Measuring the Star Formation Rate Across Epochs
Dr. Thomas Bisbas
One of the most important quantities astronomers try to measure is the so-called "Star Formation Rate" or "SFR". This quantity can reveal information about the evolutionary stages of the interstellar medium across the epochs and can provide useful insights of how stars formed during the evolution of the entire Universe. Every galaxy has its own SFR: those located at a redshift of 2-to-3 (that is when the Universe was just 3 billion years old) have the highest SFR. But how do astronomers measure this SFR? There are many different approaches which follow the same principle: measuring the radiation emitted by various ions and atoms which we can see using powerful telescopes. These intensities are then be translated to SFR. This "translation" has been the core of our work.
In Cologne we tested the ability of ionized carbon (CII, spelled as "C plus") as a SFR tracer. It emits radiation at 158 microns and is favored among all other tracers since carbon is one of the most abundant elements in the Universe. Its radiation is so strong that it can be observed from billions of light years away. We performed state-of-the-art hydrodynamical simulations of two galaxies that colide, which, in turn, produces a high number of stars; therefore the SFR increases dramatically. Our calculations clearly show that that the total luminosity of CII follows the trend of the SFR; when the SFR increases or decreases, CII also increases or decreases accordingly. There is, therefore, a CII-SFR correlation; a function that can be used to convert the observed CII luminosity to a SFR value.
In Figures 1 and 2, there is a snapshot of the hydrodynamical simulation after the two galaxies have collided and when the SFR of the system is high. Figure 1 shows a map of the total surface density of the system while Figure 2 shows a map of CII emission. As can be seen, CII is emitted brightly from dense regions of the colliding system, corresponding to places where new stars are formed. In Figure 3, we plot our simulation results versus observed data, confirming the CII-SFR correlation we find. Throughout our work, we provide such relations to the astronomical community helping to understand more how galaxies evolved during the history of the Universe.