Probing the Infrared/Radio correlation of the full IRAS Revised Bright Galaxy Sample with MeerKAT and the VLA

TL;DR

This work presents a comprehensive assessment of the infrared/radio correlation for the IRAS RBGS by combining MeerKAT 1.28 GHz data with archival VLA measurements, across isolated and interacting/merging systems, and using both literature-based and WISE mid-IR classifications. It finds a tight, nearly linear $L_{1.4\mathrm{GHz}}$–$L_{\mathrm{TIR}}$ relation with a median $q_{\mathrm{TIR}}$ around 2.63 for the total sample and 2.61 for isolated star-forming galaxies, while interacting/merging systems show lower $q_{\mathrm{TIR}}$ and larger dispersion, suggesting merger-driven processes introduce additional radio emission components. The analysis also reveals a non-linear relation between $L_{\mathrm{TIR}}$ and MIR tracers, and a clear, mass-dependent trend where more massive galaxies have lower $q_{\mathrm{TIR}}$, consistent with prior work and indicating that the IR/radio ratio is modulated by stellar mass and merger activity. Collectively, the results emphasize caution when using the IR/radio correlation as a star-formation proxy in merging systems and highlight the importance of galaxy mass and morphology in shaping the local universe's radio–IR balance.

Abstract

We study the infrared/radio correlation of galaxies in the IRAS Revised Bright Galaxy Sample using new MeerKAT observations at $\rmν= 1.28\, GHz$, complemented with VLA data. We classify the objects by primary energy source (Active Galactic Nuclei vs. Star-Forming) and take into account their merger status. With this, we aim to explore the effect of galaxy-galaxy interaction on the total-infrared (TIR)/radio correlation ($q_\mathrm{TIR}$) of star-forming galaxies by comparing the $q_\mathrm{TIR}$ distribution between isolated and interacting/merging sources. We found the median $q_\mathrm{TIR}$ to be $2.61 \pm 0.01$ (scatter = 0.16) for isolated galaxies and $2.51 \pm 0.08$ (scatter = 0.26) for interacting/merging galaxies. Our analysis reveals that interacting/merging galaxies exhibit lower $q_\mathrm{TIR}$ and higher dispersion compared to isolated galaxies, and the difference is marginally significant. Interacting/merging galaxies have redder $W2-W3$ colours, higher star formation rates (SFR) and specific SFR compared to isolated objects. We observe a significant decrease in $q_\mathrm{TIR}$ with increasing radio luminosity for isolated galaxies. Additionally, we find the median ratio of TIR ($8 \,μm < λ< 1000\, μm$) to far-infrared (FIR; $40 \,μm < λ< 120\, μm$) luminosities to be $\left<L_\mathrm{TIR}/L_\mathrm{FIR}\right>\approx2.29$. By examining the relation between $L_\mathrm{TIR}$ and the mid-infrared (MIR) star-formation rate indicator ($L_\mathrm{12\,μm}$) employed for our interacting/merging sample, we note a strong and consistent (similar non-linear behaviour) relationship between the TIR/radio and TIR/MIR ratios. Finally, we show that already at $z<0.1$, $q_\mathrm{TIR}$ exhibits a dependence on stellar mass, with more massive galaxies displaying a lower $q_\mathrm{TIR}$.

Figures (16)

• Figure 1: Aitoff sky coverage showing the 629 RBGS galaxies Sanders2003 as grey crosses. MeerKAT observed all southern RBGS galaxies except the Milky Way satellites LMC and SMC Condon2021. Galaxies north of $\rm \delta = -45 \deg$ were observed by various configurations of the VLA Condon1990Condon1996Condon1998. Despite the observed overlap, as indicated by the highlighted region, only MeerKAT flux densities are employed for analysing all sources in the southern hemisphere.
• Figure 2: MeerKAT (left) and WISE 3-colour (right) images of some of the sources in our sample depicting the different interaction stages found in this sample. The top row shows an example of an isolated galaxy (NGC $\rm 0157$), while the middle row depicts an interacting galaxy (IC $\rm 2163$/ NGC $\rm 2207$) and the last row shows a merging galaxy (NGC $\rm 4038/9; \, aka\, Arp\, 244$). The positions of the source are also overlaid on the images, identified as red and green crosses for MeerKAT and WISE measurements, respectively.
• Figure 3: Comparison of the VLA $\rm 1.49$ and $\rm 1.425$ GHz (Condon1990Condon1996, respectively) maximum-beam flux densities (y-axis) with MeerKAT 1.28 GHz (Condon2021) flux densities (x-axis) for objects with measurements in both catalogues. All the flux densities are converted to $\rm 1.4 \, GHz$ assuming a spectral index of $\rm - 0.7$.
• Figure 4: Radio spectral luminosity ($L_{1.4 \, \rm{GHz}}$) versus the total infrared luminosity ($L_\mathrm{TIR}$) for our total sample. The sources are separated into different classes as described in Section \ref{['nuclear_class']}, with "Other" referring to sources that are neither Seyferts nor LINERs based on our classification. The red triangles indicate the interacting/merging systems. The black solid line represents the overall fit to all other objects in both samples and is defined by equation \ref{['fit_linear_total']}, while the black dashed lines indicate the $\rm \pm 3 \sigma$ deviations from the best fit.
• Figure 5: Distribution of $q_\mathrm{TIR}$ against redshift (top) and radio spectral luminosity (bottom) for our southern and northern samples. The sources are similarly separated into different classes as described in Figure \ref{['firc-q']}. In both panels, the blue line represents the median of the all-sky sample, excluding measurements of sources identified as LINERs/Seyferts. The other horizontal lines represent measurements from the literature that we use to compare with our sample, as in the legend. The vertical line depicts a region with fewer points, we only use objects with luminosities above this threshold (i.e., $\rm{log}(L_{1.4\, GHz}/\rm{W\, Hz^{-1}})\, >\, 20.2$) to compute the median of our sample. Finally, the cyan line illustrates the binned average for the redshift (top) and radio spectral luminosity (bottom).