By Agnieszka Pollo, Jagiellonian University in Krakow
The expression ‘dusty universe’ is not uncommon in popular or even scientific writings, and we are certainly not suffering from lack of dust in everyday life. Grains of dust, in sizes of fractions of a micrometre, can be found not only on the Earth but also form 1% of the interstellar matter. However, to regard dusty galaxies as inert, immobile and insignificant would be a serious mistake; on the contrary, dust in cosmic space is a sign of activity, of creation and — quite literally — of life. Without interstellar dust, the Solar System itself cannot exist, let alone ourselves.
This life-giving and star-making dust has one major drawback for the observer: the dust not only fails to show up in normal light but also absorbs both normal and ultraviolet light emitted by stars — the dark lines that we often see in photos of galaxies are nothing but interstellar dust, and many stars are hidden behind them. Some galaxies are so dusty that they remain practically invisible to optical instruments despite being large and rich in bright stars.
How can we then see this scientifically valuable dust? Luckily, it becomes transparent in some parts of the infrared spectrum and can even emit light in some other parts of the infrared spectrum, thus sending back into the cosmic space the absorbed visible and ultraviolet light of stars. In such infrared light, we can examine even the dustiest parts of the universe. However, it is difficult to carry out these observations from the ground because the Earth’s atmosphere not only absorbs some infrared light but also emits some itself. Instruments aboard satellites in space therefore offer the best solution.
Since the first-ever infrared satellite — IRAS — was launched, in 1983, our vision of the universe has changed dramatically. The number of catalogued astral bodies has increased by 70% of the number of such known bodies before 1983. This shows that there are many bodies in space that are luminous in infrared light but invisible or very faint in visible light. Most of them were identified as objects belonging to our Milky Way, but a few tens of thousands of them turned out to be extragalactic: most of them were identified as relatively nearby but very dusty galaxies.
Although generations of satellites after IRAS continued scanning the sky using infrared light, the next satellite charged with a mission to undertake a more detailed and extensive survey of the entire sky using infrared light was launched only in 2006. This was the Japanese satellite AKARI. Its first data — measurements of more than a million sources visible in different ranges of the infrared spectrum — were released to the scientific community in March 2010. Additionally, AKARI observed two smaller patches of the sky in much greater detail.
The first question in dealing with these voluminous data was a fundamental one: what is it? That is, what are all these objects emitting infrared light? The possibilities were many: Stars belonging to the Milky Way? Nearby galaxies obscured by dust? Very distant galaxies the light from which moved to the infrared part of the spectrum during its long passage through the expanding universe? Active nuclei of galaxies? Or some other, even more exotic sources? Even if we assume that we are dealing with a mixture of sources, how do we tell them apart?
At the first glance, the simplest way may be to search the sky using optical light and see if we can find any bodies corresponding to the detected infrared sources, and thus infer their nature based on that information. However, this is not always possible. Looking for matching optical counterparts is a major task that requires dedicated observations. If we cannot do it, we have to rely on statistical methods — and we can test these methods using the sources that we have been able to identify. The methods used with data from IRAS were not always applicable to the AKARI data because AKARI uses longer wavelengths and also can resolve finer details than IRAS could.
Using the data from the all-sky survey, Agnieszka Pollo and Piotr Rybka from the Astronomical Observatory of Jagiellonian University in Krakow (Poland) and Tsutomu T. Takeuchi from Nagoya University in Japan designed a method to separate stars from galaxies using only those colours found in the far infrared part of the spectrum and found that the brightest infrared sources outside the disk of the Milky Way were most likely to be nearby dusty galaxies. The disk of the Milky Way is full of infrared sources: young stellar objects, ageing stars expelling dust and gas, and some sources the nature of which continues to elude us.
Analysing the data from one region of the sky that had been observed in greater detail, the AKARI Deep Field South (ADF-S), a group led by Katarzyna Malek from the Center for Theoretical Physics in Warsaw, Agnieszka Pollo, and Tsutomu T. Takeuchi also concluded that the majority of sources that shone the brightest in the far infrared part of the spectrum are nearby dusty galaxies. Many of the sources bear traces of recent intergalactic interactions. Most probably, such intergalactic interactions triggered the formation of stars in these galaxies: the newly born stars embedded in dust shine brightly; their light is absorbed by the dust and then re-emitted in the infrared light, making them visible to AKARI.
All these findings were described in a special issue — dedicated to the results of AKARI — of the journal Astronomy and Astrophysics.
Scientists all over the world are continuing to play the game of hide-and-seek with galaxies to uncover the story of interstellar dust in galaxies, which may offer clues to many mysteries, including the appearance of life itself in the Universe.
Agnieszka Pollo
Jagiellonian University in Krakow
www.atomiumculture.eu
Hay 0 Comentarios