From University of California-Riverside (US) : “Astronomers explain origin of elusive ultradiffuse galaxies”

UC Riverside bloc

From University of California-Riverside (US)

September 6, 2021
Iqbal Pittalwala

UC Riverside astronomer and colleagues use simulations to reveal how the very faint dwarf galaxies are born.


As their name suggests, ultradiffuse galaxies, or UDGs, are dwarf galaxies whose stars are spread out over a vast region, resulting in extremely low surface brightness, making them very difficult to detect. Several questions about UDGs remain unanswered: How did these dwarfs end up so extended? Are their dark matter halos — the halos of invisible matter surrounding the galaxies — special?

Now an international team of astronomers, co-led by Laura Sales, an astronomer at the University of California-Riverside (US), reports in Nature Astronomy that it has used sophisticated simulations to detect a few “quenched” UDGs in low-density environments in the universe. A quenched galaxy is one that does not form stars.

“What we have detected is at odds with theories of galaxy formation since quenched dwarfs are required to be in clusters or group environments in order to get their gas removed and stop forming stars,” said Sales, an associate professor of physics and astronomy. “But the quenched UDGs we detected are isolated. We were able to identify a few of these quenched UDGs in the field and trace their evolution backward in time to show they originated in backsplash orbits.”

Here, “in the field” refers to galaxies isolated in quieter environments and not in a group or cluster environment. Sales explained that a backsplash galaxy is an object that looks like an isolated galaxy today but in the past was a satellite of a more massive system — similar to a comet, which visits our sun periodically, but spends the bulk of its journey in isolation, far from most of the solar system.

“Isolated galaxies and satellite galaxies have different properties because the physics of their evolution is quite different,” she said. “These backsplash galaxies are intriguing because they share properties with the population of satellites in the system to which they once belonged, but today they are observed to be isolated from the system.”

On the left, one of the ultra-diffuse galaxies that was analyzed in the simulation. On the right, the image of the DF2 galaxy, which is almost transparent. Credit: NASA/ESA Hubble.

Dwarf galaxies are small galaxies that contain anywhere from 100 million to a few billion stars. In contrast, the Milky Way has 200 billion to 400 billion stars. While all UDGs are dwarf galaxies, all dwarf galaxies are not UDGs. For example, at similar luminosity, dwarfs show a very large range of sizes, from compact to diffuse. UDGs are the tail end of most extended objects at a given luminosity. A UDG has the stellar content of a dwarf galaxy, 10-100 times smaller than the Milky Way. But its size is comparable to the Milky Way, giving it the extremely low surface brightness that makes it special.

Sales explained that the dark matter halo of a dwarf galaxy has a mass at least 10 times smaller than the Milky Way, and the size scales similarly. UDGs, however, break this rule and show a radial extension comparable to that of much larger galaxies.

“One of the popular theories to explain this was that UDGs are ‘failed Milky Ways,’ meaning they were destined to be galaxies like our own Milky Way but somehow failed to form stars,” said José A. Benavides, a graduate student at the Institute of Theoretical and Experimental Astronomy in Argentina and the first author of the research paper. “We now know that this scenario cannot explain all UDGs. So theoretical models are arising where more than one formation mechanism may be able to form these ultradiffuse objects.”

According to Sales, the value of the new work is twofold. First, the simulation used by the researchers, called TNG50, successfully predicted UDGs with characteristics similar to observed UDGs. Second, the researchers found a few rare quenched UDGs for which they have no formation mechanism.

“Using TNG50 as a ‘time machine’ to see how the UDGs got to where they are, we found these objects were satellites several billion years before but got expelled into a very elliptical orbit and look isolated today,” she said.

Image shows the fall of a blue ultradiffuse galaxy into a galaxy system and its subsequent ejection as a red ultradiffuse galaxy (having already lost its gas). Credit: Vanina Rodriguez.

The researchers also report that according to their simulations, quenched UDGs can commonly make up 25% of an ultradiffuse population of galaxies. In observations, however, this percentage is much smaller.

