joannajita

by Alma González (University of Guanajuato, Mexico) and Martine Lokken (Autonomous University of Barcelona, Spain)

24 April 2025

DESI’s educational program for high school students, DESI High, is now back in session!

Designed so students can work with DESI data directly from the telescope, the program also introduces fundamental concepts in cosmology and helps develop computer programming skills. The program was a huge hit with students in online events held during the pandemic and for a couple of years thereafter. However, it has been a while since we offered an event, especially in person.

But now DESI High is coming back in full swing. We are re-factoring and improving our Jupyter notebooks, testing new platforms to host them, and more importantly, we are adding examples that illustrate how to access the now publicly-available DESI data and new notebooks.  Beyond that, we are aiming to offer new tutorial sessions on how to use such materials, both in-person and online. In February, we hosted the first two events of 2025, in Cozumel, Mexico and Catalonia, Spain.

Our first event, held on February 8th, was co-organized with the Cozumel Planetarium, CHA’AN KA’AN, who invited us to commemorate the International Day of Women and Girls in Science (February 11th). Of the approximately 20 participants who joined us on the day of the event, many were female students from the Quintana Roo area, which is located in the southern part of Mexico. Over a two-hour hybrid session, the participants interacted with DESI members, including  Andrea Múñoz from the National Autonomous University of Mexico (UNAM, Phd Student), Diego González from University of Guanajuato (UG, Msc student), Maria Pineda (UNAM, undergraduate student), John Suarez from Monterrey Institute of Technology in Guadalajara (researcher), Fanny Rodríguez (UNAM, MsC student), a non-DESI undergraduate student Daisy Torres (UG) and Alma Gonzalez (UG, researcher). Mariana Vargas (UNAM, researcher), also a DESI member, participated as well in preparations for the event.

The session began with an introduction to DESI science as well as time for the students to get to know the instructors’ motivations to get into science and what their daily life is like working in DESI. Then, the instructors and students delved into the DESI High notebooks in Spanish! For this session, we partnered with Camber Cloud, who kindly provided test accounts for our students so that they could experience the entire process of making a copy of the DESI High repository, running the notebooks, and saving their progress. While we also offer other platforms to just plug-and-play, this gave students to the opportunity to see the entire process from start to finish, which all worth it. We are expecting to have a follow up session with this team.

At the second event, on Feb 22, approximately thirty high school students from across the province of Catalonia, Spain, joined us at the Autonomous University of Barcelona campus to learn how DESI is measuring our universe’s expansion. For the cosmology edition of the “Bojos per la Física” (“crazy for physics”) annual program, three cosmologists from the Institut de Física d’Altes Energies (IFAE) led the students through the introductory DESI High tutorial notebook. This notebook uses Python code to demonstrate how measuring the spectra of distant galaxies translates into understanding the universe’s contents. The students also got an overview of how the DESI telescope works, and examined a prototype of the Guide-Focus-Acquisition (GFA) cameras that IFAE engineers developed to help guide the DESI robotic fiber array.

The day’s activities were run by DESI members Laura Casas (Phd student) and Martine Lokken (Postdoc), and Dane Cross (PhD student in the DES collaboration), who also shared what their day-to-day research life is like. Many of the students are interested in pursuing physics in the future, some in astrophysics and cosmology, and this program helped them understand the current research landscape as well as a typical day in the life of a cosmologist.

In the next few months, the DESI High team will be finalizing updates to the repository. Besides updating the current Jupyter notebooks to work with data from the DESI’s first data release (public as of 19 March), we will also introduce some new notebooks. One will feature a shorter but more straightforward introduction to Dark Energy and DESI. Another one will illustrate how we measure distances in cosmology. And a third will introduce Baryon Acoustic Oscillations! With these visually interesting, step-by-step Jupyter notebooks, we look forward to many future events that use the DESI High tutorials to spread the excitement of cosmology and DESI discoveries!

— edited by Joan Najita

Data and Analysis also in Demand

by Joan Najita (NOIRLab)

14 April 2025

DESI’s 19 March 2025 press release, which reported the likelihood that dark energy is evolving, sparked excitement and engagement across the globe, from both the general public and the scientific community. Announced by Berkeley Lab and 23 collaborating DESI institutions that published partner releases, the news led to significant media coverage.

As of 27 March 2025:

• More than 1,500 articles reporting the news had appeared worldwide in 67 countries.
• Dozens of the world’s most respected science news and media outlets covered the story.
• Articles appeared in 35 languages, with one quarter of the articles published in a language other than English.

For Berkeley Lab Science Writer Lauren Biron, who organized the press release effort for the collaboration, “It was a real privilege to be trusted with DESI’s secrets ahead of time, and to play a small part in telling DESI’s story. The collaboration put such effort and care into the results and the announcement, so it was gratifying to see DESI’s news spread far and wide. This is certainly one of the most far-reaching results I’ve ever been involved with as a science writer.”

