Astronomers are puzzled why it appears that some of the most distant galaxies in the universe are more dense with stars than expected. In this image of the Milky Way star cluster Omega Centauri, bright stars have been coloured blue, faint ones red. For more distant galaxies, though, faint stars are impossible to see. Now it turns out some of the most distant galaxies in the universe are more packed with stars than astronomers expected.
The data galaxies give astronomers comes from the light of their stars. But stars don't carry all of a galaxy's mass. According to the current model, some of the mass is invisible dark matter, which can't be measured directly.
"To estimate exactly what is the mass that they have, we always use this conversion factor, to convert light into mass," says University of Oxford astronomer Michele Cappellari. "The conversion we used for many decades was wrong, and will have to be revised," Cappellari said.
The study took a close look at 260 early galaxies and found that the distribution of stars in early galaxies is actually different from galaxies that formed at a later time in the universe — a finding that could change our understanding of how galaxies evolve.
At the center of the conundrum is an astronomical formula known as the initial mass function, or IMF. The formula helps astronomers determine the mass of stars within a galaxy, which can then be used to measure the galaxy's growth over time.
The Oxford team calculated the mass of the stars in the survey by studying the energy they radiate. They compared that number with the mass obtained by measuring the motion of the stars, which is controlled by the galaxy's gravitational pull.
Cappellari found that the relationship between visible light and stellar mass varies from galaxy type to galaxy type. The delta was greatest for the most distant galaxies, which are three times more star-filled than expected --astronomers had not been counting faint stars - like the red ones in distant galaxies like Omega Centauri.
The distribution of low- to high-mass stars was three times more massive in the older galaxies than in younger galaxies within the survey. The younger elliptical and lenticular galaxies had results similar to spiral galaxies like the Milky Way.
The researchers found that the older galaxies have a larger fraction of low-mass stars than their younger counterparts. The researchers determined that the initial mass function, long thought constant across galaxies of all types and ages, in fact varies for these older stellar groups, a finding with profound consequences.
"The IMF is necessary to convert the light we observe from galaxies into the stellar mass that all models predict," Cappellari told space.com. "Up to now, astrophysicists assumed this conversion could be performed with a universal IMF. What seems clear … is that the oldest galaxies in the universe formed their stars in much more dramatic and intense events, while spiral galaxies formed stars more quietly for the major part of their lives."
The reason for this variation remains a mystery. The cosmic enigma is "how did these star-stuffed galaxies get to be so big so early on in their lives? They need to grow faster than people thought," Cappellari said.
NASA's Hubble Space Telescope captured the image at the top of the page of 100,000 stars residing in the crowded core of globular cluster Omega Centauri, which boasts nearly 10 million stars. Globular clusters, ancient swarms of stars united by gravity, are the homesteaders of our Milky Way galaxy. The stars in Omega Centauri are between 10 billion and 12 billion years old. The cluster lies about 16,000 light-years from Earth.
This is one of the first images taken by the new Wide Field Camera 3 (WFC3), installed aboard Hubble in May 2009, during Servicing Mission 4. The camera can snap sharp images over a broad range of wavelengths.
The majority of the stars in the image are yellow-white, like our Sun. These are adult stars that are shining by hydrogen fusion. Toward the end of their normal lives, the stars become cooler and larger. These late-life stars are the orange dots in the image.
Even later in their life cycles, the stars continue to cool down and expand in size, becoming red giants. These bright red stars swell to many times larger than our Sun's size and begin to shed their gaseous envelopes.
After ejecting most of their mass and exhausting much of their hydrogen fuel, the stars appear brilliant blue. Only a thin layer of material covers their super-hot cores. These stars are desperately trying to extend their lives by fusing helium in their cores. At this stage, they emit much of their light at ultraviolet wavelengths.
When the helium runs out, the stars reach the end of their lives. Only their burned-out cores remain, and they are called white dwarfs (the faint blue dots in the image). White dwarfs are no longer generating energy through nuclear fusion and have gravitationally contracted to the size of Earth. They will continue to cool and grow dimmer for many billions of years until they become dark cinders.
Other stars that appear in the image are so-called "blue stragglers." They are older stars that acquire a new lease on life when they collide and merge with other stars. The encounters boost the stars' energy-production rate, making them appear bluer.
The average distance between any two stars in the cluster's crowded core is only about a third of a light-year, roughly 13 times closer than our Sun's nearest stellar neighbor, Alpha Centauri. Although the stars are close together, WFC3's sharpness can resolve each of them as individual stars. If anyone lived in this globular cluster, they would behold a star-saturated sky that is roughly 100 times brighter than Earth's sky.
The research was published in the April 26 issue of the journal Nature.
Image Credit: NASA/ESA/Anderson/van der Marel