| Description |
We propose for PACS spectroscopy of the O III 88 micron and N II 122
micron lines, and SPIRE far-IR photometry, to observe a sample of 16
low-redshift IR-luminous galaxies, at log L_IR = 11.6 to 12.2 Lsolar, that are
distinguished by large size and non-merger structure. In OT1 we are obtaining
PACS spectra of C II 158 and O I 63 microns for this sample. These
galaxies are interesting because they have high SFR activity spread over a
large physical area, rather than concentrated into extremely dense nuclear
regions, as in most local major merger ULIRGs. They are good analogs for
high-redshift IR-luminous galaxies, which have far-IR spectral shapes
different from local ULIRGs. At z>1, much of the star formation in massive
galaxies is at LIRG and ULIRG levels, and U/LIRGs dominate the IR luminosity
density. Understanding star forming regions in high-z IR-luminous galaxies is
necessary to understand the conditions in which most of the stars in massive
galaxies formed. In a few high-z lensed ULIRGs where C II can be observed,
C II/FIR is high, like local star-forming galaxies and unlike local ULIRGs.
C II is a major cooling line in PDRs and the far-IR lines probe the physical
conditions and UV intensity in IR-emitting regions. In our sample observed so
far in OT1, C II/FIR is high - they do not suffer the C II deficit found
in local ULIRGs. This suggests that redshift evolution in IR SEDs and line
ratios are related to the larger extent of star formation, and that this low-z
disky sample are good analogs. The O III and N II lines provide more
detailed probes of the ionization state, gas density, and ionizing SEDs in the
star-forming regions: in published samples the O III/FIR ratio shows tension
with simple models for the line deficit, and N II and O III can constrain
ionizing SED versus gas density. The SPIRE far-IR photometry will constrain
the total IR luminosity and cold dust in these galaxies, which have cool IR
colors and far-IR fluxes rising past 100 microns. |
