Slow-Growing Galaxies

We live in a Universe lit by the countless fires of a host of brilliant stellar sparklers. On clear, dark nights an observer can see literally thousands of bright stars without the aid of a telescope–yet astronomers are hindered in their study of the ancient epoch when the first stars were born because they lack direct observations. Why were some galaxies inhabiting the ancient Universe literally bursting with the fiery formation of a myriad of shining new baby stars, while others were relatively barren, and almost bereft of dancing starlight? A new study published in the October 16, 2014 issue of the journal Nature now addresses this question by making some of the most accurate measurements yet of the sluggish rates at which small, lazy galaxies dwelling in the “nearby” Universe give birth to baby stars. The new findings are helping astronomers understand how the very first stars in our Universe caught fire.

The new report uses data derived from the European Space Agency’s (ESA’s) Herschel mission, in which NASA is a partner–and both NASA’s Spitzer Space Telescope (SST) and Galaxy Evolution Explorer (GALEX) played important roles in the observations.

The new discoveries are helping astronomers to figure out how the very first generation of stars in our Universe ignited. Like the stars studied in the new research, the very first stars that were born billions of years ago caught furious fire under some very poor conditions. Nourishing “heavy metals” had not formed as yet, and baby stars need them to grow and thrive. In astronomical jargon metals are atomic elements heavier than hydrogen and helium. Long ago, soon after the Big Bang, metals had not yet had sufficient time to form.

“The metals in space help act in some ways like a fertilizer to help stars grow,” commented Dr. George Helou in an October 15, 2014 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Helou is an author of the new Nature paper and director of NASA’s Infrared Processing and Analysis Center (IPAC) at the California Institute of Technology (Caltech) in Pasadena, California. The lead author of the study is Dr. Yong Shi, who performed some of the research at IPAC before moving to Nanjing University in China.

The Birth Of Stellar Sparklers

It is generally believed that our ancient Universe was a featureless swath of darkness for a very long stretch of time. The first generation of stars likely did not burst into existence until about 100 million years or so after the inflationary Big Bang birth of the Universe almost 14 billion years ago–and nearly a billion years passed before the galaxies formed and spread throughout the ancient Cosmos. Astronomers have long pondered the question of how this dramatic transition from darkness to light finally came about.

Alas, the study of the primordial Universe is problematic because of a general lack of observations. However, astronomers have been able to examine much of the Universe’s mysterious and dark past by aiming their telescopes on remote galaxies and quasars that sent forth their dazzling light billions of years ago. The age of a shining object can be calculated by the redshift of its light, which indicates how much the Universe has expanded since the light was first produced.

Supercomputer simulations indicate that the first stars should have been born somewhere between 100 million and 250 million years after the Big Bang. They caught fire in small, amorphous protogalaxies that evolved from density fluctuations in the primordial Universe. Because the ancient protogalaxies contained no elements heavier than pristine hydrogen and helium–born in the Big Bang itself (Big Bang nucleosynthesis), the physics of star-birth in the ancient Cosmos favored the formation of bodies that were many times more massive and luminous than our Star, the Sun. Radiation sent forth by the most ancient stars ionized the ambient pristine hydrogen gas. Some stars blasted themselves to smithereens in the brilliant rage of supernovae explosions, dispersing their newly manufactured metals throughout the Universe. All elements heavier than hydrogen and helium were manufactured in the searing-hot, roiling hearts of the stars, that progressively fused heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). The oxygen we breathe, the water that we drink, the iron in our blood, the carbon that is the basis for life on our planet–all of these elements were created by the stars in their nuclear fusing hearts, or else in their supernovae deaths.

Stars of all masses–large and small–“live” out their entire normal, hydrogen-burning existence on the main-sequence. They do this by maintaining a very precious and delicate balance between two constantly battling forces–radiation pressure and gravity. A star’s radiation pressure pushes everything out and away from the star, and it keeps this huge incandescent sphere of seething, roiling gas bouncy against the horrendous crushing squeeze of its own gravity–that pulls everything in and towards the star. A star’s radiation pressure is the result of nuclear fusion–the burning of light atomic elements, such as hydrogen, into progressively heavier and heavier things. When a star, at last, runs out of its necessary supply of nuclear fuel, it has come to the end of that long stellar road, and perishes. If it is a small star, like our Sun, it goes gently into that good night by puffing its outer layers into space with relative peacefulness. If it is a massive star, however, it rages at its own inevitable death, blasts itself to pieces, and hurls its stellar material into space with fiery, brilliant fury.

