Dark Energy Spectroscopic Instrument (DESI)

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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.

DESI Imaging Leaves a Legacy at Infrared Wavelengths

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By: Aaron Meisner
November 17, 2020

It’s remarkable to think that our DESI Legacy Surveys team completed on order a thousand nights of ground-based observing from Kitt Peak and Cerro Tololo. All the while in low-Earth orbit, NASA’s WISE satellite has been steadily and reliably amassing nearly a decade of full-sky data at infrared wavelengths of 3-5 microns. WISE continuously obtains a new pair of degree-sized images every ~10 seconds, observing around the clock.

Selection of DESI’s luminous red galaxy and quasar targets requires not only optical data from telescopes like the Mayall and Blanco, but also infrared fluxes from WISE. It’s therefore crucial that DESI target selection make full use of the entire WISE data set. Once each year, we download millions of recently acquired raw WISE images to NERSC and use these to update DESI’s custom, coadded WISE maps. As of DR9, the raw WISE data set assembled at NERSC has grown to a quarter petabyte in size! Each year, upon completion of our latest WISE map-making efforts, we can once again declare that DESI has created the deepest ever full-sky maps and catalogs at mid-infrared wavelengths. DR9 incorporates seven years of WISE observations, versus five years for DR8 and just one year for DR1.

WISE has scanned the entire sky more than a dozen times, lending a strong time-domain component to the Legacy Surveys data products. Our Legacy Surveys WISE light curves for ~2 billion sources represent a totally unprecedented and as-yet little explored data set. Mining DR9’s infrared data products, especially in combination with optical Legacy Surveys photometry and DESI spectroscopy, will provide a diverse array of scientific opportunities throughout the coming years.

 

Light echoes from a Milky Way supernova, as seen in the time-domain ‘unWISE’ coadds of Legacy Surveys DR9. These custom WISE coadds also enable DESI’s selection of faint variable quasar candidates.

DESI Target Selection

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By: Adam Myers
November 4, 2020

The Dark Energy Spectroscopic Instrument will conduct spectroscopy of truly vast numbers of cosmological and astrophysical sources. These include Bright Galaxies, Emission Line Galaxies (ELGs), Luminous Red Galaxies (LRGs), Quasars, and objects in our own Milky Way Galaxy. DESI spectra are obtained by aligning optical fibers with locations on the sky, to collect light to be analyzed by dedicated spectrographs. But, how do DESI scientists know where to place those optical fibers in the first place?

Sources for the DESI key projects are targeted using images of the sky from the DESI Legacy Imaging Surveys. The Legacy Surveys include optical photometry from dedicated campaigns with the Mayall and Bok telescopes at Kitt Peak National Observatory, near Tucson, and the Blanco telescope at Cerro Tololo Inter-American Observatory near La Serena in Chile. The Legacy Surveys also incorporate infrared imaging from the WISE and NEOWISE missions, and source detections from the Gaia survey.

When envisioning the process of finding distinct objects in the sky, it is tempting to picture a bright, extended galaxy, such as this one:

Image from the Legacy Survey Viewer, using data from DR8 at right ascension of ~217.6 deg. and declination of ~11.9. See original here. credit: Legacy Surveys / D. Lang (Perimeter Institute)

But, in truth, the vast majority of DESI targets are far less spectacular to the eye, and are selected based on properties such as their color in addition to their shape. Below is the same image from the Legacy Survey Viewer above with the targets identified with circles.  You’ll see that there are many more DESI targets in this field than you might have naively expected!

Same image as above from the Legacy Survey Viewer with the targets identified. You can do this yourself by selecting “DESI Targets” in the Legacy Survey Viewer menu. credit: Legacy Surveys / D. Lang (Perimeter Institute)

To determine which of the one-and-a-half-billion or so sources in the Legacy Surveys will be the lucky few tens-of-millions targeted by DESI requires sophisticated computer algorithms to sift through sources and target objects with specific photometric properties. The publicly available software that DESI uses, which is called desitarget, comprises tens-of-thousands of lines of code and has received contributions from dozens of DESI scientists.

The DESI collaboration recently released a series of research notes detailing the currently expected targeting algorithms for the DESI five-year survey:

The target catalogs that correspond to these notes, which are drawn from Data Release 8 of the DESI Legacy Imaging Surveys, are publicly available here in a format described here.

Although it is a significant milestone to have the first official DESI target catalogs in-hand, the dedicated effort of the collaboration continues. The next data release of the Legacy Surveys (Data Release 9) will soon be used to optimize, refine and finalize the target catalogs for the DESI five-year survey, during a phase of the project known as Survey Validation.

DESI Successfully Completes Commissioning Phase

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By: Daniel Eisenstein
April 2, 2020

DESI commissioning has raced forward this winter, and we have now demonstrated the key performance parameters of the instrument! Since installation, refinement of the performance of the 8 square degree corrector, high-precision (10 micron) positioning of the fibers under active feedback, accurate calibration of the spectrographs, and on-sky commissioning of the whole user interface have been demonstrated.

All of this progress culminated in the successful demonstration in March of spectroscopy with the full DESI system of many tens of thousands of survey targets.  We have observed spectra of faint galaxies and quasars with redshift distributions and spectroscopic signal-to-noise that match well to what we expected.

Here is the infrared spectrum of one of our early luminous red galaxy targets, easily revealing the distinctive Balmer-line signature of a post-starburst galaxy at an impressive redshift z= 1.286. This galaxy is magnitude 19.9 (AB) in the z-band, about a factor of 2 brighter than our planned flux limit. DESI observed this target for 45 minutes on March 15. The spectrum has been smoothed for presentation.

Unfortunately, as it is with so many around the world, the COVID-19 outbreak is forcing us to adjust our plans.  We’re taking a break from on-sky observing until it is easier for our collaboration members to travel safely to Arizona.  But we’re fortunate that this winter’s commissioning produced so much data that we can work on it, in the meantime.  DESI will be back, we hope soon, with continued momentum toward our next goal of validating the survey design.

 

This target was selected from the Data Release 8 of the DESI Legacy Imaging Surveys (Dey et al., AJ, 157, 168, 2019); the object is shown in the center of the small image here, formed from the g, r, and z-band images.