Free Astronomy Magazine September-October 2016

SPACE CHRONICLES Dwarf galaxies are the building blocks from which larger galaxies were formed billions of years ago in the early universe. Inhabiting a sparse desert of largely emp- ty space for most of the universe's history, these two galaxies avoided that busy construction period. “These galaxies may have spent most of their history in the void,” Tollerud ex- plained. “If this is true, the void envi- ronment would have slowed their evolu- tion. Evidence for the galaxies' void address is that their hydrogen content is somewhat high relative to sim- ilar galaxies. In the past, galaxies contain- ed higher concentra- tions of hydrogen, the fuel needed to make stars. But these galaxies seem to retain that more primitive composition, rather than the enriched composition of con- temporary galaxies, due to a less vigorous history of star formation. The galaxies also are quite compact relative to the typical star-forming galaxies in our galactic neighbor- hood.” The dwarf galaxies are small and faint, so finding them is extreme- ly difficult. Astronomers spotted them by using radio telescopes in a unique survey to measure the hy- drogen content in our Milky Way. The observations captured thou- sands of small blobs of dense hy- drogen gas. Most of them are gas clouds within our galaxy, but astron- omers identified 30 to 50 of those these Hubble obser- vations, for calculat- ing how far away the galaxies are from nearby voids. Pisces A is about 19 million light-years from Earth and Pisces B roughly 30 million light-years away. An analysis of the stars' colors allowed the astronomers to trace the star forma- tion history of both galaxies. Each galaxy contains about 20 to 30 bright blue stars, a sign that they are very young, less than 100 million years old. Tollerud's team esti- mates that less than 100 million years ago, the galaxies doubled their star-formation rate. Eventually, the star formation may slow down again if the galaxies become satellites of a much larger galaxy. “The galaxies could even probably stop forming stars all together, because they will stop getting new gas to make stars,” Tollerud said. “So they will use up their existing gas. But it's hard to tell right now exactly when that would happen, so it's a reason- able guess that the star formation will ramp up at least for a while.” Tollerud's team hopes to observe other similar galaxies with Hubble. He also plans to scour the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) survey for po- tential dwarf galaxies. Future wide- survey telescopes, such as the Large Synoptic Survey Telescope (LSST) in Chile and the large radio telescope in China, should be able to find many of these puny galactic neighbors. P isces B is 29.0 million light-years away. In this image of the galaxy, the bright object with the diffraction spikes below left of center is a foreground star in our Milky Way galaxy. Several distant back- ground galaxies are also visible. [NASA, ESA, and E. Tollerud (STScI)] n blobs as possible galaxies. The re- searchers used the WIYN telescope in Arizona to study 15 of the most promising candidates in visible light. Based on those observations, Tolle- rud's team selected the two that were the most likely candidates to be nearby galaxies and analyzed them with Hubble's Advanced Camera for Surveys. Hubble's sharp vision helped the astronomers con- firm that both of them, Pisces A and B, are dwarf galaxies. The Hubble telescope is aptly suited to study nearby, dim dwarf galaxies because its sharp vision can resolve individ- ual stars and help astronomers esti- mate the galaxies' distances. Distance is important for determin- ing a galaxy's brightness, and, in