CHANG-ES

XXXIII. A 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217

¹ Hamburg University, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, 18008 Granada, Spain
³ Department of Physics, Engineering Physics, & Astronomy, Queens University, Kingston ON K7L 3N6, Canada
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, 2611 ACT, Australia
⁵ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
⁶ Department of Astronomy, University of Massachusetts, North Pleasant Street, Amherst, MA 01003-9305, USA
⁷ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁸ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

^⋆ Corresponding author; volker.heesen@hs.uni-hamburg.de

Received: 18 July 2024
Accepted: 12 September 2024

Abstract

Context. Cosmic rays may be dynamically very important in driving large-scale galactic winds. Edge-on galaxies give us an outsider’s view of radio haloes, and of their extra-planar cosmic-ray electrons and magnetic fields.

Aims. We present a new radio continuum imaging study of the nearby edge-on galaxy NGC 4217. We examine the distribution of extra-planar cosmic rays and magnetic fields. We observed it with both the Jansky Very Large Array (JVLA) in the S band (2–4 GHz) and the LOw Frequency ARray (LOFAR) at 144 MHz.

Methods. We measured vertical intensity profiles and exponential scale heights. We re-imaged both the JVLA and LOFAR data at matched angular resolution in order to measure radio spectral indices between 144 MHz and 3 GHz. Confusing point-like sources were subtracted prior to imaging. We then fitted intensity profiles with cosmic-ray electron advection models, using an isothermal wind model that is driven by a combination of pressure from the hot gas and cosmic rays.

Results. We discover a large-scale radio halo on the north-western side of the galactic disc. The morphology is reminiscent of a bubble extending up to 20 kpc from the disc. We find spectral ageing in the bubble, which allowed us to measure the advection speeds of the cosmic-ray electrons, which accelerate from 300 to 600 km s⁻¹. Assuming energy equipartition between the cosmic rays and the magnetic field, we estimate the bubble may have been inflated by a modest 10% of the kinetic energy injected by supernovae over its dynamical timescale of 35 Myr. While no active galactic nucleus (AGN) has been detected, such activity in the recent past cannot be ruled out.

Conclusions. Non-thermal bubbles with sizes of tens of kiloparsecs may be a ubiquitous feature of star-forming galaxies, and if so this would demonstrate the influence of feedback. Determining possible contributions by AGN feedback will require deeper observations.