The VLA-COSMOS 3 GHz Large Project: Average radio spectral energy distribution of highly star-forming galaxies

• Published in: Astronomy and astrophysics (Berlin) Vol. 621; p. A139
• Main Authors: Tisanić, K., Smolčić, V., Delhaize, J., Novak, M., Intema, H., Delvecchio, I., Schinnerer, E., Zamorani, G., Bondi, M., Vardoulaki, E.
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.01.2019
• Subjects: Cosmos; Energy; Frequency ranges; Galaxies; galaxies: evolution; galaxies: star formation; galaxies: statistics; Galaxy distribution; Luminosity; Power law; radio continuum: galaxies; Radio telescopes; Red shift; Rest; Spectra; Spectral energy distribution; Star formation; Survival; Survival analysis
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201834002

We construct the average radio spectral energy distribution (SED) of highly star-forming galaxies (HSFGs) up to z ∼ 4. Infrared and radio luminosities are bound by a tight correlation that is defined by the so-called q parameter. This infrared–radio correlation provides the basis for the use of radio luminosity as a star-formation tracer. Recent stacking and survival analysis studies find q to be decreasing with increasing redshift. It was pointed out that a possible cause of the redshift trend could be the computation of rest-frame radio luminosity via a single power-law assumption of the star-forming galaxies’ (SFGs) SED. To test this, we constrained the shape of the radio SED of a sample of HSFGs. To achieve a broad rest-frame frequency range, we combined previously published Very Large Array observations of the COSMOS field at 1.4 GHz and 3 GHz with unpublished Giant Meterwave Radio Telescope (GMRT) observations at 325 MHz and 610 MHz by employing survival analysis to account for non-detections in the GMRT maps. We selected a sample of HSFGs in a broad redshift range (z ∈ [0.3, 4],  SFR ≥ 100 M⊙ yr−1) and constructed the average radio SED. By fitting a broken power-law, we find that the spectral index changes from α1 = 0.42 ± 0.06 below a rest-frame frequency of 4.3 GHz to α2 = 0.94 ± 0.06 above 4.3 GHz. Our results are in line with previous low-redshift studies of HSFGs ( SFR >  10 M⊙  yr−1) that show the SED of HSFGs to differ from the SED found for normal SFGs ( SFR <  10 M⊙ yr−1). The difference is mainly in a steeper spectrum around 10 GHz, which could indicate a smaller fraction of thermal free–free emission. Finally, we also discuss the impact of applying this broken power-law SED in place of a simple power-law in K-corrections of HSFGs and a typical radio SED for normal SFGs drawn from the literature. We find that the shape of the radio SED is unlikely to be the root cause of the q − z trend in SFGs.