Around two thirds of all known galaxies in the universe are spiral galaxies – our Milky Way also belongs to this type. But when the first representatives of this type of galaxy formed is still unclear. Now astronomers have discovered the oldest known spiral galaxy with the help of the Atacama Large Millimeter / submillimeter Array (ALMA). It is around 12.4 billion light years away from us and thus already existed around 1.4 billion years after the Big Bang – in the early days of the cosmos. Their characteristics could now help to clarify the question of when and how these spiral structures first formed.
The Milky Way belongs to it, our neighboring galaxy Andromeda and many more in the local cosmos: They are all spiral galaxies with the clearly delimitable inner structures typical of these star clusters. This includes the central bulge, a zone that bulges upwards and downwards, in which the star density is particularly high and in which around a third of the total mass of the galaxy is concentrated. The supermassive black hole is also hidden in it. A flat, rotating star disk emanates from this bulge and forms the main plane of the galaxy. The stars in this disk are not randomly distributed, but are concentrated in several spiral-shaped arms. Around 70 percent of all galaxies in the local cosmos show these features and are spiral galaxies. But the further into the universe you look into the distance and thus into the past, the rarer the spiral type becomes. This raises the question of when these galaxies first formed.
Distant galaxy with two spiral arms
Now the newly discovered galaxy BRI 1335-0417 provides more information about this. They tracked down astronomers Takafumi Tsukui and Satoru Iguchi from the National Astronomical Observatory of Japan in Tokyo when they searched archive data from the Atacama Large Millimeter / submillimeter Array (ALMA) for evidence of ancient spiral galaxies. They found what they were looking for in the active star formation galaxy BRI 1335-0417. While this galaxy is shrouded in dense clouds of dust in the optical wave range, the spectrographic analyzes of the radio waves emanating from the carbon atoms contained in galactic gases revealed internal structures. According to this, this galaxy probably has two arms that extend from the galactic center around 15,000 light years into space. The features of the spectral carbon lines also suggest that the galaxy is rotating and that there is a mass concentration in the center of the structure – possibly a central bulge.
“I was thrilled because I’ve never seen such clear evidence of a rotating disk, a spiral structure, and a central mass structure in a galaxy so far away,” says Tsukui. “The quality of the ALMA data was so good that I was able to see as many details as in some nearby galaxies.” According to the measurement data, BRI 1335-0417 is around 12.4 billion years away from us. It already existed around 1.4 billion years after the Big Bang – making it the oldest known representative of the spiral galaxies, as the astronomers report. This galaxy also existed before the climax of cosmic star formation. Nevertheless, according to the data, it was already relatively large and had about as much stellar mass as today’s Milky Way. “For a galaxy in the early universe, BRI 1335-0417 was a giant,” says Tsukui.
Result of a collision?
It is still unclear how this early galaxy grew and got its spiral shape. The astronomers suspect, however, that a past collision or close encounter with another, smaller galaxy could have brought BRI 1335-0417 into its spiral form. “Cosmological simulations of the early universe show that the spiral structure can arise in a rotating disk when the disk relaxes again after a merging event,” write Tuskui and Iguchi. Another indication of such an event could also be the high rate of star formation in the early spiral galaxy, because the turbulence and gas flows generated by such a collision stimulate it. However, because the internal structure of BRI 1335-0417 and its rotational behavior now appear relatively undisturbed, this collision must have been some time ago, as the astronomers explain.
How this early spiral galaxy got its shape and how it then evolved over the course of cosmic history can only be guessed at so far. However, the researchers hope to learn more about their fate through further research. “Tracing back the roots of the spiral structure can help us better understand the history of galaxy formation,” says Iguchi. This then also provides valuable information about how our own galaxy once became what it is today.
Source: Takafumi Tsukui and Satoru Iguchi (National Astronomical Observatory of Japan, Tokyo), Science, doi: 10.1126 / science.abe9680