“This means a lot of dwarf galaxies lurking in the dark may have remained undetected to our telescopes,” Sales said. “We hope our results will inspire new strategies for surveying the low-luminosity universe, which would allow for a complete census of this population of dwarf galaxies.”

The study is the first to resolve the myriad of environments — from isolated dwarfs to dwarfs in groups and clusters — necessary to detect UDGs, and with high-enough resolution to study their morphology and structure.

Next, the research team will continue its study of UDGs in TNG50 simulations to better understand why these galaxies are so extended compared to other dwarf galaxies with the same stellar content. The researchers will use the Keck Telescope in Hawaii, one of the most powerful telescopes in the world, to measure the dark matter content of UDGs in the Virgo cluster, the closest galaxy cluster to Earth.

“Future telescopes, such as the Vera C Rubin Observatory or the Roman Space Telescope, come online in the next five to 10 years with capabilities of detecting many more of these intriguing UDGs,” Sales said.

The study was partially funded by a National Science Foundation CAREER award and a NASA Astrophysics Theory Program grant to Sales.

Sales and Benavides were joined in the research by Mario. G. Abadi of the National University of Córdoba [Universidad Nacional de Córdoba](AR); Annalisa Pillepich of the MPG Institute for Astronomy [MPG Institut für Astronomie](DE); Dylan Nelson of the Center for Astronomy Heidelberg – Institute of Theoretical Astrophysics [Institut für Theoretische Astrophysik](DE); Federico Marinacci of the University of Bologna [Alma mater studiorum – Università di Bologna](IT); Michael Cooper of The University of California-Irvine (US); Ruediger Pakmor of the Max-Planck Institute for Astrophysics in Germany; Paul Torrey of the University of Florida (US); Mark Vogelsberger of The Massachusetts Institute of Technology (US); and Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics (US) .

See the full article here .


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UC Riverside Campus

The University of California-Riverside (US) is a public land-grant research university in Riverside, California. It is one of the 10 campuses of the University of California (US) system. The main campus sits on 1,900 acres (769 ha) in a suburban district of Riverside with a branch campus of 20 acres (8 ha) in Palm Desert. In 1907, the predecessor to UC-Riverside was founded as the UC Citrus Experiment Station, Riverside which pioneered research in biological pest control and the use of growth regulators responsible for extending the citrus growing season in California from four to nine months. Some of the world’s most important research collections on citrus diversity and entomology, as well as science fiction and photography, are located at Riverside.

UC-Riverside’s undergraduate College of Letters and Science opened in 1954. The Regents of the University of California declared UC-Riverside a general campus of the system in 1959, and graduate students were admitted in 1961. To accommodate an enrollment of 21,000 students by 2015, more than $730 million has been invested in new construction projects since 1999. Preliminary accreditation of the UC-Riverside School of Medicine was granted in October 2012 and the first class of 50 students was enrolled in August 2013. It is the first new research-based public medical school in 40 years.

UC-Riverside is classified among “R1: Doctoral Universities – Very high research activity.” The 2019 U.S. News & World Report Best Colleges rankings places UC-Riverside tied for 35th among top public universities and ranks 85th nationwide. Over 27 of UC- Riverside’s academic programs, including the Graduate School of Education and the Bourns College of Engineering, are highly ranked nationally based on peer assessment, student selectivity, financial resources, and other factors. Washington Monthly ranked UC Riverside 2nd in the United States in terms of social mobility, research and community service, while U.S. News ranks UC-Riverside as the fifth most ethnically diverse and, by the number of undergraduates receiving Pell Grants (42 percent), the 15th most economically diverse student body in the nation. Over 70% of all UC-Riverside students graduate within six years without regard to economic disparity. UC-Riverside’s extensive outreach and retention programs have contributed to its reputation as a “university of choice” for minority students. In 2005, UCR became the first public university campus in the nation to offer a gender-neutral housing option. UC-Riverside’s sports teams are known as the Highlanders and play in the Big West Conference of the National Collegiate Athletic Association (NCAA) Division I. Their nickname was inspired by the high altitude of the campus, which lies on the foothills of Box Springs Mountain. The UC-Riverside women’s basketball team won back-to-back Big West championships in 2006 and 2007. In 2007, the men’s baseball team won its first conference championship and advanced to the regionals for the second time since the university moved to Division I in 2001.


