The SQUARE_BRACKET_OPENCIISQUARE_BRACKET_CLOSE/CO ratio as a tool to measure the molecular gas contents and understand the star formation histories of low mass galaxies

We request 78 hours of observing time to obtain 510 sigma detections of the CII158um line for a complete sample of 103 starforming galaxies with 9.0<log(M*/Msun)<10.2 . These observations form the bedrock of an ambitious new program to understand the gas accretion and star formation histories of low mass galaxies in the local Universe. In this stellar mass range, galaxies are dominated by cold gas rather than stars, gasphase metallicities drop below solar, and CO line luminosities quickly become a poor proxy for the total molecular gas content of a galaxy. We note that the majority of starforming galaxies in the high redshift Universe (z>23) likely lie in this regime, irrespective of their mass. By measuring CII line fluxes with Herschel/PACS, our hope is to disentangle variations in CO line flux measurements that might arise because galaxies have different gas accretion histories, from systematics in the CObased molecular gas measurements resulting from changes in the structure and chemical compositions of the molecular clouds themselves. The majority of galaxies in our sample already have HI data from Arecibo. Total star formation rates and stellar masses have been estimated using multiband GALEX and SDSS photometry. We are simultaneously applying for IRAM 30m telescope time to obtain CO line fluxes, and MMT time to obtain longslit spectroscopy to estimate gasphase metallicity and dust extinction profiles for the galaxies in our sample. At the very least, by tracking the CII/CO ratio as function of stellar mass, SFR and metallicity, we will empirically (and hence robustly) determine the global properties of galaxies for which the standard XCO factor no longer provides accurate total molecular gas masses. This will be crucial input to future studies of CO in highz galaxies. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]