Subaru telescope has discovered a minuscule galaxy, less than one hundreth the size of the Milky Way, that may be one of the early building blocks of larger galaxies that populate the Universe today. The galaxy as we see it existed only one billion years after the birth of the Universe, which is now about 13.7 billion years old.

"It is like a galaxy seed," says Dr. Taniguchi an astronomer at Tohoku University and one of the scientists in the discovery team. Galaxies like the Milky Way where we live, and other nearby galaxies have masses equivalent to over 100 billion Suns, and extend hundreds of thouands of light years. However, there is evidence that these nearby galaxies were not always this large. Astronomers think that smaller galaxies one hundred to one thousand times smaller than the Milky Way first formed and then merged together to form the larger galaxies inhabiting the Universe today.

To test such a hypothesis, astronomers have been looking for small galaxies in the distant and therefore long ago universe. The discovery of this minuscule galaxy is the first piece of evidence to show that such "galaxy seeds" actually existed in the early Universe.

The discovery team, consisting of researchers from Tohoku University , the National Astronomical Observatory of Japan, the University of Tokyo, the University of Hawaii, and the University of Maryland, used Suprime-Cam with a special filter (NB816) that only passes light with wavelengths of 810 to 822 nm to look for galaxies about 12.5 billion light years away. This combination of filter and instrument with a large telescope like Subaru is an ideal setup for finding faint distant galaxies. In addition to the fact that, background light from the night sky is not very strong at these wavelengths, Suprime-Cam's wide 34 x 27 arcsec field of view, about as large as the full moon, and the light gathering power of Subaru's 8.2 meter aperture make the search very efficient. Due to star formation, young galaxies emit very strongly at 122 nm, the Lyman alpha emission line of hydrogen. The expansion of the Universe stretches the wavelength of this emission line to 810 to 822 nm for galaxies at a redshift of 5.7.

In February 2002, the team observed the neighborhood of a distant quasar SDSSp J104433.04-012592.2 in the constellation Sextans for over 10 hours in the hope of finding galaxies at a comparable distance as the quasar. They were successful in discovering over 15 candidate galaxies. Using the the Echellette Spectrograph and Imager (ESI) on the Keck II telescope, also on Mauna Kea, the team chose to study the spectrum of the most likely distant galaxy candidate. The results showed that the hydrogen gas in the galaxy had random motions of only 20 km/s.

A galaxy confines gas with gravity. A more massive galaxy can hold on to gas with more random motions. As a result, the random motions of gas inside a galaxy is proportional to the square root of the mass of the galaxy. The gas in the newly discovered galaxy extends over 20,000 light years, but calculations show that the mass of the galaxy confining it is equivalent to only a few hundred million Suns. This is less than one percent of the mass of the Milky Way. It is more similar to the Small Magellanic Cloud that orbits around the Milky Way.

If the Universe is 13.7 billion years old as recent research suggests, this little seed of a galaxy exists 12.5 billion years away from us. This means that such galaxy seeds are forming less than a billion years after the beginning of the Universe.

"We hope to use the Lyman alpha hydrogen emission line to find galaxies at a wider range of redshifts, or distances, to see how these galaxy seeds grow as the Universe ages and to find even more distant, younger galaxies", Dr. Taniguchi says. Instruments sensitive to near-infrared light rather than visible light will be necessary to find more distant galaxies who's Lyman alpha emission line would be shifted into even longer wavelengths. "We are looking forward to the completion of Subaru's new near infrared imager and spectrograph," says Dr. Taniguchi. " It will be our window into even earlier times."

This research was published in the March 2003 edition of the Astrophysical Journal (ApJ, 585, L97-L100)

Figure 1: Images (top row) and contours (middle row) of the newly discovered galaxy LAE J1044-0123 from data taken with filters of various wavelengths. The bottom panels is the energy distribution (in magnitudes) as a function of wavelength. The galaxy shows up most prominently in the NB816 filter image. The bright object to the upper left is a galaxy 7 billion light years away.

 

Figure 2: The spectrum of LAE J1044-0123, from the ESI on the Keck II telescope taken on March 13, 2002 (Universal Time). It clearly shows a very narrow Lyman alpha emission line.