The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The kinematic diversity of stellar halos and the relation between halo transition scale and stellar mass

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: cpulsoni@mpe.mpg.de
² Excellence Cluster Universe, Boltzmannstraße 2, 85748 Garching, Germany
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, 100871 Beijing, PR China
⁵ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello, 16, 80131 Naples, Italy
⁶ School of Civil Engineering, The University of Sydney, NSW 2006, Australia
⁷ Department of Physics, Cornell University, Ithaca, 14853 New York, USA
⁸ University Observatory Munich, Scheinerstraße 1, 81679 Munich, Germany
⁹ University of Naples “Federico II”, Department of Physics “Ettore Pancini”, CU Monte Sant’Angelo, via Cinthia, 80126 Naples, Italy
¹⁰ Departamento de Astronomia, Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS 90040-060, Brazil
¹¹ Departamento de Astronomia, Instituto de Astronomia, Geofisica e Ciencias Atmosfericas da USP, Cidade Universitaria, CEP:05508900 Sao Paulo, Brazil
¹² Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, ACT 2611 Weston Creek, Australia
¹³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁴ School of Physics and Astronomy, The University of Nottingham, University Park, NG7 2RD Nottingham, UK
¹⁵ Department of Physics and Astronomy, San Jose State University, One Washington Square, San Jose, CA 95192, USA
¹⁶ University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA
¹⁷ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands

Received: 15 December 2017
Accepted: 16 July 2018

Abstract

Context. In the hierarchical two-phase formation scenario, the halos of early type galaxies (ETGs) are expected to have different physical properties from the galaxies’ central regions.

Aims. The ePN.S survey characterizes the kinematic properties of ETG halos using planetary nebulae (PNe) as tracers, overcoming the limitations of absorption line spectroscopy at low surface brightness.

Methods. We present two-dimensional velocity and velocity dispersion fields for 33 ETGs, including fast (FRs) and slow rotators (SRs). The velocity fields were reconstructed from the measured PN velocities using an adaptive kernel procedure validated with simulations, and extend to a median of 5.6 effective radii (R_e). We complemented the PN kinematics with absorption line data from the literature, for a complete description of the kinematics from the center to the outskirts.

Results. ETGs typically show a kinematic transition between inner regions and halo. Estimated transition radii in units of R_e anti-correlate with stellar mass. SRs have increased but still modest rotational support at large radii. Most of the FRs show a decrease in rotation, due to the fading of the inner disk in the outer, more slowly rotating spheroid. 30% of the FRs are dominated by rotation also at large radii. Most ETGs have flat or slightly falling halo velocity dispersion profiles, but 15% of the sample have steeply falling profiles. All of the SRs and 40% of the FRs show signatures of triaxial halos such as kinematic twists or misalignments. We show with illustrative photometric models that this is consistent with the distribution of isophote twists from extended photometry.

Conclusions. ETGs have more diverse kinematic properties in their halos than in the central regions. FRs do contain inner disk components but these frequently fade in outer spheroids which are often triaxial. The observed kinematic transition to the halo and its dependence on stellar mass is consistent with ΛCDM simulations and supports a two-phase formation scenario.