Biron attributes part of the media success to the beautiful visualizations of the DESI data created by NOIRLab, Fiske Planetarium, and DESI’s Claire Lamman: “Those maps and flythroughs of the galaxies conveyed at a glance the impressive scope of the experiment, and evoked a sense of wonder and awe at our universe. They made it easy for folks to get excited about the science.”

In creating his visualizations, color selection was important to NOIRLab’s Ron Proctor, who found the white-to-blue color scheme best at highlighting the intricate structure in the DESI dataset. The color scheme also evokes a connection to human experience. To DESI Director Michael Levi, the color scheme suggested “the white surf of the nearby galaxies and the deep sea of those faraway.” Surprised and delighted to see his work featured in the New York Times and on the cover of Le Monde, Proctor says, “It feels good to know I’ve contributed to something that is so meaningful and effective.”

The media coverage surpassed even that of last year’s April 2024 press release. While the earlier release, which reported the first hints of evolving dark energy based on the first year of DESI data, had been widely covered in the media, this year’s release, which reported results from the first 3 years of DESI data, generated more news articles (1500+ vs. 1200+) in more languages. It also resulted in much higher engagement (the number of likes, shares, and comments on those articles received online): a whopping 191,000 this year vs. 32,000 last year.

Beyond the media coverage, the response from the scientific community has also been strong, with the DESI papers reporting the science results and the Year-1 data release attracting a lot of attention.

As of 9 April 2025,

• 575 TB of data have been downloaded from the NERSC, the Berkeley Lab computing facility (with 197 TB downloaded directly from data.desi.lbl.gov, and 378 TB using globus to access the NERSC data transfer nodes).
• 15,000 new users have visited data.desi.lbl.gov.
• The 3 science papers reporting the main results have been downloaded 21,500 times from data.desi.lbl.gov/doc/papers, and potentially many more from the arXiv preprint server.
• Value Added Catalogs provided by the science collaboration in association with the data release have been downloaded 12,000 times.

With the potential to spark completely new ideas and applications than those they were originally designed to address, who knows what new discoveries will emerge from these DESI data?!

Note: Sincere thanks to Stephen Bailey (Berkeley Lab) for compiling and sharing the data usage statistics.

by Gillian Beltz-Mohrmann (Argonne National Laboratory)

In April 2024 DESI released the BAO results from its first year of data, reporting a surprising result: a slight preference for a model with a time-varying dark energy equation of state. This is in stark contrast to the standard LCDM cosmological model, which includes a constant dark energy density (i.e., not varying over time). The new preference for a time-evolving dark energy, a model known as w0waCDM, was strengthened further when the DESI data were combined with other datasets (Cosmic Microwave Background measurements and Type Ia Supernovae distances). Because the preference for w0waCDM based on the Year 1 data was moderate, DESI would have to collect more data to get a clearer picture of whether this was an anomalous result or whether we had discovered something new about the cosmos.

The December 2024 meeting of the DESI Collaboration came with an exciting prospect: the unblinding of the BAO results from the first 3 years of DESI data! This historic event would give us the first indication of whether the w0waCDM cosmology would still be preferred with the inclusion of more data.

The “live unblinding,” at which the collaboration would witness the results for the first time, was scheduled for Thursday evening. But before this could occur, a final consensus had to be reached about whether to indeed proceed with the unblinding. A Tuesday evening session was scheduled to discuss last minute analysis choices and decide whether all of the necessary checks had been passed in order to proceed with the unblinding. The stakes of the session were high: once the data was unblinded, it could not be blinded again. After a long discussion, which lasted about an hour longer than it was scheduled to, the decision was made: the unblinded analysis would proceed as scheduled!

Tuesday’s session was led by Sesh Nadathur, an STFC Ernest Rutherford Fellow at the Institute of Cosmology and Gravitation at the University of Portsmouth. Recalling the experience, Sesh said, “Tuesday was a long and exhausting day – the whole collaboration felt they had a stake in the results, and we must have had 100 people in the room or joining the call. That was a great feeling, but I also felt a lot of responsibility to ensure we took everyone with us and everyone bought into our decision to move ahead with the unblinding. At the same time, we knew the decision had to be agreed upon in that session otherwise the whole timeline would have been thrown off! By the end, my overwhelming emotion was just relief that we’d gotten there.”

The BAO analysis team spent the next two nights getting very little sleep in order to ensure the timely delivery of the results. Cristhian Garcia-Quintero, a NASA Einstein Fellow at Harvard University and a member of the analysis team, recalled, “Once we got the green light on Tuesday, I was very nervous because we needed to have the unblinded results ready to present by Thursday. There was a lot to do in a short amount of time, and everyone was waiting on the results.” Sesh added, “It was only possible to complete the analysis because of the incredible work that Cristhian and Uendert Andrade (a postdoctoral fellow at the University of Michigan) did on those two days, with massive help from many others in the months leading up to the meeting to set everything up to process all the results so quickly.”

By Thursday morning, Sesh and Cristhian were able to examine the final results for the first time. Over the next few hours, Sesh worked hard to put his presentation together in time for the evening unblinding session, a process he describes as “Actually the best experience of my career so far! It was that extremely rare combination of learning the answer to a very important scientific question – and recognizing immediately that it was very important, rather than slowly realizing this over time, which is the more usual case – and for a few hours being one of only two people in the whole world who knows it.”

