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

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 ] 158 μ m 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 ( L IR ) and IR surface density. [C ii ] emission is detected in one galaxy at high signal-to-noise (34 σ ), 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 ]/ L IR and PAH/ L IR 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.