Galaxy formation and evolution

Abstract:

Abstract A key question in extragalactic astronomy is how the star-formation rate density (SFRD) evolves over cosmic time. A powerful way of addressing this question is using radio-continuum observations, where the radio waves are unaffected by dust and are able to reach sufficient resolution to resolve individual galaxies. We present an investigation of the 1.4 GHz radio luminosity functions (RLFs) of star-forming galaxies (SFGs) and Active Galactic Nuclei (AGN) using deep radio continuum observations in the COSMOS and XMM–LSS fields, covering a combined area of ∼4 deg2. These data enable the most accurate measurement of the evolution in the SFRD from mid-frequency radio continuum observations. We model the total RLF as the sum of evolving SFG and AGN components, negating the need for individual source classification. We find that the SFGs have systematically higher space densities at fixed luminosity than found in previous radio studies, but consistent with more recent studies with MeerKAT. We attribute this to the excellent low-surface brightness sensitivity of MeerKAT. We then determine the evolution of the SFRD. Adopting the far-infrared – radio correlation results in a significantly higher SFRD at z > 1, compared to combined UV and far-infrared measurements. However, using more recent relations for the correlation between star-formation rate and radio luminosity, based on full spectral energy distribution modelling, can resolve this apparent discrepancy. Thus radio observations provide a powerful method of determining the total SFRD, in the absence of dust-sensitive far-infrared data.

Abstract:

Studying samples of young star-forming regions allows us to statistically examine the evolution of their natal gas and dust along with the associated time-scales in the volatile early stages of their lives. With the Physics at High Angular resolution in Nearby GalaxieS survey data, we analyse the diversity of spectral energy distributions (SEDs) for the rich sample of massive star-forming regions found in NGC 1365. By combining unique detections across a variety of data sets from Hubble Space Telescope (HST), James Web Space Telescope (JWST), and Atacama Large Millimeter/sub-millimeter Array (ALMA) images, we produce a catalogue of 85 star-forming regions located in the central starbursting region of NGC 1365. Prior to analysis, we observe clear saturation effects in our four JWST Mid-Infrared Instrument (MIRI) images, and implement a saturation-correction method which allows us to recover data for 23 of 32 saturation-affected regions in these images. We then perform photometry in 13 HST & JWST images which are convolved to match the resolution of MIRI/F2100W (64 pc), allowing us to probe star clusters as well as their immediate surroundings. Upon deriving their properties from SED-fitting using Code Investigating GALaxy Emission, we observe that regions selected with progressively redder wavebands are younger and generally more reddened. We also identify three SED features correlated with age: (1) sources with a positive near-infrared slope [(F300W + F360M)/(2F200W)] are by median half the age of those with negative near-infrared slopes; turnover occurs around 6 Myr, (2) the relative strength of dust emission (F2100W/F200W), and (3) polycyclic aromatic hydrocarbon emission (F335M/F300M) both show that larger such ratios correlate with younger ages. Considering our working resolution, these features are robust to the inclusion of nearby emission surrounding star clusters.