Nearby early-type galaxies with ionized gas

Author(s)

Francesca Annibali, A. Bressan, R. Rampazzo, W. W. Zeilinger, O. Vega, P. Panuzzo

Abstract

Aims. A significant fraction of early-type galaxies (ETGs) exhibit emission lines in their optical spectra. We attempt to identify the producing the emission mechanism and the ionized gas in ETGs, and its connection with the host galaxy evolution. Methods. We analyzed intermediate-resolution optical spectra of 65 ETGs, mostly located in low density environments and exhibiting spectros-copic diagnostic lines of ISM from which we had previously derived stellar population properties. To extract the emission lines from the galaxy spectra, we developed a new fitting procedure that accurately subtracts the underlying stellar continuum, and accounts for the uncertainties caused by the age-metallicity degeneracy. Results. Optical emission lines are detected in 89% of the sample. The incidence and strength of emission correlate with neither the E/S0 classification, nor the fast/slow rotator classification. By means of the classical [OIII]/Hβ versus [NII]/Hα diagnostic diagram, the nuclear galaxy activity is classified such that 72% of the galaxies with emission are LINERs, 9% are Seyferts, 12% are composite/transition objects, and 7% are non-classified. Seyferts have young luminostiy-weighted ages (≲ 5 Gyr), and appear, on average, significantly younger than LINERs and composites. Excluding the Seyferts from our sample, we find that the spread in the ([OIII], Hα, or [NII]) emission strength increases with the galaxy central velocity dispersion σc. Furthermore, the [NII]/Hα ratio tends to increase with σc. The [NII]/Hα ratio decreases with increasing galactocentric distance, indicative of either a decrease in the nebular metallicity, or a progressive "softening" of the ionizing spectrum. The average nebular oxygen abundance is slightly less than solar, and a comparison with the results obtained in Paper III from Lick indices shows that it is ≈ 0.2 dex lower than that of stars. Conclusions. The nuclear (r < r_e/16) emission can be attributed to photoionization by PAGB stars alone only for ≈ 22% of the LINER/composite sample. On the other hand, we cannot exclude an important role of PAGB star photoionization at larger radii. For the major fraction of the sample, the nuclear emission is consistent with excitation caused by either a low-accretion rate AGN or fast shocks (200-500 km s^-1) in a relatively gas poor environment (≲ 100 cm^-3), or both. The derived [SII]6717/6731 ratios are consistent with the low gas densities required by the shock models. The derived nebular metallicities are indicative of either an external origin of the gas, or an overestimate of the oxygen yields by SN models.

Organisation(s)

Department of Astrophysics

External organisation(s)

Osservatorio Astronomico, Scuola Internazionale Superiore di Studi Avanzati, National Institute of Astrophysics, Optics and Electronics (INAOE), Université Paris Saclay

Journal

Astronomy and Astrophysics

Volume

519

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/200913774

Publication date

09-2010

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/331dabf6-16ca-42fa-bbad-ee05a749c7b9