ESO Astronomers uncover a cosmic titan using VLT

An international team of astronomers using the VIMOS instrument of ESO’s Very Large Telescope have uncovered a titanic structure in the early Universe. This galaxy proto-supercluster — which they nickname Hyperion — was unveiled by new measurements and a complex examination of archive data. This is the largest and most massive structure yet found at such a remote time and distance — merely 2 billion years after the Big Bang.

A team of astronomers, led by Olga Cucciati of Istituto Nazionale di Astrofisica (INAF) Bologna, have used the VIMOSinstrument on ESO’s Very Large Telescope (VLT) to identify a gigantic proto-supercluster of galaxies forming in the early Universe, just 2.3 billion years after the Big Bang. This structure, which the researchers nicknamed Hyperion, is the largest and most massive structure to be found so early in the formation of the Universe [1]. The enormous mass of the proto-supercluster is calculated to be more than one million billion times that of the Sun. This titanic mass is similar to that of the largest structures observed in the Universe today, but finding such a massive object in the early Universe surprised astronomers.

This is the first time that such a large structure has been identified at such a high redshift, just over 2 billion years after the Big Bang,” explained the first author of the discovery paper, Olga Cucciati [2]. “Normally these kinds of structures are known at lower redshifts, which means when the Universe has had much more time to evolve and construct such huge things. It was a surprise to see something this evolved when the Universe was relatively young!

Located in the COSMOS field in the constellation of Sextans (The Sextant), Hyperion was identified by analysing the vast amount of data obtained from the VIMOS Ultra-deep Survey led by Olivier Le Fèvre (Aix-Marseille Université, CNRSCNES). The VIMOS Ultra-Deep Survey provides an unprecedented 3D map of the distribution of over 10 000 galaxies in the distant Universe.

The team found that Hyperion has a very complex structure, containing at least 7 high-density regions connected by filaments of galaxies, and its size is comparable to nearby superclusters, though it has a very different structure.

Superclusters closer to Earth tend to a much more concentrated distribution of mass with clear structural features,” explains Brian Lemaux, an astronomer from University of California, Davis and LAM, and a co-leader of the team behind this result. “But in Hyperion, the mass is distributed much more uniformly in a series of connected blobs, populated by loose associations of galaxies.

This contrast is most likely due to the fact that nearby superclusters have had billions of years for gravity to gather matter together into denser regions — a process that has been acting for far less time in the much younger Hyperion.

Given its size so early in the history of the Universe, Hyperion is expected to evolve into something similar to the immense structures in the local Universe such as the superclusters making up the Sloan Great Wall or the Virgo Supercluster that contains our own galaxy, the Milky Way. “Understanding Hyperion and how it compares to similar recent structures can give insights into how the Universe developed in the past and will evolve into the future, and allows us the opportunity to challenge some models of supercluster formation,” concluded Cucciati. “Unearthing this cosmic titan helps uncover the history of these large-scale structures.

Notes

[1] The moniker Hyperion was chosen after a Titan from Greek mythology, due to the immense size and mass of the proto-supercluster. The inspiration for this mythological nomenclature comes from a previously discovered proto-cluster found within Hyperion and named Colossus. The individual areas of high density in Hyperion have been assigned mythological names, such as TheiaEos, Selene and Helios, the latter being depicted in the ancient statue of the Colossus of Rhodes.

The titanic mass of Hyperion, one million billion times that of the Sun, is 1015 solar masses in scientific notation.

[2] Light reaching Earth from extremely distant galaxies took a long time to travel, giving us a window into the past when the Universe was much younger. This wavelength of this light has been stretched by the expansion of the Universe over its journey, an effect known as cosmological redshift. More distant, older objects have a correspondingly larger redshift, leading astronomers to often use redshift and age interchangeably. Hyperion’s redshift of 2.45 means that astronomers observed the proto-supercluster as it was 2.3 billion years after the Big Bang.

More information

This research is published in the paper “The progeny of a Cosmic Titan: a massive multi-component proto-supercluster in formation at z=2.45 in VUDS”, which will appear in the journal Astronomy & Astrophysics.

