[Paper Review] The impact on the general properties of HII galaxies in the local universe of the visibility of the [OIII]l 4363 line

C. Hoyos, Á. I. Díaz|CERN Bulletin|Nov 2, 2005

Impact of Light on Environment and Health1 references18 citations

TL;DR

This study analyzes a large sample of local HII galaxies to compare those with and without the [O III]𝜆4363 auroral emission line, finding that galaxies lacking this line are more luminous, metal-rich, and have lower ionization and older, more massive underlying stellar populations. The results suggest that high-redshift Luminous Compact Blue Galaxies (LCBGs) are more similar to local HII galaxies without the [O III]𝜆4363 line, implying they host older, more massive, and metal-rich star clusters.

ABSTRACT

We present a statistical study of a very large sample of HII galaxies taken from the literature. We focus on the differences in several properties between galaxies that show the auroral line [OIII]l4363 and those that do not present this feature in their spectra. It turns out that objects without this auroral line are more luminous, more metal-rich and present a lower ionization degree. The underlying population is found to be much more important for objects without the [OIII]l 4363 line, and the effective temperature of the ionizing star clusters of galaxies not showing the auroral line is probably lower. We also study the subsample of HII galaxies whose properties most closely resemble the properties of the intermediate-redshift population of Luminous Compact Blue Galaxies (LCBGs). The objects from this subsample are more similar to the objects not showing the [OIII]l 4363 line. It might therefore be expected that the intermediate-redshift population of LCBGs is powered by very massive, yet somewhat aged star clusters. The oxygen abundance of LCBGs would be greater than the average oxygen abundance of local HII galaxies.

Motivation & Objective

• To compare the physical properties of local HII galaxies that do and do not show the [O III]𝜆4363 auroral emission line.
• To identify a representative local control sample for high-redshift Luminous Compact Blue Galaxies (LCBGs), which are thought to be the high-z counterparts of local HII galaxies.
• To investigate whether the absence of the [O III]𝜆4363 line is a real diagnostic of higher metallicity and older stellar populations, rather than just a selection bias.
• To determine whether LCBG-like galaxies in the local universe resemble HII galaxies with or without the auroral line, for use in evolutionary modeling.
• To quantify the differences in ionization, metallicity, luminosity, and stellar population age between the two subsamples using statistical analysis of spectroscopic data.

Proposed method

• Compilation of a large spectroscopic sample of local HII galaxies from multiple literature sources, including emission line measurements and photometric data.
• Separation of the sample into two subsamples: those with detectable [O III]𝜆4363 line (Sub1) and those without (Sub2), based on spectral detection thresholds.
• Statistical comparison of key physical properties (luminosity, metallicity, ionization, extinction, stellar mass) between Sub1 and Sub2 using probability density functions and dot product analysis.
• Use of the [O III]/[O II] line ratio and the O3N2 index to estimate oxygen abundance and ionization parameter, with calibration from empirical and theoretical relations.
• Application of the dot product method to compare the LCBG-like subsample with Sub1 and Sub2, quantifying similarity in distribution shapes.
• Assumption of a flat cosmology (H₀ = 70 km s⁻¹ Mpc⁻¹) for distance and luminosity corrections.

Experimental results

Research questions

• RQ1 How do the luminosities, metallicities, and ionization parameters of HII galaxies differ between those that show and do not show the [O III]𝜆4363 auroral line?
• RQ2 Is the absence of the [O III]𝜆4363 line in HII galaxies a real indicator of higher metallicity and older stellar populations, or is it due to observational limitations?
• RQ3 To which subsample—HII galaxies with or without the [O III]𝜆4363 line—are Luminous Compact Blue Galaxies (LCBGs) at intermediate redshift most similar in terms of physical properties?
• RQ4 What are the implications of the [O III]𝜆4363 non-detection for the age, mass, and metallicity of the ionizing and underlying stellar populations in HII galaxies?
• RQ5 Can the observed differences in line ratios and luminosities be explained by metallicity effects alone, or are evolutionary effects (e.g., cluster aging) required?

Key findings

• HII galaxies without the [O III]𝜆4363 line are, on average, 0.4 dex more metal-rich than those with the line, indicating a real metallicity effect beyond selection bias.
• Galaxies without the auroral line are more luminous, with significantly higher H𝛽 luminosities, and exhibit lower ionization levels as indicated by lower [O III]/[O II] ratios.
• The underlying stellar population is more massive in galaxies without the [O III]𝜆4363 line, and the ionizing star clusters are likely older and more massive, as inferred from lower excitation and higher extinction.
• The LCBG-like subsample is statistically more similar to the subsample without the [O III]𝜆4363 line, with dot product values (e.g., 1.228 for B magnitude, 1.123 for stellar mass) indicating stronger similarity than to the subsample with the auroral line.
• The absence of the [O III]𝜆4363 line is a strong indicator of high metallicity, low ionization, and evolved stellar populations, suggesting that high-redshift LCBGs are likely powered by older, massive, and metal-rich star clusters.
• Despite higher luminosity and metallicity, LCBG-like galaxies do not show a strong correlation between metallicity and ionization, indicating that ionization differences are driven by stellar population age and mass, not metallicity alone.