[Paper Review] The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy

D. Cormier, S. C. Madden+18 more|CaltechAUTHORS (California Institute of Technology)|Feb 10, 2015

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy95 references87 citations

TL;DR

This study analyzes far-infrared fine-structure lines in 48 low-metallicity dwarf galaxies using Herschel PACS spectroscopy to reveal that their interstellar medium (ISM) has a highly porous structure with low filling factors of dense gas and high volume-filling factors of diffuse gas. The key finding is that enhanced [C ii], [O i], and [O iii] line-to-TIR luminosity ratios—driven by moderate FUV fields and low PDR covering factors—indicate a leaky ISM where UV photons escape easily, distinguishing it from metal-rich galaxies.

ABSTRACT

The far-infrared (FIR) lines are key tracers of the physical conditions of the interstellar medium (ISM) and are becoming workhorse diagnostics for galaxies throughout the universe. Our goal is to explain the differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies. We present Herschel PACS spectroscopic observations of the CII157um, OI63 and 145um, OIII88um, NII122 and 205um, and NIII57um fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of OIII/NII122 and NIII/NII122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the CII/OI63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the OI145/OI63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The OIII/OI63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found ~4 times higher in the dwarfs than in metal-rich galaxies. The high CII/LTIR, OI/LTIR, and OIII/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate FUV fields and low PDR covering factor. Harboring compact phases of low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that in metal-rich galaxies.

Motivation & Objective

• To understand how the physical conditions of the interstellar medium (ISM) in low-metallicity dwarf galaxies differ from those in metal-rich galaxies.
• To investigate the role of FIR fine-structure lines as tracers of ISM heating, cooling, and phase structure in star-forming regions.
• To determine how metallicity influences the excitation, filling factors, and luminosity ratios of key cooling lines such as [C ii] 157 μm, [O i] 63/145 μm, and [O iii] 88 μm.
• To assess the reliability of line ratios like [O iii]/[O i] and [O iii]/[N ii] as indicators of radiation field hardness and ionized gas filling factors independent of abundance variations.
• To model the observed line ratios using Cloudy radiative transfer simulations to infer the structural and physical properties of the ISM in low-metallicity environments.

Proposed method

• Acquired PACS spectroscopic observations of FIR fine-structure lines: [C ii] 157 μm, [O i] 63 and 145 μm, [O iii] 88 μm, [N ii] 122 and 205 μm, and [N iii] 57 μm in 48 low-metallicity dwarf galaxies from the Herschel Dwarf Galaxy Survey.
• Calculated line-to-TIR luminosity ratios (L_line / L_TIR) and line ratios (e.g., [O iii]/[O i], [C ii]/[N ii]) to probe ISM conditions and excitation mechanisms.
• Correlated observed line ratios with L_TIR, L_TIR/L_B, metallicity, and FIR colors to identify trends linked to ISM structure and radiation field properties.
• Used Cloudy radiative transfer models to simulate PDR and ionized gas conditions, varying parameters such as FUV field strength (G₀), gas density (n_H), and phase filling factors.
• Compared model predictions with observed line ratios to infer the covering factor of photodissociation regions (PDRs) and ionized gas phases.
• Assessed the contribution of ionized gas to [C ii] emission by comparing [N ii]/[C ii] ratios and using ionization structure models.

Experimental results

Research questions

• RQ1 How do FIR line luminosities and line-to-TIR ratios in low-metallicity dwarf galaxies compare to those in metal-rich galaxies?
• RQ2 What causes the high [O iii] 88 μm and [N iii] 57 μm emission relative to [N ii] lines in these dwarfs, and what does this imply about the ionizing radiation field?
• RQ3 To what extent do line ratios such as [C ii]/[O i] and [O iii]/[O i] reflect the filling factor of PDRs and ionized gas, respectively, in low-metallicity ISM?
• RQ4 How does the ISM structure—particularly the filling factor of dense versus diffuse gas—affect FIR line emission and the overall energy balance in dwarf galaxies?
• RQ5 Can [O iii]/[O i] serve as a robust, abundance-independent indicator of the ionized gas/PDR filling factor in low-metallicity environments?

Key findings

• The FIR lines collectively account for up to 3% of the total infrared luminosity (L_TIR), with [O iii] 88 μm being the brightest line on average, twice as bright as [C ii] 157 μm.
• The [C ii]/L_TIR ratio ranges from 0.04% to 0.7%, and the sum of [C ii] + [O i] / L_TIR ranges from 0.2% to 1.0%, indicating enhanced gas cooling relative to metal-rich galaxies.
• The [O iii]/[N ii] 122 ratio is 4 times higher in dwarfs than in metal-rich galaxies, indicating a harder radiation field and higher ionization parameter.
• The [O iii]/[O i] 63 ratio is a robust indicator of the ionized gas/PDR filling factor and is ~4 times higher in dwarfs, suggesting a larger volume-filling factor of ionized gas.
• The [C ii]/[N ii] 122 ratio is low (typically <15%), indicating minimal contribution of ionized gas to [C ii] emission, confirming [C ii] as a reliable PDR tracer.
• Radiative transfer modeling shows that the observed line ratios are best explained by a combination of moderate FUV fields and a low covering factor of dense PDR gas, resulting in a porous ISM structure with high diffuse gas volume filling factor.