March 19, 2014
A Herschel space telescope survey of cosmic dust in 323 galaxies, located 50 to 80 million light-years away, is the most complete such census that represents an important benchmark for future observations in efforts to better understand evolution of galaxies.
On the left you can see cosmic dust in observed galaxies in infrared/submillimetre wavelengths. Blue is cold dust, red is warm dust. For comparison, on the right you can see those same galaxies in visible light. Here you can't see dust, only stars. Young and hot stars are represented by blue colour and old and cooler stars are red. Younger, dust-rich spiral and irregular galaxies are in the top left and large dust-poor, red-and-dead elliptical galaxies are in the bottom right. Credit: ESA/Herschel/HRS-SAG2 and HeViCS Key Programmes/Sloan Digital Sky Survey/ L. Cortese (Swinburne University)
Cosmic dust, together with gas, plays an important role in formation and evolution of galaxies and star systems within those galaxies, yet beyond Milky Way, we don't know as much as we'd like about properties of this dust. We don't know if the type of dust a galaxy is made of determines what kind of galaxy it would be.
Thanks to Herschel space telescope survey, we now have a catalogue of 323 galaxies located 50 to 80 million light-years from Earth. These galaxies are of all three types (elliptical, spiral and irregular), with different chemical compositions, varying star formation activity and ranging from dust-poor to dust-rich. Observation were made with Herschel's various instruments in far-infrared and submilimetre wavelengths.
This catalogue shows that dust-rich galaxies are usually spiral or irregular in shape, while dust-poor galaxies are mostly elliptical. Elliptical galaxies are known to have very few new stars born since they lack necessary cold gas, so lack of dust in them is not a surprise.
By studying how much light is emitted by dust at which wavelength, scientists can now study its physical properties. In galaxies where stars are born at a higher rate, there are more massive and hot stars, which in turn should heat the surrounding dust more than their cooler counterparts. Hotter dust gives out radiation at shorter wavelengths.
However, this survey shows that the correlation between star formation rate and dust temperature is more complicated since it shows bigger variations between galaxies than expected. Chemical properties of dust could play bigger role than previously thought.
Future observations of more distant and thus older galaxies will benefit a lot from this survey which focuses on local galaxies. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile will be able to observe dust in early galaxies at the edge of observable universe. How this old dust compares to younger galaxies only 50 to 80 million light-years away will help us better understand evolution of galaxies.