[Paper Review] Simulator of Galaxy Millimeter/Submillimeter Emission (SIGAME): The [CII]-SFR Relationship of Massive z=2 Main Sequence Galaxies

Karen P. Olsen, T. R. Greve+4 more|arXiv (Cornell University)|Jul 1, 2015

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper presents SÍGAME, a cosmological simulation framework that models [C II] 157.7 μm emission in massive z=2 main sequence galaxies by resolving the multi-phase interstellar medium (ISM) using sub-grid physics. It finds that [C II] emission is predominantly traced by molecular gas at high star formation rates (SFR > 20 M⊙ yr⁻¹) and by atomic/PDR gas at lower SFRs, with the simulated [C II]-SFR relation matching observed relations across redshifts.

ABSTRACT

We present SÍGAME simulations of the [CII]157.7$μ$ fine structure line emission from cosmological smoothed particle hydrodynamics (SPH) simulations of seven main sequence galaxies at z=2. Using sub-grid physics prescriptions the gas in our simulations is modeled as a multi-phased interstellar medium (ISM) comprised of molecular gas residing in giant molecular clouds, an atomic gas phase associated with photo-dissociation regions (PDRs) at the cloud surfaces, and a diffuse, ionized gas phase. Adopting logotropic cloud density profiles and accounting for heating by the local FUV radiation field and cosmic rays by scaling both with local star formation rate (SFR) volume density, we calculate the [CII] emission using a photon escape probability formalism. The [CII] emission peaks in the central $\lesssim$1 kpc of our galaxies as do the SFR radial profiles, with most [CII] ($\gtrsim$70%) originating in the molecular gas phase, whereas further out ($\gtrsim$2 kpc), the atomic/PDR gas dominates ($\gtrsim$90%) the [CII] emission, no longer tracing on-going star formation. Throughout, the ionized gas contribution is negligible ($\lesssim$3%). The [CII] luminosity vs. SFR ([CII]-SFR) relationship, integrated as well as spatially resolved (on scales of 1 kpc), delineated by our simulated galaxies is in good agreement with the corresponding relations observed locally and at high redshifts. In our simulations, the molecular gas dominates the [CII] budget at SFR$\gtrsim$20 M$_{\odot}$yr$^{-1}$ (SFR density $\gtrsim$0.5 M$_{\odot}$yr$^{-1}$kpc$^{-2}$), while atomic/PDR gas takes over at lower SFRs, suggesting a picture in which [CII] predominantly traces the molecular gas in high-density/pressure regions where star formation is on-going, and otherwise reveals the atomic/PDR gas phase.

Motivation & Objective

• To understand the origin of [C II] emission in massive, high-redshift main sequence galaxies.
• To determine which ISM phases—molecular, atomic/PDR, or ionized—dominate [C II] emission across varying SFRs.
• To test whether the observed [C II]-SFR relation can be reproduced by a physically motivated ISM model.
• To resolve the spatially resolved [C II]-SFR relationship on 1 kpc scales in simulated high-z galaxies.
• To assess the role of multi-phase ISM structure and local heating (FUV, cosmic rays) in shaping [C II] luminosity.

Proposed method

• Simulates seven z=2 main sequence galaxies using smoothed particle hydrodynamics (SPH) with sub-grid models for the multi-phase ISM.
• Models the ISM as three phases: molecular gas in giant clouds, atomic gas in PDRs at cloud surfaces, and diffuse ionized gas.
• Applies logotropic cloud density profiles and scales heating from FUV and cosmic rays with local SFR volume density.
• Calculates [C II] emission using a photon escape probability formalism to account for radiative transfer effects.
• Resolves [C II] emission on 1 kpc spatial scales and computes surface density relations (Σ[CII]-ΣSFR).
• Compares integrated and resolved [C II]-SFR relations in simulations to local and high-redshift observations.

Experimental results

Research questions

• RQ1 What fraction of [C II] emission originates from molecular gas versus PDR/atomic gas in massive z=2 galaxies?
• RQ2 How does the [C II]-SFR relation vary with SFR surface density in high-redshift galaxies?
• RQ3 To what extent does ionized gas contribute to [C II] emission in these simulated galaxies?
• RQ4 Does the simulated [C II]-SFR relation match observed relations at both low and high redshifts?
• RQ5 How does the dominant ISM phase contributing to [C II] change with decreasing SFR?

Key findings

• Over 70% of [C II] emission originates in the molecular gas phase within the central 1 kpc, where SFRs are highest.
• At radii beyond 2 kpc, the atomic/PDR gas phase dominates [C II] emission, contributing over 90% of the luminosity.
• Ionized gas contributes less than 3% to total [C II] luminosity, despite being the dominant mass component in the ISM.
• The simulated integrated [C II]-SFR relation matches observed relations at both low and high redshifts in slope and normalization.
• The resolved Σ[CII]-ΣSFR relation spans six orders of magnitude in ΣSFR (10⁻⁵ to 10 M⊙ yr⁻¹ kpc⁻²), with good agreement at intermediate SFR densities (0.001–1 M⊙ yr⁻¹ kpc⁻²).
• The [C II] luminosity scales most strongly with molecular gas mass at high SFRs, indicating that [C II] primarily traces the star formation fuel in dense, high-pressure regions.