Galaxy harassment

Galaxy harassment is when a galaxy undergoes frequent high speed galaxy collisions within galaxy clusters. The processes of galaxy harassment can change the morphology of the galaxy. This includes such effects includes triggering starbursts, exceptionally high rates of star formation. It also causes morphological changes in the galaxy too such as spiral galaxies morphing into dwarf elliptical galaxies.^[1] Galaxy harassment is different from galactic mergers where mergers often transform disk galaxies into elliptical galaxies or spheroidal galaxies while galaxy harassment are rarely direct hits^[2] and leaves behind detectable arcs of debris. These debris arcs could also provide fuel for quasars in host galaxies.^[3]

A trio of galaxies known as SDSSCGB 10188. Galaxies that get harassed undergo many collisions with other galaxies.

Effects

The mechanisms of galaxy harassment are not significant towards the outskirts of the galaxy cluster due to the smaller number of galaxies that exist. However, towards the center of the cluster, galaxy harassment becomes a significant process altering the morphology of a galaxy.^[4] In fact, galaxy harassment drives the morphological evolution of galaxies in clusters.^[3] Galaxies towards the outskirts of the galaxy cluster that experience harassment tend to loss only <6% of the total stellar mass while galaxies towards the center of the cluster can loss up to 50% of their total stellar mass. The inclination of a disk galaxy can reduce the mass loss of a galaxy, yet amplify the thickening of the galaxies disk.^[5]

A example of the effects of galaxy harassment can be seen when comparing distant galaxy clusters past redshift distance z=0.4 to modern galaxy cluster we can see. Galaxy protoclusters at high distances are filled with small spiral galaxies showing signs of multiple phases of starburst activity but over time, galaxy harassment have transformed many of them into spheroidal galaxies.^[6]

Globular clusters

Galaxy harassment can also affect the globular clusters (GCs) surrounding galaxies. They are strongly effected by the remaining dark matter left after a harassment. When dark matter around the galaxy falls to 15% do globular clusters start to get stripped and quickly separate from galaxies. When dark matter falls to just <3%, the galaxy is close to disruption.^[7]

Examples

A example of a harassed galaxy, the Cartwheel galaxy.

There are several examples of galaxies that have experienced galaxy harassment. One example is NGC 4254, also known as Messier 99. It is a spiral galaxy located in the Virgo cluster that is believed to have been harassed by a galaxy. This caused the galaxy to experience turbulence and alter the fine scale structure of the galaxies interstellar medium (ISM) suggesting that harassment effects for than just the morphology of the galaxy.^[8]

Another example of a galaxy that was harassed in the Cartwheel galaxy, a lenticular galaxy with a noticeable ring around it making it also a ring galaxy. Its features were formed from a high speed collision with another smaller galaxy transforming it from a Milky Way like galaxy to its current shape.^[9][10]