The team behind this result was composed of O. Cucciati (INAF-OAS Bologna, Italy), B. C. Lemaux (University of California, Davis, USA and LAM – Aix Marseille Université, CNRS, CNES, France), G. Zamorani (INAF-OAS Bologna, Italy), O.Le Fèvre (LAM – Aix Marseille Université, CNRS, CNES, France), L. A. M. Tasca (LAM – Aix Marseille Université, CNRS, CNES, France), N. P. Hathi (Space Telescope Science Institute, Baltimore, USA), K-G. Lee (Kavli IPMU (WPI), The University of Tokyo, Japan, & Lawrence Berkeley National Laboratory, USA), S. Bardelli (INAF-OAS Bologna, Italy), P. Cassata (University of Padova, Italy), B. Garilli (INAF–IASF Milano, Italy), V. Le Brun (LAM – Aix Marseille Université, CNRS, CNES, France), D. Maccagni (INAF–IASF Milano, Italy), L. Pentericci (INAF–Osservatorio Astronomico di Roma, Italy), R. Thomas (European Southern Observatory, Vitacura, Chile), E. Vanzella (INAF-OAS Bologna, Italy), E. Zucca (INAF-OAS Bologna, Italy), L. M. Lubin (University of California, Davis, USA), R. Amorin (Kavli Institute for Cosmology & Cavendish Laboratory, University of Cambridge, UK), L. P. Cassarà (INAF–IASF Milano, Italy), A. Cimatti (University of Bologna & INAF-OAS Bologna, Italy), M. Talia (University of Bologna, Italy), D. Vergani (INAF-OAS Bologna, Italy), A. Koekemoer (Space Telescope Science Institute, Baltimore, USA), J. Pforr (ESA ESTEC, the Netherlands), and M. Salvato (Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany)

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a strategic partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.

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Spinning Black Hole Swallowing Star Explains Superluminous Event

An extraordinarily brilliant point of light seen in a distant galaxy, and dubbed ASASSN-15lh, was thought to be the brightest supernova ever seen. But new observations from several observatories, including ESO, have now cast doubt on this classification. Instead, a group of astronomers propose that the source was an even more extreme and very rare event — a rapidly spinning black hole ripping apart a passing star that came too close.

In 2015, the All Sky Automated Survey for SuperNovae (ASAS-SN) detected an event, named ASASSN-15lh, that was recorded as the brightest supernova ever — and categorised as a superluminous supernova, the explosion of an extremely massive star at the end of its life. It was twice as bright as the previous record holder, and at its peak was 20 times brighter than the total light output of the entire Milky Way.

An international team, led by Giorgos Leloudas at the Weizmann Institute of Science, Israel, and the Dark Cosmology Centre, Denmark, has now made additional observations of the distant galaxy, about 4 billion light-years from Earth, where the explosion took place and they have proposed a new explanation for this extraordinary event.

We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinarily bright supernova. Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star,” explains Leloudas.

In this scenario, the extreme gravitational forces of a supermassive black hole, located in the centre of the host galaxy, ripped apart a Sun-like star that wandered too close — a so-called tidal disruption event, something so far only observed about 10 times. In the process, the star was “spaghettified” and shocks in the colliding debris as well as heat generated in accretion led to a burst of light. This gave the event the appearance of a very bright supernova explosion, even though the star would not have become a supernova on its own as it did not have enough mass.

The team based their new conclusions on observations from a selection of telescopes, both on the ground and in space. Among them was the Very Large Telescope at ESO’s Paranal Observatory, the New Technology Telescope at ESO’s La Silla Observatory and the NASA/ESA Hubble Space Telescope [1]. The observations with the NTT were made as part of the Public ESO Spectroscopic Survey of Transient Objects (PESSTO).

There are several independent aspects to the observations that suggest that this event was indeed a tidal disruption and not a superluminous supernova,” explains coauthor Morgan Fraser from the University of Cambridge, UK (now at University College Dublin, Ireland).

In particular, the data revealed that the event went through three distinct phases over the 10 months of follow-up observations. These data overall more closely resemble what is expected for a tidal disruption than a superluminous supernova. An observed re-brightening in ultraviolet light as well as a temperature increase further reduce the likelihood of a supernova event. Furthermore, the location of the event — a red, massive and passive galaxy — is not the usual home for a superluminous supernova explosion, which normally occur in blue, star-forming dwarf galaxies.

Although the team say a supernova source is therefore very unlikely, they accept that a classical tidal disruption event would not be an adequate explanation for the event either. Team member Nicholas Stone from Columbia University, USA, elaborates: “The tidal disruption event we propose cannot be explained with a non-spinning supermassive black hole. We argue that ASASSN-15lh was a tidal disruption event arising from a very particular kind of black hole.

The mass of the host galaxy implies that the supermassive black hole at its centre has a mass of at least 100 million times that of the Sun. A black hole of this mass would normally be unable to disrupt stars outside of its event horizon — the boundary within which nothing is able to escape its gravitational pull. However, if the black hole is a particular kind that happens to be rapidly spinning — a so-called Kerr black hole — the situation changes and this limit no longer applies.

Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event,” concludes Leloudas. “But it is by far the most likely explanation.