Ultraluminous infrared galaxies may have an interesting home.

Left: Image from the Hubble Space Telescope’s Advanced Camera for Surveys (ACS). (Credit: Hubble Space Telescope) Right: Image from the Subaru Prime Focus Camera (Suprime-Cam). Huge, complex tidal remnants surround Arp 220. (Credit: Ehime University / NAOJ)

Three decades ago the Infrared Astronomical Satellite’s (IRAS) all-sky survey captured a new breed – the Ultraluminous Infrared Galaxy – or ULIRG.

This type of galaxy is the most luminous class in the nearby Universe and most of their energy output is located in the infrared. This means a high probability of immense star formation inside that shroud of dust. Now a team of astronomers led by Professor Yoshiaki Taniguchi (Ehime University) have used the Subaru Telescope and optical spectroscopy tools from W.M. Keck Observatory to make a new determination on ULIRGs… It is possible they may have formed from a multiple merger of four or more galaxies! 

Just how luminous are these galaxies? Try imagining the candlepower of our Sun to the twelfth power. Since ULIRGs are primarily lit by massive stars, their luminosity is very near to that of a quasar. Over the years many proposals were made as to how they got so bright – including a multiple merger event. But there were just as many unanswered questions, such as how many galaxies merged into one and what were their types? For thirty years the explanation for the origins of ULIRGs has been controversial. Now current research is hoping to answer these questions by providing a plausible, data-based explanation.

Through the use of the FOCAS (Faint Object Camera and Spectrograph) on the Subaru Telescope and the LRIS (Low Resolution Imaging Spectrometer) on the Keck II Telescope, the research team made a detailed optical study of Arp 220. What it revealed was a pair of unexpected tidal tails which expanded across more than 50,000 light-years of space. The study also uncovered intermediate-mass stars – the “leftovers” of intense star formation events known as starbursts. These events appear to have occurred in the tidal tails and the presence of H-alpha absorption lines led to their discovery. Dr. Kazuya Matsubayashi said, “I was very surprised when I found these H-alpha absorption features in the two tidal tails.”

Left: Hα image taken with the Faint Object Camera and Spectrograph (FOCAS) mounted on the Subaru Telescope. The dark parts in the figure (Hα absorption) pointed out with arrows are three post-starburst regions. Right: For reference, an R band image from Suprime-Cam. (Credit: Ehime University / NAOJ)

Starburst events are known to happen when galaxies collide, so what could account for these unusual features? According to their research, it is possible that two post-starburst galaxies could have created the two long tidal tails when they combined. However, it would require at least four galaxies to create two post-starburst galaxies! This new observation of post-starbust tidal tails suggest a brand new type of galaxy merger event in the history of Arp 220… one which consisted of at least four galaxies. It’s a scenario the astronomers hypothesize could also have happened to other galaxy groups.

There are many galaxy groups in our Universe which could have been the scene of multiple merger events. As Professor Taniguchi explains, “Some of such compact groups have already merged into one. They are the ULIRGs observed to date.” Some galaxies are associated together in a single gravitationally-bound group, and they will inevitably merge into one galaxy within several billion years.

So where will the knowledge of multiple merger events come into play? ULIRGs are expected to eventually evolve into quasars and on to giant, early-type galaxies. Now astronomers must also reckon they could have evolved from several sources and not just one massive event from two galaxies. Professor Taniguchi applied this principle to the fate of our Milky Way Galaxy: “Very recently, NASA announced that our Milky Way Galaxy will merge with the Andromeda galaxy (M31) into a giant elliptical galaxy within several billion years. Please don’t worry. That would only be a merger between two galaxies, so our Milky Way will not evolve into a ULIRG.”

Original Story Source: Subaru Telescope Press Release.

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