Cosmic dust census of the local Universe

Astronomers have recently completed the largest census of dust in the local Universe via the Herschel Reference Survey. An international team, led by Luca Cortese from Swinburne University, used ESA’s Herschel Space Observatory to study the properties of dust grains in more than 300 nearby galaxies.

When we think about the various components of galaxies dust does not immediately come to mind. However, it turns out that these tiny particles (with sizes that vary between 10-3 and 1 micron) play a vital role in the formation of stars & planets and in the overall evolution of galaxies. For example, dust grains are needed to form molecular hydrogen (see our earlier article on dust grain alignment for an explanation of how H2 forms on the surface of grains), and stars form in molecular hydrogen clouds. So without dust it would be more difficult for galaxies to form new stars.

Despite its importance, until now astronomers have had very little information on how the amount of dust and dust properties vary across the different types of galaxy in the Universe. This was mainly due to the lack of telescopes able to perform deep surveys in the far-infrared and sub-millimetre parts of the electromagnetic spectrum. Cosmic dust is heated by starlight to temperatures of only a few tens of degrees Kelvin, causing it to emit radiation in the wavelength range mainly between 100 microns and 1 millimetre – covered by the far-infrared and sub-millimetre bands.

Previous space missions such as IRAS and Spitzer only covered a small part of this frequency range, while ground-based sub-millimetre telescopes like the James Clerk Maxwell Telescope (JCMT) and the SubMillimeter Array (SMA) are not sensitive enough to observe a large number of galaxies in a reasonable amount of time, since the emission from cosmic dust is generally faint. Only with Herschel have astronomers have been able to provide the first census of cosmic dust in the local Universe.   Herschel had the largest mirror (3.5 metres) ever sent to space and three incredibly sensitive instruments.

The sample observed in the Herschel Reference Survey includes 323 galaxies spanning the entire range of morphologies (from ellipticals to spirals to irregulars) and a wide range of star formation activities.

Galaxies in the Herschel Reference Survey. These false-colour images highlight different dust temperatures, blue representing colder and red warmer dust respectively (Credit: ESA/Herschel/HRS-SAG2, HeViCS Key Programmers, L. Cortese, Swinburne University)

The two cameras on board the Herschel satellite, SPIRE and PACS, allowed astronomers to cover the entire wavelength range from 100-500 micron, which provide important information about the physical properties of cosmic dust grains. Although the SPIRE data were obtained three years ago, the team had to wait for the completion of the PACS survey last year.

Such a long wait has been worthwhile, as the combination of the PACS and SPIRE data shows that the properties of grains vary significantly as a function of galaxy type, star formation activity and metal enrichment of the interstellar medium. It now seems clear that dust is much more abundant in irregular and spiral galaxies where active star formation is still present, while it is almost completely absent in lenticular and elliptical galaxies, where star formation has stopped and the hydrogen gas has long been consumed. For example, the mass of dust is between 1% and 0.1% of the stellar mass in spirals and irregulars, decreasing to less than 0.01% in ellipticals and lenticulars.

This can be clearly seen in the two mosaics released by the team, which compare the optical and far-infrared images for galaxies in the Herschel Reference Survey. Galaxies are arranged from dust-rich in the top left to dust-poor in the bottom right. It is immediately clear that elliptical and lenticular galaxies, which are extremely rich in stars and thus shine extremely bright in the optical, are almost completely invisible when observed at sub-millimeter wavelengths.

Collage of galaxies in the Herschel Reference Survey at FIR/sub-mm wavelengths by Herschel (left) and at visible wavelengths from the Sloan Digital Sky Survey (right). The Herschel image is coloured with blue representing cold dust and red representing warm dust; the SDSS image shows young stars in blue and old stars in red. Together, the observations plot young, dust-rich spiral/irregular galaxies in the top left, with giant dust-poor elliptical galaxies in the bottom right. (Credit: ESA/Herschel/HRS-SAG2 and HeViCS Key Programmes/Sloan Digital Sky Survey/ L. Cortese, Swinburne University)

Why dust grains are no longer present in elliptical and lenticular galaxies is still unclear, but the Herschel observations have identified one of the possible physical processes able to remove dust from galaxies. These show that, when galaxies fall into a galaxy cluster, the strong pressure (known as ram-pressure) felt by the galaxy due to the hot gas (~108 K) trapped in the cluster’s potential well may be able to remove the grains from the star-forming disk of spiral galaxies, and scatter them in the intra-cluster medium.

The data obtained for the Herschel Reference Survey have been made publicly available to allow further studies of dust properties in nearby galaxies. Although the Herschel Space Telescope completed its mission in April 2013, these data will represent for quite a long time a local benchmark for studies of dust in the early Universe. In particular, the newly commissioned Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile will soon allow astronomers to carry out similar studies at very high redshift, and these Herschel data will be the local reference necessary to determine if and how the properties of grains in galaxies have changed with the age of the Universe.

For more information, see

[Luca Cortese & Sarah Maddison]

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