Laura Sales

Astronomers offer possible explanation for elusive dark-matter-free galaxies

Anonymous — Tue, 09 Feb 2021 19:09:36 +0000

Astronomers offer possible explanation for elusive dark-matter-free galaxies Anonymous (not verified) Tue, 02/09/2021 - 11:09 More News Iqbal Pittalwala | UCR News February 09, 2021

A team led by astronomers at the University of California, Riverside, has found that some dwarf galaxies may today appear to be dark-matter free even though they formed as galaxies dominated by dark matter in the past.

Galaxies that appear to have little to no dark matter — nonluminous material thought to constitute 85% of matter in the universe — complicate astronomers’ understanding of the universe’s dark matter content. Such galaxies, which have recently been found in observations, challenge a cosmological model used by astronomers called Lambda Cold Dark Matter, or LCDM, where all galaxies are surrounded by a massive and extended dark matter halo.

Dark-matter-free galaxies are not well understood in the astronomical community. One way to study the possible formation mechanisms for these elusive galaxies — the ultradiffuse DF2 and DF4 galaxies are examples — is to find similar objects in numerical simulations and study their time evolution and the circumstances that lead to their dark matter loss.

Laura Sales (seated, left) with her research group of former and current
students, including Jessica Doppel (seated, right). (UCR/Stan Lim)

Jessica Doppel, a graduate student in the UC Riverside Department of Physics and Astronomy and the first author of research paper published in the Monthly Notices of the Royal Astronomical Society, explained that in a LCDM universe all galaxies should be dark matter dominated.

“That's the challenge,” she said. “Finding analogs in simulations of what observers see is significant and not guaranteed. Beginning to pin down the origins of these types of objects and their often-anomalous globular cluster populations allows us to further solidify our theoretical framework of dark matter and galaxy formation and confirms that no alternative forms of dark matter are needed. We found cold dark matter performs well.”

For the study, the researchers used cosmological and hydrodynamical simulation called Illustris, which offers a galaxy formation model that includes stellar evolution, supernova feedback, black hole growth, and mergers. The researchers found a couple of “dwarf galaxies” in clusters had similar stellar content, globular cluster numbers, and dark matter mass as DF2 and DF4. As its name suggests, a dwarf galaxy is small, comprising up to several billion stars. In contrast, the Milky Way, which has more than 20 known dwarf galaxies orbiting it, has 200 to 400 billion stars. Globular clusters are often used to estimate the dark matter content of galaxies, especially dwarfs.

The researchers used the Illustris simulation to investigate the origin of odd dwarf galaxies such as DF2 and DF4. They found simulated analogs to dark-matter-free dwarfs in the form of objects that had evolved within the galaxy clusters for a long time and lost more than 90% of their dark matter via tidal stripping — the stripping away of material by galactic tidal forces.

“Interestingly, the same mechanism of tidal stripping is able to explain other properties of dwarfs like DF2 and DF4 — for example, the fact that they are ‘ultradiffuse’ galaxies,” said co-author Laura Sales, an associate professor of physics and astronomy at UCR and Doppel’s graduate advisor. “Our simulations suggest a combined solution to both the structure of these dwarfs and their low dark matter content. Possibly, extreme tidal mass loss in otherwise normal dwarf galaxies is how ultradiffuse objects are formed.”

In collaboration with researchers at the Max Planck Institute for Astrophysics in Germany, Sales’ group is currently working with improved simulations that feature more detailed physics and a numerical resolution about 16 times better than the Illustris simulation.

“With these data, we will be able to extend our study to even lower-mass dwarfs, which are more abundant in the universe and expected to be more dark matter dominated at their centers, making them more challenging to explain,” Doppel said. “We will explore if tidal stripping could provide a path to deplete dwarfs of their inner dark matter content. We plan to make predictions about the dwarfs’ stellar, globular cluster, and dark matter content, which we will then compare to future observations.”

The research team has already been awarded time at the W. M. Keck Observatory to help answer some of the questions pertaining to observations of dwarfs in the Virgo cluster.

Sales and Doppel were joined in the research by Julio F. Navarro of the University of Victoria in Canada; Mario G. Abadi and Felipe Ramos-Almendares of the National University of Córdoba in Argentina; Eric W. Peng of Peking University in China; and Elisa Toloba of the University of the Pacific in California.

The study was supported by grants from NASA and the National Science Foundation.

The research paper is titled “Globular clusters as tracers of the dark matter content of dwarfs in galaxy clusters.”

