Why Everyone Is Getting The Evil Eye From This Galaxy

Charles Messier included M64 in his list of things that are prone to be mistaken for comets. With better telescopes, scientists began to note how dusty the galaxy's nucleus seems in comparison to those of other neighboring galaxies, which is especially noticeable on one half due to our viewpoint. Even stranger, the majority of its stars move around the galactic center in the opposite direction as the disk's outer, hydrogen-rich disk.

These traits were first thought to be the outcome of a recent galaxy merger, but more telltale indicators of this merger remained elusive. As a result, some astronomers proposed alternate explanations for why M64 may be so peculiar. A study that will appear in The Astrophysical Journal Letters offers proof that the original interpretation was accurate from the start, with some additional information.

The authors claim that the eating of a tiny galaxy caused M64 to have a radial shell pointing to the southeast and northwest of the spiral galaxy. The overall mass of the devoured object is close to 500 million solar masses, which is the same as the SMC when taking into account material that has already moved from the galaxy's outer shell to its inner disk. Similar to the SMC, it also has a low metal content, which suggests that star development is less rapid here than in the bigger galaxies.

Even though a galaxy merger is the most logical explanation for M64's anomalies, no counter-rotating stars were found during searches. We would still anticipate detecting some stars traveling in the direction of the caught gas even if the captured galaxy was primarily made up of gas and few stars. Although it is unknown why this would happen near M64 and not other galaxies, the notion that the backward-spinning gas was grabbed from the intergalactic medium was inspired by the absence.

Red giant branch (RGB) stars were sought for because they are bright enough to be seen individually even at the M64's 14.3 million lightyear distance. These were discovered to be two radial plumes that were extending asymmetrically from the primary galaxy. The northern plume is more diffuse yet reaches a distance of 330,000 light years, which is significantly farther than the Milky Way's diameter. The southern plume is sharp and spans 150,000 light years.

The two plumes are believed to have comparable masses while having different compositions; however, the northern one is more questionable. The authors state, "We calculate a stellar mass in the southern shell as = 1.80 0.54 108 [solar masses]." "We calculate a roughly similar star mass of 1.42 0.71 108 [solar masses] in the northern plume. We calculate that M64's interior contains an additional 2.13 0.64 108 [solar masses] of accreted stellar mass using the profile of inferred accreted stars in the inner disk.

The authors come to the conclusion that the plumes are the residue of two ancient galactic routes that split apart to create M64. The southern plume, which is still more closely bunched, is from a more recent encounter, which occurred less than a billion years ago. Although mergers are a common aspect of galaxy formation—the Milky Way has undergone several mergers—the majority of galaxies don't look like M64. The characteristics of the galaxy that was eaten and the angle at which it collided with M64 were necessary to produce the results we observe.

The authors state: "We propose that its outer gas disk was recently accreted during a 52:1 merger with an SMC-mass galaxy and is now colliding with an existing inner gas disk, fueling a burst of star formation at the disk-disk interface and driving the obvious dust lanes from which it earns its name. You may access the preprint at arXiv.