Researchers estimated the effective diameter of the elliptical galaxy IC 1011 to be over 1.6 million light-years, confirming its status as the most extended of currently known galaxies. It is hypothesized that it will maintain its trend of increasing size by accreting surrounding diffuse matter. The results of this study are presented in a preprint on arXiv.org.
The size of galaxies in the Universe varies from a few tens of light-years for compact dwarfs to hundreds of thousands of light-years for massive elliptical systems. This size correlates closely with the galaxy's stellar mass and age, increasing due to active star formation and merger processes. Nevertheless, scientists do not yet have a definitive answer regarding the maximum sizes galaxies can reach, and searches for the largest systems, including central and bright cluster galaxies, are underway to answer this question.
A group of astronomers led by Carlos Marrero-de la Rosa from the Canary Institute of Astrophysics presented the results of analyzing deep images of the elliptical galaxy IC 1011. These observations were conducted between May 27 and May 30, 2022, using the WFC camera on the 2.5-meter INT telescope located at the Roque de los Muchachos Observatory. Archival data from the SDSS and DESI ground sky surveys were also utilized in the work.
IC 1011 is the brightest galaxy in the massive galaxy cluster Abell 2029, with a redshift of 0.077. Its total luminosity reaches $10^{12}$ solar luminosities, and its stellar emission was detected at a distance of 1.97 million light-years, making it one of the brightest central cluster galaxies and potentially the most extended. This galaxy hosts one of the largest galactic nuclei, which likely formed through the merger of a binary system of supermassive black holes and exhibits radio jet activity.
The main goal of the study was to estimate the galaxy's extent down to very low surface brightness values (29.5 magnitudes) and to search for any faint structures at its edges that might indicate past accretion or merger events. It was established that IC 1011 is indeed the most extended among known galaxies. Its effective radius is approximately 238 thousand light-years, and its outer radius is estimated at about 848 thousand light-years. The stellar mass within the outer radius reaches approximately $3.69 imes 10^{12}$ solar masses, while the total stellar mass in the region described by the surface brightness profile down to 29 magnitudes (with a radius of 2.127 million light-years) is $4.2 imes 10^{12}$ solar masses, including the contribution from the circumgalactic medium.
The parameters of IC 1011 align with the upper limit of the stellar mass-size relation, consistent with theoretical predictions. WFC observations revealed several faint structures near the galaxy, interpreted as traces of accretion by other galaxies and disturbances in the hot gas of the cluster. Considering previous X-ray data on core activity and jets, as well as the presence of large-scale spiral structures in the cluster center, these faint structures are considered evidence of a cluster merger with a subcluster or group of galaxies that occurred about 2–3 billion years ago. The transition from the main body of the central cluster galaxy to its outer, more rarefied envelope and further to the circumgalactic medium indicates that the galaxy continues to gain mass at its periphery.
Previously, astronomers recorded a jet system extending about seven megaparsecs from the active galaxy Porphyrion, demonstrating the ability of jets to remain stable over cosmological distances even during periods of denser universe states.
Separately, planetary scientists noted that Neptune's moon Nereid is the only intact regular satellite of the planet. Researchers concluded that Nereid formed concurrently with the ice giant, rather than being captured from outside or formed through collisions with other satellites. Its current irregular orbit is attributed to gravitational influences from Triton in the past. This information was published in the journal Science Advances.
The Milky Way may have undergone a drastic transformation in its orientation due to a significant collision that occurred approximately ten to eleven billion years ago. This hypothesis was recently proposed by astronomers from the University of Durham during the Royal Astronomical Society's National Astronomy Meeting, held in Birmingham.
The conclusion was reached through the analysis of cosmological simulations involving galaxies similar to ours, which were compared with data obtained by the Gaia mission, belonging to the European Space Agency. The main objective of this study is to elucidate why the stellar halo of the Milky Way exhibits a rotation rate considerably slower than predicted by researchers.
If this interpretation is validated, it will add a new segment to the narrative of galaxy formation and may provide clues about the evolution of dark matter. Furthermore, it suggests that the location of the Solar System within the Milky Way has not maintained a constant orientation over time.
To investigate the cause of the slow rotation of the stellar halo, scientists monitored the evolution of twenty-five galaxies with characteristics similar to the Milky Way using supercomputers over billions of years. The goal was to determine which events could result in a configuration similar to what is currently observed.
The comparison showed that galaxies whose stellar halos rotate more slowly possessed two notable characteristics: they had suffered a head-on impact with another galaxy and subsequently experienced an orientation change greater than 90 degrees in their disks, an event termed by astronomers as a 'disc flip.'
Kirill Batrakov, the researcher responsible for the study, pointed out that the history of the Milky Way itself points to the possibility of this process having occurred in our galaxy. He stated during the presentation at the Royal Astronomical Society's National Astronomy Meeting: 'We already know that the Milky Way underwent a massive head-on collision with a galaxy known as Gaia-Sausage-Enceladus. Therefore, we believe that the Milky Way's disk probably also changed orientation in the past.'
The researchers recall that Gaia-Sausage-Enceladus, nicknamed the 'Sausage Galaxy,' was a dwarf galaxy incorporated into the Milky Way between ten and eleven billion years ago. This encounter is considered the largest merger event in the early phase of our galaxy, leaving behind billions of stars in extremely elongated orbits.
According to the authors, identifying this ancient disk inversion expands the possibilities for understanding the behavior of the dark matter halo, as the findings indicate a strong correlation between the rotation of this invisible structure and that of the stellar halo.
Batrakov emphasizes that a modification of this magnitude would have altered the path of most stars in the Milky Way. He stated: 'A disk flip means that most stars in the Milky Way have traveled very different paths than those currently observed, possibly even our Sun, which indicates that our seemingly stable position in the galaxy may not have been the case throughout the entire existence of the Solar System.'
The astronomer also emphasizes that reconstructing such remote episodes aids in comparing the evolution of the Milky Way with that of other galaxies. Batrakov concluded during the dissemination of the study: 'Since we live inside the Milky Way, we can study it with a level of detail impossible in any other galaxy. Discovering that its disk changed orientation adds a new chapter to its history and shows that it is possible to reconstruct this complex past from current observations.'
