Warm ionized gas in CALIFA early-type galaxies

2D emission-line patterns and kinematics for 32 galaxies

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: jean@astro.up.pt
² Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n Aptdo. 3004, 18080 Granada, Spain
³ Institut d’Astrophysique de Paris, UMR 7095, CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁴ Centro Astronómico Hispano Alemán de Calar Alto (CSIC-MPG), C/ Jesús Durbán Remón 2-2, 4004 Almería, Spain
⁵ University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁶ Estación Experimental de Zonas Aridas (CSIC), Ctra. de Sacramento s.n., La Cañada, Almería, Spain
⁷ Sydney Institute for Astronomy, University of Sydney, NSW 2006, Australia
⁸ Millennium Institute of Astrophysics, Chile
⁹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁰ Astronomical Institute of the Ruhr-University Bochum, Universitätsstr. 150, 44580 Bochum, Germany
¹¹ RUB Research Department “Plasmas with Complex Interactions”, Universitätsstr. 150, 44580 Bochum, Germany
¹² Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis E. Erro 1, 72840 Tonantzintla, Puebla, Mexico
¹³ Departamento de Física, Universidade Federal de Santa Catarina, PO Box 476, 88040-900, Florianópolis, SC, Brazil
¹⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ Instituto de Astrofísica de Canarias, vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁶ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
¹⁷ CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain
¹⁸ CEI Campus Moncloa, UCM-UPM, Departamento de Astrofísica y CC. de la Atmósfera, Facultad de CC. Físicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain
¹⁹ Department of Physics, Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
²⁰ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
²¹ Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia

Received: 26 February 2015
Accepted: 2 November 2015

Abstract

Context. The morphological, spectroscopic, and kinematical properties of the warm interstellar medium (wim) in early-type galaxies (ETGs) hold key observational constraints to nuclear activity and the buildup history of these massive, quiescent systems. High-quality integral field spectroscopy (IFS) data with a wide spectral and spatial coverage, such as those from the CALIFA survey, offer an unprecedented opportunity for advancing our understanding of the wim in ETGs.

Aims. This article centers on a 2D investigation of the wim component in 32 nearby (≲150 Mpc) ETGs from CALIFA, complementing a previous 1D analysis of the same sample.

Methods. The analysis presented here includes Hα intensity and equivalent width (EW) maps and radial profiles, diagnostic emission-line ratios, and ionized-gas and stellar kinematics. It is supplemented by τ-ratio maps, which are a more efficient means to quantify the role of photoionization by the post-AGB stellar component than alternative mechanisms (e.g., AGN, low-level star formation).

Results. Confirming and strengthening our previous conclusions, we find that ETGs span a broad continuous sequence in the properties of their wim, exemplified by two characteristic classes. The first (type i) comprises systems with a nearly constant EW(Hα) in their extranuclear component, which quantitatively agrees with (but is no proof of) the hypothesis that photoionization by the post-AGB stellar component is the main driver of extended wim emission. The second class (type ii) stands for virtually wim-evacuated ETGs with a very low (≤0.5 Å), outwardly increasing EW(Hα). These two classes appear indistinguishable from one another by their LINER-specific emission-line ratios in their extranuclear component. Here we extend the tentative classification we proposed previously by the type i+, which is assigned to a subset of type i ETGs exhibiting ongoing low-level star-forming activity in their periphery. This finding along with faint traces of localized star formation in the extranuclear component of several of our sample galaxies points to a non-negligible contribution by OB stars to the global ionizing photon budget in ETGs. Additionally, our data again highlight the diversity of ETGs in their gaseous and stellar kinematics. While in one half of our sample, gas and stars show similar (yet not necessarily identical) velocity patterns that are both dominated by rotation along the major galaxy axis, our analysis also documents several cases of kinematical decoupling between gas and stars, or rotation along the minor galaxy axis. We point out that the generally very low (≲1 Å) EW(Hα) of ETGs requires a careful quantitative assessment of potential observational and analysis biases in studies of their wim. With standard emission-line fitting tools, Balmer emission lines become progressively difficult to detect below an EW(Hα) ~ 3 Å, therefore our current understanding of the presence and 2D emission patterns and kinematics of the diffuse wim ETGs may be severely incomplete. We demonstrate that at the typical emission-line detection threshold of ~2 Å in previous studies, most of the extranuclear wim emission in an ETG may evade detection, which could in turn cause ETGs to be classified as entirely gas-devoid systems.

Conclusions. This study adds further observational evidence for a considerable heterogeneity among ETGs with regard to the physical properties and 2D kinematics of their extended wim component, and it clearly shows that a comprehensive understanding of these systems requires IFS studies over their entire optical extent.