Galaxy merger

Description

During the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential begins changing so quickly that star orbits are greatly altered, and lose any trace of their prior orbit. This process is called “violent relaxation”. For example, when two disk galaxies collide they begin with their stars in an orderly rotation in the planes of the two separate disks. During the merger, that ordered motion is transformed into random energy (“thermalized”). The resultant galaxy is dominated by stars that orbit the galaxy in a complicated and random interacting network of orbits, which is what is observed in elliptical galaxies.

Mergers are also locations of extreme amounts of star formation. The star formation rate (SFR) during a major merger can reach thousands of solar masses worth of new stars each year, depending on the gas content of each galaxy and its redshift. Typical merger SFRs are less than 100 new solar masses per year. This is large compared to our Galaxy, which makes only a few new stars each year (~2 new stars). Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds. These collisions then induce condensations of these clouds into new stars. We can see this phenomenon in merging galaxies in the nearby universe. Yet, this process was more pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1–10 billion years ago, when there was much more gas (and thus more molecular clouds) in galaxies. Also, away from the center of the galaxy, gas clouds will run into each other, producing shocks which stimulate the formation of new stars in gas clouds. The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a major merger, and then a few billion years pass, the galaxy will have very few young stars (see Stellar evolution) left. This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched. This is the phase we call post-starbursts phase, also known as PSB phase. Although PSB phase can be triggered by multiple causes, galaxy merger of redshift around 0~2 is the most frequent cause of this phase and it can rapidly quench star formation than any other phenomenon in the universe.

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. One famous example of this simulation is probably from the merging of our Milky Way and Andromeda. The computer simulations using the information given by Hubble indicate that two billion years following the collision, the interacting galaxies will finish entirely fusing under the influence of gravity and transform into one elliptical galaxy resembling the ones frequently found in the local universe. During the last decade, we made huge progress toward the technique we used for simulating galaxy merger in the help of deep learning, particularly the convolutional neural network so it can also be utilized to deduce information regarding the timescales of merger events and as well as each of the stages that occur within the process. A research team was able to perform simulations utilizing Gadget N-body/SPH simulations to predict the relevant timescales for galaxy mergers of Sbc-Sbc and G-G types, with the first pass occurring between 0.39-1.28 Gyr after the beginning of the merger, the maximum separation at between 0.68-1.91 Gyr, and the merger event being considered to have occurred at 1.17-3.76 Gyr, depending on the subtype of the merger. Similarly such use of simulations can be extended to post-merger events as well. Another research team leveraged another simulation framework to largely divide the post-merger events into four stages and their time periods, with the entire post-merger process spanning 1.76 Gyr.

Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae. Such a library of galaxy merger simulations can be found on the GALMER website. A study led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Maryland created computer simulations in order to better understand images taken by the Hubble Space Telescope. Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining to an interaction between a giant galaxy and a tiny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles.

One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.