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|Title:||Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR||Authors:||Heesen, V.
Buie, E., II
Huff, C. J.
Perez, L. A.
Woolsey, J. G.
Rafferty, D. A.
Dettmar, R. -J.
Chyży, K. T.
Best, P. N.
Heald, G. H.
|Issue Date:||2019||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||622||First Page:||A8||Abstract:||Context. 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. Aims: We calibrate the conversion from the spatially resolved 140 MHz RC emission to the SFR surface density (Σ<SUB>SFR</SUB>) at 1 kpc scale. Radio spectral indices give us, by means of spectral ageing, a handle on the transport of cosmic rays using the electrons as a proxy for GeV nuclei. <BR /> Methods: 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<SUB>SFR</SUB> maps using the Condon relation. We compared these maps with hybrid Σ<SUB>SFR</SUB> maps from a combination of GALEX far-ultraviolet and Spitzer 24 μm data using plots tracing the relation at the highest angular resolution allowed by our data at 1.2 × 1.2 kpc<SUP>2</SUP> resolution. <BR /> Results: The RC emission is smoothed with respect to the hybrid Σ<SUB>SFR</SUB> owing to the transport of cosmic-ray electrons (CREs) away from star formation sites. This results in a sublinear relation (Σ<SUB>SFR</SUB>)<SUB>RC</SUB> ∝ [(Σ<SUB>SFR</SUB>)<SUB>hyb</SUB>]<SUP>a</SUP>, where a = 0.59 ± 0.13 (140 MHz) and a = 0.75 ± 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<SUB>ν</SUB> ∝ ν<SUP>α</SUP>), the relation becomes almost linear at both frequencies with a ≈ 0.9 and a reduced scatter of σ = 0.2 dex. We then simulate the effect of CRE transport by convolving the hybrid Σ<SUB>SFR</SUB> 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, assuming dominance of the synchrotron and inverse-Compton losses, we find diffusion coefficients of D = (0.13-1.5) × 10<SUP>28</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP> at 1 GeV. <BR /> Conclusions: A RC-SFR relation at 1.4 GHz can be exploited to measure SFRs at redshift z ≈ 10 using 140 MHz observations. Radio continuum flux densities and fits files are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://184.108.40.206) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/622/A8">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/622/A8||URI:||http://hdl.handle.net/20.500.12386/28566||URL:||https://www.aanda.org/articles/aa/abs/2019/02/aa33905-18/aa33905-18.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201833905||Bibcode ADS:||2019A&A...622A...8H||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Jan 16, 2021
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