Sesh and Cristhian spent all of Thursday deftly deflecting questions about the status of the unblinded results. Cristhian found that “Lots of people were joking with me throughout the day, asking for a sneak preview of the results.” Sesh had a similar experience: “All day people asked me subtle questions about the results and tried to read my expression to guess which way things had gone. I had to try to keep a poker face!”

By the end of the day, the two had artfully managed to keep the results concealed from everyone. Sesh even went so far as to refuse to upload his slides to the meeting website until after his presentation. He explained, “I really wanted to make the result reveal in the plenary talk as theatrical and memorable an experience for everyone as possible. This was not just about keeping the results secret until then, but also about trying to present as comprehensive an overview of the cosmology results as possible, and being prepared to answer all questions on the spot. I didn’t want people to come away with questions, and only slowly discover things in the weeks afterwards, because this could dilute the experience.”

As Sesh prepared to deliver his talk, the energy in the room was palpable. Cristhian recalled, “Once we were in the unblinding session, I could see the anticipation in everyone’s faces and feel it in the room. The biggest difference compared to the Year 1 unblinding was that in Year 1 the dark energy results came as a surprise, but in Year 3 there was a lot of expectation from the collaboration.” As an attendee at the meeting, I can honestly say that I have never been so excited about a conference presentation. It was a privilege to bear witness to the unveiling of results that had the potential to reveal something new and strange about the Universe.

Building the drama of the moment, Sesh made us wait for the results while he explained the science behind the BAO measurement (and then added a couple of extra slides just for fun). Finally, we arrived at the moment we had all been waiting for: the big reveal of the constraints on the dark energy equation of state. Was the preference for evolving dark energy from the Year 1 analysis a fluke? Or is DESI was onto something big?

At last, the results were revealed: the preference for a time-evolving dark energy was confirmed! With the increase in data from Year 1 to Year 3, the constraints on the cosmological parameters governing the dark energy equation of state were tightened, strengthening the results and making history in the process. The findings provide further indication that a cosmological constant is not the origin of cosmic acceleration, but rather that dark energy is a kind of dynamically evolving fluid that pervades all of space. With this result, a whole new era of cosmology begins.

Reflecting on the time since the unblinding, Sesh said, “The past three months getting the results ready for publication have been the hardest I’ve worked — a stretch of about 120% effort! I was anticipating this of course. With a subject is as sensitive and important as this, we’ve really had to make sure every argument is absolutely watertight and backed up by multiple calculations and tests. It’s a very important and impactful result, but one that is naturally going to face a lot of skepticism. Working to present the arguments in the most logical and convincing form, and crafting a paper that people will hopefully also enjoy reading — that’s a challenge I have actually enjoyed thinking about. I hope we have achieved it!”

— Edited by Joan Najita

To learn more:

Read about how blinding works and what it’s like to work this way in an interview with Sam Brieden, Uendert Andrade, and Juan Mena-Fernández.

Read about the Year 3 BAO results in the LBNL press release.

Joan Najita (NOIRLab)

10 March 2025

“Perhaps only in a world of the blind will things be what they truly are.” — José Saramago, Blindness

We’re all human. Science is a human endeavor, carried out by humans with all their imagination, creativity…and biases. Sometimes we explore the Universe without expectations, but often we have an idea of what we may find (a hypothesis, a hunch). These ideas motivate and guide us in our exploration. But they can also introduce confirmation bias, “the tendency to process information by looking for, or interpreting information that is consistent with…existing beliefs.” (Britannica). In its quest to make precision cosmological measurements, DESI uses a process called “blinding” in its analysis methodology in order to avoid the inaccuracies that can arise from confirmation bias.

We sat down with DESI scientists Sam Brieden (U. Edinburgh), Uendert Andrade (U. Michigan), and Juan Mena-Fernández (LPSC, Grenoble) to learn about how blinding works, the rationale behind it, and what it’s like to work this way.  

Q: What is the general purpose of blinding in research?

Sam: Research is never carried out in a vacuum, it is always bound to existing knowledge and expectations. Every researcher or group of researchers is subject to external influences that may affect the way an analysis is performed. For example, a new analysis result that is in agreement with current consensus and does not contradict any prior results, is more likely to be accepted by the community. On the other hand, if a new result is unexpected, it is more likely to raise eyebrows. Due to this “pressure”, researchers unconsciously tend to examine unexpected results more closely and, say, include more cross-checks and caveats, or a less optimistic error estimation. In other words, there is a trend in research that expected results tend to be treated with less scrutiny than required, while unexpected results may be tweaked until they agree better with expectations. In “The Neglect of Experiment,” Allan Franklin describes the process this way:

“Although each experiment was honestly made, they were, except for the first, conducted in light of previous results. In any experiment, the sources of error, particularly systematic error, may be hidden and subtle. This is particularly true of…technically difficult experiments. The question of when to stop the search for sources of error is then very important. One psychologically plausible end point is when the result ‘seems’ right.”

