It doesn’t take much of an imagination to see why M64 is better known as the Evil Eye galaxy. Sitting in the abyssal vacuum of space it seems to cast a sinister glare over the cosmos, a cloud of dust framing its visible periphery like a dark bruise.
The galaxy is even stranger than it seems. The hydrogen-rich outer disk rotates in the opposite direction to the inner disk of the galaxy’s stars, suggesting differences in their origins.
Now, astronomers have discovered that the outer disk’s hydrogen gas comes from a smaller, gas-rich satellite dwarf galaxy, an M64 that was recently cannibalized and wrapped in it. It’s the material from that smaller galaxy obscuring the outskirts of M64.
The discovery could give us a glimpse into the future of our own Milky Way galaxy. Estimates of the fragmented satellite’s mass and content suggest it was remarkably similar to the Small Magellanic Cloud, a dwarf galaxy of the Milky Way that will one day be absorbed into the larger mass.
These findings, led by astronomer Adam Smercina of the University of Washington, have been accepted in The Astrophysical Journal Lettersand are available on the preprint server arXiv.
“We find evidence for a spectacular scaling function and other tidal structures suggesting sustained late-stage radial fusion,” write the researchers.
“We estimate the stellar mass of the precursor galaxy at 500 million solar masses, with a metallicity of [M/H] ≃ −1 – very similar to the mass and metallicity of the Small Magellanic Cloud.
“The mass of M64’s counter-rotating outer gas disk is also similar to the gas mass of the Small Magellanic Cloud, suggesting that the most likely origin of M64’s unique counter-rotating disk was a recent merger with a gas-rich satellite very similar to the Small Magellanic Cloud.”
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Galaxy mergers are considered a very important part of the growth and evolution of galaxies. These events not only increase a galaxy’s mass, they imbue it with new material and gravitational interactions that increase star formation, keeping the galaxy alive with new generations of stars.
The Milky Way has merged with other galaxies several times in its long history of 13.6 billion years, and we can see other galaxies in the Universe at different stages of the merger process.
Since galactic cannibalism is not uncommon, it seemed reasonable to think that the strange gas at the edge of M64 was the result of one of these events.
However, previous studies found surprisingly few signs of a recent galaxy-galaxy interaction, leading to the suggestion that the hydrogen disk had just been gobbled up from space around the galaxy, or from a brief fly-by encounter with another galaxy called Coma P.
However, evidence for any of these scenarios was also lacking; so Smercina and his colleagues enlisted the Subaru telescope’s Hyper Suprime-Cam to study the space around M64, some 1.4 million light-years away, looking for signs of the origin of the outer disk in lone stars far from the main disk of the galaxy.
Their observations revealed what they believe are clear traces of a galactic merger. They found one galactic halo – a diffuse sphere of dark matter, gas and stars that encapsulates spiral galaxies, which are thought to form and grow by galaxy mergers.
But they also found evidence of a grenade. This is a density of stars produced by the gravitational interaction between two merging galaxies. As the galaxies orbit each other, their gravitational interaction creates resistance that causes stars to be pushed together, somewhat like the crests of ripples in a pond.
Comparison with simulations showed that the distribution of scale around M64 is consistent with what is known as a “minor” merger; that is, the absorption of a satellite dwarf galaxy by the larger host.
The motion of the gas suggests that this satellite had a partially retrograde orbit around M64. And the dusty center and dark streaks of dust are the result of the ongoing fusion as material collides with M64’s inner gas disk.
To confirm this scenario, the researchers used data from Hubble to estimate the mass and composition of the absorbed galaxy. They deduced a stellar mass of about 500 million solar masses, consistent with the 300 million Unpleasant a billion solar masses of the Small Magellanic Cloud.
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The mass of the hydrogen disk around M64 is also about 500 million solar masses, which again, the researchers say, is consistent with the 400 million solar masses of gas in the Small Magellanic Cloud. Taken together, the clues seem pretty convincing to point to galactic cannibalism recent enough to have left these marks – within the past billion years.
“The probable origin of the Evil Eye’s counter-rotating gas is finally revealed, after decades of curiosity,” write the researchers.
“We suggest that its outer disk of gas recently accreted during a 52:1 merger with a small Magellanic cloud-mass galaxy and is now colliding with an existing inner disk of gas, fueling a burst of star formation at the disk-disk interface and driving the visible dust lanes from which it takes its name. Future observational and theoretical studies will help test this idea.”
Meanwhile, M64 may help us understand how stellar halos are formed. Being able to calculate the past orbital motion of a now-depleted satellite galaxy could help reproduce M64’s halo evolution, providing valuable insight into the growth of these mysterious structures.
The team’s research has been accepted The Astrophysical Journal Lettersand is available at arXiv.