How and when did galaxies with hundreds of billions of stars form and evolve? The sun, which is the center of the solar system in which we live, is also only one of the countless stars contained within a galaxy. In brief, it can be said that we need to understand the evolution of galaxies to understand the world we live in.
One of the most effective methods of studying galaxy evolution is "element investigation." In short, this involves investigating the chemical composition of galaxies. In astronomy, observing distant galaxies means investigating galaxies in the past universe. (This is because the velocity of electromagnetic waves such as visible light and radio wave, or the speed of light, is finite -- approximately 0.3 million km per second. It takes time for the light to reach us and the light carries the information of celestial objects at the time when the light itself was emitted, then we can obtain the information of the past.) In fact, constituents of galaxies at various periods of the universe have been studied by visible light observations of distant galaxies using optical telescopes such as the Subaru Telescope.
However, galaxies in the phase of active star formation are covered in large amounts of dust which block visible light. Furthermore, for galaxy constituent investigation, it is necessary to observe very dark galaxies located at extreme distances. For these reasons, it has been difficult to investigate chemical composition of active star-forming galaxies at a great distance with visible light observations.
Conception of This Research
Tohru Nagao (Kyoto University), who heads the research team, explains that "We focused attention on millimeter waves, the radio waves with the wavelength of about 1 mm, with which we could study galaxies without being blocked even if there was a lot of dust, and decided to observe galaxies called "submillimeter galaxies" which have intense star formation activity in the process of evolving."
However, actual observation is not easy. In order to derive the galaxies' elemental abundances, it is necessary to detect emission lines emitted by multiple elements and examine their strength ratio. To detect the weak signals, high sensitivity is inevitably necessary. Last year, the research team observed a sub-millimeter galaxy called "LESS J033229.4-275619 (abbreviated to LESS J0332 hereafter)" that is about 12.4 billion light-years away from Earth in the direction of the constellation Fornax using the APEX telescope operated by the European Southern Observatory, and successfully detected a carbon emission line. However, due to the lack of sensitivity of the existing millimeter-wave telescopes, they have been unable to detect any emission lines except from carbon atom and carbon-bearing molecules. This is because carbon is one of the abundant elements in the universe and the emission lines from carbon atom or carbon-bearing molecules are rather stronger than those from other elements, in general. Therefore they could not investigate the constituents of this galaxy.
ALMA, the international radio astronomy observatory under construction in the Chilean Andes, has solved this sensitivity problem . Tohru Nagao and other staff proposed observing a nitrogen emission line from the sub-millimeter galaxy LESS J0332 with ALMA and comparing it with the already detected carbon emission line. More than 900 proposals from around the globe were submitted to ALMA for its first scientific observation period and reviewed by experts in various astronomical fields. The team led by Nagao finally won the ALMA observation time by surviving the competition with the oversubscription rate of nine.
Observations with ALMA
Observations were intermittently carried out from October 2011 to January 2012. Consequently, an emission line from nitrogen in the galaxy LESS J0332 was successfully detected.
As seen in the numbers shown at the vertical-axis, the brightness of the nitrogen emission line was less than one tenth of the brightness of the carbon emission line. The total observation time was 14.5 hours for detection of the carbon emission line with APEX. On the other hand, the total observation time with ALMA was only 3.6 hours to detect the weak nitrogen emission. From this, it is clear that ALMA already has quite high sensitivity, 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.
Nagao, the project leader, said that "At first, I thought this observation would be very hard because the originally expected strength of the nitrogen emission line was very weak. So, I was very excited to find a clear nitrogen emission line. The power of ALMA overwhelmed me." Hatsukade, one of the research staff and an expert in millimeter-wave observations, commented, "I was very surprised with the excellent sensitivity of ALMA. ALMA has more antennas than conventional interferometers for millimeter/submillimeter waves so its images are less likely to be artificially influenced. Therefore, it was possible to get very clear images even in such a short time of observation. It would not have been possible to get these results without ALMA."
Element Investigation of a Galaxy at about 12.4 Billion Light-Years Away
The research team analyzed the element abundance of LESS J0332, comparing the brightness ratio of the observed emission lines from nitrogen and carbon with theoretical calculations. The results show 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 (consisting of almost only hydrogen and helium), but rather similar to that of the sun in the current universe (in which 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 we observe the galaxy located in the young universe at 1.3 billion light 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 Nagao, explaining the significance of the findings.
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