Diagnosing the interstellar medium of galaxies with far-infrared emission lines

Abstract

Context. Atomic fine structure lines have been detected in the local Universe and at high redshifts over the past decades. The [C ii] emission line at 158 µm is an important observable as it provides constraints on the interstellar medium (ISM) cooling processes. Aims. We develop a physically motivated framework to simulate the production of far-infrared line emission from galaxies in a cosmological context. This first paper sets out our methodology and describes its first application: simulating the [C ii] 158 µm line emission in the local Universe. Methods. We combine the output from EAGLE cosmological hydrodynamical simulations with a multi-phase model of the ISM. Gas particles are divided into three phases: dense molecular gas, neutral atomic gas, and diffuse ionised gas (DIG). We estimate the [C ii] line emission from the three phases using a set of Cloudy cooling tables. Results. Our results agree with previous findings regarding the contribution of these three ISM phases to the [C ii] emission. Our model shows good agreement with the observed L [C ii]-star formation rate (SFR) relation in the local Universe within 0.4 dex scatter. Conclusions. The fractional contribution to the [C ii] line from different ISM phases depends on the total SFR and metallicity. The neutral gas phase dominates the [C ii] emission in galaxies with SFR ∼ 0.01-1 M yr −1 , but the ionised phase dominates at lower SFRs. Galaxies above solar metallicity exhibit lower L [C ii] /SFR ratios for the neutral phase. In comparison, the L [C ii] /SFR ratio in the DIG is stable when metallicity varies. We suggest that the reduced size of the neutral clouds, caused by increased SFRs, is the likely cause for the L [C ii] deficit at high infrared luminosities, although EAGLE simulations do not reach these luminosities at z = 0.

Measuring the HI content of individual galaxies out to the epoch of reionization with [CII]

“…This line originates primarily from the cold, neutral gas so finding a way to calibrate it to trace the overall H i gas content is an important goal. While the line has previously been used as a proxy for the molecular gas content of galaxies (Zanella et al 2018;Madden et al 2020), observations (Pineda et al 2014;Croxall et al 2017;Díaz-Santos et al 2017;Cormier et al 2019;Tarantino et al 2021) and simulations (Franeck et al 2018;Pardos Olsen et al 2021;Ramos Padilla et al 2021) show that typically the molecular phases make only a minor contribution to the total [C ii] line luminosity. The association of the [C ii]-158 µm line with the diffuse ionized or neutral atomic gas is further supported by the extent of its emission being greater than the star-forming disk, the CO component, and the dust continuum in both low-and high-redshift galaxies (Madden et al 1997;Carniani et al 2018;Fujimoto et al 2019;Harikane et al 2020;Herrera-Camus et al 2021).…”

CII*/CII ratio in high-redshift DLAs: ISM phase separation drives the observed bimodality of [CII] cooling rates

“…Finally, observations and simulations show that ionized gas can also provide a large fraction (at least in our Galaxy) of C observed in emission (e.g. Roy et al 2017;Ramos Padilla et al 2021).…”

“…Lehner et al 2004) as well as modelling (e.g. Ramos Padilla et al 2021) all indicate that a large fraction of [C ] 158𝜇m emission can be associated with ionized medium, there are doubts that the same holds for DLAs (e.g. based on the modeling of ion abundances Wolfe et al 2004).…”

sígame v3: Gas Fragmentation in Postprocessing of Cosmological Simulations for More Accurate Infrared Line Emission Modeling

“…We can compare our [C II]-SFR relation with that found through similar techniques and hence better understand the decisive differences between the models. Figure 11 shows the [C II]-SFR relations found by SÍGAME for the SIMBA-100 galaxies with the 100MPC_AREPOPDF run (green contour lines) together with the relations derived by Popping et al (2019) and Ramos Padilla et al (2021). From the 100MPC_AREPOPDF run, 3 for all lines considered here, with the fine-structure lines sorted in order of increasing critical density.…”

“…Selecting only MS galaxies does not change this picture significantly. Our simulated galaxies lie above the simulated sample of Popping et al (2019), but in apparent extension of the samples of Ramos Padilla et al (2021), made with cosmological simulations from the Evolution and Assembly of Galaxies and their Environments (EAGLE) project (Crain et al 2015;Schaye et al 2015). In the latter study, GMCs are created in postprocessing following Olsen et al (2015), and CLOUDY is used to derive line intensities as in newer versions of SÍGAME. One interesting avenue of further study would be to apply SÍGAME to EAGLE, to fully understand how intrinsic differences between EAGLE and SIMBA affect this comparison.…”