Free Astronomy Magazine July-August 2016

SPACE CHRONICLES D EI- MOS (DEep Imaging Multi-Ob- ject Spec- trograph) is a vis- ible-wave- length, faint-ob- ject, multi-slit imaging spectro- graph in operation at the Nasmyth focus of the Keck II tele- scope since 2002. [W. M. Keck Observa- tory] enough to create three different images of the object. “Keck Obser- vatory's telescopes are simply the best in the world for this work,” said Marusa Bradac, a professor at University of California, Davis, who led the team. “Their power, paired with the gravitational force of a massive cluster of galaxies, allows us to truly see where no human has seen before.” “Because you see three of them and the characteristics are exactly the same, that means it was lensed,” said Marc Kassis, staff astronomer at Keck Observatory who assists the discovery team at night. “The other thing that is particularly interesting is that it is small. The only way they would have seen it is through lens- ing. This allowed them to identify it as an ordinary galaxy near the edge of the visible Universe.” “If the light from this galaxy was not magnified by factors of eleven, five and two, we would not have been able to see it,” said Kuang- Han Huang, a team member from UC Davis. “It lies near the end of the reionization epoch, during which most of the hydrogen gas between galaxies transitioned from being mostly neutral to being mostly ion- ized (and lit up the stars for the first time). That shows how gravitational lensing is important for understand- ing the faint galaxy population that dominates the reionization photon production.” The galaxy’s magnified images were originally seen separately in both Keck Observatory and Hubble Space Telescope data. The team collected and combined all the Keck Observa- tory/DEIMOS spectra from all three images, confirming they were the same and that this is a triply-lensed system. “We now have good con- straints on when the reionization process ends – at redshift around 6 or 12.5 billion years ago – but we don’t yet know a lot of details about how it happened,” Huang said. “The galaxy detected in our work is likely a member of the faint galaxy population that drives the reionization process.” “This galaxy is exciting because the team infers a very low stellar mass, or only one percent of one percent of the Milky Way galaxy,” Kassis said. “It’s a very, very small galaxy and at such a great distance, it’s a clue in answering one of the funda- mental questions astronomy is trying to understand: What is caus- ing the hydrogen gas at the very beginning of the Universe to go from neutral to ionized about 13 billion years ago. That’s when stars turned on and matter became more complex.” n