Uendert: Working with blinded data can feel both liberating and challenging. On the one hand, it frees researchers from preconceived notions about the results, encouraging a more objective and unbiased analysis. On the other hand, it requires a strong trust in the analysis pipeline and the blinding process itself, for which we have to creatively design tests and then carry them out. The anticipation of discovering the true cosmological parameters after unblinding adds an exciting layer of mystery to the research.

Q: What is it like to experience the “unblinding”? Is it exciting? What emotions do you have?

Juan: I have experienced several unblindings, and these are always extremely exciting. The unblinding is typically done in a video call meeting in which everyone in the collaboration can connect, and so there are a lot of people watching. This usually makes me feel nervous, especially if I’m deeply involved in the analysis (partly because the analysis could have taken months or even a year to carry out). There are lots of things to prepare for an unblinding, such as slides to present the project and scripts that will create the unblinded plots and output the results. The most exciting part is that moment in which you press the button to run your script and obtain the final unblinded measurement!

Uendert: For me, as you might imagine, the moment of unblinding is filled with a mix of excitement, anticipation, and a bit of nervousness. It’s the culmination of years of hard work, and there’s a tangible sense of revealing the Universe’s secrets. The experience is akin to opening a highly anticipated gift; you’re eager to see what’s inside but also hoping it meets your expectations. Regardless of the outcome, it’s a pivotal moment that deepens our understanding of the cosmos.

Sam: I was really excited in anticipation of the internal full-shape unblinding event on 12 June 2024 — which revealed the results that were eventually published on 19 November 2024 — since I developed both the Blinding and the full-shape (i.e., ShapeFit) methodologies during my PhD. Coincidentally, my wife and I were expecting a child, and we went to hospital on the same day, so I missed the DESI unblinding in favor of another (and for me personally even more exciting) unblinding event!

Q: What aspect of the DESI results are you looking forward to next?

Juan: I’m always looking forward to seeing the constraints in cosmological parameters inferred from the data, in DESI or in any other experiment.

Uendert: I’m particularly looking forward to seeing how the DESI results will refine our understanding of the dark energy equation of state and its potential evolution over cosmic time. The scale and precision of DESI’s dataset offer an unprecedented opportunity to probe the dynamics of the Universe’s expansion and the nature of dark energy. This could lead to groundbreaking insights into one of the most profound mysteries in cosmology.

Cosmological measurements often make use of fun and interesting ideas that depart from our everyday experience. Here cosmologist Jamie McCullough explains and helps us visualize several of the concepts involved in plans to measure the distribution of dark matter in the universe and probe the growth of cosmic structure over time.

by Jamie McCullough

26 February 2025

When DESI measures the spectrum of light from a galaxy – i.e., the intensity of light as a function of wavelength or energy – we learn a lot about the physics of what is happening both inside the galaxy and in the space between the galaxy and us.

A particularly important property we can measure is a galaxy’s redshift. As galaxies recede from us in an expanding universe, their light is pulled to less energetic wavelengths, or redshifted, by the same doppler effect that makes train whistles pitch low as they travel away from us. In an expanding universe, more distant objects are redshifted more than nearby objects. As a result, if we know how the universe is expanding, we can learn the distance to the galaxy from a measurement of its redshift. With the ability to measure the spectra of as many as 5000 galaxies at a time, DESI is uniquely suited to measuring the distances to galaxies across the night sky and thereby mapping the visible universe. You can see redshift in action for a galaxy in the figure below. Here the vertical dashed line marks a bright emission line of oxygen in star-forming galaxies. The number in the upper left corner is the redshift z = v/c where v is the recession velocity and c is the speed of light.

However, the visible universe accounts for only a small fraction of the matter we know exists. The vast majority doesn’t interact with light in any known way – it’s dark matter. The spatial distribution of this dark matter is of great interest, because it drives the movement of galaxies in the universe. Mapping the dark matter — which is thought to be arranged in long filaments, i.e., in a dark cosmic web — requires more information than DESI alone can provide. If we combine the galaxy distance measurements from DESI with the measurement of galaxy shapes from imaging surveys (like, for example, the Dark Energy Survey (DES), the Kilo-Degree Survey (KiDS), and the Hyper Suprime-Cam (HSC) survey), we can map the dark structure using a method called weak gravitational lensing. This method relies on an effect from general relativity that massive objects warp the geometry of space and time. As a result, an image of a distant galaxy will be distorted as light from the galaxy passes massive objects on its way to us.

To understand how weak lensing works, we can first look at the case of a more straightforward example. In what’s called “strong lensing,” we can readily see this warping in action, as we find images of background galaxies stretched and distorted tangentially around a very massive foreground object like a galaxy cluster. This stretching is very similar to the optical effect you might see looking through the thick base of a wine glass. With a thicker piece of glass we see more refraction, just as we see more distortion behind more massive galaxy clusters. You can see this effect in the simulation below and in this other simulation, in which a moving massive galaxy passes first in front of a grid of shapes, and then in front of the Hubble Deep Field. We see that these strong lenses can stretch a background galaxy’s light into continuous rings called “Einstein rings”. They can even produce more than one image of the same background galaxy! What happens all depends on how the background and foreground objects line up.

