CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium

Abstract

We investigate the CO excitation of normal star forming disk galaxies at z=1.5 using IRAM PdBI observations of the CO[2-1], CO[3-2] and CO[5-4] transitions for 4 galaxies, including VLA observations of CO[1-0] for 3 of them, with the aim of constraining the average state of H2 gas. Exploiting prior knowledge of the velocity range, spatial extent and size of the CO emission we measure reliable line fluxes with S/N>4-7 for individual transitions. While the average CO Spectral Line Energy Distribution (SLED) has a sub-thermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO[5-4] emission is four times stronger than assuming MW excitation. This demonstrates the presence of an additional component of more excited, denser and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is lower than in local (U)LIRGs and high-redshift SMGs, correlating closely with the average intensity of the radiation field and with the star formation surface density, but not with the SF efficiency (SFE). The CO[5-4] luminosity correlates linearly with LIR over 4 orders of magnitudes, with z=1.5 BzK galaxies following the same trend as local spirals and (U)LIRGs and high redshift star bursting SMGs. The CO[5-4] luminosity is thus empirically related to the dense gas, and might be a more convenient way to probe it than standard high--density tracers that are much fainter than CO. We see excitation variations among our sample galaxies, that can be linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, where the more highly excited part of the galaxy corresponds to the location of massive SF clumps. This supports to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies.