Hα imaging observations of early-type galaxies from the ATLAS^3D survey^★,★★

¹ Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
e-mail: giuseppe.gavazzi@mib.infn.it
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
e-mail: mfossati@mpe.mpg.de
³ Universitäts-Sternwarte München, Schenierstrasse 1, 81679 München, Germany
⁴ Institute for Computational Cosmology, and Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
e-mail: michele.fumagalli@durham.ac.uk
⁵ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

Received: 30 April 2017
Accepted: 14 September 2017

Abstract

Context. The traditional knowledge of the mechanisms that brought to the formation and evolution of early type galaxies (ETG) in a hierarchical Universe was challenged by the unexpected finding by ATLAS^3D that 86% ETGs show signs of a fast rotating disk at their interior, implying an origin common to most spiral galaxies, followed by a quenching phase, while only a minority of the most massive systems are slow rotators and were likely to be the products of merger events.

Aims. Our aim is to improve our knowledge on the content and distribution of ionised hydrogen and their usage to form stars in a representative sample of ETGs for which the kinematics and detailed morphological classification were known from ATLAS^3D.

Methods. Using narrow-band filters centered on the redshifted Hα line along with a broad-band (r-Gunn) filter to recover the stellar continuum, we observed or collected existing imaging observations for 147 ETG (including members of the Virgo cluster), representative of the whole ATLAS^3D survey.

Results. 55 ETGs (37%) were detected in the Hα line above our detection threshold (HαEW ≤ −1 Å) and 21 harbour a strong source (HαEW ≤ −5 Å) .

Conclusions. The strong Hα emitters appear associated with mostly low-mass (M_* ~ 10¹⁰ M_⊙) S0 galaxies which contain conspicuous stellar and gaseous disks, harbouring significant star formation at their interior, including their nuclei. The weak Hα emitters are almost one order of magnitude more massive, contain gas-poor disks and harbour an AGN at their centers. Their emissivity is dominated by [NII] and does not imply star formation. The 92 undetected ETGs constitute the majority in our sample and are gas-free systems which lack a disk and exhibit passive spectra even in their nuclei. These pieces of evidence reinforce the conclusion of Cappellari (2016, ARA&A, 54, 597) that the evolution of ETGs followed the secular channel for the less massive systems and the dry merging channel for the most massive galaxies at the center of clusters of galaxies.