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arXiv · 2002.08371

Measuring the Heating and Cooling of the Interstellar Medium at High redshift: PAH and [C II] Observations of the Same Star Forming Galaxies at z~2

Jed McKinney·Alexandra Pope·Lee Armus·Ranga-Ram Chary·Tanio Diaz-Santos·Mark E. Dickinson·Allison Kirkpatrick
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Abstract

Star formation depends critically on cooling mechanisms in the interstellar medium (ISM); however, thermal properties of gas in galaxies at the peak epoch of star formation (z ~ 2) remain poorly understood. A limiting factor in understanding the multiphase ISM is the lack of multiple tracers detected in the same galaxies, such as Polycyclic Aromatic Hydrocarbon (PAH) emission, a tracer of a critical photoelectric heating mechanism in interstellar gas, and [C II] 158μm fine-structure emission, a principal coolant. We present ALMA Band 9 observations targeting [C II] in six z ~ 2 star-forming galaxies with strong Spitzer IRS detections of PAH emission. All six galaxies are detected in dust continuum and marginally resolved. We compare the properties of PAH and [C II] emission, and constrain their relationship as a function of total infrared luminosity (LIR) and IR surface density. [C II] emission is detected in one galaxy at high signal-to-noise (34σ), and we place a secure upper limit on a second source. The rest of our sample are not detected in [C II] likely due to redshift uncertainties and narrow ALMA bandpass windows. Our results are consistent with the deficit in [C II]/LIR and PAH/LIR observed in the literature. However, the ratio of [C II] to PAH emission at z ~ 2 is possibly much lower than what is observed in nearby dusty star-forming galaxies. This could be the result of enhanced cooling via [O I] at high-z, hotter gas and dust temperatures, and/or a reduction in the photoelectric efficiency, in which the coupling between interstellar radiation and gas heating is diminished.

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Jed McKinney, Alexandra Pope, Lee Armus, Ranga-Ram Chary, Tanio Diaz-Santos, Mark E. Dickinson, Allison Kirkpatrick. 2020-02-19. Measuring the Heating and Cooling of the Interstellar Medium at High redshift: PAH and [C II] Observations of the Same Star Forming Galaxies at z~2. https://doi.org/10.3847/1538-4357%2Fab77b9

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