A Cosmic Contrail: Clues from a Flyby in NGC 3627
In their new study, Zhao and collaborators present the discovery of a galactic-scale contrail in the spiral galaxy NGC 3627. Unlike contrails left by airplanes on Earth, this one is a thin structure of gas and dust stretching more than 8,000 light-years across space. It is far larger than similar structures seen in the Milky Way and could provide rare evidence of a massive object passing through a galaxy disk.
Setting the Stage: Why Contrails Matter
Galaxies are vast systems made of stars, gas, and dark matter, and they are where most star formation in the universe happens. Inside their disks, gas typically follows the curved pattern of spiral arms. Yet, astronomers sometimes find long, straight features called molecular contrails. These are puzzling because they don’t match the normal motions of gas. Previous research suggested that such contrails might be carved when a dense object, like a black hole or a small galaxy, plows through the interstellar medium. If true, contrails become powerful tools for spotting otherwise hidden companions around galaxies.
A Surprising Find in NGC 3627
NGC 3627, located about 31 million light-years away in the constellation Leo, is a barred spiral galaxy with active star formation. Using observations from two major surveys, PHANGS-ALMA for molecular gas and PHANGS-JWST for infrared dust, Zhao’s team noticed a strikingly narrow feature in the galaxy. This contrail is at least 6 kiloparsecs long (almost 20,000 light-years) but only about 200 parsecs wide. Its gas shows an unusually flat velocity gradient compared to the nearby spiral arms, and star formation within it is surprisingly weak. These traits set it apart from the galaxy’s normal structures.
Evidence for a Flyby Event
The team argues that the contrail likely formed when a compact object, such as a massive black hole or the nucleus of a dwarf galaxy, shot through the disk at speeds of at least 300 km/s. As it passed, its gravity compressed the warm, diffuse hydrogen gas, causing it to cool rapidly and condense into molecules of CO, the material now visible as the contrail. Measurements show the gas is turbulent at speeds higher than the sound speed of warm gas, a strong sign of shock and compression from such an encounter. The galaxy’s magnetic fields also appear aligned with the contrail, suggesting that they were shaped as the gas contracted.
Timescales and Mass Constraints
By comparing the contrail’s width and velocity dispersion, Zhao estimates its lifetime to be only about 20 million years. This short timescale implies the flyby event was recent. Calculations suggest the flyby object must have had a mass of about ten million times that of the Sun, consistent with either a massive black hole or the dense central core of a dwarf galaxy. Detecting the responsible object directly is difficult, however, since faint dwarfs at NGC 3627’s distance are nearly invisible with current surveys.
Implications and Future Searches
This discovery positions contrails as a new way to find hidden massive objects orbiting or passing through galaxies. Candidates include small galaxies too faint to spot directly, rogue black holes, or even exotic compact systems like the recently discovered “little red dots” in the distant universe. Future deep surveys, such as with the Rubin Observatory’s LSST, could help trace any stellar remnants of the object along the contrail. Meanwhile, higher-resolution ALMA observations might uncover the precise kinematics of the gas, giving further clues about the perturber’s identity.
Source: Zhao
