By RYAN O’HERN
Eight hundred and eighty million years after the Big Bang, when the young universe was mostly populated by small galaxies, one galaxy – HFLS3 – achieved a size 10 to 30 times larger than any other galaxy known. This mass of gas and dust was forming new stars 2000 times faster than the Milky Way is currently.
This massive and highly active galaxy, discovered by Prof. Dominik Riechers, astronomy, will enable scientists to better understand the processes that gave the universe the structure it has today.
The model for the evolution of the universe currently used by astronomers suggests that just after the Big Bang, matter was nearly evenly distributed throughout space. Over time, gravitational forces gathered this matter into a net-like structure, according to Riechers. At points where the threads of the net intersected, greater quantities of mass collected. Once a critical quantity of mass was collected at such a point, the process of star formation would begin and a galaxy would be born.
In the context of this model, the galaxy HFLS3 is an oddity, according to Riechers. This is because, at only 880 million years old, the universe would not have had enough time to produce many objects of such great size through the process of gravity gathering together mass. It is probable that some astronomical event occurred to bring this quantity of mass together. One possibility is that HFLS3 was formed through the collision of two smaller galaxies, according to Riechers.
The other unique feature of HFLS3, besides its age and size, is its rapid rate of star formation. According to Riechers, the amount of gas present in a region and the rate at which it is brought together by gravity determines the rate at which stars form. Small galaxies with the same structure as HFLS3 have lower star formation rates, Riechers said. It is HFLS3’s large size that allows it to create stars so quickly in relation to other galaxies.
“A galaxy like this doesn’t exist today,” Riechers said.
To find HFLS3, Riechers and his team analyzed data collected by the Herschel Space Observatory, an orbital telescope launched into space in 2009. The observatory collected 800 hours of data on infrared radiation emitted by distant objects. From this data, Riechers identified around 100 candidates that might be distant, and thus potentially ancient galaxies, by examining the characteristics of the light they emit. Infrared radiation and red light can be an indicator of great age in extragalactic objects, according to Riechers.
Astronomers can evaluate the distance from a distant object by determining the redshift of the light that the object emits. Redshift is the reddening of light due to increase in its wavelength. This is an example of the Doppler effect, a physical process in which the wavelength of waves emitted by an object are stretched if the object is moving away from an observer or compressed if it is moving towards an observer, according to Riechers.
In the case of extragalactic objects, redshift is not due to the type of motion humans experience in daily life, but to the expansion of space-time itself. For a distant object, the redder the light it appears to emit, the further it is from the Earth. Objects appear to be moving away from the Earth because the universe itself is expanding, increasing the distance between the Earth and other objects, according to Riechers.
Astronomers can translate the amount of redshift into an approximate distance and then estimate a distant object’s age. Because light travels at a fixed speed, the time taken for light to reach Earth from a specified distance can be calculated. From such calculations, Riechers and his colleagues determined the age of HFLS3, Riechers said.
One difficulty in determining if distant objects are galaxies is that distant objects can appear red from either redshift, or simply by being cold objects, according to Riechers. An example of a cold object is a red star, which is at a relatively lower temperature in comparison to blue or white stars. Yellow stars, such as the Sun, have a temperature in between those two extremes.
Riechers determined the rate of star formation in galaxy HFLS3 by analyzing the radiation emitted by the gas and dust clouds in which the stars form.
“[HFLS3] has so much gas and dust that almost all the light directly from the stars is blocked,” Riechers said.
The radiation emitted by these gas clouds allows Riechers to determine the types of elements present in the gas cloud and the types of processes that occur within them.
The rate of star formation detected in HFLS3 is so large that it is considered to be a starburst galaxy, a type of astronomical object with unusually active star formation.
In most galaxies, the rate of star formation steadily rises over time, reaches a stable level and plateaus until star formation dies off. Galaxies in a starburst region, such as HFLS3, experience an exponential increase in the rate of star formation that rapidly rises, reaches a peak and just as rapidly dies off once the gas fueling star formation is used up, according to Riechers.
To continue his study of objects present in the early universe, Riechers requires a larger infrared telescope that can resolve dimmer, and thus smaller, objects. To enable this research, Cornell is leading a group of universities from the US, Canada and Germany in building the Cerro Chajnantor Atacama Telescope in the desert mountains of Northern Chile. Construction on CCAT will begin later this year and is expected to be completed in 2018.
Until then, Riechers will continue to examine the data from the Herschel Observatory for other potential extragalactic objects. If more objects with similar characteristics to HFLS3 are discovered, the current model of the universe’s development will need to be adjusted to account for the presence of more massive and highly active galaxies in the early universe, according to Riechers.