However, the typical (weak lensing) distortion that a galaxy’s light experiences on its way to us is not as dramatic as in the above examples. Warping from large-scale structure changes galaxy shapes much less, on the order of a mere percent. As we can see in the prior simulations, the amount of distortion depends very strongly on the distance to the background object and to the lensing structure, so understanding those distances is crucial. We expect that if we measure enough of these galaxy shapes and their correlations with one another, we can trace those dark cosmic filaments dominated by dark matter. You can see a toy model of weak lensing below, where a hypothetical background grid of perfect circles becomes displaced, magnified, and sheared about the hidden structure shaded in blue.

If we measure the distances and observed shapes of these galaxies and correlate them with one another, we can devise a relationship for how alike any two shapes are as a function of their separation (𝜉±). With these weak gravitational lensing measurements and different cosmological models, we can find the one that best explains our observations and produces constraints on how clumpy our universe is (as measured by a quantity called S8) and how much matter there is in the universe (as measured by a quantity called 𝛺m).

With the incredibly precise distances we get from DESI and the increasingly precise measurements we are now getting for galaxy shapes, we can perform these lensing measurements better than ever before – making it a very promising way to probe the growth of cosmic structure over time and learn about all the contents of our universe — both the luminous and the dark!

— Edited by Joan Najita

20 February 2025

BaoBan, DESI’s ambassador for Education and Public Outreach, recently dropped in on this year’s Tohono O’odham Nation Rodeo Parade, greeting parade spectators from high atop the Kitt Peak National Observatory (KPNO) parade float. The DESI project is being carried out at KPNO, which is located atop I’oligam Du’ag, in the homeland of the Tohono O’odham Nation. A coyote from the wilds of Arizona, BaoBan also took a star turn at last year’s Rodeo Parade.

At this year’s parade, held on 1 February 2025 in Sells, Arizona, BaoBan appeared on the KPNO parade float alongside images of Tohono O’odham employees and those who have supported the Observatory over the past 60 years. Decorated with colorful images of stars and planets, the float joined in on the parade theme “Celebrating Our O’odham Superheroes” with BaoBan sporting his own superhero cape. NOIRLab volunteers also walked alongside the float clad in vibrant capes and masks.

Now in its 86th year, the Tohono O’odham Nation Rodeo and Fair is an important tradition that celebrates Tohono O’odham culture and history. In addition to the all-Indian rodeo, the event featured live music, fairground rides, exhibitions and food booths. At the NOIRLab information booth, volunteers, including DESI astronomers, invited fair attendees to view the Sun through telescopes and to visit Kitt Peak to learn more about projects such as DESI. A special thanks to BaoBan and all volunteers for their help making this event a “super” success!

Joan Najita (NOIRLab)

8 February 2025

One of the remarkable things we’ve learned about galaxies over the past few decades is that they often come with a special surprise in the middle — a huge black hole. Weighing in at more than a million times the mass of the Sun, these “supermassive black holes” are found at the centers of almost all galaxies similar to or more massive than the Milky Way. Why do galaxies have these black holes? And how did they get there? Do much lower mass galaxies also have central black holes? These questions have motivated the search for black holes in low-mass “dwarf galaxies,” systems that may provide vital clues regarding the origin of black holes in the Universe.

A recent paper, led by Ragadeepika Pucha (U. Utah), takes a big step in this direction. Analyzing DESI spectra of dwarf galaxies, the researchers identified candidate black holes that are actively accreting matter from their surroundings. The feeding process produces a characteristic spectrum of bright atomic emission lines that the researchers used to identify these systems as Active Galactic Nuclei (AGN). The study detected AGN signatures in over 2000 dwarf galaxies, more than tripling the number of known dwarf galaxies with candidate black holes. The detections, which extend to lower galaxy masses and higher redshifts (i.e., further back in time) than previously probed, suggest that black holes may commonly exist even in these very low-mass galaxies.

We sat down with Raga to learn more about this remarkable result.

Questions for Raga:

Q: Why was it important to increase the census of dwarf AGN? Was this difficult before DESI?

A: Dwarf galaxies are the most abundant galaxies in the Universe, and understanding their growth is crucial for piecing together the puzzle of galaxy formation and evolution. Because their lower mass (and therefore lower gravity) makes it more challenging for them to hang on to their gas, the energy output from the AGN, which can eject gas from a galaxy, can potentially have a major impact on a dwarf galaxy’s ability to continue to form stars. What happens exactly remains an open question. To explore this complex interplay between dwarf galaxy evolution and black hole evolution, we first need to establish a robust statistical sample of dwarf AGN candidates.

