Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR

V. Heesen; E. Buie; C. J. Huff; L. A. Perez; J. G. Woolsey; D. A. Rafferty; A. Basu; R. Beck; E. Brinks; C. Horellou; E. Scannapieco; M. Brüggen; R. -J. Dettmar; K. Sendlinger; B. Nikiel-Wroczyński; K. T. Chyży; P. N. Best; G. H. Heald; R. Paladino

doi:10.1051/0004-6361/201833905

paper

Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR

arXiv:1811.07968 · doi:10.1051/0004-6361/201833905

Abstract

Radio continuum (RC) emission in galaxies allows us to measure star formation rates (SFRs) unaffected by extinction due to dust, of which the low-frequency part is uncontaminated from thermal (free-free) emission. We calibrate the conversion from the spatially resolved 140 MHz RC emission to the SFR surface density ( $Σ_{SFR}$ ) at 1 kpc scale. We used recent observations of three galaxies (NGC 3184, 4736, and 5055) from the LOFAR Two-metre Sky Survey (LoTSS), and archival LOw-Frequency ARray (LOFAR) data of NGC 5194. Maps were created with the facet calibration technique and converted to radio $Σ_{SFR}$ maps using the Condon relation. We compared these maps with hybrid $Σ_{SFR}$ maps from a combination of GALEX far-ultraviolet and Spitzer 24 $μ m$ data using plots tracing the relation at $1.2 \times 1.2$ -kpc $^{2}$ resolution. The RC emission is smoothed with respect to the hybrid $Σ_{SFR}$ owing to the transport of cosmic-ray electrons (CREs). This results in a sublinear relation $(Σ_{SFR})_{RC} \propto [(Σ_{SFR})_{hyb}]^{a}$ , where $a = 0.59 \pm 0.13$ (140 MHz) and $a = 0.75 \pm 0.10$ (1365 MHz). Both relations have a scatter of $σ = 0.3 dex$ . If we restrict ourselves to areas of young CREs ( $α > - 0.65$ ; $I_{ν} \propto ν^{α}$ ), the relation becomes almost linear at both frequencies with $a \approx 0.9$ and a reduced scatter of $σ = 0.2 dex$ . We then simulate the effect of CRE transport by convolving the hybrid $Σ_{SFR}$ maps with a Gaussian kernel until the RC-SFR relation is linearised; CRE transport lengths are $l = 1$ -5 kpc. Solving the CRE diffusion equation, we find diffusion coefficients of $D = (0.13$ - $1.5) \times 1 0^{28} c m^{2} s^{- 1}$ at 1 GeV. A RC-SFR relation at $1.4$ GHz can be exploited to measure SFRs at redshift $z \approx 10$ using $140$ MHz observations.

arXiv comment9 figures, 6 tables and 17 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal

Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR

Abstract

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