The far-infrared/radio correlation and radio spectral index of galaxies in the SFR-M$_∗$ plane up to z2

B. Magnelli, R. J. Ivison, D. Lutz, I. Valtchanov, D. Farrah, S. Berta, F. Bertoldi, J. Bock, A. Cooray, E. Ibar, A. Karim, E. Le Floc'h, R. Nordon, S. J. Oliver, M. Page, P. Popesso, F. Pozzi, D. Rigopoulou, L. Riguccini, G. RodighieroD. Rosario, I. Roseboom, L. Wang, S. Wuyts

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We study the evolution of the radio spectral index and far-infrared/radio correlation (FRC) across the star-formation rate – stellar masse (i.e. SFR–M∗) plane up to z ~ 2. We start from a stellar-mass-selected sample of galaxies with reliable SFR and redshift estimates. We then grid the SFR–M∗ plane in several redshift ranges and measure the infrared luminosity, radio luminosity, radio spectral index, and ultimately the FRC index (i.e. qFIR) of each SFR–M∗–z bin. The infrared luminosities of our SFR–M∗–z bins are estimated using their stacked far-infrared flux densities inferred from observations obtained with the Herschel Space Observatory. Their radio luminosities and radio spectral indices (i.e. α, where Sν ∝ ν−α) are estimated using their stacked 1.4 GHz and 610 MHz flux densities from the Very Large Array and Giant Metre-wave Radio Telescope, respectively. Our far-infrared and radio observations include the most widely studied blank extragalactic fields – GOODS-N, GOODS-S, ECDFS, and COSMOS – covering a total sky area of ~2.0 deg2. Using this methodology, we constrain the radio spectral index and FRC index of star-forming galaxies with M∗ > 1010 M⊙ and 0 <z< 2.3. We find that α1.4 GHz610 MHz does not evolve significantly with redshift or with the distance of a galaxy with respect to the main sequence (MS) of the SFR–M∗ plane (i.e. Δlog (SSFR)MS = log  [ SSFR(galaxy) /SSFRMS(M∗,z) ]). Instead, star-forming galaxies have a radio spectral index consistent with a canonical value of 0.8, which suggests that their radio spectra are dominated by non-thermal optically thin synchrotron emission. We find that the FRC index, qFIR,displays a moderate but statistically significant redshift evolution as qFIR(z) = (2.35 ± 0.08) × (1 + z)−0.12 ± 0.04, consistent with some previous literature. Finally, we find no significant correlation between qFIR and Δlog (SSFR)MS, though a weak positive trend, as observed in one of our redshift bins (i.e. Δ [ qFIR ]/Δ [ Δlog (SSFR)MS ] = 0.22 ± 0.07 at 0.5 <z< 0.8), cannot be firmly ruled out using our dataset.
Original languageEnglish
Article numberA45
Number of pages18
JournalAstronomy & Astrophysics
Early online date15 Dec 2014
Publication statusPublished - 1 Jan 2015


  • galaxies: evolution
  • galaxies: formation
  • galaxies: starburst
  • galaxies: high-redshift
  • infrared: galaxies

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    Magnelli, B., Ivison, R. J., Lutz, D., Valtchanov, I., Farrah, D., Berta, S., Bertoldi, F., Bock, J., Cooray, A., Ibar, E., Karim, A., Le Floc'h, E., Nordon, R., Oliver, S. J., Page, M., Popesso, P., Pozzi, F., Rigopoulou, D., Riguccini, L., ... Wuyts, S. (2015). The far-infrared/radio correlation and radio spectral index of galaxies in the SFR-M$_∗$ plane up to z2. Astronomy & Astrophysics, 573, [A45].