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NSF CAREER Award supports astronomer’s quest to further understand the universe

Anonymous — Tue, 04 Feb 2020 16:46:38 +0000

NSF CAREER Award supports astronomer’s quest to further understand the universe Anonymous (not verified) Tue, 02/04/2020 - 08:46 More News IQBAL PITTALWALA February 03, 2020 https://www.eurekalert.org/pub_releases/2020-02/uoc--nca020320.php

Laura Sales first fell in love with astronomy when she was in middle school in Argentina, her country of birth. Today, she is an assistant professor of physics and astronomy at UC Riverside and the recipient of the National Science Foundation’s prestigious Faculty Early Career Development (CAREER) Award, one of the most coveted recognitions a young faculty member can receive.

The five-year award supports early-career faculty who have the potential to serve as academic role models in research and education and build a firm foundation for a lifetime of leadership in integrating education and research.

When she received news of the award, Sales couldn’t help recalling how in middle school her geography teacher made her class study the solar system, with close attention paid to each planet.

“I was immediately hooked,” she said. “I remember going through the chapter on the solar system quickly. I did not want it to end. I even walked home from school reading the rest of the book.”

Soon after, a biweekly collectible edition on space, comprised of a magazine and a VHS tape on the mysteries of the universe, began arriving at bookstores. Sales managed to purchase the entire collection.

“I watched each video several times,” she said. “The videos inspired me to study the universe for the rest of my life.”

With the nearly $720,000 CAREER Award she has received, Sales will be doing just that. Specifically, she will study dark matter — a mysterious nonluminous material in space — that is understood to constitute 85 percent of the matter in the universe.

“The nature of dark matter is unknown and understanding what it is made of is one of the most important leading challenges in physics, astrophysics, and cosmology,” Sales said.

The CAREER Award will allow Sales to study the dark matter content of dwarf galaxies found in major concentrations of galaxies astronomers refer to as clusters. Dwarf galaxies are small galaxies that contain between a thousand to a few billion stars. In contrast, the Milky Way has 100-400 billion stars. Dwarf galaxies are the most abundant galaxy type in the universe and often orbit larger galaxies.

“The most successful theory we have about dark matter is the Lambda Cold Dark Matter, or LCDM, model, which makes predictions that agree remarkably well with our observations and experiments on astronomical scales,” Sales said. “As we move toward smaller scales and low-mass galaxies, however, these predictions begin to deviate from observations. Only by observing small galaxies can we learn more about dark matter and refine and correct our theoretical models.”
Sales explained that the specific nature of dark matter defines not only the behavior of stars in galaxies, but also dictates how the universe evolved from tiny fluctuations in mass after the Big Bang to the spectacular collection of galaxies, groups, clusters, filaments, and other structures that populate the universe.

Her research team also plans to use hydrodynamical simulations to determine the many paths by which dwarf galaxies start and stop forming stars in high-density environments.

“Such simulations can successfully bring us a step closer to understanding how exactly the faintest galaxies form and evolve in the universe, particularly in complex high-density environments such as clusters,” she said.

Laura Sales (second from left) is seen here, from left to right, with graduate students Omid Sameie, Ethan Jahn, Lydia Elias, and Jessica Doppel. (UCR/Stan Lim)

Sales is most interested in addressing one question that relates to the different morphologies of the smallest galaxies. She explained that observations show that the stellar mass of a galaxy correlates well with the size of the galaxy. For example, more massive galaxies are more extended than smaller ones.

“This has been the standard bread-and-butter of galaxy formation models for decades,” she said. “This completely breaks down, however, if we look at dwarf galaxies that have about a thousand times fewer stars than our Milky Way. For these dwarfs, the mass in stars says nothing about their sizes. Two dwarfs with similar stellar content can be up to 100 times different in radius. We suspect that dark matter and star formation play a leading role in defining this, but we don't really know. Our simulations will attempt to answer this particular question also.”

Sales received her doctoral degree at the Universidad Nacional de Córdoba in Argentina. Subsequently, she held prestigious postdoctoral fellowships at the Max Planck Institute for Astrophysics in Germany and at the Center for Astrophysics at Harvard University. She joined the UCR faculty in 2015. Two years later she received a Hellman Fellowship.

The CAREER Award Sales received will support two graduate students and summer internships for several undergraduate students. Sales’ group will also run a summer program for high school students in the Riverside area; the students will take and analyze astronomical images. The grant will also make possible the purchase of state-of-the-art computational nodes to run demanding numerical simulations.

