ISM through the [CII] Mirror”


This bite was written by Prachi Khatri. Prachi Khatri is a Ph.D. Student at the Argelander Institute for Astronomy in Bonn, Germany, working on galaxy evolution. She uses cosmological hydrodynamic simulations to study molecular gas, carbon chemistry and star formation in high redshift galaxies. Aside from astronomy, she enjoys writing poetry, reading fiction, cooking and enjoying the sun.

title: Diagnosis of the interstellar medium of galaxies with far-infrared emission line I. The [CII] 158 µm line at e.g ∼ 0

author: AF Ramos Padilla1, L Wang, S Ploeckinger, FFS van der Tak and SC Trager

Institution of the first author: Kapteyn Astronomical Institute, University of Groningen, and SRON Netherlands Institute for Space Research, Groningen, The Netherlands

status: Published in A&A (freely accessible on arXiv)

The interstellar medium (ISM) is the material that fills the space between the stars in a galaxy and consists mostly of hydrogen, helium, trace metals and dust. The ISM can be divided into different ones phases depending on the physical conditions, each with a characteristic density, temperature, ionization percentage and chemical composition. Because of these differences, the phases show variations in their emission properties. The phases are roughly classified as atomic, molecular or ionized based on the predominant state of the most abundant element – hydrogen in that phase. As the names suggest, this state is atomic hydrogen (HI), molecular hydrogen (H2) and ionized hydrogen (H+ or HII) for the three phases.

Among the numerous emission lines which originate from the ISM, the [CII] line at 158 µm is one of the brightest in star-forming galaxies and arises from the 2P3/22P1/2 Fine structure Transition to the ground state of singly ionized carbon, C+. Look here for the fine structure of hydrogen. It is a major cooling line in the ISM and is often used as an indicator of star formation activity in nearby galaxies. However, the effectiveness of this line as a tracer of physical conditions in the ISM is hampered by the fact that it originates from multiple phases of the ISM – atomic, molecular and ionized. To circumvent this problem, the authors of today’s article have developed a framework to model this [CII] Emission from the three different phases and study their relative contributions as a function of galaxy properties such as metallicity and star formation rate (SFR).

Painting the (simulation) walls, also known as post-processing

Cosmological simulations of galaxies often don’t resolve the ISM very well – the simulation treats a blob of gas of mass mres (∼ 104 − 106 m) as a single particle. Then it resolves the movement and evolution of that particle without considering the density structure and chemical composition within the blob. After running the simulation up to the desired point in time (or redshift), the missing details are “painted” onto the blobs based on physically motivated analytical prescriptions, empirical relationships from observations, or scaling relationships from numerical simulations of smaller scales. This process is referred to as “post-processing” a simulation.

In this paper, the authors predict the [CII] Luminosity of simulated galaxies in post-processing. Their method is summarized in Figure 1. For each simulated galaxy, each gas particle is first split into neutral and ionized components based on ionization equilibrium. Ionization equilibrium means that the number of ionizations and recombination locally balance each other out. Then the strength of the Interstellar Radiation Field (ISRF) of near and distant stars is calculated at the location of each gas particle. Next, the mass in the neutral component is distributed into multiple spherical clouds, each of which has an onion-like structure (as shown in Figure 1). The chemical composition of the cloud layers varies with depth in the cloud and is calculated based on the density and metallicity of the neutral gas and the ISRF.

This image shows a complex flowchart that details how carbon and hydrogen emissions from clouds can be modeled using galaxy simulations.
Figure 1: The framework for predicting the [CII] brightness L[CII] for simulated galaxies in the EAGLE simulation suite. DIG stands for Diffuse Ionized Gas. (Figure 2 from the paper.)

In this way one obtains the proportions of the neutral atomic, molecular and diffusely ionized components for each gas particle in the galaxy. the [CII] Emission from each of these components is included in the calculation cloudy (a spectral synthesis code that predicts the emission of the ISM for a wide range of physical conditions). Summing up the contributions from the three phases for all gas particles gives the total [CII] brightness L[CII] the galaxy.

As an application of their model, the authors predict the [CII] Luminosity of today’s galaxies (ie at redshift e.g ∼ 0) in EAGLE Series of cosmological hydrodynamic simulations and examine the contribution of each phase L[CII]. Here is a Video one of the EAGLE simulations. They note that the relative contributions depend on the galaxy-averaged SFR (see Figure 2). As the SFR increases, the contribution of the neutral phases (atomic + molecular) increases (particularly the molecular phase), while that of the diffuse ionized gas phase (DIG) decreases.

This image shows the modeled contribution of diffuse ionized gas, atomic gas, and molecular gas.  At low star formation rates, diffuse ionized gas dominates, but at higher star formation rates, atomic and molecular gases dominate.
Figure 2: The contribution of the three ISM phases to this L[CII] as a function of the SFR of the galaxy. The shaded bars at the left and right ends correspond to SFR bins with fewer than ten galaxies. Note that as the SFR increases, the DIG bars shorten while the molecular bars lengthen. Overall, at SFR ≳ 0, the contribution of the neutral (atomic + molecular) phases dominates.01 m/Year. (Adapted from Figure 5 in the publication).
This image shows C+ luminosity as a function of metallicity for diffuse ionized gas, atomic gas, and molecular gas.  It also attempts to correlate the modeled values ​​with those observed in local galaxies.
Figure 3: The ratio of L[CII] from each phase to the SFR as a function of the average gas-phase metallicity of the galaxy, Z. For DIG, this ratio is almost constant with Zwhile it increases for the atomic and molecular phases at subsolar metallicities (adapted from Figure 8 in the paper).

You keep looking, like them L[CII]/SFR ratio of each phase varies as a function of the average metallicity of the gas phase of the galaxy, Z. As can be seen in Figure 3, this ratio is nearly constant for DIG (albeit with a large spread), while for the other two phases it increases with decreasing metallicity, although it plateaus for the molecular phase at Z ∼ 0.3Z.

Through their analysis, the authors show that their framework helps to traverse the parameter space of galaxy features and to delineate how the origin of the [CII] Line varies with these properties. Extending such studies to high redshift galaxies can provide invaluable insight into the origin of [CII] in these systems. These can also guide efforts on the observation front to use the effectively [CII] Mirrors to infer the physical conditions in the ISM of distant galaxies.

Astrobite edited by Abby Wagoner
Credit for selected images: SOFIA


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