blog

Paul Martini, DESI Instrument Scientist

May 17, 2021

The discovery of cosmic acceleration over twenty years ago captured the imagination of the general public and professional scientists alike, and the cause of this acceleration remains one of the greatest, unsolved questions in science. Something mysterious, such as a new particle, a new force of nature, or some property of space itself, is overcoming the gravitational attraction of all of the mass in the universe. While we commonly call the origin of this acceleration ‘dark energy,’ naming it did not solve the mystery, nor have the last two decades of progressively larger and more sophisticated sky surveys. 

Enter the DESI survey. For the last decade, hundreds of scientists and engineers have worked to build a new instrument and design a survey capable of constructing the most precise measurement of cosmic expansion history ever. On Friday, May 14 this work came to fruition with the formal launch of survey observations. And over the next five years, DESI will measure approximately 35 million galaxies and quasars that extend across about 12 billion years of cosmic history. These observations will measure an order of magnitude more targets than the largest survey to date, and complete these observations in a small fraction of the time. 

DESI aims to survey about 14,000 square degrees of the night sky, which corresponds to most of the sky visible from Arizona. We will do this with over 15,000 unique observations that we call tiles. Each of these tiles is a unique configuration of the 5000 robotically-positioned fiber optic cables designed to collect the light of specific galaxies and quasars distributed across the 3.2 degree diameter field of view of the instrument. 

The image below shows a small fraction of this field of view that includes the nearby Coma cluster of galaxies, one of the largest concentrations of galaxies in the local universe. The circles in the bottom panel illustrates the high density of the spectroscopic observations obtained with a single tile. Nearly 10,000 of these tiles are designed for good to great conditions, such as when the Moon does not appreciably affect the darkness of the night sky. In these cases, we observe our faintest targets, including luminous red galaxies, emission-line galaxies, and quasars. The remaining tiles target brighter galaxies and Milky Way stars, sources that can be observed when the moon is brighter and there may be moderate cloud cover.

As the DESI survey will be so much larger than previous surveys, we anticipate measuring cosmic acceleration and other parameters with substantially smaller statistical errors. Yet in order to fully realize these gains, and not be limited by systematic effects, it is especially important for us to carefully plan all aspects of the experimental design and maximize the reproducibility of our work. One aspect of this is that we have built substantial software and hardware systems to help us to achieve the same sensitivity with all of our tiles, in spite of the fact that they will be observed on nights spread over five years, and obtained under nights with varying amounts of atmospheric turbulence or seeing, and varying amounts of cloud cover. Other aspects include superb and repeatable algorithms that handle every aspect of observing, ranging from the selection of potential targets, to the assignment of targets to the individual fibers, to the measurement of their properties. 

We have spent the last many months building, testing, and refining all of these aspects of the survey with increasingly sophisticated and extensive observations. During this Survey Validation phase, we have obtained over 1 million unique redshifts, which has already made DESI the second largest spectroscopic redshift survey in the world before it formally begins. Over the coming months, we expect that the number of new galaxies and quasars we measure will continue to expand, much like the universe itself!

Daniel Eisenstein, Harvard University

April 28, 2021

On April 5, 2021, DESI turned to the second and final phase of its Survey Validation (SV) work, the so-called “1% survey”.  Whereas the first phase of SV was aimed at getting long and definitive observations of a broad superset of the planned spectroscopic targets, the 1% survey aims to demonstrate that we can operate the facility in a model closely matched to that planned for the 5-year survey.

The 1% survey is being conducted with target selection similar to what is planned for the main survey and with exposure times only mildly longer.  Importantly, we aim to observe each region of sky more times than in the main survey so that we achieve a higher fiber-assignment completeness, the fraction of targets that are assigned a fiber.

A single exposure with DESI provides about 600 fibers per square degree, but the survey target lists contain about 3500 targets per square degree for the dark/grey conditions (when the moon is small or set) and about 2000-2500 targets per square degree for bright conditions (when the moon is fuller).  In the main survey, we will return to a point on the sky numerous times to observe about 80% of the targets, but allowing some incompleteness so that we can efficiently move on to new regions.  In the 1% survey, we plan to return to each region at least 10 times for dark-time targets and 8 times for bright-time targets, each with a distinct target list, so as to observe all but a few percent of the targets.

So far, we have started observations on 16 regions, each 7 square degrees and each including both dark-time and bright-time targets.  We expect to finish this program in May.  With this data set, we will be able to get a first estimate of the 3-dimensional clustering of the galaxy and quasar samples, which will then allow the collaboration to tune the models of simulated galaxy positions in our cosmological mock catalog.

With the observations moving at normal survey pace from one target set to the next, the rate of gathering new redshifts has increased dramatically.  In the first 8 nights, we acquired over 400,000 separate successful redshift measurements of 350,000 unique sources.

