Abstract
The radio and far-infrared luminosities of star-forming galaxies are tightly correlated over several orders of magnitude; this is known as the far-infrared radio correlation (FIRC). Previous studies have shown that a host of factors conspire to maintain a tight and linear FIRC, despite many models predicting deviation. This discrepancy between expectations and observations is concerning since a linear FIRC underpins the use of radio luminosity as a star-formation rate indicator. Using LOFAR 150MHz, FIRST 1.4 GHz, and Herschel infrared luminosities derived from the new LOFAR/H-ATLAS catalogue, we investigate possible variation in the monochromatic (250$\mathrm{\mu m}$) FIRC at low and high radio frequencies. We use statistical techniques to probe the FIRC for an optically-selected sample of 4,082 emission-line classified star-forming galaxies as a function of redshift, effective dust temperature, stellar mass, specific star formation rate, and mid-infrared colour (an empirical proxy for specific star formation rate). Although the average FIRC at high radio frequency is consistent with expectations based on a standard power-law radio spectrum, the average correlation at 150MHz is not. We see evidence for redshift evolution of the FIRC at 150MHz, and find that the FIRC varies with stellar mass, dust temperature and specific star formation rate, whether the latter is probed using MAGPHYS fitting, or using mid-infrared colour as a proxy. We can explain the variation, to within 1$\sigma$, seen in the FIRC over mid-infrared colour by a combination of dust temperature, redshift, and stellar mass using a Bayesian partial correlation technique.
Highlights
IntroductionThe far-infrared luminosities of star-forming galaxies have long been known to correlate tightly and consistently with synchrotron radio luminosity across many orders of magnitude in infrared and radio luminosities, independent of galaxy type and redshift (van der Kruit 1971; de Jong et al 1985; Condon et al 1991; Yun et al 2001; Bell 2003; Bourne et al 2011).The existence of some relation should not be surprising since
It is reasonable to suppose that the calorimetry interpretation must be at least partially inaccurate and that there should be some observable variation in the far-infrared radio correlation (FIRC) over the diverse population of star-forming galaxies
We find that the FIRC measured with LOFAR is described by L1L5O0FAR = 10−0.77±0.19L205.907±0.01 with an intrinsic width of 0.89 ± 0.02 dex
Summary
The far-infrared luminosities of star-forming galaxies have long been known to correlate tightly and consistently with synchrotron radio luminosity across many orders of magnitude in infrared and radio luminosities, independent of galaxy type and redshift (van der Kruit 1971; de Jong et al 1985; Condon et al 1991; Yun et al 2001; Bell 2003; Bourne et al 2011).The existence of some relation should not be surprising since. The far-infrared luminosities of star-forming galaxies have long been known to correlate tightly and consistently with synchrotron radio luminosity across many orders of magnitude in infrared and radio luminosities, independent of galaxy type and redshift (van der Kruit 1971; de Jong et al 1985; Condon et al 1991; Yun et al 2001; Bell 2003; Bourne et al 2011). The timescale of the electron synchrotron cooling that produces the radio emission is thought to be longer than the timescale for the escape of those electrons (Lisenfeld et al 1996a; Lacki et al 2010) for normal spirals, and starlight is only partially attenuated in the UV (Bell 2003). Due to their strong magnetic fields, we expected starburst galaxies to be good calorimeters and have a correlation with a slope that is much closer to one than other star-forming galaxies (Lacki et al 2010)
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