A spatially resolved radio spectral study of the galaxy M 51^★

¹ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: lovorka.gajovic@hs.uni-hamburg.de
² Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstrasse 150, 44801 Bochum, Germany
³ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
⁷ Departamento de Física de la Tierra y Astrofísica, Instituto de Física de Partículas y del Cosmos, IPARCOS, Universidad Complutense de Madrid (UCM), 28040, Madrid, Spain
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Coor. de Astrofísica, Luis Enrique Erro No.1, Tonantzintla, Puebla, México C.P. 72840, Mexico
¹⁰ Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, A.P. 3-72, C.P. 58089, Mexico
¹¹ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

Received: 13 November 2023
Accepted: 16 June 2024

Abstract

Context. Radio continuum emission from galaxies at gigahertz frequencies can be used as an extinction-free tracer of star formation. However, at frequencies of a few hundred megahertz, there is evidence for low-frequency spectral flattening.

Aims. We wish to understand the origin of this low-frequency flattening better, and to this end, we performed a spatially resolved study of the nearby spiral galaxy M51. We explored the different effects that can cause a flattening of the spectrum towards lower frequencies, such as free–free absorption and cosmic-ray ionisation losses.

Methods. We used radio continuum intensity maps between 54 and 8350 MHz at eight different frequencies, with observations at 240 MHz from the Giant Metrewave Radio Telescope presented for the first time. We corrected for the contribution from thermal free–free emission using an H α map that was corrected for extinction with 24 μm data. We fitted free–free absorption models to the radio spectra to determine the emission measure (EM) as well as polynomial functions to measure the non-thermal spectral curvature. We also obtained a new extinction-corrected H α intensity map from the Metal-THINGS survey using integral field unit spectroscopy.

Results. The non-thermal low-frequency radio continuum spectrum between 54 and 144 MHz is very flat and even partially inverted, particularly in the spiral arms; in contrast, the spectrum at higher frequencies is typical for a non-thermal radio continuum spectrum. However, we did not find any correlation between the EMs calculated from radio and from H α observations; instead, the non-thermal spectral curvature weakly correlates with the H I gas-mass surface density. This suggests that cosmic-ray ionisation losses play an important role in the low-frequency spectral flattening.

Conclusions. The observed spectral flattening towards low frequencies in M51 is caused by a combination of ionisation losses and free–free absorption. The reasons for this flattening need to be understood in order to use sub-gigahertz frequencies as a tracer of star formation.