The first generation of massive, highly luminous stars changed the dynamics of the Universe by heating and ionizing the ambient gases. The most ancient of stellar sparklers manufactured and then dispersed the first batch of heavy elements–metals–out into space when they went supernova, thus paving the happy way for the eventual formation of solar systems like the one we inhabit. The furious collapse of some of the first stars may have seeded the growth of supermassive black holes that formed in the dark hearts of galaxies and became the fierce power sources for quasars–which are very brilliant and active galactic nuclei. Therefore, the most ancient stars made possible the evolution of the Universe that we live in today–everything from galaxies to planets and people.

How Slow-Growing Galaxies Shed Light On Stars Most Ancient

The lazy duo of slow-growing galaxies used in the Nature study, dubbed Sextans A and ESO 146-G14, lack heavy metals, just like our ancient and remote Universe–only they are considerably closer to us and, therefore, much easier to see! Sextans A is located approximately 4.5 million light-years from Earth, and ESO 146-G14 is over 70 million light-years away.

These relatively petite galaxies are late-bloomers. Somehow, the duo managed to make their way through the history of the Universe while remaining pristine–they never created their own batch of heavy metals. Heavy metals not only enable stars to be born–they are also are created by the stars.

“The metal-poor galaxies are like islands left over from the early Universe. Because they are relatively close to us, they are especially valuable windows to the past,” Dr. Helou explained in the October 15, 2014 JPL Press Release.

Alas, studying star-birth in metal-poor environments such as these can be complicated. The two galaxies, though relatively nearby, are still difficult to see and quite dim, glowing with only a feeble light. Dr. Shi and his international team attacked the problem with a multi-wavelength approach. The data derived from ESA’s Herschel, captured the longest infrared wavelengths of light, and enabled the astronomers to observe the cool dust in which stars are enshrouded. The dust serves as a “proxy” for the total amount of gas floating around in the region–and gas is the stuff that stars are made of. To the prying “eyes” of other telescopes, this dust is invisible and cold. However, Herschel has the ability to capture its faint glow.

The National Radio Astronomy Observatory’s (NRAO’s) Jansky Very Large Array observatory, located near Socorro, New Mexico, and the Australia Telescope Compact Array observatory, near Narrabri, provided supporting radio-wavelength measurements of the gas lurking in the two metal-poor galaxies.

In the meantime, archival data derived from SST and GALEX were used to observe the rate of star-birth. SST sees shorter-wavelength infrared light, which emanates from dust that is warmed by fiery neonatal stars. GALEX images capture ultraviolet light rushing out from the brilliantly sparkling baby stars themselves.

Putting all the pieces of this puzzle together, the team of astronomers were able to determine that the lazy galactic duo are plodding sluggishly along, forming new baby stars at rates about 10 times slower than their more normal, active counterparts.

“Star formation is very inefficient in these environments. Extremely metal-poor nearby galaxies are the best way to know what went on billions of years ago,” Dr. Shi explained in the JPL Press Release.

The heavy metals existing in the galactic constituents of today’s Cosmos help star-birth to flourish via cooling effects. In order for a star to be born, a sphere of gas needs to tumble in on itself with the invaluable aid of its own gravity. Eventually, the star-stuff has to grow sufficiently dense for atoms to fuse and catch fire, creating sparkling starlight. But as this cloud of gas collapses, it grows hotter and hotter and puffs back out again, thus counteracting the process. Heavy metals cool everything off by radiating away the searing-heat, thus enabling the cloud to condense into a new baby star.

It is a mystery how stars dwelling in the ancient Universe were able to accomplish this without the necessary cooling benefits provided by heavy metals. Studies like this one are important because they shed light on stars most ancient–the very first shining stellar sparklers to dance around the Cosmos–thus giving astronomers a valuable glimpse into our Universe’s ancient, mysterious past.