Low-Frequency Turnover Star Forming Galaxies I: Radio Continuum Observations and Global Properties

Abstract

The broad-band radio spectral energy distribution (SED) of star-forming galaxies (SFGs) contains a wealth of complex physics. We aim to determine the physical emission and loss processes causing radio SED curvature and steepening to see which observed global astrophysical properties are correlated with radio SED complexity. We have acquired radio continuum data between 70 MHz and 17 GHz for a sample of 19 southern local (z < 0.04) SFGs. Of this sample 11 are selected to contain low-frequency (< 300 MHz) turnovers (LFTOs) in their SEDs and eight are control galaxies with similar global properties. We model the radio SEDs for our sample using a Bayesian framework whereby radio emission (synchrotron and free-free) and absorption or loss processes are included modularly. We find that without the inclusion of higher frequency data, single synchrotron power-law based models are always preferred for our sample; however, additional processes including free-free absorption (FFA) and synchrotron losses are often required to accurately model radio SED complexity in SFGs. The fitted synchrotron spectral indices range from -0.45 to -1.07 and are strongly anticorrelated with stellar mass suggesting that synchrotron losses are the dominant mechanism acting to steepen the spectral index in larger nearby SFGs. We find that LFTOs in the radio SED are independent from the inclination. The merging systems in our SFG sample have elevated specific star formation rates and flatter fitted spectral indices with unconstrained LFTOs. Lastly, we find no significant separation in global properties between SFGs with or without modelled LFTOs. Overall LFTOs are likely caused by a combination of FFA and ionisation losses in individual recent starburst regions with specific orientations and interstellar medium properties that, when averaged over the entire galaxy, do not correlate with global astrophysical properties.

34 pages, 18 figures, 12 tables. Accepted for publication in PASA

References in corpus (15)

• A z=0 Multi-wavelength Galaxy Synthesis I: A WISE and GALEX Atlas of Local Galaxies
• LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation
• The Radio Spectral Energy Distribution and Star Formation Rate Calibration in Galaxies
• Connecting Far-Infrared and Radio Morphologies of Disk Galaxies: Cosmic-Ray Electron Diffusion After Star Formation Episodes
• The WISE Extended Source Catalogue (WXSC) I: The 100 Largest Galaxies
• Synchrotron spectral index and interstellar medium densities of star-forming galaxies
• Nearby galaxies in LoTSS-DR2: insights into the non-linearity of the radio-SFR relation
• Deciphering the radio-star formation correlation on kpc-scales I. Adaptive kernel smoothing experiments
• Extragalactic Peaked-Spectrum Radio Sources at Low-Frequencies are Young Radio Galaxies
• Measurements of the Hyperfine Structure of Atomic Energy Levels in Co II
• The Long and the Short of It: The Benefits and Leverage of Ultraviolet-Radio Galaxy Fitting
• Low frequency radio continuum imaging and SED modeling of 11 LIRGs: radio-only and FUV to radio bands
• Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54 MHz
• CONGRuENTS (COsmic-ray, Neutrino, Gamma-ray and Radio Non-Thermal Spectra). I. A predictive model for galactic non-thermal emission
• CONGRuENTS (COsmic-ray, Neutrino, Gamma-ray and Radio Non-Thermal Spectra). II. Population-level correlations between galactic infrared, radio, and γ-ray emission

Cited by in corpus (3)

• CHILLING: Continuum Halos in LVHIS Local Irregular Nearby Galaxies - Radio continuum spectral behavior of dwarf galaxies
• MEDUSA I. Tracing magnetic field structures in tidal arms of the dwarf-dwarf merger NGC 1487
• Estimating Galaxy Star Formation Rates from Murchison Widefield Array Radio Continuum Emission