Into The Submillimeter: The Early Universe’s Formation

A new study looked at 52 submillimeter galaxies to help us understand the early ages of our Universe. Image: University of Nottingham/Omar Almaini

In order to make sense of our Universe, astronomers have to work hard, and they have to push observing technology to the limit. Some of that hard work revolves around what are called sub-millimeter galaxies (SMGs.) SMGs are galaxies that can only be observed in the submillimeter range of the electromagnetic spectrum.

The sub-millimeter range is the waveband between the far-infrared and microwave wavebands. (It’s also called Terahertz radiation.) We’ve only had the capability to observe in the sub-millimeter range for a couple decades. We’ve also increased the angular resolution of telescopes, which helps us discern separate objects.

The submillimter wavelength is also called Terahertz Radiation, and is between Infrared and Microwave Radiation on the spectrum. Image: By Tatoute, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6884073
The submillimter wavelength is also called Terahertz Radiation, and is between Infrared and Microwave Radiation on the spectrum. Image: By Tatoute, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6884073

SMGs themselves are dim in other wavelengths, because they’re obscured by dust. The optical light is blocked by the dust, and absorbed and re-emitted in the sub-millimeter range. In the sub-millimeter, SMGs are highly luminous; trillions of times more luminous than the Sun, in fact.

This is because they are extremely active star-forming regions. SMGs are forming stars at a rate hundreds of times greater than the Milky Way. They are also generally older, more distant galaxies, so they’re red-shifted. Studying them helps us understand galaxy and star formation in the early universe.

ALMA is an array of dishes located at the Atacama Desert in Chile. Image: ALMA (ESO/NAOJ/NRAO), O. Dessibourg

A new study, led by James Simpson of the University of Edinburgh and Durham University, has examined 52 of these galaxies. In the past, it was difficult to know the exact location of SMGs. In this study, the team relied on the power of the Atacama Large Millimeter/submillimeter array (ALMA) to get a much more precise measurement of their location. These 52 galaxies were first identified by the Submillimeter Common-User Bolometer Array (SCUBA-2) in the UKIDSS Ultra Deep Survey.

There are four major results of the study:

  1. 48 of the SMGs are non-lensed, meaning that there is no object of sufficient mass between us and them to distort their light. Of these, the team was able to constrain the red-shift (z) for 35 of them to a median range of z-2.65. When it comes to extra-galactic observations like this, the higher the red-shift, the further away the object is. (For comparison, the highest red-shift object we know of is a galaxy called GN-z11, at z=11.1, which corresponds to about 400 million years after the Big Bang.
  2. Another type of galaxy, the Ultra-Luminous Infrared Galaxy (ULIRG) were thought to be evolved versions of SMGs. But this study showed that SMGs are larger and cooler than ULIRGs, which means that any evolutionary link between the two is unlikely.
  3. The team calculated estimates of dust mass in these galaxies. Their estimates suggest that effectively all of the optical-to-near-infrared light from co-located stars is obscured by dust. They conclude that a common method in astronomy used to characterize astronomical light sources, called Spectral Energy Distribution (SED), may not be reliable when it comes to SMGs.
  4. The fourth result is related to the evolution of galaxies. According to their analysis, it seems unlikely that SMGs can evolve into spiral or lenticular galaxies (a lenticular galaxy is midway between a spiral and an elliptical galaxy.) Rather, it appears that SMGs are the progenitors of elliptical galaxies.
The Pinwheel Galaxy (M101, NGC 5457) is a stunning example of a spiral galaxy. This study determines that there likely is no evolutionary link between sub-millimeter galaxies and spiral galaxies. Image: European Space Agency & NASA. CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=36216331

This study was a pilot study that the team hopes to extend to many other SMGs in the future.

Early “Elemental” Galaxy Found 12.4 Billion Light Years Away

This is definitely a story about a galaxy long ago and far away. An international team of researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) has observed a “submillimeter galaxy” located about 12.4 billion light-years away. Their observations have revealed that the elemental composition of this galaxy in the early universe, at only 1.3 billion years after the Big Bang, was already close to the current elemental composition of the Universe. This means that intense star formation was taking place at that early point in the Universe’s history.

A submillimeter galaxy is a type of galaxy which has intense star formation activity and is covered by large amounts of dust. Since dust blocks observations in visible light, using ALMA’s millimeter wavelength capabilities can penetrate and see though dust clouds. In addition, ALMA also has extraordinary sensitivity, which is capable of catching even extremely faint radio signals. This is one of the most distant galaxies ALMA has ever observed.

The team was able to examine the chemical composition of the galaxy, called LESS J0332, and detected an emission line that contained nitrogen. To do this, they compared the brightness ratio of the observed emission lines from nitrogen and carbon with theoretical calculations. Their results showed that the elemental composition of LESS J0332, especially the abundance of nitrogen, is significantly different from that of the Universe immediately after the Big Bang – which consisted of almost only hydrogen and helium — but was much more similar to that of our Sun today, where a variety of elements exist abundantly.

It took 12.4 billion years for the emission lines from LESS J0332 to reach us, which means that the team was able to observe the galaxy located in the young universe at 1.3 billion years after the Big Bang.

“Submillimeter galaxies are thought to be relatively massive galaxies in the growth phase. Our research, revealing that LESS J0332 already has an elemental composition similar to the sun, shows us that the chemical evolution of these massive galaxies occurred rapidly made in the early universe, that is to say, in the early universe active star formation occurred for a short period of time,” said Tohru Nagao from Kyoto University, co-author of the paper.

The observations were made with ALMA, even though construction is not yet completed; only 18 antennas were used in this observation, while ALMA will be equipped with 66 antennas when completed.

This research was published in the “Letters” section of the journal, “Astronomy & Astrophysics.”

Lead image caption: Artist impression of the submillimeter galaxy LESS J0332 observed the ALMA at the 5000-meter altitude plateau. [Credit: NAOJ]

Source: ALMA