Strong UV Radiation Detected in the Most Distant Galaxies
February 24, 2009
A team of astronomers from the National Astronomical Observatory of Japan, Osaka Sangyo University, Tohoku University, and Laboratoire d’Astrophysique de Marseille, France have used the Subaru Telescope in Hawai’i to detect strong ultraviolet (UV) radiation from galaxies that lie about 12 billion light-years away. This radiation, which ionizes hydrogen atoms, marks the end of the “cosmic dark age” and heralds the so-called “epoch of reionization” that occurred about a billion years after the beginning of the universe. Although this cosmic reionization was spurred by ionizing radiation from the first generation of galaxies, there have been only two definite detections reported so far. As a result, it has been unclear how much galaxies contributed to the cosmic reionization.
Thanks to the unprecedented performance of the prime-focus camera on the Subaru telescope, the team detected ionizing radiation from 17 galaxies at once. This result suggests that galaxies with strong ionizing radiation could well have played an important role in the cosmic reionization and helps astronomers understand the processes that led to the end of the cosmic Dark Age.
The Role of Ionizing Radiation in the Early Universe
One minute after the Big Bang occurred, the universe consisted of protons, neutrons and electrons rushing outward through the hot, expanding universe. As temperatures in the newborn universe cooled, the protons and electrons combined to form hydrogen atoms. At that epoch, there were no stars or galaxies – simply a soup of atoms and particles. This is often referred to as “the cosmic dark age”. It ended when the first astronomical objects in the universe emitted radiation with a wavelength shorter than 91.2 nanometers. (*1) This radiation had enough energy to separate a hydrogen atom into a proton and an electron (i.e., it ionized hydrogen atoms). Such light is called “ionizing radiation”.
As more astronomical objects formed, their radiation ionized hydrogen throughout the expanding universe (intergalactic space) by about 1 billion years after the Big Bang (about 12.5 billion years ago). Astronomers call this period when the universe first “lit up” the “cosmic reionization” or “epoch of reionization.” Since that time, most of the universe has been kept continually ionized (Fig. 1) as it formed the variety of objects and structure such as galaxies and their associated stars and planets.
Although astronomers have detected several objects that existed in the epoch of the cosmic reionization (*2), the process of reionization at that time remains a puzzle. For example, it is not clear what kinds of objects provide the source of the reionization. Quasars are one possible candidate. These are luminous objects hosting supermassive black holes. However, there appear to be too few quasars existing in the epoch of the reionization to ionize the whole universe.
On the other hand, galaxies -- which consist of stars and gas -- are so numerous that they might well have played a primary role in the cosmic reionization even though individual galaxies are much dimmer than quasars. However, the amount of ionizing radiation from galaxies is not well-defined and this presents a serious problem in understanding their contribution to reionization. So far there have been only two cases where ionizing radiation has been detected from individual galaxies (*3). Thus, the galaxies’ contribution to the cosmic reionization has remained unclear.
Characteristics and Importance of the Present Research
From September 10-14, 2007, the Subaru telescope searched distant galaxies for ionizing radiation. The approach was different from previous research on ionizing radiation. In the most of previous studies, spectroscopy (which can separate light from objects into its component wavelengths) was used. Observers can use spectroscopy to measure the relative strength of objects in each wavelength, but this process is not suited for identifying faint signals. In the September 2007 observation, the team made imaging observations, which are very similar to ordinary photographs. The use of a special filter (*4, Fig. 2) designed to collect only ionizing radiation emitted from galaxies at a certain distance from the Earth made it possible to measure the strength of ionizing radiation much more efficiently (compared to spectroscopic observations).
Moreover, Subaru’s prime-focus camera has the widest field of view (about 0.5 degree) of all world’s large telescopes. That enables the observation of a large number of galaxies at once. The observed sky area was a field called SSA22 (*5) where a giant cluster of young galaxies was discovered at a distance of about 12 billion light-years. The team was able to examine ionizing radiation from 198 galaxies at once in one single field of view using the prime focus camera.
With this observation, ionizing radiation was detected from 17 of the galaxies. This number greatly exceeds the number of previous detections (only two). It shows that wide-field imaging observation with the Subaru telescope is an extremely powerful tool for searching out this ionizing radiation from distant galaxies.
Interestingly, some of those detected galaxies show more ionizing radiation than theoretical predictions suggest they should be emitting. This indicates that young galaxies in the distant universe (at least some of them) emit more ionizing radiation than expected and that they may have played an important role in the cosmic reionization. In addition, for some galaxies the spatial positions of ionizing radiation appears to be offset from brightest part of the galaxies (Figure 3). This may be a clue to understanding how ionizing radiation escapes from galaxies.
These new discoveries should also have an important impact on the theory of galaxy evolution, which describes how stars formed in the earliest galaxies and how galaxies have come to exist in the beautiful shapes we observe in the current universe.
