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

April 2020 • 2020ApJ...892..119M

Authors • McKinney, Jed • Pope, Alexandra • Armus, Lee • Chary, Ranga-Ram • Díaz-Santos, Tanio • Dickinson, Mark E. • Kirkpatrick, Allison

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 μm 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 (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]/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.

Links

• PREPRINT http://arxiv.org/abs/2002.08371
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2020ApJ...892..119M
• ELECTR https://doi.org/10.3847%2F1538-4357%2Fab77b9
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2020ApJ...892..119M
• PDF https://iopscience.iop.org/article/10.3847/1538-4357/ab77b9/pdf
• PDF https://stacks.iop.org/0004-637X/892/119/pdf
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2020ApJ...892..119M
• DATA https://hst.esac.esa.int/ehst/#/pages/search;bibcode=2020ApJ...892..119M