Historically, this has been a real challenge. Because low-mass galaxies are faint, it has been difficult to measure the spectra of a large sample and identify dwarf AGN candidates. But DESI is changing the landscape with its ability to measure spectra of many objects simultaneously. The early DESI data included spectroscopy of nearly 115,000 dwarf galaxies, from which we uncovered the largest sample of dwarf AGN candidates to date. Part of the success is the result of the smaller fiber size of DESI, which makes it easier to  focus on the light from the central region of the galaxy, where the AGN is, and ignore the starlight from the rest galaxy. As a result, we were able to identify fainter dwarf AGN candidates than in previous studies.

Q: It looks like you find that dwarf galaxies also commonly host black holes. Do these results tell us anything about the origin of supermassive black holes in general?

A: A major question about supermassive black holes is how they formed. What mass did the black hole start out with? What was its “seed” mass? Because lower mass galaxies are likely to harbor lower mass black holes, they may provide a bridge between stellar mass black holes (< 100 solar masses) — which are familiar to us from X-ray binaries and LIGO gravitational wave sources — and supermassive black holes at the centers of large galaxies. These “intermediate mass black holes”, while elusive to date, are theorized to be the “seeds” of supermassive black holes and the relics of the first black holes formed in the Universe.

With DESI, we’ve found the lowest mass black holes in galaxies to date. Since black holes can only grow over time and cannot disintegrate into smaller ones, these findings suggest that the black holes we are observing may be analogs of the primordial black holes that formed in the early Universe, i.e., the “seed black holes” of supermassive black holes. A very small number of our sources may even be primordial black holes, having persisted through the ages with little to no evolution.

Q: What do you find most interesting about the results?

A: We find that nearly 2% of dwarf galaxies host active black holes, a significant increase compared to the ~0.5% reported in earlier studies. This is an exciting result, as it suggests that we have been missing a substantial number of undiscovered black holes. It opens the possibility that even more black holes are concealed within these low-mass galaxies.

The ability to detect an active black hole depends on several factors, including the black hole’s mass, the availability of gas in its vicinity, the accretion rate, and the sensitivity of the instruments used to detect the emission from the resulting AGN. As a result, for a given galaxy mass, the black holes we observe will tend to be either the most massive or those with the highest accretion rates, depending on the specific telescope and instrument used.

Our findings show that the fraction of galaxies hosting actively accreting black holes increases with galaxy mass, reaching nearly 100% for the most massive galaxies. This suggests that when a massive galaxy has an active black hole, it is readily detected by DESI. In contrast, in lower-mass galaxies, the emission from ongoing star formation can mask or dilute the AGN signal, making it harder to detect their faint AGN. This does not imply that low-mass, star-forming galaxies do not host black holes, but rather that we are currently identifying all the actively accreting black holes that are detectable with our instruments.

Q: What drew your interest to this topic?

A: When I began my PhD, my primary goal was to delve into the field of galaxy formation and evolution. I was particularly drawn to dwarf galaxies, as they are the most common kind of galaxy in the universe, yet they remain poorly understood. What are their histories? Do they follow the same evolutionary path as more massive galaxies? Or does the energy released by their active black holes (if they have one) play a significant role in shaping their growth?

By sheer coincidence, my advisors, Stephanie Juneau and Arjun Dey, encouraged me to join the DESI collaboration, which turned out to be the perfect opportunity to dive deeper into this research. They were incredibly supportive of the idea to use DESI data to search for dwarf AGN candidates as a first step in understanding the evolution of dwarf galaxies in the universe.

An unexpected bonus was realizing that this project also ties into one of the most fundamental and exciting questions in present day astronomy: the formation of supermassive black hole seeds. The chance to simultaneously explore the evolution of dwarf galaxies and the origins of supermassive black holes has been deeply motivating. The interconnection between these two lines of inquiry, and the potential to advance our understanding on both fronts, has been a truly rewarding aspect of my research journey.

Q: What’s next for you?

A: With the largest sample of dwarf AGN and IMBH candidates now at our disposal, we are poised to tackle some of the most pressing questions in the study of supermassive black hole seed formation and the co-evolution of dwarf galaxies and their central black holes. My upcoming projects will focus on examining the relative effects of AGN versus star-formation feedback in dwarf galaxies, exploring the energetics related to these feedback mechanisms, and characterizing the population of dwarf AGN candidates identified through multi-wavelength and multi-diagnostic approaches.

My DESI collaborators and I also plan to investigate the demographics of these black holes, employing modeling techniques to study whether all galaxies have black holes (or what fraction do), as well as the black hole mass function in the universe. We will expand these analyses to include DESI Year 3 data, which will further enhance the scale and scope of our research.

Joan Najita (NOIRLab)

One of the current tantalizing mysteries of cosmology is the “Sigma-8 tension,” or the persistent disagreement between the predicted and observed amounts of “clumpiness” of matter in the Universe. Briefly, the small density fluctuations in the early Universe (as recorded in the cosmic microwave background or CMB) can be used to predict the expected matter density fluctuations at later times, up to the present day. While the observed clumpiness of the matter density distribution at later times agrees well with expectations from our current cosmological model, observations consistently find less clumpiness than predicted by the CMB. The difference may indicate exciting new physics, or more prosaically, systematic effects.

