It was unexpected: near the Andromeda galaxy, yet studied for centuries, amateur astronomers discovered a huge cloud of gas. And it doesn’t look like any known astronomical object!
Its first known written mention dates back to 964, more than a millennium before: the Andromeda galaxy, also designated as M31, was one of the first deep sky objects to be studied in detail. Visible to the naked eye on particularly clear nights in thenorthern hemispherenorthern hemispherethis galaxy could be designated as a “neighbor” of ours, being the spiral galaxyspiral galaxy closest to the Milky WayMilky Way. So much so that it has been the subject of numerous publications, scrutinizing it and detailing its composition. But a new study published in RNAAS reports an astonishing discovery, and above all very unexpected: a strange cloudcloud of gazgaz never observed before is on the outskirts of this galaxy. Never seen !
Late detection caused by its composition
It all started when astronomersastronomers amateurs have carried out various optical surveys in the direction of the Andromeda galaxy, located at 2.55 millionlight yearslight years from U.S. Through the use of different imaging systems, they targeted several lines ofemissionemissionof which Hα, characteristic of thehydrogenhydrogen ionized gas, the main component of most gas clouds, and others less present in nebulae. With 22 nights of observations between August and October 2022 on “various dark observation sites in Lorraine, France”, they obtained Hα and O III emission spectra, characteristic of theoxygenoxygen doubly ionized, denoted O2+.
And that’s where they saw a large O III emission nebulosity, located one degree southeast of the Andromeda galaxy. To confirm their discovery, the astronomers took new images, centered on another position. The same cloud appeared, thus revealing that it was not an equipment artifact or was not from a starstar or of M31 itself. Finally, new confirmations came from different places in the United States, obtained thanks to other astronomers.
We do not know its origin
As the study explains, this cloudiness “has no obvious emission equivalent in the readings at X-raysX-rays, UVUVoptical, infrared and radioradio » et “has not been previously reported in the literature”. A question then remains unanswered: why was this strange cloud not detected before? Astronomers say this is because of its low brightness and wide angular size, making it difficult to detect with most imaging systems.
The authors then discuss the possible origin of the cloud, without concluding on a preferred scenario. It could be a planetary nebula, because the filamentous structures of the nebulosity resemble it. But the abundance ratio of doubly ionized oxygen to ionized hydrogen implies a temperature above 60,000 KelvinsKelvinswhich no surrounding astronomical object could generate.
Then there remains the hypothesis of a remanentremanent of supernovasupernova which, this time, may well create the right ratio of abundance. But such a scenario does not explain the absence of coincident radio or UV emissions. Finally, an interaction between the andromeda galaxyandromeda galaxy and the Milky Way could create cloudiness similar to that observed. But there again, problem: the cloud is far too close to M31, in its very stellar halo! The study concludes on the need to know the radial velocity of the cloud which would make it possible to know more about its interactions with M31 and its halo, it is currently in the process of being obtained.
