The Far-Infrared Radio Correlation at low radio frequency with LOFAR/H-ATLAS

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 150 MHz, FIRST 1.4 GHz, and Herschel infrared luminosities derived from the new LOFAR/H-ATLAS catalogue, we investigate possible variation in the monochromatic (250 µm) 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 150 MHz is not. We see evidence for redshift evolution of the FIRC at 150 MHz, 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 1σ , seen in the FIRC over mid-infrared colour by a combination of dust temperature, redshift, and stellar mass using a Bayesian partial correlation technique.

Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole

“…We notice immediately that, despite differences in the location of our sources on the FIRC compared to Ivison et al (2010) (likely due to our FIR estimation method and selection effects, as discussed below), none of our sources appear significantly below the FIRC, and therefore we may assume that our sources do not host radio-loud AGN, or at least that their spectra are dominated by contributions from star formation. Selecting only the brightest submillimetre sources from a flux-limited sample introduces selection effects such that we bias our sample towards the FIR-bright, radio-faint population; therefore, we do not attempt to comment on the distribution of our sources with regards to the normalisation of the FIRC and its dependence on other galaxy properties, such as redshift, temperature, and stellar mass, as extensively studied by, for example, Yun et al (2001), Ivison et al (2010), Smith et al (2014), andRead et al (2018). The depth of the radio surveys used in our sample also impacts the FIRC here: Many older studies are based on much shallower surveys, and the FIRC is plotted only for objects detected in both bands.…”

“…Ramasawmy et al: Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole Ivison et al 2010;Smith et al 2014). The Low Frequency Array (LOFAR; van Haarlem et al 2013) has opened up new ways of studying galaxies in the radio, and a number of studies have used LOFAR's capabilities to investigate this relationship between star formation and radio luminosity in the low-frequency regime -for example Gürkan et al (2018), Read et al (2018), Smith et al (2020), andWang et al (2019a). However, these studies generally investigate the statistical properties of large samples of galaxies, in optically selected samples at low redshift (z 2), rather than probing the shapes of individual radio spectra.…”

LoTSS/HETDEX: Optical quasars

“…Recently, Gürkan et al (2018) investigated the low-frequency radio luminosity to star formation rate relation and far-IR to radio correlation (FIRC) in local star-forming galaxies selected based on their optical emission lines, using LOFAR-144 MHz measurements to probe radio and Herschel-250 µm to probe farIR. Read et al (2018) further used the same sample to investigate FIRC as a function of redshift, effective dust temperature, stellar mass, specific star formation rate, and mid-infrared colour. In Fig.…”

Placing LOFAR-detected quasars in C iv emission space: implications for winds, jets and star formation

“…Whatever the origin of the emission, properly characterizing the radio-quiet population is critical for understanding the connection between quasar outflows and star formation. In addition, the well-known correlation between far-infrared luminosity and radio luminosity, the far-infrared radio correlation (Helou, Soifer & Rowan-Robinson 1985;Yun, Reddy & Condon 2001;Calistro Rivera et al 2017;Gürkan et al 2018;Read et al 2018) means that radio luminosity can be used as a star formation rate estimator in certain cases; understanding the level at which this correlation is contaminated from AGN-driven mechanisms is again important.…”