Radiative Transfer Modeling Calibrates Star Formation Diagnostics in Galaxies Beyond Redshift 0.5

Luminous infrared galaxies represent crucial environments for the formation of stars at redshifts greater than 0.5, yet accurately measuring their star formation rates remains a significant challenge. L. Robinson, D. Farrah, and A. Efstathiou, alongside colleagues including A. Engholm, E. Hatziminaoglou, and M. Joyce, have investigated the reliability of commonly used indicators , specifically, polycyclic aromatic hydrocarbon (PAH) features and neon emission lines , for tracing star formation within these obscured galaxies. Their research, employing radiative transfer modelling and observations of 42 local ultraluminous infrared galaxies, addresses the uncertainty surrounding whether calibrations of these features are influenced by a galaxy’s overall luminosity or the presence of an active galactic nucleus. The team demonstrates that while PAH and [Ne II] emission predominantly originate in star-forming regions, the [Ne III] line receives contributions from both star formation and AGN activity, ultimately refining existing relationships and revealing a systematic underestimation of star formation rates in lower luminosity systems. These findings are particularly relevant for ongoing and future studies of star-forming galaxies at redshifts less than 3 utilising the James Webb Space Telescope.

The research team meticulously investigated the relationship between the luminosity of polycyclic aromatic hydrocarbons (PAHs) and neon lines with star formation rates, focusing on 42 local Ultraluminous Infrared Galaxies (ULIRGs). Employing radiative transfer modeling alongside archival observations, the study addresses a long-standing uncertainty regarding whether calibrations of these features as star formation rate tracers are influenced by the overall luminosity of starburst activity or the presence of Active Galactic Nuclei (AGNs). The study reveals that PAH and [Ne II] emissions predominantly originate in star-forming regions, experiencing minimal contamination from AGN or host galaxy contributions.

However, the [Ne III] line exhibits a more complex origin, arising from a combination of both star formation and AGN activity. Researchers established new relationships between the luminosities of PAH and [Ne II], and their correlation with both starburst luminosity and overall star formation rate. These findings are particularly significant as the team discovered that existing calibrations for lower luminosity systems underestimate star formation rates in local ULIRGs by as much as one order of magnitude. This work establishes the 6.2 and 11.2 micron PAH features, alongside the [Ne II] line, as robust tracers of star formation within ULIRGs.

Importantly, the team found no evidence that a more powerful AGN directly impacts the relationship between star formation rate and PAH or neon luminosity, although increased AGN activity can diminish the detectability of PAH emission. The team’s detailed radiative transfer modeling allowed for a nuanced understanding of emission origins, separating contributions from star formation and AGN activity. This careful analysis enabled the development of more precise calibrations for estimating star formation rates, addressing a critical need in the study of galaxy evolution.

The derived relationships are expected to be invaluable for interpreting observations of star-forming and composite galaxies at redshifts up to three, furthering our understanding of stellar mass assembly in the early universe. These findings represent a significant advancement in the field, providing astronomers with refined tools for probing obscured star formation in distant galaxies. By accurately quantifying star formation rates, scientists can better constrain models of galaxy evolution and unravel the processes that drive the formation of stars and the growth of supermassive black holes. Researchers selected these ULIRGs from the HERschel ULIRG Reference Survey, prioritizing objects at redshifts below 0.3 with strong IRAS 60μm flux densities exceeding 2 Jansky, creating a nearly unbiased sample of low-redshift ULIRGs. Spitzer Space Telescope data, specifically from the InfraRed Spectrograph (IRS), provided measurements of the 6.2 and 11.2 micron polycyclic aromatic hydrocarbon (PAH) features, carefully integrating flux above a spline-interpolated continuum to avoid contamination from silicate absorption. Scientists employed the CYGNUS radiative transfer code, detailed in Efstathiou et al. (2022) and Varnava & Efstathiou (2024), to decompose the total infrared luminosity of each ULIRG into contributions from the starburst, the Active Galactic Nucleus (AGN), and the host galaxy.

This innovative technique allowed for a direct comparison of PAH and Neon line luminosities with individual component luminosities, a significant advancement over previous studies. The team obtained physical properties, including total and anisotropy-corrected IR luminosities (Lo Tot, Lc Tot), starburst luminosity (LSb), AGN luminosity (Lo AGN, Lc AGN), host luminosity (LHost), and starburst star formation rate (MSb), directly from the CYGNUS modeling results. The research pioneered a robust statistical analysis using the Kendall-τ correlation coefficient to assess relationships between MIR line properties and physical characteristics, establishing significance with a p-value threshold of 0.05. To quantify these relationships, scientists implemented a log-linear model, enabling conversion between observed luminosities and component luminosities, expressed as LObservable = α · LComponent + β. This methodology allowed the study to demonstrate that the 6.2 micron and 11.2 micron PAH features, alongside the [Ne II] line, serve as reliable tracers of star formation rates in ULIRGs, while also revealing that AGN activity does not significantly alter these relationships, though it can hinder PAH emission detection. The work’s meticulous methodology, separating luminosity contributions and utilizing advanced statistical analysis, provides crucial insights into obscured stellar mass assembly at redshifts greater than 0. The research team meticulously investigated the relationship between luminosities of Polycyclic Aromatic Hydrocarbons (PAHs) and Neon lines with star formation rate, utilising radiative transfer modelling and archival observations of 42 local ULIRGs. Experiments revealed that PAH and [Ne II] emissions predominantly originate in star-forming regions, with minimal contribution from the Active Galactic Nucleus (AGN) or host galaxy, establishing these features as reliable indicators of star formation. Measurements confirm that the [Ne III] line exhibits a mixed origin, arising from both star formation and AGN activity, highlighting the complex interplay of energy sources within these galaxies.

The study presents new relations between L PAH and L NeII, correlating these luminosities with both starburst luminosity and star formation rate. Data shows that existing calibrations for lower luminosity systems, with L IR approximately 10 10 -10 12 L sun, underestimate star formation rates in local ULIRGs by up to approximately 1 dex, a significant discrepancy requiring revised estimation techniques. Results demonstrate that the 6.2 micron and 11.2 micron PAH features, alongside the [Ne II] line, function effectively as tracers of star formation rates within ULIRGs. The work establishes a robust framework for analysing mid-infrared spectral features, providing a foundation for understanding stellar mass assembly in the early universe and the combined influence of star formation and AGN activity. This research provides vital tools for astronomers seeking to unravel the complexities of galaxy evolution and the processes driving star birth in the most luminous galaxies known.