A new paper led by Tanveer Karim explores the Sigma-8 tension using data from the DESI Legacy Imaging Survey. Similar to previous studies, the new paper also finds significantly less clumpiness in the galaxy distribution than predicted by the CMB (i.e., the red and blue shapes are below the green shape in the lower left panel of the figure). However (and interestingly), the results also depend on how the effect of intervening dust in our own galaxy is taken into account. In other words, our view of the Universe from within a galaxy means that dust in the Milky Way can block light from fainter distant galaxies, altering the apparent clumpiness of the galaxy distribution. Much like explorers of old, astronomers need to “brush away” this surface dust to reveal the cosmological relics of interest beneath. Here the authors make this correction using a new map of Milky Way dust, derived from DESI data itself. The correction reduces the tension (blue shape), but the clumpiness of the galaxy distribution still differs from the CMB prediction (by 3-sigma).  We sat down with Tanveer to learn more about the results.

Questions for Tanveer:

Q: How do you interpret these results? Is your measurement of Sigma-8 significantly different from the CMB value? And should we be concerned? (Or maybe excited about the prospect of new physics?)

A: As a bit of background, our results add to the “sigma-8 tension” story in two ways. Firstly, we find that the tension is already present quite a long time ago, at a redshift of z ~ 1.1, when the Universe was not yet affected by dark energy (in LCDM cosmology). Secondly, our study examines the clustering of lower-mass blue galaxies rather than massive red ones. That is, we study emission-line galaxies (ELGs), which are similar to the mass of the Milky Way. Previous studies have used luminous red galaxies (LRG) (Sailer 2407.04607, Kim 2407.04606, White 2111.09898, Kitanidis 2010.04698) or unWISE galaxies (Farren 2309.05659, Krolewski 2105.0342) that are typically 100-1000 times more massive than ELGs.

So, what do our results mean? The more exciting interpretation is that we are perhaps seeing hints of something new (not the traditional constant dark energy) happening at z ~ 1.1. But if we consider the LRG and unWISE galaxies as well, then our result is the outlier. What could explain this? The key could be that the previous studies studied massive red galaxies while we are probing the clustering of lower-mass blue galaxies, like our Milky Way. Why does this matter? While the CMB traces the clustering of dark matter, galaxy clustering studies are observing normal matter. To compare these, we need to understand how galaxies form in dark matter clumps and how well different galaxy populations trace the dark matter. So naturally, galaxy formation and other processes can come into play. Our understanding of Milky-Way-mass galaxies is limited at such high redshift, so perhaps our results not only point to the impact of systematics but also signatures of unknown ELG galaxy physics!

Q: How do your results relate to other studies of clumpiness? The Sigma-8 tension is also reported by studies that use completely different measures of the clumpiness of the Universe (e.g., weak lensing, galaxy cluster counting). Does the Milky Way dust distribution also affect these studies? Or are these other studies affected by different systematic effects?

A: That’s an interesting question that has not been explored at length yet, although there may be a possible connection. Papers such as (https://arxiv.org/pdf/1808.03294) and (https://arxiv.org/abs/2306.03926) have shown that certain extinction maps actually retain imprints of the large-scale structure of galaxies, because they use far-infrared light to map dust. While most of the far-infrared light is produced by dust in the Milky Way, distant star-forming galaxies also contribute. If their emission is incorrectly attributed to Milky Way dust, the process of correcting for Milky Way dust will incorrectly imprint a signature of distant star-forming galaxies on images of the sky, which may affect these other measures to some extent.

In any case, and as far as I am aware, our paper is the first to show exactly how much these effects change the effect of Milky Way dust changes our cosmological interpretations. As for the other galaxy clustering measurements, I think one could argue that since the earlier studies were using more massive galaxies, they were less prone to extinction systematics. But a reanalysis of such works will be important to definitively rule out the role of Milk Way dust. After all, Milky Way dust has impacted cosmological results in the past, such as the false detection of the primordial B mode in the CMB by the BICEP telescope!

Q: Does dynamic dark energy play a role here? As you say in your paper, the negative pressure of dark energy inhibits the growth of large-scale structures over time, countering the effect of gravity. Earlier in 2024 DESI reported that dark energy may be dynamic and weakening. Does this effect matter in your study? If it does, do you take this development into account?

A: I am puzzled by the recent DESI Key Paper results—in a very positive way—as I am sure many of the DESI collaborators are! While I do not have a clear answer on how dynamic dark energy relates to a detection of Sigma-8 disagreement at z ~ 1.1, it is a line of questioning we should consider seriously in interpreting the upcoming Y3 dataset (next major dataset for DESI). The Y3 dataset will be much cleaner than the Y1 data in our current paper, and I expect it will show a more robust detection of the ELG-CMB lensing cross-correlation. We should consider the impact of dynamic dark energy in this context, where the simplest extension of our current study would be to measure not only Sigma-8 and Omega_matter but also the parameters describing the dark energy equations of state (w0, and wa).

