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α-enhanced astrochemistry

Astrochemistry has been widely developed as a power tool to probe physical properties of the interstellar medium (ISM) in various conditions of the Milky Way Galaxy, and in near and distant galaxies. Most current studies conventionally apply linear scaling to all elemental abundances based on the gas-phase metallicity. However, these elements, including carbon and oxygen, are enriched differentially by stellar nucleosynthesis and the overall galactic chemical evolution, evident from α-enhancement in multiple galactic observations such as starbursts, high-redshift star-forming galaxies, and low-metallicity dwarfs.

The left panel above shows the correlation between metallicity ([O/H]) and the ratio between carbon and oxygen ([C/O]). Note that these quantities are logarithmic and normalized to the solar value, hence the negative values. The dotted horizontal line “Linear (reference)” is the commonly assumed scaling of C/O versus metallicity that is frequently adopted for calculations. However, observations of low-metallicity dwarf galaxies (pentagons) show that this is not the case and that in fact they are oxygen-enhanced. Note that the galaxy “GN-z11” is one of the most distant galaxies ever observed at a redshift of z~11. On the right panel, it is plotted how the dust-to-gas mass ratio (relative to the solar value) scales with metallicity. Once again, the dotted line shows the frequently adopted scaling by various groups, although observations show a sub-linear connection between the dust-to-gas ratio and the metallicity. 

In this project I study the impact of an α-enhanced ISM gas on the abundances of the three phases of carbon (C+, C, CO) dubbed as “the carbon cycle”. The study is carried out by performing astrochemical modelling using 3D-PDR in one-dimensional and three-dimensional clouds with ISM environmental parameters appropriately selected to mimic the conditions of different galaxy types. The models use C/O and dust-to-gas ratio taken from observations, thus making them the most accurate up to now. 

As a general finding, we see that in an α-enhanced ISM gas, CO, C and C+ will all decrease in both abundances and line emission, though with differential biases. Our results predict that an α-enhanced ISM gas will be brighter in CO lines than in [CI], even when the cosmic-ray ionization rate is high. This is in line with the recent discoveries of [CI]-dark galaxies, while the cause of the missing [CI] lines remain a mystery. We call for caution when using [CII] 158μm and [CI](1-0) as alternative H2-gas tracers for both diffuse and dense α-enhanced gas.

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