CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium (1409.8158v2)

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 <U> 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.

CO Excitation of Normal Star-Forming Galaxies at Redshift 1.5

This paper investigates the CO excitation properties of normal star-forming disk galaxies at a redshift of approximately 1.5, using IRAM Plateau de Bure and VLA observations to analyze transitions of CO[2-1], CO[3-2], CO[5-4], and CO[1-0]. The paper explores the excitation of molecular Hydrogen (H$_2$) gas and aims to delineate the features of the gas phase in these high-redshift galaxies.

Key Findings

1. Sub-Thermal Excitation in Low-J CO Transitions: The galaxies exhibit a sub-thermal CO Spectral Line Energy Distribution (SLED) similar to the Milky Way up to the CO[3-2] transition. This suggests that the excitation of the CO emissions in these galaxies at low-J levels is not significantly higher than what is observed in nearby spirals.
2. Enhanced CO[5-4] Emission: CO[5-4] emission is significantly stronger, by a factor of four, than what would be predicted using Milky Way excitation levels, indicating the presence of a denser and potentially warmer molecular gas component. However, this enhancement is less pronounced when compared to local (U)LIRGs and high-redshift starbursting SMGs.
3. Correlation with $<U>$ and Star Formation Surface Density: The elevated CO[5-4] to lower-J CO emission ratios correlate well with the average intensity of the radiation field ($<U>$) and star formation surface density, but not with star formation efficiency (SFE). This suggests that the molecular gas heating and actual star formation activity must be directly linked.
4. Linear Scaling of L$_{\text{CO[5-4]}}$ with L$_{\text{IR}}$: The CO[5-4] luminosity displays a linear relationship with the bolometric infrared luminosity over four orders of magnitude, indicating that this transition may be an effective tracer of dense star-forming gas.
5. Excitation Variations and Structural Implications: Differences in excitation levels among the sample galaxies can be attributed to their evolutionary states and clumpiness observed in rest-frame optical images. Spatially resolved excitation variation suggests that massive star-forming clumps may be responsible for high excitation CO emissions in these galaxies, supporting models where giant clumps are central to elevated CO excitation.

Implications

The presence of a more excited CO component in $z=1.5$ galaxies implies an evolution of interstellar medium (ISM) properties with redshift, correlating with previously noted increases in $<U>$ and gas fractions up to $z=2$. As cosmic time progresses, these changes might indicate transformations in the physical conditions of molecular clouds, perhaps driven by evolving star formation activities and ISM dynamics.

Future Prospects

This paper bears implications for understanding the physical properties underpinning star formation in galaxies across cosmic time. The variance in CO excitation between different galaxy types and redshifts accentuates the necessity of tracing high-J CO lines to accurately characterize dense gas reservoirs in galaxies. High-resolution CO SLED data from future observations, particularly with facilities such as ALMA, are poised to refine our understanding and characterization of galaxy evolution significantly. These observations could further elucidate the links between galaxy structural features and molecular gas properties, fostering a more profound grasp of star formation's environmental dependencies.