The space telescope Euclid, launched last July 1st, will observe billions of galaxies, going back in the history of the universe up to ten billion years ago. So many and so distant are necessary to unravel the mysteries of cosmic acceleration, or why the expansion of the universe, for a few billion years now, has been proceeding at an increasingly rapid pace. A problem that has plagued cosmology for almost a quarter of a century. But before taking on such a challenge, like someone preparing to run a marathon one day, the most recent space eye added to the European Space Agency’s fleet has begun training close to home.
The most distant object among the five immortalized in first images of EuclidAnd the Perseus galaxy cluster, a good 240 million light years from us. A very respectable distance – the light of its galaxies began when dinosaurs had recently appeared on Earth – which however pales in comparison to the billions of light years from which the most distant objects appear, timidly peeping out in the background. The image, in addition to a thousand galaxies belonging to the cluster, shows more than one hundred thousand even more distant galaxies, many of which had never been observed before.
“We want to observe extremely faint and small galaxies, called dwarf galaxies,” he comments Jean-Charles Cuillandre, researcher of the Euclid collaboration at CEA Paris-Saclay, France. «They are dominated by older stars that shine in infrared light. According to cosmological simulations, the universe should contain many more dwarf galaxies than we have found so far. With Euclid we will be able to see them, if they really exist in such large numbers as expected.”
Among Euclid’s objectives, mapping the distribution and shape of a very large number of galaxies will allow us to understand how dark matter, whose only indirect effect is perceived on what we see, has shaped the cosmos we observe today. Over billions of years, under the attraction of gravity, dark matter has formed filamentary structures, giving rise to the so-called cosmic spider web that permeates the universe, in whose densest nodes are clusters of galaxies such as that of Perseus . “If dark matter didn’t exist, galaxies would be evenly distributed throughout the universe,” adds Cuillandre.
The light from distant galaxies bears the marks of everything it has encountered on its path, including dark matter, which distorts the shape of these galaxies: it is the effect of weak gravitational lens. Euclid will observe many galaxy clusters like this one, scanning a portion of the sky thirty thousand times the size of this image, and thus providing a 3D view of the distribution of dark matter in the universe. The evolution of this map throughout the history of the cosmos also contains important clues about dark energy, one of the main suspects behind the accelerated expansion of the universe.
Riot of galaxies
Closer to home, “only” eleven million light years from us, Euclid has taken a splendid view spiral galaxy. The experts call it Ic 342 or Caldwell 5, but its nickname is “the hidden galaxy” because, due to its position in the sky, it is almost completely obscured by the dust of our galaxy, the Milky Way, and can only be observed in wavelengths of the infrared. Euclid is not the first space telescope to have observed it: Hubble had also photographed its nucleus, but until now it was impossible to study the star formation history of the entire galaxy.
«This is the brilliant thing about Euclid’s images. In just one shot, he can see the entire galaxy in all its wonderful details,” he explains Leslie Hunt, researcher at the National Institute of Astrophysics (Inaf) in Florence. «This image might seem normal, as if every telescope could make an image like this, but that’s not true. The peculiarity is that we have a wide view that covers the entire galaxy, but we can also zoom in to distinguish individual stars and star clusters. This makes it possible to trace the history of star formation and better understand how stars formed and evolved over the lifetime of the galaxy.”
The spiral Ic 342 resembles the Milky Way, but not most galaxies in the universe, which are small and irregularly shaped. It is from these dwarf galaxies that larger galaxies, like ours, took shape. Euclid has already photographed one: the dwarf irregular galaxy NGC 6822 which, “just” 1.6 million light years from Earth, belongs to the Local Group, the cluster of galaxies to which the Milky Way belongs.
First discovered in 1884 and identified as a “remote star system” by Edwin Hubble in 1925, this galaxy has been imaged many times, most recently by the JWST space telescope. Yet this is the first high-resolution image of the entire galaxy, taken by Euclid in just one hour of observation. «Studying low metallicity galaxies [ovvero che contengono piccole quantità di elementi più pesanti dell’idrogeno e dell’elio, ndr] like NGC 6822 in our galactic neighborhood, we can discover how galaxies evolved in the early universe,” adds Hunt.
Ever smaller, ever closer
Euclid is not even joking in our “cosmic neighborhood”: the glittering image ofglobular cluster NGC 6397, about 7800 light years from Earth, is the first to enclose in a single “shot” the nucleus and the external regions of this agglomeration which brings together hundreds of thousands of stars. Globular clusters are among the oldest objects in the universe, and for this reason they preserve memory of the star formation history of the galaxies that host them.
NGC 6397, located in the disk of the Milky Way, is the second closest globular cluster to us. “Currently no other telescope besides Euclid can observe the entire globular cluster and at the same time distinguish its faint stellar members in the outer regions from other cosmic sources,” he explains Davide Massari, Inaf researcher in Bologna. Since the core of a globular cluster contains many stars, the brightest ones tend to obscure the fainter, low-mass ones. But it is precisely these stars that hold the secrets of the cluster’s previous interactions with the Milky Way, sometimes leaving trails – called “tidal tails” that extend well beyond the cluster. “We expect all globular clusters in the Milky Way to have them, but so far we have only seen a few,” adds Massari.
“If there are no tidal tails, then there may be a halo of dark matter around the globular cluster, preventing outer stars from escaping. But we don’t expect dark matter halos around smaller-scale objects like globular clusters, but only around larger structures like dwarf galaxies or the Milky Way itself.” However, if Euclid’s observations reveal tidal tails in globular clusters like NGC 6397, it would be possible to calculate very precisely how these stellar clusters orbit our galaxy. “And this will tell us how dark matter is distributed in the Milky Way.”
Even closer to home – we are about 1375 light years away – Euclid has revisited an icon of astronomical photography: the Horsehead Nebula. Discovered in 1888 by Williamina Fleming by inspecting the photographic plates of the Harvard Observatory with the naked eye, this dark cloud with an unmistakable shape hides among the gas and dust of the Orion molecular cloud, not far from the star Alnitak, one of the three that form the “belt” of the mythological hunter in the famous constellation.
Also in this case, the new Euclid image is astonishing for its clear vision over such a vast field, which was also achieved with a single observation lasting about an hour. Among the thick blankets of cosmic dust, new stars take shape and, with them, planetary systems. “We are particularly interested in this region, because star formation occurs under very special conditions,” he explains Eduardo Martín Guerrero de Escalante dell’Instituto de Astrofisica de Canarias a Tenerife.
The nebula is in fact illuminated by the intense radiation coming from Sigma Orionis, a very bright star located above the Horsehead, just outside the image. Sigma Orionis itself belongs to a cluster containing more than one hundred stars, of which the Gaia mission has already revealed many new members, «but in this image of Euclid», notes Martín, «we already see new candidates among stars, brown dwarfs and objects of planetary mass, so hopefully Euclid will give us a more complete picture.”
We’re almost home. And here too, in this stellar nursery of the Milky Way, the deepest cosmos is on display. Distant galaxies with disparate shapes can be glimpsed in the background, beyond the curtain of the nebula, especially in the highest part of the image. The results of this “cosmic training” are promising. Even after a turbulent startEuclid seems in all respects ready for the marathon: “capture” billions of galaxies near and far, measure their properties and their tendency to aggregate, and finally face the secrets of the dark universe.
They are MediaInaf TvLeslie Hunt illustrates the spiral galaxy Ic 342:
They are MediaInaf TvLeslie Hunt illustrates the dwarf galaxy NGC 6822:
They are MediaInaf TvDavide Massari illustrates the globular cluster NGC 6397:
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