Creating ultra-diffuse galaxies

A optical image of GMP 4348, an ultra-diffuse galaxie in the Coma group of galaxies. The scale bar indicates a distance of thirty two thousand light-years. Astronomers concluded that eleven of these galaxies were affected by ram-stripping gas, which caused them to stop star formation and grow, making them ultra-diffuse.
Ultra-diffuse galaxies have low luminosities and comparatively few stars. They also show less star-formation activity than normal galaxies with similar sizes. UDGs are common in galaxy clusters. They come in many shapes and sizes. Some of them are smooth and round like dwarf ellipticals galaxies. Others have distorted shapes due to tidal disturbances. These galaxies are not only interesting, but also important for astronomers. Their diffuse structures are useful in models that seek to find information about dark matter halos.

It is not clear how UDGs evolved. Sometimes they resemble dwarf elliptical galacticessmall, faint and distant elliptical galaxyes that formed early in cosmic time but did not merge with larger galaxies. This suggests that some UDGs may have primordial roots. They may have been inflated from their dwarf-galaxy sizes by supernovae, which may have stopped further star formation. Some astronomers believe UDGs are "failed galactic systems", but tidal interactions or other peculiar properties may have also played a role. CfA astronomers Dan Fabricant, Igor Chilingarian and Sean Moran used the Binospec instrument of the MMT to study faint, diffuse galaxies within the Coma and Abell 2147 clusters. The eleven objects were diffuse, low-luminosity and had little star-formation. They also had stars that averaged 1.5 billion years old, which indicates that these galaxies are currently in a post-starburst stage. The objects have also had recent encounters to another galaxy, and all of them show signs of recent star formation.

Astronomers were able to use the deep optical spectra from these post-starburst galaxyes to model the stellar history and kinematics of each object. The spectra showed the existence of rotating stellar discs, which are believed to contain as much as 95% of dark matter. Scientists propose that these galaxies were formed around twelve billion years ago and started making stars. Between two hundred million and one billion years ago, star formation was stopped by bursts of star-formation triggered in part by ram pressure from interactions with other galaxies within their clusters. It appears that all eleven objects formed this way. Because the galaxy sample size was statistically significant for all the clusters studied, the team concluded that ram pressure processes caused the galaxies to puff up and eventually to become UDGs. About half of all UDGs in the world today likely arose from similar processes.

Nature Astronomy published the reearch.

Continue reading Astronomers explain the origin of ultradiffuse galaxies

Further information: Kirill. A. Grishin and colleagues, Transforming low-mass gas-rich galaxies into extremely diffuse galaxies using ram pressure, Nature Astronomy (2021). Information from Nature Astronomy Kirill A. Grishin and colleagues, Transforming low-mass gas disky galaxies to ultra-diffuse galaxies using ram pressure (2021). DOI: 10.1038/s41550-021-01470-5