The SAURON project - XVI. On the sources of ionization for the gas in elliptical and lenticular galaxies

Abstract

Following our study on the incidence, morphology and kinematics of the ionized gas in earlytype galaxies, we now address the question of what is powering the observed nebular emission. To constrain the likely sources of gas excitation, we resort to a variety of ancillary data we draw from complementary information on the gas kinematics, stellar populations and galactic potential from the SAURON data, and use the SAURON-specific diagnostic diagram juxtaposing the [O III] λ5007/Hβ and [N I] λλ5197, 5200/Hβ line ratios. We find a tight correlation between the stellar surface brightness and the flux of the Hβ recombination line across our sample, which points to a diffuse and old stellar source as the main contributor of ionizing photons in early-type galaxies, with post-asymptotic giant branch (pAGB) stars being still the best candidate based on ionizing balance arguments. The role of AGN photoionization is confined to the central 2-3 arcsec of an handful of objects with radio or X-ray cores. OB-stars are the dominant source of photoionization in 10 per cent of the SAURON sample, whereas for another 10 per cent the intense and highly ionized emission is powered by the pAGB population associated to a recently formed stellar subcomponent. Fast shocks are not an important source of ionization for the diffuse nebular emission of early-type galaxies since the required shock velocities can hardly be attained in the potential of our sample galaxies. Finally, in the most massive and slowly or non-rotating galaxies in our sample, which can retain a massive X-ray halo, the finding of a spatial correlation between the hot and warm phases of the interstellar medium (ISM) suggests that the interaction with the hot ISM provides an additional source of ionization besides old ultraviolet-bright stars. This is also supported by a distinct pattern

Characterizing the chemically enriched circumgalactic medium of ∼38 000 luminous red galaxies in SDSS DR12

“…The observed LINER-like spectra can be explained by photo-ionization due to post-asymptotic giant branch (post-AGB) stars (e.g., Binette et al 1994). This is motivated by recent observations that have uncovered spatially extended LINER signals in passive red galaxies with a surface brightness profile shallower than r −2 (e.g., Sarzi et al 2006Sarzi et al , 2010Yan & Blanton 2012;Singh et al 2013). The shallow, extended surface brightness profiles are inconsistent with the gas being ionized by a central point source, but indicate a spatially distributed ionizing source.…”

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…While most LINERs appear to be driven by old stellar populations (e.g., Sarzi et al 2010) and by shocks in ULIRGs (e.g., Soto & Martin 2010, 2012, in some cases they can be associated with low-luminosity AGNs (e.g., Ho 2008). Yuan et al (2010) have recently used a new semi-empirical optical spectral classification to classify IR-selected galaxies based on three diagrams: Kewley et al (2006) to separate starburst galaxies, starburst/AGN composite galaxies, Seyfert 2s, and LINERs.…”

The SAMI Galaxy Survey: spatially resolving the environmental quenching of star formation in GAMA galaxies

“…For those that are not inclined to our line of sight, the contribution to the total flux is generally low enough as to not be detected. Galaxies with spatially extended line emission excited by old stellar populations (Sarzi et al 2010;Belfiore et al 2016) do not satisfy the EWHα criterion.…”