&Bullet; physics 14, 81
Astronomers have identified spiral arm features in a galaxy from the early universe, suggesting that these arms may evolve faster than some theories predict.
When we picture a galaxy, we often think of it with beautiful spiral arms. How these spiral features form, however, remains unclear. Are spiral arms the last step in a long, steady development, or do they come and go in cycles like “galactic seasons”? One way to answer these questions is to study the earliest spiral galaxies in the universe. A new study that appears in Science has identified the earliest spiral arm features observed to date in a galaxy. The timing of the arms’ appearance – 1.4 billion years after the Big Bang – suggests that spiral galaxies can evolve faster than some galactic models predict.
Spiral galaxies are widespread in the nearby universe, with around 70% of the galaxies observed, including our Milky Way, bearing spiral arms. But the further the astronomers look into the distance – or correspondingly back in time – the proportion of spirals seems to decrease. “It may be due to the limitation of observation – we need a high angular resolution and sensitivity to identify spiral structures in observed images,” says co-author Takafumi Tsukui, a doctoral student at SOKENDAI, the Graduate University for Advanced Studies in Japan.
But it could be that spiral structures cannot form in young galaxies, as the currently prevailing theory for spiral arms, the spiral density wave theory, suggests. This theory suggests that arms can only arise in galaxies that have had time to settle in stable, rotating disks. “Theorists believe that spiral arms take a long time to form,” says co-author Satoru Iguchi, an astronomer with the National Astronomical Observatory of Japan. How long exactly is still a matter of dispute.
Earlier work discovered spiral arm features in a galaxy 11.3 billion years ago. Since the galaxy must have formed sometime after the Big Bang (13.8 billion years ago), the spiral arm formation would have to be faster than about 2 billion years. Tsukui and Iguchi cut that time window for formation by roughly half by identifying spiral arm morphology in an even earlier galaxy. The astronomers examined archive data from the Atacama Large Millimeter / submillimeter Array (ALMA) – a radio observatory in Chile. In particular, they examined a handful of galaxies whose light emission contains lines of ionized carbon found in clouds of gas around young stars. One of the galaxies, BRI 1335-0417, appeared to have two spiral arms extending about 15,000 light years from the center.
The spiral features appear noticeable at carbon wavelengths because the ion emission is concentrated in regions of dense stellar activity, such as spiral arms. To confirm the presence of the arms, Tsukui and Iguchi continued to analyze data from this galaxy and determine the velocity of material in different regions of the galaxy. This velocity map showed that BRI 1335-0417 has a spinning disk as one would expect for a spiral galaxy. The map also showed that the velocity is uniform along the arms, suggesting that these are bound structures within the galaxy.
The galaxy expert Andreas Burkert from the Ludwig Maximilians University in Munich says it is interesting that a spiral galaxy appears so early in the universe when galaxies frequently merge with other galaxies. He explains that spiral arms are believed to be formed from gravitational instabilities within a galaxy disk. But these instabilities cannot grow into arms until the disc has achieved some kind of balance between gravity, pressure, and shear forces. “Fine-tuning is still necessary,” says Burkert, who apparently succeeded in the observed disc after analyzing it by Tsukui and Iguchi. “It is surprising, however, that this young disc has regulated itself to the disc equilibrium state,” says Burkert. Whether or not this early regulation is rare he cannot say, as simulations of the early universe are not yet detailed enough to calculate the dynamics within galactic disks.
But maybe equilibrium conditions aren’t necessary, says Jerry Sellwood of the University of Arizona. Sellwood has worked on spiral arm models for 40 years and today believes that spiral arms can form in less quiet conditions. “The process of galaxy formation is chaotic,” he explains. And this clutter creates disturbances that can induce spiraling patterns that come and go at a rapid rate of fluctuation. “The Milky Way has probably had about 25 different spiral patterns in its lifetime,” Sellwood says. This recurring spiral model could easily explain a galaxy with spiral arms within a billion years of its formation.
However, Sellwood says another possibility is that the spiral arms of BRI 1335-0417 were created from a galactic encounter. “It looks a lot to me like a companion being whizzed by the galaxy under observation,” he says. Such tidal interaction could explain the unusually strong spiral arms in the ALMA observations. Sellwood believes BRI 1335-0417 could be an analogue of the stunningly beautiful Whirlpool galaxy from the early Universe, a nearby spiral galaxy that is clearly being “shaken” by a small companion galaxy.
Tsukui and Iguchi consider a tidal origin for BRI 1335-0417, but consider this scenario unfavorable as it usually creates long tidal tails that are not observed. Higher resolution images from BRI 1335-0417 could help reveal the circumstances behind the weapons demonstration in this early galaxy.
Michael Schirber is the corresponding editor for physics based in Lyon, France.