While mapping the Triangle galaxy, a team of astronomers discovered that its structure is different depending on the age of the stars observed and that it has changed dramatically in the recent past.
The Triangle galaxy, also called Messier 33 (M33), is the third most massive galaxy in the Local Group (after the Andromeda galaxy and the Milky Way) and the smallest spiral galaxy in this group (although the Small and Large Magellanic Clouds may have been spirals prior to their encounter with the Milky Way).
M33, located about 2.7 million light-years from us, is probably a satellite – the largest – of the Andromeda galaxy, which makes it the analog for the latter of the Large Magellanic Cloud for the Milky Way. However, unlike the Magellanic Clouds, which are irregular and distorted by each other and by the Milky Way, the Triangle galaxy is an elegant fluffy (or flocculant) dwarf spiral with many arms.
Different structures depending on the age of stars
The Triangle Galaxy presents a unique opportunity to study spiral galaxies. Smaller star-forming dwarfs tend to be irregular. Large, unperturbed dwarf spirals like M33 could therefore help us understand how these patterns form and change.
To this end, astronomers examined M33 with the Hubble Space Telescope as part of the Panchromatic Hubble Andromeda Treasury : Triangle Extended Region (Phatter), an extension of the study of the Andromeda galaxy called Panchromatic Hubble Andromeda Treasury (Phat). In Phatter’s case, the researchers created a mosaic of the galaxy’s central roughly 10,000 light-years by combining 54 individual Hubble fields, revealing 22 million individual stars in M33.
The researchers divided these stars into four age groups: those that are less than 100 million years old, those that are several hundred million years old, those that are around 1 billion years old and finally, those that are several billion years. Where the younger stars traced the usual fluffy multi-armed spiral, the two clusters of older stars traced a large two-armed spiral with a central bar. Intermediate-aged stars fall between the two.
As Adam Smercina of the University of Washington’s Astronomy Department summed it up in a Jan. 11 press conference during the winter meeting of theAmerican Astronomical Society in Seattle, the structure of M33 really depends on the stellar population you are looking at ».
The researchers saw the first signs of this hidden structure in 2021. The more detailed analysis, which they are now writing in a new paper, confirms the two-armed barred pattern and links it to larger-scale galaxy structure. .
A galaxy that has recently changed in appearance
Old stars may not have been born by tracing their current structure; something might have happened after they were born to rearrange them. Multi-wavelength observations of the stars and gas in the periphery of M33 indicate that the galaxy has a large distortion and the spiral arms also show a kind of asymmetry: for Smercina, ” all evidence points to M33 hitting or interacting with something not too long ago “. Tidal forces from this encounter could have rippled the disc, stimulating a two-armed spiral. Stars born long after the event would not have been rearranged, which would explain why today’s disk looks so different.
Another possibility is that the two-armed spiral is a dominant long-lived pattern recently obscured by gas and younger stars. The fact is that the old arms have not totally disappeared: ” Young stars are forming there. It’s just that they also form in other places », explains Smercina. For some reason, the gas might have taken on a different structure than the underlying waves propagating through the disk.
Work is underway to study Phatter stars spectroscopically to determine how they move through space. Any structure in this movement could help understand why M33 changed its face.
