A project of the Dark Energy Survey collaboration

DECam Images


CSI: Early Universe


For this installment of Cosmic Scene Investigation, we travel to one of the earliest collisions of large-scale structures in the known universe.

A splatter of red (denoting galaxies) lies at the center of this image, and extends toward the lower left. These are the remnants of a cosmic collision. Aeons ago, one group plunged through another at millions of miles per hour, leaving in its wake a wreckage. The galaxy cluster ‘El Gordo‘ is all that remains of this raucous event, which took place less than a billion years after the universe started.

From the deserts of Chile, the Atacama Cosmology Telescope was the first to detect this prodigious system. NASA’s Chandra X-ray Observatory, the European Southern Observatory’s Very Large Telescope, and NASA’s Spitzer Telescope have also collected forensic evidence across the energy spectrum, from the infrared to the X-ray. All put together, we see a system similar to the infamous Bullet Cluster: a pair of clumps converted to a churning, violent amalgam of hot gas, dust and light.

An extremophile in the truest sense, El Gordo is the earliest-occurring cluster of its caliber. Its hot gas is burning at 360 million degrees Fahrenheit (200 million degrees Celsius), and it weighs in at a million billion times the mass of Earth’s sun. Compare this to the Virgo cluster of galaxies, the celestial city that holds our Milky Way and its neighbors. El Gordo’s mass is about the same, but it is over a hundred times hotter.

Dark energy is the name given to that substance, that energy, that is making spacetime spread out faster and faster. In the early universe, the small chunks that make up El Gordo were able to overcome dark energy (if it even existed then) and move toward each other to produce this cosmic crash scene. How many more like it are out there? The case remains open.

To read more about the curious case of the big old cluster, see the peer-reviewed paper and the press release from Chandra X-ray Observatory.

Written by: Det. B. Nord [FNAL]

Image by: Det.’s Nikolay Kuropatkin and Martin Murphy [FNAL]


Lights Burning and Churning Through Time


The light is going the distance.

Some celestial (light-producing) objects are farther away than others. The three larger galaxies in today’s image are nearby, located in the Fornax galaxy cluster. As you can see, light comes from other galaxies much farther away: the smaller points are galaxies, too. We measure the patterns in the galaxies that are far away from us and compare that to the patterns of the nearby galaxies. Differences in this pattern give us indispensable clues about how dark energy has affected space-time over the last several billions of years.

And some of the bright points (galaxies) are behind the large galaxy near the center (NGC1374), yet we can see them. What does that tell us about the brightness in different parts of galaxies? If you look closely, we can see background galaxies (those behind) everywhere—except a small patch near the center. This is due to the fact that the centers of galaxies are brighter than their outskirts.

Light from distant regions of space can travel up to billions of light-years, to accidentally land here on Earth — to land on a small spot on the side of a mountain in Chile, where the Dark Energy Camera is waiting to capture it. To measure the expansion of the universe, the Dark Energy Survey needs to measure light from hundreds of millions of galaxies, all differing in their colors, brightnesses and locations. These properties will betray their patterns, which we must decipher.

Light travels the distance, and we’re hunting it all the way.

Written by:   Det. B. Nord [FNAL]

Image by:   Det.’s Nikolay Kuropatkin [FNAL], Martin Murphy [FNAL]


360,000 Minutes … How Do You Measure the Sky


On August 31, 2013, the Dark Energy Survey (DES) began its exploration of new realms of the observable universe. For about 100 nights per year, during the next five years, the Blanco Telescope in Chile will scan the sky using the newly commissioned Dark Energy Camera. With 62 highly sensitive detectors (containing about 500 million pixels), DECam will collect light from hundreds of millions of galaxies and thousands of supernovae from billions of years ago, their light leaving faint traces of a cosmos expanding ever faster. Whether the cause is a mysterious dark energy or a change in our understanding of gravity, DES endeavors to discern the nature of our accelerating universe.

We spent the last year commissioning the telescope and camera, performing calibrations and tests to make it science-ready. The 62 detectors are shown in today’s image, taken from one of last year’s preparatory observations. Clicking on this image will take you to an interactive page where you can zoom in on each chip and look at what DES scientists see after each picture of the sky is taken.  We will gather thousands of images like this in the coming years.

We now begin a new journey, with many seasons of discovery ahead of us: in photons, after sunsets, past midnight, with lots of coffee… In pixels, in light-years, that’s how we measure the sky.

Written by: Det. B. Nord [FNAL]

Image by: Nikolay Kuropatkin [FNAL], Martin Murphy [FNAL]




From bright blue spirals to golden and red ellipsoids, our deep night sky is dotted with nearly innumerable unique galaxies, all teeming with stars and planets. We will use their colors, brightnesses, shapes and even how they are distributed throughout the fabric of space-time to uncover the secrets of dark energy. The veritable cornucopia seen in this image is just one example of the pictures we’re taking.

