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

Abstract

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. Using the measured 150MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies. 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. Spectra of our galaxies are generally flatter at lower compared to higher frequencies but 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 M51-like galaxies, the free-free absorption effects can be seen only below 30MHz and in the global spectra just below 20MHz, while in the spectra of starburst galaxies, like M82, the flattening due to absorption is instead visible up to higher frequencies of about 150MHz. Locally, within galactic disks, the absorption effects are distinctly visible in M51-like galaxies as spectral flattening around 100-200MHz in the face-on objects, and as turnovers in the edge-on ones, while in M82-like galaxies there are strong turnovers at frequencies above 700MHz, regardless of viewing angle. Our modelling 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.

arXiv commentAccepted for publication in Astronomy and Astrophysics (corrected typo in the title)

References in corpus (39)

• LOFAR: The LOw-Frequency ARray
• SINGS: The SIRTF Nearby Galaxies Survey
• The Interstellar Environment of our Galaxy
• Radio Properties of Infrared Selected Galaxies in the IRAS 2 Jy Sample
• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue
• The Asymmetric Wind in M82
• The far-infrared/radio correlation and radio spectral index of galaxies in the SFR-M* plane up to z 2
• LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation
• The Far-Infrared--Radio Correlation at High Redshifts: Physical Considerations and Prospects for the Square Kilometer Array
• The Radio Spectral Energy Distribution and Star Formation Rate Calibration in Galaxies
• LOFAR/H-ATLAS: A deep low-frequency survey of the Herschel-ATLAS North Galactic Pole field
• The LOFAR window on star-forming galaxies and AGN - curved radio SEDs and IR-radio correlation at $0<z<2.5$
• A new interpretation of the far-infrared - radio correlation and the expected breakdown at high redshift
• The LOFAR Multifrequency Snapshot Sky Survey (MSSS) I. Survey description and first results
• The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
• A Detailed Study of the Radio--FIR Correlation in NGC6946 with Herschel-PACS/SPIRE from KINGFISH
• Winds, Clumps, and Interacting Cosmic Rays in M82
• CHANG-ES IX: Radio scale heights and scale lengths of a consistent sample of 13 spiral galaxies seen edge-on and their correlations
• Radio synchrotron spectra of star-forming galaxies
• M82 - A radio continuum and polarisation study I. Data reduction and cosmic ray propagation
• Radio haloes in nearby galaxies modelled with 1D cosmic-ray transport using SPINNAKER
• The Optical Structure of the Starburst Galaxy M82. II. Nebular Properties of the Disk and Inner-Wind
• Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows
• Advective and diffusive cosmic ray transport in galactic haloes
• Synchrotron spectral index and interstellar medium densities of star-forming galaxies
• The Spectral Energy Distribution of Powerful Starburst Galaxies I: Modelling the Radio Continuum
• Integrated Radio Continuum Spectra of Galaxies
• Evaluating the Calorimeter Model with Broadband, Continuous Spectra of Starburst Galaxies Observed with the Allen Telescope Array
• Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz
• The galaxy-wide distributions of mean electron density in the HII regions of M51 and NGC 4449
• Magnetic fields and gas in the cluster influenced spiral galaxy NGC4254 - I. Radio and X-rays observations
• Interpreting the Low Frequency Radio Spectra of Starburst Galaxies: A Pudding of Strömgren Spheres
• Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891
• Spectral energy distribution and radio halo of NGC 253 at low radio frequencies
• GMRT 333 MHz observations of 6 nearby normal galaxies
• Modelling the cosmic ray electron propagation in M 51
• LOFAR MSSS: The Scaling Relation between AGN Cavity Power and Radio Luminosity at Low Radio Frequencies
• What drives galactic magnetism?

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