Correlations among multiwavelength luminosities of star‐forming galaxies

It has been known for two decades that a tight correlation exists between global far‐infrared (FIR) and radio continuum (1.4 and 4.8 GHz) fluxes/luminosities from star‐forming galaxies, which may be explained by formation activities of massive stars in these galaxies. For this very reason, a correlation might also exist between X‐ray and FIR/radio global luminosities of galaxies. We analyse data from the ROSAT All‐Sky Survey and from IRAS to show that such correlation does indeed exist between FIR (42.5–122.5 μm) and soft X‐ray (0.1–2.4 keV) luminosities L X and L FIR in 17 normal star‐forming galaxies (NSFGs), including 16 late‐type galaxies and one host‐dominant Seyfert galaxy, as well as in 14 active star‐forming galaxies (ASFGs) consisting of starburst‐dominant Seyfert galaxies. The quantitative difference in such correlations in NSFGs and in ASFGs may be interpreted in terms of evolutionary variations from classic starburst galaxies to starburst‐dominant Seyfert galaxies. Meanwhile, some low‐luminosity active galactic nuclei (LLAGNs) tend to exhibit such a correlation that we infer for star‐forming galaxies, implying that star‐forming activities might be more dominant in LLAGNs. In contrast, AGN‐dominant Seyfert galaxies do not show such a L X versus L FIR correlation; this is most likely related to accretions towards supermassive black holes (SMBHs) in galactic nuclei. In order to establish a physical connection between the L X – L FIR correlation and global star formation rate (SFR) in galaxies, we empirically derive both L X – L B and L FIR – L B relations with the blue‐band luminosity L B roughly representing the mass of a galaxy. It appears that the more massive galaxies are, the more star formation regions exist in these galaxies. The global SFR is not only associated with the mass of a galaxy but also closely related to the level of star‐forming activities therein. We propose a relation between soft X‐ray luminosity and SFR in star‐forming galaxies. In order to probe the L X – L FIR relation, we construct an empirical model in which both FIR and X‐ray emissions consist of two components with one being closely associated with star formation and the other one not. Based on this model, we infer a linear relation between FIR/soft X‐ray in star formation regions and radio luminosities, and get a linear relation between L X and L FIR for star‐forming regions.