[Paper Review] Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work

Flores-Fajardo, Nahiely, Morisset, Christophe+2 more|arXiv (Cornell University)|Apr 4, 2011

Galaxies: Formation, Evolution, Phenomena102 citations

TL;DR

This paper proposes that hot low-mass evolved stars (HOLMES) are the dominant ionizing source for the extraplanar diffuse ionized gas (eDIG) in NGC 891, explaining the observed increase in [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα with galactic latitude. Using a finely meshed grid of photoionization models combining HOLMES and massive OB stars, the study finds that electron density decreases with height, HOLMES dominate at high z only under solar metallicity, and N/O increases with z, indicating enrichment by HOLMES progenitors.

ABSTRACT

We revisit the question of the ionization of the diffuse medium in late type galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The most important challenge for the models considered so far was the observed increase of [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha with increasing distance to the galactic plane. We propose a scenario based on the expected population of massive OB stars and hot low-mass evolved stars (HOLMES) in this galaxy to explain this observational fact. In the framework of this scenario we construct a finely meshed grid of photoionization models. For each value of the galactic latitude z we look for the models which simultaneously fit the observed values of the [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha ratios. For each value of z we find a range of solutions which depends on the value of the oxygen abundance. The models which fit the observations indicate a systematic decrease of the electron density with increasing z. They become dominated by the HOLMES with increasing z only when restricting to solar oxygen abundance models, which argues that the metallicity above the galactic plane should be close to solar. They also indicate that N/O increases with increasing z.

Motivation & Objective

• To resolve the long-standing puzzle of why emission line ratios like [O iii]/Hβ and [N ii]/Hα increase with distance from the galactic plane in late-type galaxies.
• To test whether hot low-mass evolved stars (HOLMES), previously overlooked, can explain the ionization of the extraplanar diffuse ionized gas (eDIG).
• To construct a self-consistent photoionization model using both massive OB stars and HOLMES as ionizing sources in NGC 891.
• To constrain the electron density, metallicity, and N/O abundance ratio in the eDIG as a function of galactic latitude z.
• To assess whether HOLMES can account for the observed line ratios without requiring additional ionization mechanisms like shocks or cosmic rays.

Proposed method

• Constructed a finely meshed grid of photoionization models using combined ionizing radiation fields from massive OB stars and HOLMES, based on known stellar population models for NGC 891.
• For each galactic latitude z, selected models that simultaneously reproduce observed [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα ratios.
• Treated oxygen abundance (O/H) and nitrogen-to-oxygen ratio (N/O) as free parameters to explore chemical degeneracy in the eDIG.
• Used the observed Hβ line to correct for reddening and stellar absorption, enabling accurate flux ratio measurements.
• Compared model outputs with deep spectroscopic data from Otte et al. (2001) along a slit perpendicular to the galactic plane.
• Conducted a sensitivity analysis in the appendix to assess the impact of small grains and [Ne iii]/[Ne ii] line ratios, confirming robustness of main conclusions.

Experimental results

Research questions

• RQ1 Can the observed increase in [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα with galactic height in NGC 891 be explained by a combination of massive OB stars and HOLMES as ionizing sources?
• RQ2 What is the relative contribution of HOLMES to the ionizing radiation field at increasing distances from the galactic plane?
• RQ3 Does the observed variation in line ratios imply a change in electron density, metallicity, or N/O abundance with height?
• RQ4 Is the metallicity of the eDIG consistent with solar values, as suggested by HOLMES dominance at high z?
• RQ5 Can HOLMES alone explain the ionization of the eDIG without invoking shocks, cosmic rays, or other non-stellar mechanisms?

Key findings

• Solutions exist for all galactic latitudes z, indicating that the model can reproduce the observed line ratios across the entire extraplanar region.
• Electron density systematically decreases with increasing z, consistent with ionization by a harder radiation field at larger heights.
• Under the assumption of solar oxygen abundance, HOLMES become the dominant ionizing source at high z, suggesting the eDIG metallicity is close to solar.
• The N/O ratio increases with z, indicating progressive enrichment of the eDIG by HOLMES progenitors over time.
• The model successfully reproduces the observed [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα ratios without requiring additional ionization mechanisms.
• Discrepancies in [Ne iii]/[Ne ii] ratios are attributed to uncertainties in stellar spectral energy distributions at high energies, not to flaws in the HOLMES-based scenario.