eDIG-CHANGES

III. The lagging eDIG revealed by multi-slit spectroscopy of NGC 891

¹ Department of Astronomy, Xiamen University, 422 Siming South Road, Xiamen 361005, China
² Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
³ Department of Astronomy and Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁴ Department of Physics, University of Wisconsin–Whitewater, 800 West Main Street, Whitewater, WI 53190, USA
⁵ Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109-1107, USA
⁶ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁷ Department of Physics & Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
⁸ CSIRO, Space and Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁹ School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
¹⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA

^⋆ Corresponding author; pandataotao@gmail.com

Received: 3 August 2024
Accepted: 1 October 2024

Abstract

Context. The kinematic information of the extraplanar diffuse ionized gas (eDIG) around galaxies provides clues to the origin of the gas.

Aims. The eDIG-CHANGES project studies the physical and kinematic properties of the eDIG around the CHANG-ES sample of nearby edge-on disk galaxies.

Methods. We use a novel multi-slit narrow-band spectroscopy technique to obtain the spatial distribution of the spectral properties of the ionized gas around NGC 891, which is often regarded as an analogue of the Milky Way. We developed specific data reduction procedures for the multi-slit narrow-band spectroscopy data taken with the MDM 2.4 m telescope. The data presented in this paper cover the Hα and [N II]λλ6548, 6583 Å emission lines.

Results. The eDIG traced by the Hα and [N II] lines shows an obvious asymmetric morphology, being brighter in the northeastern part of the galactic disk and extending a few kiloparsecs above and below the disk. Global variations in the [N II]/Hα line ratio suggest additional heating mechanisms for the eDIG at large heights beyond photoionization. We also construct position-velocity (PV) diagrams of the eDIG based on our optical multi-slit spectroscopy data and compare them to similar PV diagrams constructed with the H I data. The dynamics of the two gas phases are generally consistent with each other. Modelling the rotation curves at different heights from the galactic mid-plane suggests a vertical negative gradient in turnover radius and maximum rotation velocity, with magnitudes of approximately 3 kpc kpc⁻¹ and 22 − 25 km s⁻¹ kpc⁻¹, respectively.

Conclusions. Our measured vertical gradients of the rotation curve parameters suggest significant differential rotation of the ionized gas in the halo, often referred to as the lagging eDIG. Systematic study of the lagging eDIG, using the multi-slit narrow-band spectroscopy technique developed in our eDIG-CHANGES project, will help us to better understand the dynamics of the ionized gas in the halo.