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astrophysics

Lost in space

Brazilians discover four star nurseries outside the galaxies

In a corner of the Universe where it was thought there was nothing, a team of astrophysicists from Brazil and France identified four new nurseries of hot young stars, with ages between 3.2 million and 5.6 million years (see the luminous blue dots highlighted on the next page). Rich in metals and compounds of hydrogen charged with electric particles, these regions are located at apparently empty points of intergalactic space existing in the environs of a compact group of five galaxies called Stephan’s Quintet, 280 million light-years distant from the Earth. One light-year is equivalent to the space covered by light in one year, something like 10 trillion kilometers.

The nurseries are to be found in the tail of gas of one of the galaxies of Stephan’s Quintet, NGC 7319, but are located about 70,000 light years from its spiral arms, where the formation of stars usually occurs. This means that the new regions are literally outside the galaxies. Their tens or maybe hundreds of stars are therefore “loose” in the Cosmos, outside their captive place, as if they were space orphans or hermits. They are stars without galaxies. Close to the size of a galaxy, which may contain billions of stars, like the Milky Way, the new nurseries are an imperceptible dot in the Universe.

The discovery of H II regions, a technical name given to the places where new stars originate, in zones external to the galaxies, is extremely rare and recent. “Up until two years ago, astronomers believed that that the formation of stars, especially the younger ones, took place only inside the galaxies”, says Brazilian researcher Claudia Mendes de Oliveira, from the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of São Paulo (IAG-USP), one of the authors of the discovery of the nurseries, reported in a scientific article published in the April 10 issue of The Astrophysical Journal. “Only in their insides would there be a sufficiently high density of gas for this to happen.”

In the intergalactic medium, the density of the gas would be too low to lead to the formation of stars. It should be, but it is not – to judge from the work written by the Franco-Brazilian team and by other recent scientific articles. The identification of four nurseries of young and hot stars in the environments of Stephan’s Quintet – achieved with the use of instruments installed in the Gemini North Observatory, in Hawaii – represents the second evidence of weight in favor of the idea that there are indeed H II regions outside the galaxies. The first arose in 2002, when a group of researchers from Europe, Australia and Japan discovered a solitary H II region in the vicinity of the constellation Virgo.

After the publication of these pioneering works, two other international groups found another six star nurseries outside galaxies. By the look of it, there are more groups of young orphan stars wandering through intergalactic space than any astrophysicist ever imagined. “The presence H II regions outside the galaxies is not so rare as we used to think up until now”, explains Laerte Sodré Junior, also from the IAG-USP, another author of the discoveries of the nurseries next to Stephan’s Quintet. “Actually, we were faced by a new mechanism for forming stars.”

Also with their signatures in the article in The Astrophysical Journal are a Brazilian, Eduardo Cypriano, who lives in Chile today, a French woman, Chantal Balkowski, from the Paris Observatory. The team’s researches enjoyed finance from FAPESP and support from the National Council for Scientific and Technological Development (CNPq) and Centers of Excellence Support Program (Pronex). Finding H II regions in the middle of nowhere is still not conclusive proof that there is star formation in this stretch of space, outside the galaxies.

The stars can be there today, but this does not mean that they have always been there. Their birthplace may have been the inside of a nearby galaxy and, later, for some reason, probably collisions between galaxies, these stars may have been expelled into the intergalactic medium. An analogy would be the situation of a Brazilian who, for some reason, leaves his country and moves to the United States. This does not make him an American. At the most, it makes him an American resident.

In the case of the four star nurseries identified in the environs of Stephan’s Quintet, the astrophysicists have evidence to hand to defend the hypothesis that these H II regions must have originated precisely in the place where they are currently to be found: their extreme youth in astronomical terms. The four star nurseries are regarded as excessively young to have arisen at some point in the Universe (the inside of one or more neighboring galaxies) and afterwards migrated to another (the intergalactic medium). This kind of movement calls for more time to happen than the average age attributed to the new H II regions, 4.6 million years. The nurseries must therefore have been born where they are today.

The scenario seemed to be perfect, but for one detail. How do the researchers explain the high metallicity measured in these young stars that were formed outside the galaxy, paradoxically in an environment almost devoid of this kind of chemical element? After all, the quantity of metals present in the new H II regions is of the same order as that found in the inside of galaxies and stars, like the Sun.

“The truth is that we do not yet have a good answer to this question”, Claudia admits. “But we are proposing a scenario to account for this situation.” For the astrophysicists, the H II regions originated in the almost empty intergalactic medium from material already recycled and enriched with metals that, about 100 million years back, had been ejected from the inside of Stephan’s Quintet, due to collisions between its galaxies. Accordingly, their constituent element was not just the primordial gas in the intergalactic medium, of low density and poor in metals. It was above all the heavier and metallic material that was set loose from the neighboring galaxies.

The scientists suspect that the star nurseries may be associated with clouds of cold gas (neutral hydrogen) that came loose from Stephan?s Quintet and are to be found today in the tail of the NGC 7319 galaxy. Due to some more recent instability, occurring a few million years ago, these gas clouds gave origin to the H II regions in the intergalactic medium”, Sodré Junior explains. Now that they have discovered the existence of star nurseries in the empty part of the Universe, the researchers are going to have to accompany the evolution of these strange formations. They think that these stars, loose in space, may one day originate one of the most energetic events in the Cosmos: explosions that generate supernovae, stars that are capable of temporarily shining more than an entire galaxy.

The Project
Evolution of Galaxies in Groups and Agglomerates; Modality Thematic Project; Coordinators Laerte Sodré Junior and Claudia Mendes de Oliveira; Investment R$ 207,588.40

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