The plot shows a visualization of one of the 16 regions, which has 13 dark-time visits, showing a map of the galaxy locations in Mpc on the plane of the sky.  Only galaxies between redshift 0.90 and 0.95 are shown, with the luminous red galaxy targets plotted in red and the emission-line galaxy targets in blue.  One can see the cosmic web of large-scale structure, with walls, filaments, and voids, as well as the tendency of red galaxies to cluster together more than blue galaxies.  This is about 1 part in 20,000 of the final survey size!

Daniel Eisenstein, Harvard University

April 19, 2021

Since mid-December, DESI has been intensively performing its Survey Validation (SV) observations. With SV, we seek to optimize the 5-year survey design using on-sky observations of our target classes. We of course based our designs on what was known before DESI, but DESI is a big enough step forward that one can’t be sure until one verifies the performance with the instrument itself!

One of the challenges for Survey Validation is that we want to verify our survey plans with targets that are fainter and spread over wider areas of the sky than have previously been observed. So we need to make our own truth tables by observing our targets with much longer exposures than we plan for the survey, so that the correct answers become obvious. We then can split the observations into portions to see what a survey-length exposure would yield. 

From mid-December through the end of March, DESI observed 161 separate tiles as part of the first phase of its survey validation program, collecting over 1600 separate spectroscopic exposures. We combined 4.3 million separate observations of 465,000 distinct examples of our target classes, with an average of 95 minutes per target, obtaining redshifts for 412,000 objects. This is already one of the largest extragalactic spectroscopic data sets ever collected, including 108,000 redshifts above 0.8.

The whole DESI collaboration has mobilized to analyze these on-sky data, because rapid answers are needed to define the main survey. The results have been very encouraging; we’ll share more examples in future blog posts.

Brian Bauer, Daniel Allspach, and Noah Franz

April 16, 2021

We are a working group of three undergraduate students at Siena College. We started with DESI under Dr. John Moustakas in February 2020. We were intrigued by the opportunity to work with such a large collaboration with so many scientific possibilities. Since then, Noah Franz and Brian Bauer have focused on identifying strong gravitational lenses in DESI spectra by using Python to separate the source and lens galaxy spectra then analyzing the success. Daniel Allspach has been working to fit stellar templates to model DESI spectra continua and determine galactic demographics and outflow classifications. Contributing to these projects has provided us with invaluable insight and experience into working with large collaborations.

After working with DESI, attending Zoom telecons, and learning how a collaboration functions, what we enjoyed most of all is the welcoming nature of everyone involved. Without this feeling of acceptance we would have found it difficult to integrate into a project effectively. Although receiving emails in the wee hours of the morning can be a little odd at first, we were able to quickly adapt and become accustomed to the practices and methods employed by DESI. Being included in any aspect of the project, especially as an undergraduate, is an honor, yet we were continually challenged to dive deeper and join as much as we can. A welcoming environment provides the perfect jumping off point for new members and that is truly the best part about the DESI collaboration.

With over 700 contributing members, DESI is a large collaboration, and, as undergraduate researchers we found entering the community of mostly graduate students and PhDs intimidating and overwhelming. While some of the DESI updates and zoom meetings can be daunting, after spending time reading the literature and learning the acronyms we became accustomed to the jargon. Once we surpassed this learning curve, we were able to start our own research projects. In many ways this was our introduction into large scale collaborative astronomy and cosmology: many things are new to us. Having the DESI environment to help us orient and navigate this research environment has been incredibly helpful. DESI has given us a fantastic first experience not just with research but also working with a large collaboration. As a result, we all were inspired to continue research going into the future.

John Moustakas, Siena College

February  3, 2021

Now, the typical temperature of these star-forming regions is a balmy 10,000 degrees Kelvin, hotter than the surface of the Sun! So how does the region get rid of this extra energy and cool down? Well, as atoms like oxygen whip around in random directions, they will occasionally crash into one another. When this happens, some of the kinetic energy of motion goes into “exciting” one of the atom’s electrons into a higher energy state. But the electron doesn’t stay excited very long!

After about a second, the excited electron spontaneously “jumps down” to a lower energy level, simultaneously emitting a photon of energy (i.e., light) in the process. Subsequently, this photon escapes from the star-forming region, robbing it of the original energy pumped in by the hot young star and helping it cool down. In the actively star-forming emission-line galaxies which DESI is hunting, we observe oxygen “shining” in this way at two close but distinct energies or wavelengths, 372.71 and 372.98 nanometers. (One nanometer is one billionth of a meter, so the wavelength of this light is about 170 times smaller than the width of human hair!)

This pair of lines is called the “oxygen two doublet” (the “two” confusingly means that the oxygen atom is missing one electron) and it is written using the symbol “[O II].” In December 2020, DESI began observations as part of its “Survey Validation” phase, and one important question this phase aims to answer is, “How efficiently will DESI be able to pre-select (or “target”) emission-line galaxies?” So far, the answer to this question has been a resounding, “Really well!” To illustrate the kind of tremendous data DESI is obtaining, Dr. Julien Guy of Lawrence Berkeley National Lab has created an animation of the [OII] doublet in a sample of roughly 3400 emission-line galaxies which DESI observed in December, a tiny fraction of the more than 35 million galaxies DESI will observe during its 5-year survey.

credit: DESI Collaboration

This movie shows the strikingly clear [OII] doublet in these galaxies, which brings DESI one step closer to being able to uncover the mystery of dark energy.

Jeremy Tinker and Zheng Zheng

January 19, 2021

DESI member and noted rapper David Weinberg, from Ohio State University, was awarded the Dannie Heineman Prize for Astrophysics. He shares the award with Robert Lupton of Princeton University. The award celebrates the massive contributions both researchers have made to ushering in the era of large-scale three-dimensional mapping of the universe through the spatial distribution of the galaxies within it, primarily through their work with the Sloan Digital Sky Survey (SDSS).

As a project, DESI owes a debt of gratitude to SDSS. SDSS was the first truly large-scale galaxy redshift survey, using a CCD camera and fiber-fed spectrographs. With first light in 2000, its goal— realized in 2007 with the completion of SDSS-II— was to map nearly a quarter of sky by obtaining distances (redshifts) of the million brightest galaxies and quasars in the universe. In late 1980s, when the SDSS was merely an idea being batted around the Paris Conference Room of the Chicago O’Hare Hilton, David was a graduate student at Princeton, working with SDSS’s visionary founder Jim Gunn. He both literally and figuratively got started on the ground floor of large-scale spectroscopic surveys.

David became a full SDSS member in 1992, and went on to serve as the Scientific Spokesperson for SDSS-II, a position that demands a myriad of critical tasks related to the design, organization, promotion, and execution of the project. When the SDSS-III collaboration was created in 2008, David was chosen to be the Project Scientist. In recent years David has brought his expertise to the DESI collaboration, being one of the chief architects of the Bright Galaxy Survey component of the project as well as serving as the inaugural BGS working group co-chair. 

While a member of SDSS, David made invaluable contributions to survey design, galaxy target selection, and eventual analysis of the maps of cosmic structure. David was an early adopter and developer of the halo occupation model to describe the distribution of galaxies in space, who introduced the now commonly used term Halo Occupation Distribution (HOD). The model relates galaxies to clumps (halos) in the matter distribution in the universe. The SDSS data led to the pioneering application of the HOD framework to interpret the clustering of galaxies in space, with clustering trend naturally explained and galaxy-halo connection informatively inferred. The HOD model has ever since been widely adopted to analyze galaxy clustering data, becoming a powerful tool in learning about galaxy formation and cosmology and in creating simulated galaxy catalogs for various purposes in large galaxy surveys. 

In addition to mapping the universe with galaxies, David was also a pioneer in a novel method of determining the matter distribution of the universe: the Lyman-alpha forest. This method uses bright quasars as cosmological flashlights. Cool gas along the pathway to our telescopes absorbs some of the quasars’ light, leaving wiggles (Lyman-alpha forest) in the quasar spectra. Such wiggles reveal the spatial distribution of cool gas, which tracks dark matter. Thus, the spectra of quasars taken in SDSS and in DESI provide complementary maps of cosmic structure. 

As a scientist, David wears many hats in addition to survey astronomy. He has produced highly influential work on hydrodynamical simulations of galaxy formation, he has worked on the life cycles of active galactic nuclei, and more recently he has used SDSS’s spectra of stars to perform “chemical cartography” within our own Milky Way Galaxy. His 169-page review article on “Observational Probes of Cosmic Acceleration”, with nearly 900 citations, has become the standard reference in guiding our observational efforts toward revealing the nature of cosmic acceleration.

In addition to his research-oriented scientific pursuits, David has a long-standing collaboration with the MacArthur Award winning sculptor Josiah McElheny. Together, they have created cosmologically-inspired glass sculptures that have been exhibited all over the world. David’s role is in making the designs representative of the physics of the universe. For example, the piece An End to Modernity depicts the history of the universe from the Big Bang to the present day, emphasizing the evolution of cosmic structure and the epoch of galaxy formation.

Since starting at Ohio State in 1995, David has been advisor and mentor to 17 graduate students. Many of his former students, including the two of us, are themselves members of SDSS and DESI, and are using these data to mentor and train their own students, drawing on the lessons learned while working with David. Needless to say, David’s profound influence on cosmological redshift surveys will be felt for many academic generations. Congratulations to David for this well-deserved recognition of his continuing impact on astronomy, cosmology, and its presentation to the public.