THE STAR FORMATION IN RADIO SURVEY: GBT 33 GHz OBSERVATIONS OF NEARBY GALAXY NUCLEI AND EXTRANUCLEAR STAR-FORMING REGIONS

Abstract

We present 33 GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope (GBT) as part of the Star Formation in Radio Survey (SFRS). Among the sources without evidence for an AGN, and also having lower frequency radio data, we find a median thermal fraction at 33 GHz of ≈76% with a dispersion of ≈24%. For all sources resolved on scales 0.5 kpc, the thermal fraction is even larger, being 90%. This suggests that the rest-frame 33 GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate indicator. Taking the 33 GHz star formation rates as a reference, we investigate other empirical calibrations relying on different combinations of warm 24 µm dust, total infrared (IR; 8 − 1000 µm), Hα line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33 GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33 GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of σ α NT 0.13.

Radio and Mid-Infrared Properties of Compact Starbursts: Distancing Themselves From the Main Sequence

“…We note that the coefficient here is 1.23 times larger than, but within the scatter of, the empirically derived relation between IR and radio free-free star formation rates (Murphy et al 2012). …”

Radio Continuum Observations of Local Star-Forming Galaxies Using the Caltech Continuum Backend on the Green Bank Telescope

“…Four of the galaxies in our sample were previously observed with the CCB by Murphy et al (2012), three of which were detected (M101 was reported as an upper limit by Murphy et al (2012) and is a 2.6σ marginal detection when the four sub-bands were averaged in our observations). Only one galaxy in our sample, Tol 35, was not detected when averaging the four sub-bands' fluxes.…”

“…There is a well-established tight correlation between FIR and radio flux in star-forming galaxies (e.g., Helou et al 1985;Murphy et al 2006Murphy et al , 2012. When plotted on a log-log scale, the relationship between radio continuum and FIR flux for star-forming galaxies appears linear.…”

“…Since e These galaxies have previously published radio continuum observations. M12 denotes galaxies detected with the CCB by Murphy et al (2012) at 33 GHz. T72 denotes galaxies that were observed, but not detected, by Tovmassian (1972) at 9.5 mm (31.6 GHz).…”

Planck2015 results

“…These are lower than the estimate of 0.2 M yr −1 for the average SFR from analyses of stellar populations by Harris & Zaritsky (2009) and Rezaeikh et al (2014), but agree well with the recent star formation rate of 0.06 M yr −1 calculated by Whitney et al (2008) based on young stellar objects; these authors also give a range of SFR estimates of 0.05-0.25 M yr −1 from infrared and Hα data. However, all of these estimates correspond to an SFR over different timescales (e.g., Murphy et al 2012), as well as being subject to systematic effects; a more detailed study would be necessary to disentangle these effects. For the SMC, the amplitude at 10 GHz of 11 Jy from the Commander and LSF models gives an SFR of 1.1 × 10 −2 M yr −1 ; the LSF-CMB model gives a slightly higher value of 1.3 × 10 −2 M yr −1 from a flux density of 12.8 Jy.…”