“Our research will foster important collaborations with other institutions, in particular, with the Max Planck Institute in Germany, Harvard University, and the Massachusetts Institute of Technology,” Sales said.

Iqbal Pittalwala

Black holes stunt growth of dwarf galaxies

Anonymous — Thu, 24 Oct 2019 22:42:30 +0000

Black holes stunt growth of dwarf galaxies Anonymous (not verified) Thu, 10/24/2019 - 15:42 More News IQBAL PITTALWALA | UCR News October 11, 2019 https://news.ucr.edu/articles/2019/10/11/black-holes-stunt-growth-dwarf-galaxies

Astronomers at the University of California, Riverside, have discovered that powerful winds driven by supermassive black holes in the centers of dwarf galaxies have a significant impact on the evolution of these galaxies by suppressing star formation.

Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)

Dwarf galaxies are small galaxies that contain between 100 million to a few billion stars. In contrast, the Milky Way has 200-400 billion stars. Dwarf galaxies are the most abundant galaxy type in the universe and often orbit larger galaxies.

The team of three astronomers was surprised by the strength of the detected winds.

“We expected we would need observations with much higher resolution and sensitivity, and we had planned on obtaining these as a follow-up to our initial observations,” said Gabriela Canalizo, a professor of physics and astronomy, who led the research team. “But we could see the signs strongly and clearly in the initial observations. The winds were stronger than we had anticipated.”

Canalizo explained that astronomers have suspected for the past couple of decades that supermassive black holes at the centers of large galaxies can have a profound influence on the way large galaxies grow and age.

“Our findings now indicate that their effect can be just as dramatic, if not more dramatic, in dwarf galaxies in the universe,” she said.

Study results appear in The Astrophysical Journal.

The researchers, who also include Laura V. Sales, an assistant professor of physics and astronomy; and Christina M. Manzano-King, a doctoral student in Canalizo’s lab, used a portion of the data from the Sloan Digital Sky Survey, which maps more than 35% of the sky, to identify 50 dwarf galaxies, 29 of which showed signs of being associated with black holes in their centers. Six of these 29 galaxies showed evidence of winds — specifically, high-velocity ionized gas outflows — emanating from their active black holes.

“Using the Keck telescopes in Hawaii, we were able to not only detect, but also measure specific properties of these winds, such as their kinematics, distribution, and power source — the first time this has been done,” Canalizo said. “We found some evidence that these winds may be changing the rate at which the galaxies are able to form stars.”

From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. (UCR/Stan Lim)

Manzano-King, the first author of the research paper, explained that many unanswered questions about galaxy evolution can be understood by studying dwarf galaxies.

“Larger galaxies often form when dwarf galaxies merge together,” she said. “Dwarf galaxies are, therefore, useful in understanding how galaxies evolve. Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies. Thus, they serve as fossils by revealing what the environment of the early universe was like. Dwarf galaxies are the smallest galaxies in which we are directly seeing winds — gas flows up to 1,000 kilometers per second — for the first time.”

Manzano-King explained that as material falls into a black hole, it heats up due to friction and strong gravitational fields and releases radiative energy. This energy pushes ambient gas outward from the center of the galaxy into intergalactic space.

“What’s interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies rather than by stellar processes such as supernovae,” she said. “Typically, winds driven by stellar processes are common in dwarf galaxies and constitute the dominant process for regulating the amount of gas available in dwarf galaxies for forming stars.”

Astronomers suspect that when wind emanating from a black hole is pushed out, it compresses the gas ahead of the wind, which can increase star formation. But if all the wind gets expelled from the galaxy’s center, gas becomes unavailable and star formation could decrease. The latter appears to be what is occurring in the six dwarf galaxies the researchers identified.

“In these six cases, the wind has a negative impact on star formation,” Sales said. “Theoretical models for the formation and evolution of galaxies have not included the impact of black holes in dwarf galaxies. We are seeing evidence, however, of a suppression of star formation in these galaxies. Our findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies.”

Next, the researchers plan to study the mass and momentum of gas outflows in dwarf galaxies.

“This would better inform theorists who rely on such data to build models,” Manzano-King said. “These models, in turn, teach observational astronomers just how the winds affect dwarf galaxies. We also plan to do a systematic search in a larger sample of the Sloan Digital Sky Survey to identify dwarf galaxies with outflows originating in active black holes.”

The research was funded by the National Science Foundation, NASA, and the Hellman Foundation. Data was obtained at the W. M. Keck Observatory, and made possible by financial support from the W. M. Keck Foundation.