An international team of researchers from nearly 20 research institutions around the world, led by Chinese astronomers, used observations from the Hubble Space Telescope (HST), the Canada-France-Hawaii Telescope (CFHT), the Gemini North Telescope and the Burrell Schmidt Telescope to discover the complete process of dwarf galaxies being disintegrated and peeled apart in the dense environment of galaxy clusters and gradually evolving into ultra-dense dwarf galaxies. It reveals the mystery of the origin of ultra-compact dwarf galaxies that has puzzled astronomers for a long time. The findings were published in the November 9, 2023 issue of the scientific journal Nature. Kaixiang Wang, a Ph.D. student in the Department of Astronomy and the Covli Institute of Astronomy and Astrophysics, School of Physics, Peking University, is the first author and co-corresponding author of the paper.
With the development of astronomical observation equipment and technology, astronomers have discovered many galaxies with extreme properties, such as ultra-compact dwarf galaxies (UCDs) and ultra-diffuse galaxies (UDGs). The discovery of extreme galaxies has enriched the large galaxy families, and also challenged the existing theories of galaxy formation and evolution, and the study of these extreme galaxies is of great significance for the improvement of the theory of galaxy formation and evolution.
For a long time, galaxies and star clusters were considered two distinct types of objects. Galaxies are born and grow in dark matter halos, and have a more complex history of star formation. Star clusters arise from the massive molecular clouds that lie within galaxies. Around 2000, ultra-dense dwarf galaxies were discovered for the first time, and their properties are somewhere between galaxies and star clusters, blurring the originally clear boundaries between galaxies and clusters. The origin of ultra-compact dwarf galaxies has also been controversial, with one side believing that super-compact dwarf galaxies originated from massive globular clusters, while the other side believes that super-compact dwarf galaxies are central nuclear clusters left over from the process of accretion and merger of ordinary nucleated dwarf galaxies. In recent years, although more and more research results support the origin of some ultra-compact dwarf galaxies from stripped dwarf galaxies, the specific process of evolution of dwarf galaxies into ultra-compact dwarf galaxies has not been confirmed by observation.
In recent years, using observations from the next-generation Virgo Cluster Survey (NGVS), the research team has found about 600 ultra-compact dwarf galaxy candidates in the Virgo Cluster. Through spectral identification and comprehensive and detailed morphological analysis, it was found that about 15% of ultra-dense dwarf galaxies are surrounded by faint stellar halos. These ultra-dense dwarf galaxies are highly correlated with the newly defined class of "strongly nucleated dwarf galaxies" in terms of morphology, color, spatial distribution and other characteristics, and the strong-nucleated dwarf galaxies are likely to be an intermediate stage in the evolution of dwarf galaxies into ultra-compact dwarf galaxies. In addition, researchers have found extremely faint "S"-shaped stream structures around certain strong-core dwarf galaxies and ultra-compact dwarf galaxies, which is direct evidence that they are being disintegrated by the tides.
Figure 1: The two columns on the left are optical images of some strong-nured dwarf galaxies and ultra-dense dwarf galaxies discovered in the study. On the right is their spatial distribution in the central region of the Virgo Cluster, which is mostly concentrated around the giant elliptical galaxy M87, indicating that the dense environment at the center of the cluster plays an important role in their formation.
"The angular diameter of the ultra-compact dwarf galaxies in the Virgo Cluster is close to the limit of the resolution of large terrestrial telescopes, and the surface brightness of their stellar halos and tidal tails is hundreds of times darker than that of the night sky," Wang said. Therefore, the observation of ultra-dense dwarf galaxies and their surrounding faint structures has very high requirements for the spatial resolution and exposure depth of the observed data, and the later data processing process also needs to be detailed enough. In this work, we observed for the first time the stages of the formation of ultra-compact dwarf galaxies, like time slices, depicting the transformation from ordinary dwarf galaxies to ultra-compact dwarf galaxies (and even star clusters).
The co-corresponding author of the paper, former postdoctoral fellow in the Department of Astronomy of the School of Physics of Peking University and now associate researcher of the School of Physics and Astronomy of Shanghai Jiao Tong University, Liu Chengze, said: "The main research object of this work is ultra-dense dwarf galaxies with faint star halos, and the previous work on this special ultra-compact dwarf galaxy is mostly limited to individuals. This is the first time that morphological studies and surface brightness profile analysis of this special type of ultra-dense dwarf galaxy based on complete samples have been carried out, and the results are statistically significant.
Interestingly, the research team found that the most diffuse galaxy in the universe (the super-diffuse galaxy UDG) and the most compact galaxy (the ultra-dense dwarf galaxy UCD) may be homologous, both of which evolved from ordinary galaxies. Hyperdiffuse galaxies can be an intermediate stage in the evolution of dwarf galaxies into ultra-compact dwarf galaxies. When a large amount of dark matter in a galaxy is first stripped away by the tides, the distribution of stars within the galaxy becomes more diffuse than before. When the outer stars of the galaxy are gradually stripped away, the nuclear cluster that leaves the center becomes an ultra-compact dwarf galaxy. This study further refines the evolution trajectories from some dwarf galaxies to ultra-compact dwarf galaxies: ordinary nucleated dwarf galaxies→ nucleated hyperdiffuse galaxies/low-surface brightness galaxies→ strong-nucleated dwarf galaxies→ ultra-compact dwarf galaxies with faint stellar halos→ ultra-compact dwarf galaxies.
Figure 2: Observed schematic diagram of the stages of the disintegration of normal dwarf galaxies to form ultra-compact dwarf galaxies, which will last about 2 billion to 3 billion years. A significant number of the hundreds of ultracompact dwarf galaxies around the M87 galaxy may have come from this mechanism. Drafting: Wang Kaixiang; Background image credit: NASA/R. Gendler.
Another co-corresponding author, Eric Peng, a former tenured associate professor in the Department of Astronomy at Peking University's School of Physics and now an astronomer at the Optical-Infrared Astronomy Laboratory (NOIRLab) in the United States, added: "Studying the origin of ultra-compact dwarf galaxies is important for understanding the formation and evolution of galaxies. In addition, ultra-compact dwarf galaxies are also important probes for the archaeological study of galaxy clusters. The Virgo Cluster may have experienced a relatively large merger event in the last 2 billion to 3 billion years, with hundreds of galaxies falling near the center of the Virgo Cluster, galaxy M87. The halos of some dwarf galaxies were stripped away during the merger, and the nucleus of the galaxy with a compact center evolved into an ultra-dense dwarf galaxy. At the same time, stripped stars disperse around M87 and become part of the intracluster light (ICL). We estimate that about 30% of the stellar mass in the outer star halo of M87 comes from these merged dwarf galaxies, which provides an important reference for understanding the merger growth history of giant elliptical galaxies.
It is understood that the research team will continue to use a number of international large astronomical telescopes, including the Webb Space Telescope (JWST) and the Hubble Space Telescope, to conduct follow-up observations of ultra-dense dwarf galaxies, and look forward to revealing more scientific mysteries related to them.