Prospects for Further Developments
This observation took advantage of the unique capability of the Subaru telescope and allowed significant progress in the research of the ionizing radiation from galaxies in the early universe. The results should provide important clues to help astronomers understand what kind of objects are responsible for the cosmic reionization and the processes that spurred that reionization. Follow-up observations – including those done at new observing facilities – will allow astronomers to clarify what actually happened at the epoch of the cosmic reionization, a period we are still working to understand.
These results will be published in the February 2009 issue of the Astrophysical Journall, as "Detections of Lyman Continuum from Star-forming Galaxies at z~3 through Subaru/Suprime-Cam Narrow-band Imaging", I. Iwata, A. K. Inoue, Y. Matsuda, H. Furusawa, T. Hayashino, K. Kousai, M. Akiyama, T. Yamada, D. Burgarella, and J.-M. Deharveng, 2009, The Astrophysical Journal, Volume 692, Issue 1
Note 1: 1 nanometer (nm) is 0.000000001 meter or 0.000001 centimeter (cm). Visible light covers a wavelength range between 380 nm and 780 nm. In standard astronomical observations (using optical telescopes) we observe non-ionizing photons (which have wavelengths longer than 91.2 nm). Their energy is lower than ionizing radiation.
Note 2: "Cosmic Archeology Uncovers the Universe’s Dark Ages", Subaru Press Release on Sep. 13, 2006 ; "Universe Re-ionized 900 Million Years After Birth", Subaru Press Release on May 25, 2006
Note 3: Shapley, A. E. et al. 2006, Astrophysical Journal 651, 688. In addition, there is a report of a detection of ionizing radiation by combining spectral data from many galaxies into one to amplify the signal (Steidel, C. C. et al., 2001, Astrophysical Journal, 546, 665). These are on galaxies at about 12 billion light-years away. For galaxies closer to the Earth, their redshifts (see note 4) are smaller and their ionizing radiation cannot be observed from the ground due to atmospheric absorption. These require space-borne observation. Although there is one report of the detection of ionizing radiation from such a nearby galaxy, there are arguments about its reality and no conclusion has been drawn.
Note 4: The expansion of the universe makes wavelengths of light from distant objects appear “redder”. This “redshift” is used as an indicator of the distance to astronomical objects. Ionizing radiation is originally ultraviolet radiation with wavelength shorter than 91.2 nm, but it is stretched by the expansion of the universe to an observed wavelength that is longer. Thus, for ionizing radiation with a wavelength of 90 nm from galaxies at 12 billion light-years, the observed wavelength is about 360 nm.
Note 5: SSA22 is located in Aquarius (Figure 4). The results on the cluster of galaxies in SSA22 using the Subaru telescope include Matsuda, Y. et al. 2004, Astronomical Journal , 128, 569 and Hayashino, T. et al. 2004, Astronomical Journal, 128, 2073. There was also a Subaru web-release on the results for the field (July 26, 2006; http://www.naoj.org/Pressrelease/2006/07/26/index.html).
Note 6: (Supplemental Note) the Relationship between the Present Observation and the Cosmic ReionizationThe galaxies observed in this study are about 12 billion light-years away from Earth, and we are seeing them as they existed when the universe was about 2 billion years old. However, the epoch of cosmic reionization is considered complete by about 1 billion years after the beginning of the universe. So the galaxies observed in this study are those that existed at the epoch after the completion of cosmic reionization. It would be very useful if we could observe ionizing radiation from galaxies as they radiated it during the actual epoch of the cosmic reionization. Unfortunately, that is impossible; hydrogen atoms between us and distant galaxies absorb ionizing radiation emitted during the cosmic reionization and render it invisible to us. Thus, we can observe ionizing radiation only from galaxies at about 2 billion years after the Big Bang (Inoue, A. K. and Iwata, I. 2008, Monthly Notice of Royal Astronomical Society, 387, 1681).
We know that the properties of galaxies that existed 2 billion years after the beginning of the universe (e.g., that they form stars more actively than galaxies in the current universe) are similar to those of galaxies in the epoch of the cosmic reionization. Thus, it is quite important to study many young galaxies at the epoch of 2 billion years and to know the relationship between the strength of their ionizing radiation and other properties, such as their sizes, masses and ages. With such correlations and observations of the properties of galaxies at the epoch of cosmic reionization, it would be possible to estimate the amount of ionizing radiation from galaxies at that time, and to clarify how important the galaxies were to the process of cosmic reionization.
Figure 1: A brief history of the universe, including the cosmic reionization.
Figure 2: The filter used in the September 2007 observation. Because this filter does not transmit light visible to humans, it reflects an image of the ceiling in the room where it was photographed.
Figure 4: The position of SSA22. Created with the StellaNavigator by AstroArts Inc.