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Breakthrough Discovery: Revealing Cosmic Re-ionization Using Black Holes as Background Lights

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Breakthrough Discovery: Revealing Cosmic Re-ionization Using Black Holes as Background Lights

The Universe experienced a big phase change in its early stage, from neutral to ionized. This process is called “cosmic re-ionization,” which is, however, one of the remaining mysteries. It has been difficult to map out this cosmic re-ionization observationally, because as we approach the dark age of the Universe, over 99% of the UV light is absorbed by the neutral hydrogen.

One of the innovative approaches to revealing cosmic re-ionization is to use the bright light of distant super massive black holes (SMBHs) as a background torch to shine on the dark age of the Universe. The bright light of distant SMBHs allows us to measure the intervening absorption, which imprints the re-ionization history of the Universe. However, distant SMBHs are extremely rare. It has been a challenge to find them in quantity.

Our research team has advanced the field in the following two aspects: (1) A student and I have discovered one of the most distant SMBHs at z~6.6 (12.9 billion years ago). This SMBH was so bright that we mapped out the cosmic re-ionization up to z~6.4, using a single SMBH; (2) Collaborating with an international team, we have reached a milestone in finding 100 SMBHs at z~6, where only ~10 were known previously. With much improved quantity, we have, for the first time, statistically ruled out that radiations from SMBHs are not the source of the cosmic re-ionization. Our result is an important step in understanding what astronomical objects have ionized the Universe.

 
Tomotsugu Goto
 
The Universe experienced a big phase change in its early stage, from neutral to ionized. This process is called “cosmic re-ionization,” which is, however, one of the remaining mysteries in cosmology. It has been difficult to map out this cosmic re-ionization observationally, because as we approach the earlier, more neutral Universe, over 99% of the UV light is absorbed by the neutral hydrogen, escaping the pursuit of even the largest telescopes around the world. This is the reason why the epoch is called “the dark age of the Universe.”
 
How do we observe the dark age when little light is coming out? One of the innovative approaches to revealing the cosmic re-ionization is to use the bright light of distant super massive black holes (SMBHs) as a background torch to shine through the dark age of the Universe. The bright light of distant SMBHs allows us to measure the intervening neutral hydrogen absorption, which imprints the re-ionization history of the Universe, i.e., the more neutral, the more absorption. However, distant SMBHs are extremely rare. It has been a challenge for astronomers to find them in quantity.
 
Our research team has advanced the field in the following two aspects:
  1. Discovery of one of the most distant SMBHs at z~6.6 (12.9 billion years ago). Distant SMBHs are extremely rare. But how rare are they? They could only be found one in a billion objects, a probability which is less than winning a lottery. A student and I have carefully selected SMBH candidates out of billions of objects in our unprecedentedly large survey, for spectroscopic confirmation with one of the largest telescopes in the world. We used the Subaru 8m telescope at the Maunakea observatory in Hawaii (Fig. 1) for spectroscopic confirmation. On the first night of observation, immediately after we started observing, we successfully discovered a distant SMBH. It was an exciting moment for us to see a widely broadened hydrogen line, which is a sign of ~ billion solar mass black holes. It was the moment when our years of preparation finally paid off. This SMBH was so bright that we mapped out the cosmic re-ionization up to z~6.4, using a single SMBH. 
  2. In collaboration with an international team including US and Japan, we also focus on hunting for fainter SMBHs than previous ones in the distant Universe. Our large project has observed for 300 nights at the Subaru telescope to survey an unprecedentedly large area of 1400 deg2 with the depth of an 8m telescope. Such deep and wide survey data never existed in the past. After two years of the survey, we have already found 100 SMBHs at z~6 (Fig.2), and the number is increasing, where only ~10 were known previously. Since finding 100 SMBHs at z~6 is a milestone of our project, the result was released to the press in US, Japan, Germany, and Taiwan. In Taiwan, more than 30 newspapers and websites reported the result (Fig.3).

Using these 100 SMBHs, we have, for the first time, statistically ruled out that radiation from SMBHs is not the source of the cosmic re-ionization. Our result is an important step in understanding what astronomical objects have ionized the Universe. Another student of mine has been analyzing the 100 SMBHs data to reveal the cosmic re-ionization (Fig.4). Please stay tuned. 100 SMBHs will shed light on the dark age of the Universe soon. 

Figure 1. My students observing SMBHs at the Subaru telescope
Figure 1. My students observing SMBHs at the Subaru telescope

Figure 2. SMBHs we have newly discovered (image credit: National Astronomy Observatory of Japan)
Figure 2. SMBHs we have newly discovered (image credit: National Astronomy Observatory of Japan)

Figure 3. Our discovery of 100 SMBHs was reported by a Taiwanese Newspaper
Figure 3. Our discovery of 100 SMBHs was reported by a Taiwanese Newspaper
 
Figure 4. A student and I analyzing the new SMBHs to reveal cosmic re-ionization
Figure 4. A student and I analyzing the new SMBHs to reveal cosmic re-ionization

 

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