The fate of stars born in gas-rich high redshift galaxies

Abstract: One of the greatest challenges astronomers are facing today is understanding how galaxies form. We know that the Milky Way is a spiral disk galaxy, similar to many others we see in the sky, therefore the Milky Way is a benchmark for understanding the inner workings of disk galaxies. The task of unraveling the mystery of the formation of the Milky Way has been given to astronomers, who attempt to construct models of the Galaxy’s evolution based on its present appearance. These models need to account for not only the large-scale gravitational forces involved in assembling the Galaxy, but also the chemical composition and motion of one of its primary components: the stars. It turns out that the chemistry and kinematics of the stars hold clues to how the Galaxy was made and how it has changed through time. Constraining the different properties of stars, one can subdivide the spiral disk of the Galaxy into a thin and thick disk. The thick disk mostly includes older stars that have a bigger spread in vertical velocity (= kinematically hot), whereas the thin disk has younger stars that have a smaller spread (= kinematically cold). But how did these two disks form and why are they so different? There are multiple theories going around on how the thick disk has been formed. Some models assume a continuous evolutionary transition in the formation of the thick disk and the thin disk. Others believe that a component of the thick disk had its origin in a merger event where two galaxies collided. In this work we explored the possibility of the thin and thick disk properties to be formed in a simulation without any influence from outside the Galaxy. When the Milky Way was younger it consisted of more gas, so stars had more opportunities to get kinematically hot by the turbulence of the gas. This leads to older stars having a higher velocity dispersion at birth than the stars born more recently, but also their orbital properties such as eccentricity change. Using simulations of Milky Way-like galaxies we calculated this birth velocity dispersion at different times in our galaxy’s past. Many scientists focus on the secular heating, the physical process after the star is born where the star gets more kinematically hot due to interactions with gas clouds, spiral arms of the Galaxy etc., of the star to explain the difference between the thick and thin disk. In this project, we had another approach, and looked at the heating by the turbulence at birth. In figure 1 we see the relationship between the birth velocity dispersion and age indicated by the black line, the gray area illustrates the margin of error of this relation. The colored blocks indicate different gas fraction environment the stars are born in. Using this figure we can see that turbulent heating plays a very big role in the general heating of older stars. There is relations between age and birth velocity dispersion, stars born in a higher gas fraction galaxy have a higher vertical velocity dispersion. Showing us that at least a part of the question: "How did the different disks form?", can be answered using turbulent heating.