In a publication recently accepted in Astronomy and Astrophysics, an international team involving French and Canadian researchers using Canada-France-Hawaii Telescope (CFHT) have studied Malin 1, a nearby galaxy that has been known only since the 1980s and shows an extremely large disk of gas and stars. The new observations of Malin 1, a prototype of the class of “giant low surface brightness galaxies” allowed the team to obtain new results in contradiction with one of the hypotheses concerning the formation of this type of galaxy.
Low surface brightness galaxies are galaxies fainter than the brightness of the ambient night sky. Their mass to light ratios are very low — meaning very little of the mass of the galaxy comes from objects we can see — like stars or bright regions of gas. Up to 95 percent of the mass of these galaxies appears to come from dark matter, making them the perfect petri dishes for the study of dark matter.
Because they are very diffuse and faint, giant low surface brightness galaxies, while massive, are difficult to observe and are still poorly understood. They may represent a significant percentage of the galaxies in the universe. Because they are so faint, it is possible astronomers missed such objects in galaxy surveys.
It is important to study them and understand their formation and evolution. This is now possible owing to the new generation of telescopes and modern detectors, with higher sensitivity to low surface brightness than in the past.
The team’s research presents for the first time deep images obtained at six different wavelengths, from the ultraviolet of the GUViCS project to the optical and near-infrared obtained in the context of the Next Generation Virgo Survey with MegaCam on CFHT. Originally, these large observational campaigns were planned to study the Virgo cluster, but they also allow us to study objects in the background like Malin 1. The images offer a new view of this spectacular galaxy, the largest galactic disk known, with a diameter above 250 kilo-parsec. In comparison, our Milky Way is only about 30 kpc wide and a kilo-parsec is equal to 3262 light years.
The team of researchers extracted from the data the variation of the luminosity with the distance to the center of the galaxy, as well as the how the galaxy looks in different wavelengths of light. The colors of galaxies strongly depend on the star formation history. The comparison of the observations with predictions of various models allowed the team to estimate for the first time what must have been the history of star formation in the giant disk of Malin 1. It suggests that the giant disk has been in place for several gigayears, and that star formation proceed at a regular long-term rhythm despite the very low density.
This result is important as it contradicts a scenario proposed a few years ago predicting that these giant galaxies are formed during violent interactions. Moreover, in the context of the cosmological formation of galaxies, numerous interactions should have perturbed the disk of Malin 1. The formation of such a structure and its survival for very long time offers then a challenge for the simulations of the formation of galaxies run by astronomers.
What is the future of Malin 1? The giant disk contains a large quantity of gas in which star formation will keep proceeding at a low rate for billions of years, increasing progressively the stellar mass of the galaxy, unless another galaxy comes in the picture to interact with Malin 1 and totally change its destiny. Few galaxies, however, may play this role as Malin 1 is a relatively isolated galaxy.
