The applicability of FIR fine-structure lines as Star Formation Rate tracers over wide ranges of metallicities and galaxy types

Abstract: We analyze the applicability of far-infrared fine-structure lines [CII] 158 micron, [OI] 63 micron and [OIII] 88 micron to reliably trace the star formation rate (SFR) in a sample of low-metallicity dwarf galaxies from the Herschel Dwarf Galaxy Survey and compare with a broad sample of galaxies of various types and metallicities in the literature. We study the trends and scatter in the relation between the SFR (as traced by GALEX FUV and MIPS 24 micron) and far-infrared line emission, on spatially resolved and global galaxy scales, in dwarf galaxies. We assemble far-infrared line measurements from the literature and infer whether the far-infrared lines can probe the SFR (as traced by the total-infrared luminosity) in a variety of galaxy populations. In metal-poor dwarfs, the [OI] and [OIII] lines show the strongest correlation with the SFR with an uncertainty on the SFR estimates better than a factor of 2, while the link between [CII] emission and the SFR is more dispersed (uncertainty factor of 2.6). The increased scatter in the SFR-L([CII]) relation towards low metal abundances, warm dust temperatures, large filling factors of diffuse, highly ionized gas suggests that other cooling lines start to dominate depending on the density and ionization state of the gas. For the literature sample, we evaluate the correlations for a number of different galaxy populations. The [CII] and [OI] lines are considered to be reliable SFR tracers in starburst galaxies, recovering the star formation activity within an uncertainty of factor 2. [Abridged]

• The paper demonstrates that [O I] 63µm and [O III] 88µm lines strongly correlate with star formation rates in metal-poor galaxies, yielding uncertainties below a factor of 2.
• It reveals that variations in ISM conditions, such as lower metallicity and warmer dust temperatures, increase scatter in the [C II] SFR relation.
• Extended analysis shows that while [C II] and [O I] 63µm lines work well as SFR indicators in starbursts, their reliability drops in composite, AGN, and ULIRG environments.

The Applicability of FIR Fine-Structure Lines as Star Formation Rate Tracers

This paper evaluates the potential of far-infrared (FIR) fine-structure lines—[C II] 158µm, [O I] 63µm, and [O III] 88µm—as reliable tracers of the star formation rate (SFR) across a broad range of galaxy types and metallicities. Using data from the Herschel Dwarf Galaxy Survey and integrating findings from the literature, the research seeks to assess the robustness of these spectral lines as proxies for SFR, particularly in environments with varied interstellar medium (ISM) conditions.

Key Findings

1. Correlation with SFR in Metal-Poor Galaxies: The study finds that in metal-poor dwarf galaxies, the [O I]$_{63}$ and [O III]$_{88}$ lines correlate strongly with the SFR, with uncertainties on SFR estimates being less than a factor of 2. In contrast, [C II] exhibits a larger scatter, indicating potential difficulty in using it as a reliable SFR tracer in such environments.
2. Variation with ISM Conditions: The increased scatter in the SFR-[C II] relation, especially at lower metallicities, warmer dust temperatures, and in regions with large diffuse ionized gas contributions, suggests an intricate dependence on ISM conditions. These variations may be attributed to changes in the dominant cooling mechanism, shifting from C-based lines to other coolants as ISM properties evolve.
3. Extended Sample Insights: For a broader galaxy sample beyond dwarfs, both [C II] and [O I]$_{63}$ serve as reliable SFR tracers within an uncertainty factor of about 2 in starburst galaxies. However, for composite and AGN sources, the lines' reliability diminishes, with uncertainties rising to a factor of 2.3.
4. ULIRGs and High-Redshift Galaxies: In ULIRGs, SFR calibrations using these lines indicate deficits compared to starbursts/AGN due to an apparent decoupling between line emission and total-infrared luminosity. At high redshifts, the forthcoming challenge lies in obtaining more detections for [O I]$_{63}$ and [O III]$_{88}$ lines to establish robust SFR calibrations.

Implications for Astrophysics

The differences in line emission reliability as SFR proxies highlight a complex interplay between star formation activity, ISM properties, and FIR line coolant effectiveness. The findings suggest a need to consider ISM conditions comprehensively when applying SFR calibrations based on FIR lines. In high-redshift and extreme environments like ULIRGs, alternative or supplementary tracers might be necessary. The insights can refine our understanding of star formation evolution across cosmic times under varying galactic environments.

Future Directions

Future analyses should focus on expanding the dataset, particularly for high-redshift galaxies, to reinforce the statistical significance of the correlations observed. Additionally, further investigation into other potential FIR and submillimeter lines as SFR indicators in diverse ISM conditions could offer complementary insights. The role of different heating and cooling processes influencing line emissions in various phases of the ISM also warrants deeper exploration to develop more robust and universally applicable SFR tracers.