The low-frequency flattening of the radio spectrum of giant H II regions in M 101
Artikel i vetenskaplig tidskrift, 2025

Context. In galaxies, the flattening of the spectrum at low radio frequencies below 300 MHz has been the subject of some debate. A turnover at low frequencies could be caused by multiple physical processes, which can yield new insights into the properties of the ionised gas in the interstellar medium. Aims. We investigate the existence and nature of the low-frequency turnover in the H II regions of M 101. Methods. We study the nearby galaxy M 101 using the LOw Frequency ARray (LOFAR) at frequencies of 54 and 144 MHz, Apertif at 1370 MHz, and published combined map from the Very Large Array (VLA) and Effelesberg telescope at 4850 MHz. Results. The spectral index between 54 and 144 MHz is inverted at the centres of H II regions. We find a significant low-frequency flattening at the centres of five out of six H II regions that we selected for this study. Conclusions. The low frequency flattening in H II regions of M 101 can be explained with two different free-free absorption models. The flattening is localised in a region smaller than 1.5 kpc and can only be detected with high resolution (better than 45″). The detection of low frequency flattening has important consequences for using radio continuum observations below 100 MHz to measure extinction-free star-formation rates.

Galaxies: ISM

Galaxies: individual: M 101

Radio continuum: galaxies

Radiation mechanisms: general

H II regions

Författare

Lovorka Gajovic

Universität Hamburg

Volker Heesen

Universität Hamburg

Marcus Brüggen

Universität Hamburg

H. W. Edler

Netherlands Institute for Radio Astronomy (ASTRON)

Universität Hamburg

Björn Adebahr

Netherlands Institute for Radio Astronomy (ASTRON)

Ruhr-Universität Bochum

Thomas Pasini

Istituto di Radioastronomia

Francesco de Gasperin

Istituto di Radioastronomia

Aritra Basu

Thüringer Landessternwarte Tautenburg

Max-Planck-Gesellschaft

Marek Weżgowiec

Uniwersytet Jagiellonski w Krakowie

Cathy Horellou

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

Dominik Bomans

Netherlands Institute for Radio Astronomy (ASTRON)

Helga Dénes

Universidad Yachay Tech

Dany Vohl

Netherlands Institute for Radio Astronomy (ASTRON)

Universiteit Van Amsterdam

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 695 A41

Ämneskategorier (SSIF 2025)

Astronomi, astrofysik och kosmologi

DOI

10.1051/0004-6361/202453003

Mer information

Senast uppdaterat

2025-03-21