Misc,

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

, , , , , , , , , , , , , , , , , and .
(2014)cite arxiv:1409.8158Comment: 18 pages, 17 figures, 4 tables, submitted to A&A.

Abstract

We investigate the CO excitation of normal star forming galaxies at $z=1.5$ using IRAM PdBI observations of the CO2-1, CO3-2 and CO5-4 transitions for 4 galaxies, and VLA observations of CO1-0 for 3 of them, measuring 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 CO3-2, we show that the average CO5-4 emission is 4 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 CO5-4 to lower-J CO emission is however lower than in local (U)LIRGs and high-redshift starbursting SMGs, and appears to correlate closely with the average intensity of the radiation field $<U>$ and with the star formation surface density, but not with SF efficiency (SFE). This suggests that the overall CO excitation is at least indirectly affected by the metallicity of the ISM. The luminosity of the CO5-4 transition is found to correlate linearly with the bolometric infrared luminosity over 4 orders of magnitudes, with BzK galaxies following the same linear trend as local spirals and (U)LIRGs and high redshift star bursting sub-millimeter galaxies. The CO5-4 luminosity is thus related to the dense gas, and might be a more convenient way to probe it than standard high--density tracers. We see excitation variations among our sample galaxies, linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, the more highly excited part corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies.

Tags

Users

  • @miki

Comments and Reviews