Forty years ago, astronomers using new sensitive imaging techniques discovered a class of large, faint galaxies that they called low-surface-brightness galaxies. Giant galaxies with low brightness (gLSBG) are a subset whose masses are comparable to the Milky Way, but whose radii are ten times larger than four hundred thousand light-years. These gLSBGs pose a problem for astronomers: although they are massive, galactic disks are (kinematically speaking) relatively inactive. The usual paradigm for the formation of high-mass galaxies imagines that they develop from galaxy mergers, a process that moves the disk and must make it kinematically active. Moreover, most gLSBGs were not found in other environments in other galaxies, suggesting that the collisions may not have been important in their formation.
The question of how gLSBGs are formed is a matter of active debate. Two popular models are offered. In the first, non-catastrophic scenario, the slow accumulation of gas on the galaxy leads to its growth. Alternatively, the catastrophic scenario, a merger has occurred in the past; the main advantage of this model is that it fits into the current framework for galaxy formation. CfA astronomer Igor Chilingaryan and his colleagues have completed sensitive optical observations of seven gLSBGs, taking spectra across the full diameters of these weak giant systems and combining their results with archival optical and radio measurements of atomic hydrogen emissions. Their new report is the latest in a series of results for gLSBG.
Astronomers used the large data set to test these two scenarios; they also consider a third option, in which galaxies form within an unusually shallow halo of dark matter and its gravitational influence. (All galaxies are thought to have halos of dark matter; the halo of the Milky Way contains ten times as much mass as the stars.) They conclude that all three scenarios appear to work, but in different situations. For most of their sample, the most likely process is growth growth by gradual growth after the initial formation of the galaxy. For the other gLSBGs, the main merger scenario better explains the observations, although in a few cases they found that the halo of rare dark matter may also play a role. Scientists have also found that at least six of their seven gLSBGs are active galactic nuclei (AGNs), but their supermassive black hole nuclei are much less massive than those in normal galaxies of similar mass, suggesting that mergers, even if were involved in the formation of gLSBGs, must have been relatively modest.
Dark matter and massive galaxies
Anna S Saburova et al, Observational insights into the origin of low-brightness giant galaxies, Monthly notices of the Royal Astronomical Society (2021). DOI: 10.1093 / mnras / stab374
Provided by the Harvard-Smithsonian Center for Astrophysics
Quote: Giant galaxies with low surface brightness (2021, June 4) extracted on June 5, 2021 from https://phys.org/news/2021-06-giant-low-surface-brightness-galaxies.html
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