Subaru Discovers an Exploding Galaxy at the Edge of the Universe
August 8, 2002
A collaboration led by astronomers from Tohoku University in Japan has used Subaru Telescope to discover a galaxy rapidly forming stars when the Universe was less than a billion years old. Images and spectra from the Subaru and Keck telescopes reveal that the galaxy has a high-speed outflow of hydrogen gas believed to be caused by a massive burst of star formation. The galaxy is more than 14 billion light years from Earth, and is currently the most distant galaxy known to harbor such a phenomenon. Studying such distant galaxies is made possible by combining careful technique and large telescopes, and is expected to reveal how galaxies are born and evolve in the early stage of the Universe.
The newly discovered galaxy is so far away that the light which is now reaching Earth began its journey 14 billion years ago. Only large telescopes like Subaru can collect enough light to observe these very faint and distant galaxies to study what the Universe was like in its infancy. Because the Universe is expanding, the light emitted from these distant galaxies is stretched out to longer wavelengths as it travels toward us, a phenomenon astronomers call redshift. The further an object is from us, the greater its redshift.
Hoping to find star-forming galaxies at the farthest reaches of the Universe, the team (including Tohoku University graduate student Masaru Ajiki and associate professor Yoshiaki Taniguchi, researchers at the National Astronomical Observatory of Japan (NAOJ), the University of Tokyo, the University of Hawaii, and the University of Maryland) installed a special filter (NB816) into Subaru's prime focus camera, Suprime-Cam, and observed a region of sky about the size of the full moon around a distant quasar named SDSSp J104433.04-012502.2 in the constellation Sextant. Quasars are believed to be objects powered by black holes about a billion times the mass of the Sun and, due to their extreme luminosities, can be readily discovered even at very large distances. The special filter was designed to transmit only a very narrow color range of light around 800 nanometers which, for galaxies at the same distance as the quasar, includes an emission line (called Lyman-alpha) which is produced when gas is heated by vigorous star formation.
After a 10-hour observation in February 2002, more than 15 galaxies were discovered which could only be seen in the image taken through the narrow filter. One of these galaxies was then observed with Subaru's Faint Object Camera and Spectrograph (FOCAS) in March 2002, which confirmed that it was a very distant galaxy. Additional, more detailed, observations with the Echellette Spectrograph and Imager (ESI) on the Keck II telescope supported the Subaru results and indicated that hydrogen gas is flowing from the galaxy at speeds of several hundred kilometers per second.
Stars more than about 10 times the mass of our Sun end their lives in vast explosions called supernovae. When many such stars are being formed in a short space of time, the combined force of these explosions can cause gas to be ejected out of the galaxy. Such an outflow, called a "superwind", can be seen in the nearby galaxy Messier 82. If the outflow seen in this young galaxy is also produced by a superwind, it would be the most distant example of this phenomenon yet discovered and would mean that large-scale star formation must have begun within several hundred million years after the start of the Universe.
By discovering more distant galaxies and studying them in detail, astronomers expect to learn what galaxies were like when the Universe was very young, and understand how they are born and evolve. Says Professor Taniguchi, "For the past 15 years while Subaru was being planned and built, Keiichi Kodaira [former Director of NAOJ and now President of the Graduate University for Advanced Studies] and I have had a dream to look at distant galaxies, more than 10 billion light years away. At last that dream has come true."
These results will be published in the September 1, 2002 issue of The Astrophysical Journal.
Figure 1 : Thumbnail images and spectral energy distribution of the galaxy, named LAE J1044-0130 (LAE stands for Lyman-alpha emitter). The upper row shows images of the galaxy in different filters and the middle row shows contour maps of the images above them. The galaxy, at the center of the circle, appears clearly in the image taken with the special NB816 filter, but is barely detectable in the other filters. The brighter object to the upper left of the galaxy is a foreground galaxy only 7 billion light years away. The lower panel plots the amount of light detected in each filter in units of magnitude as a function of wavelength. (Note that wavelengths of each filter do not align with the image panels above.) This plot, called the spectral energy distribution, shows that the galaxy is brightest in the wavelength region of the special filter, NB816.
Figure 2 : Spectra of LAE J1044-0130 taken by FOCAS on the Subaru Telescope on March 11, 2002 UT (upper panel) and March 13, 2002 UT (lower panel). The spectrum in the upper panel clearly detects the emission line. In the higher resolution spectrum in the lower panel, the line shows a slight broadening on its longer wavelength side (right). This "red wing" suggests the presence of superwinds in the galaxy.
Figure 3 : Spectrogram and spectra obtained with Keck II Telescope's ESI on March 15, 2002 UT. The spectrogram in the upper panel shows emission as a function of wavelength and position along a slit placed to detect light from the galaxy LAE J1044-0130 and the foreground galaxy. The galaxy LAE J1044-0130 is particularly bright at a wavelength of 8130 angstroms. The spectrum in the middle panel clearly shows the line and its red wing. Comparing the spectrum in the middle panel with the spectrum of the emission lines in the night sky (OH air glow) in the bottom panel confirms that the detection is real.