September 28, 2005
Dr. Jacqueline Fischer
Remote Sensing Division, Naval Research Lab
"Far-Infrared Diagnostics of Galaxies: From ISO to Herschel"
TALK ABSTRACT:
Infrared Space Observatory (ISO) spectroscopic studies of galaxies gave us a taste of the diversity of IR spectroscopic signatures of galaxies and their potential to characterize stellar populations and the local interstellar medium. For the first time, far-infrared spectroscopic observations of ultraluminous infrared galaxies were obtained. The spectra of these galaxies are characterized by molecular absorption lines, weak emission lines from photodissociation regions (PDR), and absence of far-infrared (wavelengths > 40 microns) lines from ionized regions. I will discuss molecular absorption in the nearest ultraluminous galaxy Arp 220 and PDR and photoionization models of the ultraluminous galaxies and the interpretation that non-PDR contributions to the FIR continuum are responsible for the apparent [CII] deficiency. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [CII] emission, or optically thick far-infrared emission, offer possible physical origins for additional non-PDR components. Such conditions may also be responsible for the observed suppression of PAH and FIR fine-structure emission from ionized gas, as well as the warmer FIR colors found in ULIRGs and may be a signature of hidden active galactic nuclei or very compact ionized regions.
The Herschel Space Observatory is slated to be launched into an L2 orbit by ESA in 2007 with a projected lifetime of more than 3 years. With a passively cooled 3.5-meter telescope, it will conduct photometry and spectroscopy in the far-infrared and submillimeter spectral regions to investigate the evolution of stars and galaxies. I will describe its instrumental capabilities and potential to provide a rich and high signal-to-noise database of local to high redshift galaxies for understanding the emission line deficit in warm, heavily obscured moderate luminosity to ultraluminous infrared galaxies. Combined with ground-based, SOFIA, and Spitzer studies, Herschel will be able to study starburst evolution in galactic disks, gaseous abundance variations and gradients among Hubble types and the effects of active galactic nuclei on the central regions of these galaxies.