[Paper Review] ISM properties in low-metallicity environments I. mid-infrared spectra of dwarf galaxies

S. C. Madden, F. Galliano+2 more|arXiv (Cornell University)|Oct 4, 2005

Astrophysics and Star Formation StudiesPhysics and Astronomy81 references170 citations

TL;DR

This study analyzes mid-infrared (MIR) spectra of low-metallicity dwarf galaxies using ISO/ISOCAM to investigate the interstellar medium (ISM) properties. It finds that PAH emission is faint globally and spatially, while [Ne iii] and [S iv] lines dominate, indicating hard radiation fields; a strong correlation between PAH/VSG ratios and [Ne iii]/[Ne ii] ratios reveals that radiation field hardness, not just low metallicity, drives PAH destruction in these environments.

ABSTRACT

We present new ISOCAM mid-infrared spectra of three starbursting nearby dwarf galaxies, NGC1569, IIZw40, NGC1140 and the 30Dor region of the LMC and explore the properties of the ISM in low-metallicity environments, also using additional sources from the literature. We analyse the various components of the ISM probed by the mid-infrared observations and compare them with other Galactic and extragalactic objects. The MIR spectra of the low-metallicity starburst sources are dominated by the [NeIII] and [SIV] lines, as well as a steeply rising dust continuum. PAH bands are generaly faint, both locally and averaged over the full galaxy, in stark contrast to dustier starburst galaxies, where the PAH features are very prominant and even dominate on global scales. The hardness of the modeled interstellar radiation fields for the dwarf galaxies increases as the presence of PAH band emission becomes less pronounced. The [NeIII]/[NeII] ratios averaged over the full galaxy are strikingly high, often >10. Thus, the hard radiation fields are pronounced and pervasive. We find a prominent correlation between the PAHs/VSGs and the [NeIII]/[NeII] ratios for a wide range of objects, including the low metallicity galaxies as well as Galactic HII regions and other metal-rich galaxies. This effect is consistent with the hardness of the interstellar radiation field playing a major role in the destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGs and metallicity correlation, also found by Engelbracht et al. (2005) for a larger survey. Combined effects of metallicity and radiation field seem to be playing important roles in the observed behavior of PAHs in the low metallicity systems.

Motivation & Objective

• To characterize the mid-infrared interstellar medium (ISM) in low-metallicity dwarf galaxies using high-resolution ISOCAM spectroscopy.
• To determine the relative contributions of dust, gas lines, and PAHs to the MIR emission in these systems.
• To investigate how low metallicity and hard radiation fields affect the excitation and survival of polycyclic aromatic hydrocarbons (PAHs).
• To compare ISM properties in low-metallicity dwarfs with Galactic H II regions and metal-rich starbursts to identify universal trends.
• To assess the reliability of broad-band MIR photometry for interpreting ISM components in low-metallicity environments.

Proposed method

• Acquisition of ISOCAM mid-infrared spectra (5–16 µm) for three starbursting dwarf galaxies: NGC 1569, NGC 1140, II Zw 40, and the 30 Dor region.
• Spectral decomposition to isolate contributions from ionic emission lines (e.g., [Ne iii] λ15.56 µm, [S iv] λ10.51 µm), dust continuum (from very small grains), and PAH features.
• Spatially resolved analysis of NGC 1569 and 30 Dor using 2.4 pc and 106 pc resolution to map the distribution of ISM components.
• Calculation of [Ne iii]/[Ne ii] line ratio as a proxy for radiation field hardness across the sample.
• Correlation analysis between PAH/VSG emission ratios and [Ne iii]/[Ne ii] ratios across diverse objects, including Galactic H II regions and metal-rich galaxies.
• Comparison with literature data and dust SED modeling to assess dust abundance and extinction effects in low-metallicity systems.

Experimental results

Research questions

• RQ1 How do the mid-infrared spectral characteristics of low-metallicity dwarf galaxies differ from those of more metal-rich starburst or spiral galaxies?
• RQ2 What is the relative contribution of PAHs, very small grains (VSGs), and ionic emission lines to the MIR flux in low-metallicity environments?
• RQ3 To what extent does the hardness of the interstellar radiation field, rather than metallicity alone, control PAH destruction in these galaxies?
• RQ4 How do spatially resolved MIR maps of NGC 1569 and 30 Dor reveal the relationship between radiation field, ionized gas, and dust emission?
• RQ5 Is there a universal correlation between PAH/VSG ratios and [Ne iii]/[Ne ii] ratios across different metallicity and star formation environments?

Key findings

• The MIR spectra of low-metallicity dwarf galaxies are dominated by the [Ne iii] λ15.56 µm and [S iv] λ10.51 µm lines, with a steeply rising dust continuum, while PAH bands are faint both locally and globally.
• The [Ne iii]/[Ne ii] line ratio is exceptionally high in low-metallicity galaxies, often exceeding 10, indicating a pervasive and hard interstellar radiation field.
• A strong correlation exists between PAH/VSG ratios and [Ne iii]/[Ne ii] ratios across a wide range of objects, including low-metallicity dwarfs, Galactic H II regions, and metal-rich starbursts, spanning three orders of magnitude.
• In the 15 µm ISOCAM broad band, up to 10% of the flux can arise from the [Ne iii] line alone, and in active galaxies like II Zw 40, the 6.7 µm band captures roughly equal contributions from PAHs and hot dust continuum.
• In NGC 1569 and 30 Dor, the [Ne iii] emission forms a sharp ridge closer to the central ionizing source than [Ne ii], and PAH peaks avoid the hard radiation field peak, while VSGs prefer it.
• The spatial extent of [Ne ii] and [Ne iii] lines in NGC 1569 traces Hα ejecta, indicating strong coupling between ionizing radiation and ionized gas dynamics.