Free Astronomy Magazine July-August 2015

SPACE CHRONICLES T hese six snapshots taken by NASA's Hubble Space Telescope show how galaxies similar in mass to our Milky Way evolved over time. The images reveal that Milky Way-like galaxies grow larger in size and in stellar mass over billions of years. The image at far right reveals a compact, youthful galaxy as it looked 11.3 billion years ago, when our universe was only about 2.5 billion years old. The bluish-white glow reveals that the fledgling galaxy is undergoing a wave of star birth, as its rich reservoir of gas compresses under gravity, creating myriad stars. At 10.3 billion years ago (image at center), the firestorm of star birth is reaching its peak. The stellar "baby boom" churned out stars 30 times faster than the Milky Way does today. The galaxy's yellowish color most likely indicates ongoing star formation that is being obscured by dust and gas. Eventually, the galaxies exhaust their star-making gas. The galaxy at 8.9 billion years ago has developed a spiral shape, and the oldest stars reside in its central region. Nearly 3 billion years later, a similar galaxy has grown even larger. The galaxy is dominated by mostly older stars, which can be seen in its reddish appearance. [NASA, ESA, C. Papovich (Texas A&M Univer- sity), H. Ferguson (STScI), S. Faber (University of California, Santa Cruz), and I. Labbé (Leiden University)] in the star-forming boom as more massive stars ended their lives early and enriched the galaxy with material that served as the building blocks of planets and even life on Earth. As- tronomers don't have baby pictures of our Milky Way's formative years to trace the history of stellar growth. In- stead, they compiled the story from studying galaxies similar in mass to our Milky Way, found in deep surveys of the universe. The farther into the universe astronomers look, the fur- ther back in time they are seeing, be- cause starlight from long ago is just arriving at Earth now. From those sur- veys, stretching back in time more than 10 billion years, researchers as- sembled an album of images contain- ing nearly 2,000 snapshots of Milky Way-like galaxies. The new census provides the most complete picture yet of how galaxies like theMilkyWay grew over the past 10 billion years into today's majestic spiral galaxies. The multi-wavelength study spans ultraviolet to far-infrared light, com- bining observations from NASA's Hubble and Spitzer space telescopes, the European Space Agency's Herschel Space Observatory, and ground-based telescopes, including the Magellan Baada Telescope at the Las Campanas Observatory in Chile. "This study al- lows us to see what the Milky Way may have looked like in the past," said Casey Papovich of Texas A&M Univer- sity in College Station, lead author on the paper that describes the study's results. "It shows that these galaxies underwent a big change in the mass of its stars over the past 10 billion years, bulking up by a factor of 10, which confirms theories about their growth. And most of that stellar-mass growth happened within the first 5 billion years of their birth." The new analysis reinforces earlier research that showed Milky Way-like galaxies began as small clumps of stars. The di- minutive galaxies built themselves up by swallowing large amounts of gas that ignited a firestorm of star birth. The study reveals a strong correlation between the galaxies' star formation and their growth in stellar mass. Ob- servations revealed that as the star- making factories slowed down, the galaxies' growth decreased as well. "I think the evidence suggests that we can account for the majority of the buildup of a galaxy like our MilkyWay through its star formation," Papovich said. "When we calculate the star-for- mation rate of a Milky Way galaxy and add up all the stars it would have produced, it is pretty consistent with the mass growth we expected. To me, that means we're able to understand the growth of the 'average' galaxy with the mass of a Milky Way galaxy." The astronomers selected the Milky Way-like progenitors by sifting through more than 24,000 galaxies in the entire catalogs of the Cosmic Assembly Near-infrared Deep Extra- galactic Legacy Survey (CANDELS), taken with Hubble, and the FourStar Galaxy Evolution Survey (ZFOURGE), made with the Magellan telescope. They used the ZFOURGE, CANDELS, and Spitzer near-infrared data to study the galaxy stellar masses. The Hubble images from the CANDELS survey also provided structural infor- mation about galaxy sizes and how they evolved. Far-infrared light ob- servations from Spitzer and Herschel helped the astronomers trace the star- formation rate. n