TY - JOUR
T1 - Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR
AU - Heesen, V.
AU - Buie, E.
AU - Huff, C. J.
AU - Perez, L. A.
AU - Woolsey, J. G.
AU - Rafferty, D. A.
AU - Basu, A.
AU - Beck, R.
AU - Brinks, E.
AU - Horellou, C.
AU - Scannapieco, Evan
AU - Brüggen, M.
AU - Dettmar, R. J.
AU - Sendlinger, K.
AU - Nikiel-Wroczyński, B.
AU - Chy, K. T.
AU - Best, P. N.
AU - Heald, G. H.
AU - Paladino, R.
N1 - Funding Information:
Acknowledgements. We thank the anonymous referee for an insightful and helpful report. LOFAR, the LOw Frequency ARray designed and constructed by ASTRON, has facilities in several countries, which are owned by various parties (each with their own funding sources) and are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy. This research was performed in the framework of the DFG Forschergruppe 1254 Magnetisation of Interstellar and Intergalactic Media: The Prospects of Low-Frequency Radio Observations. The data used in this work were in part processed on the Dutch national e-infrastructure with the support of SURF Cooperative through grant e-infra 160022 & 160152. BNW acknowledges support from the Polish National Centre of Sciences (NCN), grant no. UMO-2016/23/D/ST9/00386. PNB is grateful for support from the UK STFC via grant ST/M001229/1. EB, CJ, LP, JW, and ES were supported by the National Science Foundation under grant OISE 1458445, “Measuring Cosmic Magnetism with the Low Frequency Radio Array”.
Publisher Copyright:
© ESO 2019.
PY - 2019/2/1
Y1 - 2019/2/1
N2 - 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 (ΣSFR) 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. 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 ΣSFR maps using the Condon relation. We compared these maps with hybrid ΣSFR 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 2 resolution. Results. The RC emission is smoothed with respect to the hybrid ΣSFR owing to the transport of cosmic-ray electrons (CREs) away from star formation sites. This results in a sublinear relation (ΣSFR)RC [(ΣSFR)hyb] a , 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; Iν ν α ), 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 ΣSFR 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 28 cm 2 s -1 at 1 GeV. Conclusions. A RC-SFR relation at 1.4 GHz can be exploited to measure SFRs at redshift z 10 using 140 MHz observations.
AB - 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 (ΣSFR) 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. 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 ΣSFR maps using the Condon relation. We compared these maps with hybrid ΣSFR 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 2 resolution. Results. The RC emission is smoothed with respect to the hybrid ΣSFR owing to the transport of cosmic-ray electrons (CREs) away from star formation sites. This results in a sublinear relation (ΣSFR)RC [(ΣSFR)hyb] a , 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; Iν ν α ), 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 ΣSFR 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 28 cm 2 s -1 at 1 GeV. Conclusions. A RC-SFR relation at 1.4 GHz can be exploited to measure SFRs at redshift z 10 using 140 MHz observations.
KW - Cosmic rays
KW - Galaxies: magnetic fields
KW - Galaxies: star formation
KW - Radiation mechanisms: non-thermal
KW - Radio continuum: galaxies
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U2 - 10.1051/0004-6361/201833905
DO - 10.1051/0004-6361/201833905
M3 - Article
AN - SCOPUS:85062047418
VL - 622
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
SN - 0004-6361
M1 - A8
ER -