[Paper Review] The ionized gas in the CALIFA early-type galaxies I. Mapping two representative cases: NGC 6762 and NGC 5966

C. Kehrig, A. Monreal‐Ibero+19 more|arXiv (Cornell University)|Feb 2, 2012

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy66 references74 citations

TL;DR

This study uses integral field spectroscopy from the CALIFA survey to analyze the ionized gas in two early-type galaxies, NGC 6762 and NGC 5966, revealing spatially homogeneous low-excitation emission with LINER-like line ratios. In NGC 6762, photoionization by post-AGB stars explains the gas; in NGC 5966, an extended ionized gas structure suggests a possible ionization cone or decoupled rotating disk, requiring a nuclear source beyond pAGB stars.

ABSTRACT

As part of the CALIFA survey, we conducted a thorough 2D analysis of the ionized gas in two E/S0 galaxies, NGC6762 and NGC5966, aiming to shed light on the nature of their warm ionized ISM. We present optical IFS obtained with the PMAS/PPAK spectrophotometer. To recover the nebular lines, we subtracted the underlying stellar continuum from the observed spectra using the STARLIGHT code. In NGC6762, the ionized gas and stellar emission display similar morphologies, while the emission line morphology is elongated in NGC5966, spanning ~6 kpc, and is oriented roughly orthogonal to the major axis of the stellar continuum ellipsoid. Whereas gas and stars are kinematically aligned in NGC6762, the gas is kinematically decoupled from the stars in NGC5966. A decoupled rotating disk or an ionization cone are two possible interpretations of the elongated ionized gas structure in NGC5966. The latter would be the first ionization cone of such a dimension detected within a weak emission-line galaxy. Both galaxies have weak emission-lines relative to the continuum [EW(Ha)< 3 A] and have low excitation, log([OIII]5007/Hb) < 0.5. Based on optical diagnostic ratios ([OIII]5007/Hb,[NII]6584/Ha,[SII]6717,6731/Ha,[OI]6300/Ha), both objects contain a LINER nucleus and an extended LINER-like gas emission. The emission line ratios do not vary significantly with radius or aperture, which indicates that the nebular properties are spatially homogeneous. The gas emission in NGC6762 can be best explained by photoionization by pAGB stars without the need of invoking any other excitation mechanism. In the case of NGC5966, the presence of a nuclear ionizing source seems to be required to shape the elongated gas emission feature in the ionization cone scenario, although ionization by pAGB stars cannot be ruled out.(abridged)

Motivation & Objective

• To investigate the nature of warm ionized ISM in early-type galaxies using high-resolution spatially resolved spectroscopy.
• To determine the ionization source(s) responsible for low-excitation nebular emission in ETGs with weak Hα lines.
• To assess whether ionization cones or kinematically decoupled gas disks can explain extended ionized gas morphologies in ETGs.
• To test the hypothesis that post-AGB stars alone can account for ionization in quiescent ETGs.

Proposed method

• Obtained optical integral field spectroscopy (3745–7300 Å) with PMAS/PPAK at Calar Alto Observatory, covering 1 arcmin × 1 arcmin fields.
• Modelled and subtracted the stellar continuum using the STARLIGHT spectral synthesis code to isolate pure emission-line spectra.
• Measured nebular line ratios ([O iii]λ5007/Hβ, [N ii]λ6584/Hα, [S ii]λ6717,6731/Hα, [O i]λ6300/Hα) to classify ionization mechanisms.
• Compared observed line ratios with diagnostic diagrams to identify ionization sources (e.g., LINER, AGN, shocks).
• Analyzed spatial and kinematic alignment between ionized gas and stellar components to infer dynamical history.
• Assessed energetic consistency between ionizing luminosity and observed gas emission to constrain ionization mechanisms.

Experimental results

Research questions

• RQ1 What is the origin of the extended, low-excitation ionized gas in NGC 6762 and NGC 5966?
• RQ2 Can post-AGB stars alone explain the observed ionization in these ETGs, or is an additional source required?
• RQ3 Is the elongated ionized gas structure in NGC 5966 best explained by an ionization cone or a kinematically decoupled rotating disk?
• RQ4 How do the spatially resolved line ratios in these galaxies compare to standard diagnostic diagrams for ionization sources?
• RQ5 What is the role of nuclear activity or shocks in shaping the ionized gas morphology in weak emission-line ETGs?

Key findings

• Both NGC 6762 and NGC 5966 exhibit weak Hα emission with equivalent widths ≤ 3 Å and low excitation, as indicated by log([O iii]λ5007/Hβ) ≤ 0.5.
• The emission-line ratios are spatially homogeneous across both galaxies, indicating consistent ionization conditions over large radii.
• In NGC 6762, the gas and stellar emission are morphologically and kinematically aligned, supporting photoionization by post-AGB stars as the dominant mechanism.
• In NGC 5966, the ionized gas forms an elongated structure (~6 kpc) orthogonal to the stellar major axis and is kinematically decoupled, suggesting a non-stellar origin.
• The energetic budget in NGC 5966 implies an additional ionizing source beyond post-AGB stars, with a possible nuclear radio source indicating AGN activity.
• An ionization cone scenario is plausible for NGC 5966, marking the first potential detection of such a feature in a weak emission-line galaxy.