and 170 μm) will complement the science possible with the integral field spectroscopy, because HERSCHEL will be able to resolve for the first time individual dust enshrouded activity knots, i.e. the place where star formation is triggered, to locate the starburst regions and their relative contributions in interacting systems. Due to the unprecedent spatial resolution HERSCHEL provides at FIR and sub-millimeter wavelengths and the improved sensitivities of its instruments, we expect that a significant part of HERSCHEL observing time will be spent on the study of high redshift galaxies. Detailed studies of nearby templates, as illustrated here, will significantly contribute in understanding in detail the physics governing the diagnostic diagrams and the change in spectral characteristics which are the basic tools for studying unresolved galaxies at high redshift. © 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim