Dark Energy Spectroscopic Instrument (DESI)

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DESI begins its “1% survey”

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by: Daniel Eisenstein
April 28th, 2020

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.

DESI Collaboration. Daniel Eisenstein (Harvard University)

 

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!

 

DESI embarks on survey validation

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by: Daniel Eisenstein
April 19th, 2020

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. 

A small region from the Dark Energy Camera Legacy Survey (DECaLS), roughly 0.1 degree across, overlaid with redshifts from DESI survey validation spectroscopy. Visible here are representatives from DESI’s major extragalactic targets: two quasars (QSO) beyond redshift 2; luminous red galaxies at redshift 0.8; an emission-line galaxy at redshift 1.1, fainter than r-band magnitude 23; and two brighter galaxies at lower redshift. The grey circle marks the location of a fiber placed on a blank position to monitor the emission from the sky.

 

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.

 

An Undergrad Perspective on DESI

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By: Brian Bauer, Daniel Allspach, and Noah Franz
April 16th, 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.    

From left to right is Noah Franz, Brian Bauer and Daniel Allspach.

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. 

 

International day of women, women in DESI

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By: Education and Public Outreach group collaborators (Shanthala Gorur, Michael Wilson, Alma González and many others)
March 8, 2021

To commemorate the 2021 International day of Women, March 8th, we wanted to highlight and show the work of the many women working in DESI.

We asked the women in DESI to share with us their advise for the new generations to come, and the work they are doing in DESI.

We start this post with a collage from the pictures and a small selection of the responses.

This post will continue with some more of the particular responses, on twitter, stay tuned.

 

SPANISH VERSION

Hunting the Oxygen Doublet in Distant Galaxies

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By: John Moustakas 
February  3, 2021

In its quest to uncover the mysteries of dark energy, DESI will measure precise redshifts for more than 15 million emission-line galaxies or “ELGs.”  Although they are incredibly distant and faint, DESI will take advantage of a distinctive feature in the light emitted by these galaxies—a feature called the “oxygen two doublet”, represented by the symbol “[OII].”

 But what is this so-called doublet and where does it come from? And why oxygen? To answer these questions we need to dig into some astrophysics!

 Did you know that after hydrogen and helium, oxygen is by far the most abundant element in the universe? For example, in our solar system there are one-and-a-half times more oxygen atoms than carbon atoms and nearly twenty times more oxygen atoms than iron atoms! Because it is so common, oxygen is also an incredibly important element in galaxies, especially in the gas between stars, what astronomers call the “interstellar medium.” In fact, oxygen is one of the primary ways for galaxies to “cool down.”

 Let’s take a quick look at what this means. In galaxies, new stars generally form at the centers of cold, dense clouds of gas and dust. Occasionally, a very massive star will be born—a hot, bright star which can be anywhere from two to one hundred times more massive than the Sun. These monster stars pump enormous amounts of high-energy photons (i.e., light) into the surrounding gas, stripping away the electrons which are normally bound to atoms and creating a spectacular “soup” of fast-moving electrons, protons, and positively charged heavier atoms like oxygen surrounding the new star. Astronomers call these vibrant, short-lived sites of activity in galaxies “HII regions” or “star-forming regions”.

30 Doradus Across the Spectrum (Credit: Q. Daniel Wang (NWU), UM/CTIO, UIT, ROSAT.)

 

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.

 

Congratulations to David Weinberg

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By: Jeremy Tinker and Zheng Zheng
January 19th, 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.

From left to right: (a) An image of the gas distribution around a nascent galaxy forming in a supercomputer simulation, taken from the paper “How Do Galaxies Get Their Gas?” (b) The map of the Main Galaxy Survey of the SDSS, color-coded by the stellar age of each galaxy (image created by Mike Blanton). (c) The Last Scattering Surface, by Josiah McElheny, with scientific consulting by David Weinberg.

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.