Speaking of these earlier reports, it’s interesting that an echo of our current results is also found there. While the November 2024 Full-Shape and Redshift Space Distortion (RSD) results measure Sigma-8 with many tracers and find it to be consistent with the CMB, if you look at Figure 1 of the full-shape paper (https://arxiv.org/pdf/2411.12022), you will see that the ELGs on their own are also consistent with a lower Sigma-8! The two studies are done in different ways — the full shape result derives from the 3D clustering of ELGs, while our analysis is carried out in 2D using ELGs selected in a different way — it is interesting that our two independent methods yield similarly low Sigma-8s. This naturally links back to the first question and leads me to wonder, if not a signature of dark energy, could we be on the verge of understanding how these early star-forming galaxies were interacting with their dark matter haloes?

Q: How did you decide to work on this project? Were you surprised by the results?

A: After I finished my initial work on the ELG target selection as a first and second-year graduate student, I was interested in exploring how to use the DESI ELG sample to study cosmology. Extensive discussions with my thesis advisor, Daniel Eisenstein, and collaborators of this project, Sukhdeep Singh and Mehdi Rezaie, helped me understand that the high-redshift star-forming galaxies could be the key to unlocking the early large-scale structures. I never thought that I would have to learn so much about “local” structures, such as the Milky Way dust mapmaking and the Sagittarius Stream, to learn about the very distant cosmos, so it was both shocking and exciting to see how the science of the distant Universe and that of our local neighborhood is becoming more and more intertwined.

Q: What’s next for you?

A: As an Arts & Sciences Postdoctoral Fellow at the University of Toronto, I am currently finishing up a similar analysis using the same ELGs, but this time cross-correlating with the cosmic infrared background, to quantify the star-formation rate of these galaxies and their galaxy-dark matter halo connection. I am excited to see what more we can learn about the ELGs and whether a better understanding of their physics can help us better interpret the CMB lensing cross-correlation results. My ELG work also made me fall in love with these early star-forming galaxies, and so I am currently co-leading the Lyman-Break Galaxies (LBGs) Topical Team in the Dark Energy Science Collaboration (DESC). The hope is that with the upcoming Rubin Observatory, we will explore all the up to z ~ 5.5 using LBGs. The coming years of wide-field high-redshift surveys will be the era of ELGs and LBGs, and I am thrilled to see what these galaxies will teach us about the infancy of our Universe.

Joan Najita (NOIRLab)

DESI reaches 50M milestone

On 18 December 2024, DESI reached a new milestone, having measured the spectra of 50 million astronomical sources (36.3 million galaxies and quasars, and 13.7 million stars) over 819 nights of observations. The milestone is remarkable for both its speed and scope. As described by DESI team member Arjun Dey (NOIRLab), “When we originally proposed the DESI project, we forecast that we would measure spectra of about 38 million sources (30 million galaxies and quasars and 8 million stars) over the 5-year survey. We have now already exceeded that mark in just 68% of our official survey time.”  While the 50M milestone has been reached quicker than expected, many of the spectra were obtained in “bright time”, when the moon is up. The main “dark time” portion of the DESI survey is still underway. Currently ahead of schedule, it is expected to complete toward the end of 2025.

Q: You mentioned that stuff on small scales usually forgets about the cosmic web. Why doesn’t that happen here?

A: While galaxy orientations tend to “forget” their history in a very general sense, here we show that some memory is in fact preserved! This is likely a result of how the galaxies formed. My mental picture is matter (gas, dust, galaxies) being channeled along cosmic filaments and their angular momentum in that direction is preserved in the motions (and therefore positions) of the galaxies relative to each other and to the densest nearby regions, the large clusters / nodes where filaments meet.

Q: Can we make a map of dark matter using your technique?

A: Yes! Or, more specifically, a map of the tidal forces created by dark matter. This is similar to how weak lensing makes a “map” of dark matter between us and distant galaxies.

Q: What drew your attention to this topic? And why multiplets in particular?

A: Initially I studied the orientations of galaxies as a source of error in cosmological surveys. (For example, we expect the intrinsic alignments of galaxies to bias measurements of redshift-space distortions for DESI.) But eventually I became interested in their underlying cause. It’s  fascinating that things on relatively “small” scales (galaxies) can be connected to the largest structures in the universe!

Q: How does DESI help with this problem?

A: Because DESI measures the distances to galaxies, it helps us better determine which galaxies are close to each other along the line of sight. That helps us better identify multiplets and gives us better 3D information about their positions. These improvements help us find subtle correlations between galaxies and the cosmic web.

Q: What was your reaction to the results? Were you surprised?

A: Although I expected we would find some correlation, I was surprised at how clear the signal was! And I was also pleasantly surprised to find that we could detect a correlation in the most “difficult” sample: the faint, blue, distant galaxies. This is exciting because we do not see any evidence of alignment of these galaxies as individuals.

Q: What’s next on your horizon?

A: Getting my PhD! I’m excited to graduate and start postdoctoral research, where I hope to collaborate with theorists and experts in galaxy dynamics in order to better understand the modeling and applications of multiplet alignment.

More information about Claire’s recent work is available in this doodle summary of the actual paper and in this Halloween lunch talk (Claire’s presentation begins at 35:15).