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May 28, 2023



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The GTC’s dreams


The first images from the GTC and its OSIRIS instrument are not just scientific, they are poetic. A windmill galaxy, an Eskimo nebula, a swarm of galaxies called Corona Borealis, a supernova factory, the evocative Eyes of Markarian galaxy group... There is no doubt about it: Astronomy should be written in verse. When astronomers are observing at night they should do as the poet Saint-Pol-Roux did before he went to sleep. Put a sign on the door that says: “poet at work.”

Truthfully, it is through dreams that science has uncovered many of its mysteries; the scientist goes to bed with a problem and in the morning he wakes up with the solution. The GTC does the same thing for us: we go to bed full of existential question about our place in the world and in the morning as we make breakfast we can see the most fascinating and revealing vision of the Universe. The telescope, which was just inaugurated this summer, started work as an open instrument for science in March. Many proposals have been received for observations, from investigating planets far from the Solar System to examining primordial galaxies. Scientists have used the first results from the telescope to surprise us too, with a gift in the form of a photo album.

To everything the machine

For astronomers, like poets, the hidden is more interesting than the visible. To penetrate beyond the narrow band of reality that our senses show us, scientists have had to develop new prostheses and accessories so that they can capture the invisible. Technology has hugely accentuated our ability to see the Universe in all its shapes, distances and colours. OSIRIS, the first instrument to be installed at the GTC, is the ultimate showcase for the power of engineering to detect visible light. It can observe several objects at once in a wide field of view, with great clarity, and it can perform spectroscopy by splitting visible light into different wavelengths. This is all possible because of an innovative system of tuneable filters that is unique on a large diameter telescope.

OSIRIS’ powerful abilities will allow it to see very weak objects and to analyse their chemistry,  temperature, density and speed of rotation. With all of this power OSIRIS will attempt to produce a map of the deep sky, going beyond the horizons of what we currently know. It is ironic that the first instrument to work on the GTC carries the name of the Egyptian god of the far distance who, according to mythology, also taught men about civilisation, agriculture and laws. OSIRIS has already shown us some surprising fragments of the Universe.

The power of the image

Astronomy is expressed in two languages. Its native language is the laws of physics and mathematics. At other times, though, it has to use another vehicle of communication: the image. Nothing is better for understanding complex ideas than to turn them into a representation. Scientists have tested OSIRIS’ optical quality and checked its coordination with the mirrors, mechanics, control mechanisms and electronics of the GTC. This has resulted in a series of astronomical images which, although they are of no value for science, nonetheless clearly demonstrate  the enormous potential of the instrument.   

The first image taken by scientists was a spiral, one of the main shapes that nature uses to grow in a controlled way and an omnipresent symbol in the history of nearly all human cultures. This one is M51, also known as the Windmill Galaxy; it is a magnificent spiral 23 million light years from the earth and one of the brightest galaxies in the firmament. What is surprising is not seeing how easily the Universe adopts the shape of a wastepipe, but rather the exposure time that OSIRIS needed to obtain the image: two minutes, which is equivalent to four hours at a one metre diameter telescope. 

A photo album

OSIRIS has also produced other strange photographs. One of them is of the Eyes of Markarian, two galaxies – NGC 4435 and NGC 4438 – which are 50 million light years from the earth and which affect one another through their gravitational pull. The reason for the name is obvious when you see the image, but it is still disturbing to look at it through a telescope and realise that the Universe really is watching you. To capture the details of such a beautiful view we had to combine three wide band filter images from OSIRIS.

Using the same technique and at an exposure of just 200 seconds, a portrait has been produced of galaxy NGC 7331, which is a “twin” of the Milky Way but much brighter and more massive, and also of NGC2770, a true “supernova factory” which is 100 million light years away. This is one of the nearest galaxies exhibiting this type of phenomenon, which is triggered when the nucleus of a massive star collapses. During the last decade two supernovas and a source of X ray emissions have been detected in this region, presaging a spectacular firework display in the future.  

Tuneable filters are used to select light in a very narrow spectral range. It is because of them that the telescope has been able to observe objects like the Eskimo Nebula, but by selecting the Hydrogen Balmer Alpha emission line rather than by photographing it directly; this is no insult, but rather a means of stripping objects in the sky and bringing out their colours. It has also been possible to capture an image of the family of galaxies in cluster Abell 2065 which are part of the Corona Borealis Supercluster. What is fascinating in this case is that OSIRIS has been able to capture images of more than a dozen members of this family, most of them galaxies populated by old stars, in a moment of domestic harmony over a thousand million light years away.

The symbiotic relationship between the GTC and OSIRIS is turning out to be not just a very powerful key for unlocking the deepest secrets of the cosmos but also a magnificent tool for getting close to the Universe so that we can marvel at its beauty. This is the essence of Astronomy:  Depicting echoes of the past by using ancient, faint threads of light from objects that no longer exist. It is like waking up from a dream that we can’t remember but we can still feel.

Iván Jiménez Montalvo

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