In order to form stars, one first requires a supply of cold gas
This seemingly trivial statement is wrapped up in many other complex physical processes which govern the supply of gas into a galaxy. We hope to understand the efficiency with which a galaxy can convert its hot, ionised hydrogen into the clouds of dense molecular hydrogen that form the stars whose light shines across the Universe and into our telescopes.
When one looks to a large population of galaxies, it would seem that fundamental properties such as their gas content and rates of star formation show some intrinsic link to their morphology. In particular, the strong trends that are observed between important physical properties (e.g., gas and dust content, star formation) show a dramatic change of slope when shifting from galaxies which are dominated by a disk-like component to those whose central bulge is dominant. It has become quite common to see this “transition” given as evidence for the direct effect of a dominant galactic bulge regulating the ability of hydrogen gas to collapse and form stars. However, is it really true that galaxies with equal mass in their disks will have a different star formation activity if one hosts a bulge and the other does not? We see that the correlation is there, but we cannot yet say whether this link is truly causal or rooted in some other underlying physical mechanisms.
In order to answer these questions, one must be able to accurately disentangle the contributions from the disk and bulge components to the total stellar mass of a galaxy and investigate scaling relations at a “fixed” disk mass.
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