LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

Abstract

Aims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one α high , calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.

Chang-Es

“…The observed radio spectral indices are in good agreement with expected values for synchrotron emission. The low-frequency radio spectral index below 1.5 GHz is slightly steeper (with a difference of 2σ) than the value of α = −0.57 ± 0.10 4 found by Chyży et al (2018). Also, we see only a small break in the radio continuum spectrum with ∆α = α low − α high = 0.09 ± 0.05, whereas Chyży et al (2018) found ∆α = 0.18 ± 0.20.…”

“…The low-frequency radio spectral index below 1.5 GHz is slightly steeper (with a difference of 2σ) than the value of α = −0.57 ± 0.10 4 found by Chyży et al (2018). Also, we see only a small break in the radio continuum spectrum with ∆α = α low − α high = 0.09 ± 0.05, whereas Chyży et al (2018) found ∆α = 0.18 ± 0.20. This suggests different CR electron propagation and energy losses in NGC 4013 compared to other spiral galaxies.…”

“…Quoted uncertainties for results byChyży et al (2018) refer to the standard deviation rather than the smaller standard deviation of the mean.…”

Millisecond Pulsars Modify the Radio-SFR Correlation

“…We further adopt typical Milky Way parameters to estimate the energy loss timescales. Regarding the electron spectrum, we assume an −2.0 spectrum up to a few GeV, and −2.4 above that, which is consistent with observed radio spectral index of galaxies (e.g., [11]). Under these assumptions, we obtain MSP 1 ( 9)…”

The LOFAR LBA Sky Survey

“…LoLSS data make it possible to detect the turnover from free-free emission with fairly low emission measures, probing low-density (5 cm −3 ) warm ionised gas which may be prevalent in the mid-plane of galaxies (Mezger 1978). Even though one particular statistical study using LOFAR HBA at 144 MHz hinted that free-free absorption plays a minor role (Chyzy et al 2018), the contribution from cooler (T < 1000 K) ionised gas remains largely uncertain (Israel & Mahoney 1990;Emig et al 2020). LoLSS will probe a critical turnover frequency in SED modelling that can characterise ionised gas properties and distinguish its contributions from CRe propagation effects.…”