Warum ist Malin 1 sechsmal breiter als die Milchstraße

Malin 1 spans a staggering 650,000 light-years, making it approximately six times wider than the Milky Way. While it initially appears to be a standard spiral galaxy about 30,000 light-years wide, extremely long-exposure imaging reveals intricate, dim extensions that push its boundaries to an immense scale.

The difficulty of defining galactic boundaries

Defining a galaxy is notoriously difficult because these objects lack the distinct edges found on planets or moons. Instead, a galaxy's distribution of stars simply thins out as distance from the center increases. This attenuation creates fuzzy boundaries that are easily lost in the foreground of stars within our own Milky Way. Astronomers often rely on "surface brightness"—the amount of light falling within a small square arcsecond—to estimate where a galaxy's main body ends and the background sky begins.

Measuring true physical dimensions also requires accounting for distance via redshift. While uncertainties in the expansion history of the universe can complicate measurements at distances of many billions of light-years, we can still reliably identify outliers that defy standard galactic proportions.

Giant low surface brightness galaxies defy merger logic

Malin 1 is the first "giant low surface brightness" galaxy ever discovered, and it presents a significant structural mystery. Typically, galaxies reach massive sizes through mergers with other galaxies, a process that leaves behind visible distortions in their large-scale structures. Malin 1, however, maintains delicate, undisturbed spiral arms despite its enormous width. It also appears relatively isolated, lacking the nearby galactic neighbors required to fuel such growth through traditional accretion.

Another massive outlier is UGC 2885, also known as Rubin's galaxy, which sits 230 million light-years away and measures nearly 450,000 light-years in width. Like Malin 1, its overgrown size and relative isolation suggest that our current models for galactic evolution may be missing a key mechanism for isolated, massive growth.

While cosmic collisions in galaxies like the Tadpole or Condor can temporarily stretch spiral arms into long tidal tails, these "train wrecks" are exceptions rather than the rule. The existence of stable, massive, and isolated spirals suggests that the upper limits of galactic scale are much higher than previously thought.

Source: How big can a galaxy get?
Domain: scientificamerican.com