New Metal-poor Galaxies: The Youngest in the Universe?

Alexei Y. Kniazev Max-Planck-Institute for Astronomy, Germany,; (+49 6221 528-263)
Michael A. Strauss, Princeton University Observatory, (609)258-3808
Masataka Fukugita, University of Tokyo, Japan, (+81 471 36 3155)
Gary S. Ruderman, Public Information Officer, The Sloan Digital Sky Survey,, (312) 320-4794

Pictures of the SDSS extremely metal poor galaxies (XMPGs)
SDSS J093401.92+551427.9 (alias I Zw 18);
SDSS J110553.76+602228.9 (alias SBS 1102+606);
SDSS J121546.56+522313.9 (alias CGCG 269-049)
SDSS J020549.13-094918.1 (alias KUG 0203-100)
can be found at:
CREDIT: The Sloan Digital Sky Survey (Kniazev et al.)

HEIDELBERG, Germany - (August 18, 2003) -- A new era of investigation into extremely metal poor galaxies (XMPGs) is underway using data from the Sloan Digital Sky Survey.

Using spectroscopic information from the Sloan Digital Sky Survey (SDSS), the research team of German, American, Russian and Japanese investigators analyzed the metal content of more than 250,000 galaxies and found eight new and very rare galaxies. XMPGs hold important clues to understanding galaxy evolution as a whole.

After the Big Bang 14 billion years ago, the only atoms in the Universe were hydrogen and helium. All heavier elements, known as metals in astronomical terms, were produced in the superheated hearts of emerging stars. As galaxies aged, the stars continued to be born and to evolve, increasing the amount of metal-rich material in the galaxies. Therefore galaxies in the present-day Universe tend to be metal-rich.

SDSS team scientists explained that the amount of "metals" in the interstellar medium depends on the time of star-formation process as elements heavier than hydrogen are ejected from stars. Youngest galaxies should have the least "metal" content. So researchers surveyed the 250,000-galaxy SDSS database to select extremely metal poor ones (XMPGs).

The newly discovered galaxies only have 1/20 of the metal content of the Sun, which is an unusual finding for such nearby, contemporary galaxies. The paper based on the findings is scheduled to appear in the August 20 issue of The Astrophysical Journal Letters.

Alexei Kniazev and Eva Grebel from the Max-Planck Institute for Astronomy in Germany, leaders of the SDSS research team, studied the metal content of the these galaxies by measuring the concentration of elements in the excited gases in the galaxies. The gases tell the make up of the star-forming material and of the material ejected from the stars.

"It is quite unexpected to find metal-deficient galaxies in the present Universe. Could these enigmatic XMPGs be galaxies that only formed recently?" asked Kniazev. Unlike normal galaxies, a young galaxy could be less than a billion years to only a few hundred million years and would be expected to have a lowe r concentration of metals. For comparison, the oldest stars in the Milky Way Galaxy are probably 13 billion years old.

"But it's not clear that low metallicity is indeed an indicator of a young age," cautioned Kniazev.

There are two basic types of extremely metal-poor galaxies. One type belongs to the class of blue compact galaxies, small galaxies in isolated environments that are currently undergoing a major starburst. The other, general class of galaxies includes disk galaxies and irregular or dwarf irregular galaxies.

The findings raise a number of questions. Although most galaxies formed billions of years ago, is it possible that some small fraction of galaxies is just forming now? Is it possible that some primordial clouds of hydrogen never tried to form stars but for some reason started to do it very recently?

Grebel surmised that some of the new objects could have lost many of their metals. Starburst winds - formed when large numbers of massive stars are born - may have blown the metals into space. Or the galaxies may have evolved so slowly with very low star formation rates that they have small amounts of metals. Some of the previously known XMPGs have been shown to be ancient galaxies that are evolving slowly. Other XMPGs are undergoing vigorous starbursts.

Another question is how the XMPGs are distributed relative to normal galaxies. "Theory tells us that XMPGs should be in isolated environments, such as voids. Our next job is to see whether this is correct for the objects we're discovered," added researcher Lei Hao from Princeton University Observatory.

The SDSS coverage (one-quarter of the northern sky), the homogeneity of its results and the depth of its field make it a unique database to search for these rare objects. "The very small number of XMPGs found in this huge sample underlines how rare and unusual these galaxies are," Grebel added.

The importance of searching for XMPGs as the candidates for "nearby young galaxies" began more than 30 years. The earliest XPMG study used glass photogra phic plates by Leonard Searle and Wallace L.W. Sargent in the early 1970s. By comparison, the SDSS metal-poor research team has their pick of hundreds of times more galaxies per square degree than did Searle and Sargent.

"Extrapolating to the full anticipated SDSS survey area (of approximately 10,000 square degrees) we expect to detect at least 20 additional XMPGs," SDSS research team members explained.

"The general idea is to study some of them in great detail and their general properties as a group. This was practically impossible to do in previous studies because galaxies were found randomly," added Kniazev.

The SDSS discoveries are an important step toward assembling a sufficiently large sample to study the properties of XMPGs as a group and toward exploring the full range of galaxy metallicities, the researchers said.

"We plan to answer 'how' and 'why' such rare galaxies appeared and in doing so we hope to understand slightly more the total galaxy formation process," Kniazev said.

Other members of the discovery team were: Michael A. Strauss of the Princeton University Observatory; Jonathan Brinkmann of the Apache Point Observatory in Sunspot, NM., and Masataka Fukugita of Japan's Institute for Cosmic Ray Research at the University of Tokyo.


Alexei Y. Kniazev Max-Planck-Institute for Astronomy, Konigstuhl 17, D-69117 Heidelberg, Germany; (49 6221 528-263), and Special Astrophysical Observatory, Nizhnij Arkhyz, Karachai-Circessia, 369167, Russia

Eva K. Grebel, Max-Planck-Institute for Astronomy, (49-6221-528-225)

Lei Hao, Princeton University Observatory, Peyton Hall, Princeton, NJ 08544-1001; (609)258-8063

Michael A. Strauss, Princeton University Observatory, (609)258-3808

Jonathan Brinkmann, Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349 505-437-6822

Masataka Fukugita, Institute for Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwa, Kashiwa City, Chiba 277-8582, Japan, +81 471 36 3155