Understanding how stars are formed in our Universe is one of the main objectives of Astrophysics. In order to answer this question, typically Astronomers rely on the Observed light they detect in their telescopes. Nowadays, this light is analyzed by sophisticated computational methods which give us estimates of the star formation within these galaxies and the Universe. This procedure has been so fundamental for the community that typically theoretical models and simulations of galaxy formation are tuned to reproduce these "observables". A recent study led by Dr. Antonios Katsianis, a researcher at the Tsung-Dao Lee Institute, Shanghai Jiao Tong University, has compiled observations of million galaxies and constructed a simple model to study the evolution of the star formation in the Universe. The results are elegant mathematically and Physically motivated. However, severe problems are reported regarding the practices used in the field.
The lead author of this study reports a Crisis in Extragalactic Astrophysics. In order to obtain properties of galaxies like star formation rate or stellar mass different researchers have been relying on different models and assumptions. Two big surprises were that 1) we have actually been reporting results in tension with each other for almost a decade since we all have been using different methods and tools (the sophisticated computational methods mentioned in the first paragraph). 2) Besides that, most researchers in the Field leave very small room for a mistake in their results, the tension persists and none of us can eventually confirm who is correct (especially for distant galaxies) since we do not know the true result.
Caption: The evolution of the rate of star formation in the Universe resembles a Gamma distribution
Thus, the authors decided to separate the different indicators/datasets of galaxy star formation rates to different categories. a) a UV data set with dust corrections b) an IR dataset. The two indicators produced results for star formation that have some key differences. However, the first main finding of the paper was that both indicators suggest an evolution for the cosmic star formation rate that can be described by only 2 parameters and has a form that resembles a Gamma(a,b) distribution. This mathematical form has been widely used in numerous Fields like Biology (spread of infectious diseases with time) and Engineering (to model the deterioration of buildings with time).
The authors have noted that there are severe problems not only with Observations but also with Simulations. The limitations of the latter are typically related to computational limitations and modeling. Thus, the authors decided to keep it simple and constructed a model with pen and paper. The model just adopted some parameters from the standard model of cosmology and some insights from simulations and both the shape and the 2 parameters (!) that describe the observed evolution of the star formation rate of the Universe emerged! This represents a benchmark since in the past most parameterizations of the Cosmic star formation rate have been empirical fits to the data while the free parameters were not directly connected to the Physics of galaxies (for reference the most common models in the literature have been employing 4 parameters determined empirically). The group’s findings are Published at the prestigious Astrophysical Journal ( https://iopscience.iop.org/article/10.3847/1538-4357/ac11f2 ) .
Caption: The evolution of the star formation in the Universe derived from simple assumptions. Inputting the parameters from Cosmology (like Pcrit – critical density of the Universe) reproduces the observations.
Prof. Xiaohu Yang said, "The most innovative part of this new model is its physical motivation. Previous SFRD functions are modeled as a function of redshift, an indicator of the size of Universe. While this new Gamma SFRD model, as a function of cosmic time, is motivated by the star formation physics itself, e.g., the gas accretion and depletion time scale". Prof. XianZhong Zheng said, "The measurements of cosmic star formation rate density using different tracers can now be understood better. In addition, Star formation in a galaxy involves complex processes. But the global star formation of the Universe seems to be governed by some simple rules described by a form that resembles a Gamma distribution and just 2 parameters. This is really a surprise! " The researchers will continue to explore the implications of their findings to other related fields. Dr. Antonios Katsianis stated: "Star formation is one of the most important processes in the Universe so numerous phenomena are expected to be studied and simplified giving room to new findings and further understanding for our Cosmos".
