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.
Description
[1409.8158] CO excitation of normal star forming galaxies out to $z=1.5$ as regulated by the properties of their interstellar medium
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