Nav: Home

The XXL Survey: First results

December 15, 2015

Astronomy & Astrophysics is publishing a special feature on the first results of the XXL Survey. XXL is a large survey of the X-ray sky with the XMM-Newton space observatory. It aims to detect a few hundreds of clusters back to a time when the age of the Universe was about half its present value (z~1). With this information, astronomers will be able to infer the evolutionary properties and spatial distribution of the massive structures of the universe and, subsequently, to test various cosmological properties.

The distribution of matter in the Universe has a history: very small density perturbations, which were present at the time of recombination (just after the cosmic microwave background (CMB) was emitted) grew under the competing actions of gravity and expansion of the space-time. This progressively formed a remarkable network of filaments, sheets, and voids. Over time, matter has flowed along these filaments: overdensities have become denser and low-density regions emptier. Tracking the evolution of the cosmic structure traces back this process. It corroborates the cosmological constraints inferred from the CMB (z~1000) and from the distribution of matter at much later times (z~0-4). Clusters of galaxies, which are the most massive objects in the Universe, probe the largest scales of the matter distribution. Located at the nodes of the cosmic web, they are huge reservoirs of hot gas that fills the space between their galaxies. The temperature of this gas is a few tens of millions degrees, and it consequently emits X-rays.

The XXL X-ray observations were performed between 2011 and 2013 by XMM-Newton and cover two extragalactic regions of 25 deg2 each (named XXL-N and XXL-S). The total observing time was 6 million seconds, making XXL the largest XMM program undertaken since the 1999 launch of the observatory. The analysis of the data required sophisticated detection algorithms because the cluster signals are faint.

Any cosmological analysis based on clusters needs the total mass of each cluster (the sum of its galaxies, gas, and dark matter). This is challenging and requires information assembled from all over the entire electromagnetic spectrum. To characterize the properties of the detected clusters and active galactic nuclei (AGNs), the XXL Survey is therefore accompanied by coordinated observations with the largest ground-based and space observatories, covering all wavelengths from X-ray to radio. Of these, the ESO large program (2013-2015) is of particular relevance because it measures the redshifts of the clusters, and therefore locates them in the 3D space. Clusters out to a redshift of ~0.5 were observed at the ESO NTT with the EFOSC2 spectrograph and the more distant ones (0.5:1.2) with the FORS2 instrument on the VLT. Detailed investigations of the galaxy dynamics and properties of individual clusters have been undertaken at the William Herschel Telescope. The redshifts of the many detected AGNs are also being systematically measured with the AAOmega instrument on the Australian Astronomical Telescope. In addition, radio follow-up of the X-ray AGNs is being undertaken with the GMRT, VLA, and ATCA telescopes.

The XXL survey detects some 450 clusters and 25 000 AGNs. The series of XXL articles published in this special feature releases catalogs of the brightest 100 clusters and 1000 AGNs, and presents a number of scientific results for the low-mass range which, to date, has been barely explored. Among these, Pompei et al. describe one of the five newly discovered superclusters. The inferred X-ray luminosity evolution shows that clusters in the past were scaled replicates of the local ones, as a function of mass (Giles et al.) The number density of clusters is lower than predicted by the Planck CMB cosmology, but this trend is similar to that found for the Planck cluster of galaxies sample for a different cluster mass range (Pacaud et al.). More work on cluster masses, involving information from other wavelengths, will help understand this enigma.

The next cosmological analysis of the full XXL cluster sample (release date in 2017) will involve five times as many clusters, enabling better constraints on cluster physics, hence providing mass estimates at the level required for precision cosmology.

Astronomy & Astrophysics

Related Universe Articles:

The largest virtual Universe ever simulated
Researchers from the University of Zurich have simulated the formation of our entire Universe with a large supercomputer.
Does the universe have a rest frame?
Physics is sometimes closer to philosophy when it comes to understanding the universe.
Ancient signals from the early universe
For the first time, theoretical physicists from the University of Basel have calculated the signal of specific gravitational wave sources that emerged fractions of a second after the Big Bang.
Big data for the universe
Astronomers at Lomonosov Moscow State University in cooperation with their French colleagues and with the help of citizen scientists have released 'The Reference Catalog of galaxy SEDs,' which contains value-added information about 800,000 galaxies.
The universe is expanding at an accelerating rate -- or is it?
Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery, in the late 1990s, that the universe is expanding at an accelerating pace.
Visualizing the universe
Computer scientists from the University of Utah will be working with researchers from New York University's Tandon School of Engineering and the American Museum of Natural History (AMNH) to develop OpenSpace, an open-source 3-D software for visualizing NASA astrophysics, heliophysics, planetary science and Earth science missions for planetariums and other immersive environments.
Insights into the dawn of the universe
What did the universe look like just after the Big Bang?
Exploring the mathematical universe
A team of more than 80 mathematicians from 12 countries has begun charting the terrain of rich, new mathematical worlds, and sharing their discoveries on the Web.
The expansion of the universe simulated
The universe is constantly expanding. But how does our universe evolve?
Multilingual Universe from 'Mitaka'
The door to the digital Universe has been flung open!

Related Universe Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".