[Paper Review] Nearby early-type galaxies with ionized gas.IV. Origin and powering mechanism of the ionized gas

F. Annibali, A. Bressan+4 more|arXiv (Cornell University)|Apr 9, 2010

Galaxies: Formation, Evolution, Phenomena130 references84 citations

TL;DR

This study investigates the origin and ionization mechanisms of ionized gas in 65 nearby early-type galaxies (ETGs), using intermediate-resolution optical spectroscopy to disentangle stellar continuum and emission lines. It finds that 89% show emission lines, with 72% classified as LINERs, primarily powered by low-accretion-rate AGN or shocks, while nebular metallicity is ~0.2 dex lower than stellar metallicity, suggesting external gas origin or underpredicted oxygen yields in models.

ABSTRACT

[ABRIDGED] With the aim of constraining the source of excitation and the origin of the ionized gas in early-type galaxies (ETGs), we analyzed optical spectra of a sample of 65 ETGs mostly located in low density environments. Optical emission lines are detected in 89% of the sample. The incidence and strength of emission do not correlate either with the E/S0 classification, or with the fast/slow rotator classification. Comparing the nuclear r

Motivation & Objective

• To understand the origin and excitation mechanism of ionized gas in early-type galaxies (ETGs), which are traditionally considered gas-poor and inactive.
• To resolve the long-standing puzzle of why a significant fraction of ETGs exhibit optical emission lines despite lacking strong star formation.
• To distinguish between photoionization by post-AGB stars, low-accretion-rate AGN, and shock excitation as the dominant ionization sources.
• To determine the metallicity of the ionized gas and compare it with stellar metallicities to infer gas origin.
• To assess the role of environmental and structural factors (e.g., velocity dispersion, kinematic peculiarities) in shaping ionization properties.

Proposed method

• A new spectral fitting procedure was developed to accurately subtract the underlying stellar continuum, accounting for age-metallicity degeneracy in stellar population synthesis.
• Emission lines were extracted from spectra in annuli of increasing galacto-centric radius to map spatial variations in line strength and ratios.
• Diagnostic diagrams, including [OIII]/Hβ vs. [NII]/Hα, were used to classify ionization mechanisms (LINER, Seyfert, Composite, etc.).
• Nuclear emission (r < re/16) was compared with extended emission to assess radial trends in excitation and metallicity.
• Nebular oxygen abundance was derived and compared with stellar metallicities from prior analysis (Paper III) to assess gas origin.
• Shock models were tested using [SII]6717/6731 ratios to constrain gas density and shock velocity.

Experimental results

Research questions

• RQ1 What is the dominant ionization mechanism responsible for the observed optical emission lines in early-type galaxies?
• RQ2 How does the incidence and strength of ionized gas emission correlate with galaxy structural or kinematic properties such as velocity dispersion or rotation class?
• RQ3 What is the metallicity of the ionized gas, and how does it compare to the metallicity of the underlying stellar population?
• RQ4 To what extent can post-AGB star photoionization explain the observed nuclear emission in LINERs and Composites?
• RQ5 Is the ionized gas in ETGs likely of internal or external origin, based on metallicity and kinematic data?

Key findings

• Optical emission lines were detected in 89% of the sample, with 57% showing strong emission (EW(Hα + [NII]) > 3 Å), indicating a high incidence of ionized gas.
• 72% of emitting galaxies are classified as LINERs, 9% as Seyferts, 12% as Composites/Transitions, and 7% as non-classified, with Seyferts showing younger luminosity-weighted ages (≤5 Gyr) than other types.
• The [NII]/Hα ratio increases with central velocity dispersion (σc), and emission line strength spreads more widely in high-σc galaxies, suggesting a link between galaxy mass and ionization complexity.
• The [NII]/Hα ratio decreases with galacto-centric radius, indicating either decreasing nebular metallicity or a 'softening' of the ionizing spectrum at larger radii.
• The average nebular oxygen abundance is slightly below solar, and is approximately 0.2 dex lower than the stellar metallicity, suggesting either external gas accretion or underprediction of oxygen yields in supernova models.
• For the majority of LINERs/Composites, photoionization by post-AGB stars alone cannot explain the nuclear emission; instead, low-accretion-rate AGN, fast shocks (200–500 km/s), or a combination of both are more consistent with the data.