How many galaxies and how many types do you see?  (Galaxies are fuzzy with various shapes, while stars are spherical.)


The Ancient Universe, Untouched

Om-Cen Composite

The past can be far, far away, but sometimes it is so close to home. Long ago, the first stars lit up, and hydrogen burned inside them. The hydrogen fused and became helium, which in turn fused into yet heavier elements: through nuclear reactions, the cores of stars birthed all the elements that make up our world. When extremely massive stars with these heavier elements exploded, they sent forth into the universe the stuff that would become new stars, as well as planets, and you and me. These first stars are far away in time and space, but there lives an ancient collection of stars in our galactic backyard that simply grew old slowly and quietly.

Omega Centauri is composed of stars that have elements relatively light in weight—from hydrogen and helium to silicon and neon. However, they are lacking in heavy elements, like iron—the same iron found in our blood and in the steel of our buildings and machines. These stars fused atoms in their cores, but they never grew massive enough to explode, so they just burned on, slowly but surely.

Though Ptolemy cataloged Omega Centauri as a single star 2,000 years ago, it is a dense cluster of several million, very old stars. This globular cluster has been orbiting the Milky Way for 12 billion years, nearly the entire age of the universe. Omega Cen is about 15,000 light-years away from the Galaxy, but just a dozen light-years in diameter itself. What’s more, its millions of stars are separated from each other by just a tenth of a light-year. This is roughly equivalent to a golf ball full of very fine sand sitting at the edge of a football pitch. The nearest star to our solar system is Proxima Centauri, just 4 light-years away—still over 10 times the distance between stars in Omega Cen.

Rediscovered by Edmond Halley (of eponymous cometary fame), Omega Cen (a.k.a., NGC 5139) is a globular cluster located in the direction of the Centaurus constellation. Teeming with millions of ancient furnaces, it is the largest and oldest of the 150 globular clusters orbiting the Milky Way.

Omega Cen is visible with the naked eye and  can appear as large as the full moon.  It lives at Right Ascension, 13 : 26.8 (h:m) and Declination, -47 : 29 (deg:m), should you choose to seek it out yourself.  The image above shows the full cluster in the frame and zoom-in of a small section in the right frame.

Written by: Det. B. Nord [FNAL]
Image Credit: Det.’s M. Murphy and N. Kuropatkin [FNAL]

A Dark and Lonely Universe


It is early in the life of the universe. Cold clumps of inanimate matter are randomly distributed throughout the cosmos. In this randomness, some places have more matter, more stuff, than other places: some regions are denser than others.  Over time – millions and billions of years – the force of gravity causes these dense clumps to accumulate more and more matter, often taking from the already-emptier places. Essentially, when it comes to the growth of structure (from planets to galaxies and all the way up to the largest scales of the universe), the rich get richer, and the poor get poorer.

Fast forward to the present and we can see the results of this evolution.

Some galaxies were born in very rich environments; and while they started blue, today they are red.  These  galaxies, like some we’ve seen in earlier posts, become red because of all the massive stuff running around nearby them, disrupting the formation of their stars.  These red galaxies have come to live in clusters.

Other galaxies that don’t live near a lot of stuff don’t have that problem. They live in the field, and they can still give birth to stars, because other galaxies aren’t whizzing by them. These galaxies will remain blue for a long, long time as they drift along in their lonely, relatively empty piece of the universe.

NGC 1090 is one such field galaxy that lives 135 million light-years from Earth – equivalent to about 1.5 million billion round trips between Earth and the Moon.  It resides in the Cetus constellation and lies near a group of galaxies, M77.  However, NGC 1090 is not gravitationally bound to M77: it is completely unassociated; it is alone.

In the far, far, far future, dark energy may continue to pull objects farther and farther away from each other, and it may do so faster and faster – despite gravity’s attractive force.  Eventually, all galaxies could live in their own lonely regions of the universe.

If you’d like to track down the lonely NGC 1090 yourself, it sits on the celestial sphere at RA (02h 46m 33.9s) and DEC (-00° 14′ 49″)

Written by: Det. B. Nord [FNAL]

Image credit: Det. M. Murphy [FNAL],  Det. N. Kuropatkin [FNAL]


A Universe of Possibilities


Over the course of billions of years, a new home is built. It will eventually house stars, planets and perhaps civilizations. The force of gravity and the conservation of momentum can transform a dense cloud of cold dust and gas into a menagerie of stars and myriad opportunities for life. The stuff of stars is the stuff of us.

This particular distant galactic home, NGC 1398, lives in the Fornax cluster of galaxies 65 million light-years away (or one billion round trips between New York City and Los Angeles). It is farther away from us each day, moving away at 1400 kilometers per second–over 3 million miles per hour. For comparison, the NASA space shuttle during launch only moves at 35,000 miles per hour.

