Correlations among multiwavelength luminosities of star-forming galaxies

Abstract

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 LB 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 Lx and L FIR for star-forming regions.