| Description |
One of the biggest challenges in galaxy evolution is to understand how star formation is
powered in high-redshift star-forming galaxies. Recent discovery of unusually strong
(EW > 500A) Halpha emission in majority of z>4 star-forming galaxies has provided
indirect evidence that significant fraction of high-redshift star formation is fueled by
mechanisms other than gas-rich merger, suggesting that cold gas accretion is an important
source of cold, neutral gas in high-redshift galaxies. FIR cooling lines such as
CII 158 micron line, produced by the UV stellar radiation like Halpha, afford
an efficient probe to study physical properties of the interstellar medium in star-forming
galaxies. Therefore, we propose to observe CII emission line from local counterparts
of high-redshift strong Halpha Emitters to investigate the properties of cold neutral
gas in these galaxies and understand how star formation is powered. We request total
11.1 hours of Herschel/PACS observation using Line Spectroscopy AOT for 20 targets,
detecting the expected CII line with line S/N of >3. Unlike the IR-luminous galaxies
or QSOs, HAEs are typical star-forming galaxies at high redshifts thus CII observation
of local HAEs would provide the first measurement of CII and CII/FIR ratio of
typical high-redshift star-forming galaxies. Thanks to the existing data including
spectroscopy, the relationship between the observed CII and Halpha, FIR luminosity
can be investigated as a function of metallicity, stellar population, extinction, etc.
Such relationship would be able to tell whether CII/FIR ratio can be used as an indicator
that discriminates merger-driven star formation and cold-flow driven star formation. Using
the last opportunity of Herschel to access rest-frame FIR cooling lines of local galaxies,
this observing program would provide a detailed physical framework for interpretation of
future large CII surveys in galaxies at higher redshifts using existing or planned
submm to radio facilities. |