At 135,000 light-years in diameter, NGC 1398 is just slightly larger than the galaxy we call home, the Milky Way. Like our home galaxy, it has come to burn with the light of a hundred million suns and who knows how many civilizations.

If you’d like to look for this southern hemisphere gem yourself, it lives at RA/DEC: (03 38.9, -26 20).

We close with a question: Who first wrote the now-famous equation that estimates the probability of life in the universe?

Written by: Det. B. Nord [FNAL]
Image Credit: Erin Sheldon [BNL], Martin Murphy [FNAL]


A Family of Galaxies


In a sea of darkness, innumerable points of light come into focus. While there are some stars visible here, nearly all the red, blue and yellow objects are galaxies. In the lower left and upper right, some of the blue galaxies show off their beautiful spirals (similar to our own Milky Way). The blue color and the clear structures (bars, spiral arms, dense clouds) within these galaxies betray their basic nature: newly birthed stars emanate the blue color and the very existence of the structures means that these galaxies live in a pristine environment where gravity is the dominant force acting to clump things together. All in all, these spiral galaxies are thus relatively young.

In stark contrast, the yellower and redder galaxies have stopped forming new stars and are often referred to as ‘red and dead.’ These galaxies have endured turbulent lives: having been rocked by collisions with other galaxies, they are too hot to form stars. What’s more, all the work that gravity did to make structures within has been washed away leaving just bright cores, diffuse outskirts and elliptical shapes (and so they are named ellipticals).

Several elliptical galaxies appear clustered (just left of center of the image). These galaxies are gravitationally bound as a group or cluster. This cluster has 43 galaxies (can you find them all?), and it was one of the clusters discovered by George Abell and collaborators during the 1970’s and 80’s as part of the Southern Sky Abell Catalog (published posthumously in 1989).  Amazingly, these clusters were discovered and measured with the human eye using photographic plates, rather than the electronics that the Dark Energy Survey uses, and they are part of the earliest comprehensive collection of optically observed clusters started by George Abell in 1958.

Our cluster here is Abell Catalog No. 3151 (out of over 4000), located in the Fornax Constellation. It lives just about one billion light years from Earth (actually quite close compared to the most distant clusters) and spans about five million light years from end to end.

What has dark energy done to this cluster?  Galaxies within a cluster gravitate toward one another, because galaxies have mass.  In contrast, dark energy stretches the fabric of space-time, upon which galaxies reside.  Thus, dark energy directly opposes gravity.  Dark energy can affect larger objects like clusters of galaxies, but isn’t strong enough to pull apart stars, solar systems or galaxies (where gravity is much stronger).

If gravity serves to pull massive objects (like galaxies) toward one another, then dark energy will pull them away from each other. There are millions and millions of families of galaxies across the universe, and dark energy will make them smaller.

What’s more, you’ll find small colored streaks randomly dispersed throughout the image. These come from very high-energy (fast!) cosmic rays that hit the camera’s detectors but briefly, leaving small imprints in the image. These cosmic ray streaks are just one type of artifact that have to be cleaned out of the images before cosmological questions can be asked. In future posts, we’ll discuss more about the data reduction process.

If you want to find Abell 3151 yourself, look at coordinates RA (03:38:16.61) and DEC (-28:50:32.28) in units of Degrees:Minutes:Seconds; it may be hard to see if you’re in the the Northern Hemisphere.

What other kinds of cosmological structures do you think will be affected by dark energy? Why is gravity stronger than dark energy amidst smaller structures?
Written by: Det. B. Nord, PhD [Fermi National Accelerator Laboratory]
Image Credit: DECam via Det. Erin Sheldon, PhD [Brookhaven National Laboratory]
Email us at darkenergydetectives@gmail.com if you have clues, questions or comments about anything cosmology.


Time to Meet the Neighbors!


Spiral Galaxy NGC 0895 was discovered by William Herschel in 1785. Herschel created the first maps of the Milky Way galaxy by observing and drawing the stars. Herschel also saw galaxies outside the Milky Way, but he didn’t know what they were, so he only referred to them generically as nebulae. That was the common term at the time for diffuse, extended objects – including actual nebulae, which are the gaseous remains of exploded stars.

Galaxy NGC 0895 is located in the constellation Cetus, about 110 million light years away – still a fraction (about 0.2 percent) of the observable universe. The star nearest to us, Alpha Centauri, is 4.3 light years away, and the nearest spiral galaxy, Andromeda, is 2.5 million light years away.

We can tell how many stars are forming by how blue the galaxy appears through the camera lens. Blue galaxies contain many young, newly formed stars. The golden object in the upper right is a redder galaxy, which has many more older red stars, and fewer still forming.

If you want to find NGC 0895 yourself, it is located at coordinates (RA 02 21 36.5, Dec -05 31 16).

This image was taken with the Dark Energy Camera, and shows us this galaxy in sharper detail than we have ever seen it. Check back here every Monday for another image and another story from the Dark Detectives at DES.