At the turn of the 20th century, Southern California was a major producer of citrus, the region’s primary agricultural export. The industry developed from the country’s first navel orange trees, planted in Riverside in 1873. Lobbied by the citrus industry, the UC Regents established the UC Citrus Experiment Station (CES) on February 14, 1907, on 23 acres (9 ha) of land on the east slope of Mount Rubidoux in Riverside. The station conducted experiments in fertilization, irrigation and crop improvement. In 1917, the station was moved to a larger site, 475 acres (192 ha) near Box Springs Mountain.

The 1944 passage of the GI Bill during World War II set in motion a rise in college enrollments that necessitated an expansion of the state university system in California. A local group of citrus growers and civic leaders, including many University of California-Berkeley(US) alumni, lobbied aggressively for a UC-administered liberal arts college next to the CES. State Senator Nelson S. Dilworth authored Senate Bill 512 (1949) which former Assemblyman Philip L. Boyd and Assemblyman John Babbage (both of Riverside) were instrumental in shepherding through the State Legislature. Governor Earl Warren signed the bill in 1949, allocating $2 million for initial campus construction.

Gordon S. Watkins, dean of the College of Letters and Science at University of California-Los Angeles, became the first provost of the new college at Riverside. Initially conceived of as a small college devoted to the liberal arts, he ordered the campus built for a maximum of 1,500 students and recruited many young junior faculty to fill teaching positions. He presided at its opening with 65 faculty and 127 students on February 14, 1954, remarking, “Never have so few been taught by so many.”

UC-Riverside’s enrollment exceeded 1,000 students by the time Clark Kerr became president of the University of California system in 1958. Anticipating a “tidal wave” in enrollment growth required by the baby boom generation, Kerr developed the California Master Plan for Higher Education and the Regents designated Riverside a general university campus in 1959. UC-Riverside’s first chancellor, Herman Theodore Spieth, oversaw the beginnings of the school’s transition to a full university and its expansion to a capacity of 5,000 students. UC-Riverside’s second chancellor, Ivan Hinderaker led the campus through the era of the free speech movement and kept student protests peaceful in Riverside. According to a 1998 interview with Hinderaker, the city of Riverside received negative press coverage for smog after the mayor asked Governor Ronald Reagan to declare the South Coast Air Basin a disaster area in 1971; subsequent student enrollment declined by up to 25% through 1979. Hinderaker’s development of innovative programs in business administration and biomedical sciences created incentive for enough students to enroll at UC-Riverside to keep the campus open.

In the 1990s, the UC-Riverside experienced a new surge of enrollment applications, now known as “Tidal Wave II”. The Regents targeted UC-Riverside for an annual growth rate of 6.3%, the fastest in the UC system, and anticipated 19,900 students at UC-Riverside by 2010. By 1995, African American, American Indian, and Latino student enrollments accounted for 30% of the UC-Riverside student body, the highest proportion of any UC campus at the time. The 1997 implementation of Proposition 209—which banned the use of affirmative action by state agencies—reduced the ethnic diversity at the more selective UC campuses but further increased it at UC-Riverside.

With UC-Riverside scheduled for dramatic population growth, efforts have been made to increase its popular and academic recognition. The students voted for a fee increase to move UC-Riverside athletics into NCAA Division I standing in 1998. In the 1990s, proposals were made to establish a law school, a medical school, and a school of public policy at UC-Riverside, with the UC-Riverside School of Medicine and the School of Public Policy becoming reality in 2012. In June 2006, UC-Riverside received its largest gift, 15.5 million from two local couples, in trust towards building its medical school. The Regents formally approved UC-Riverside’s medical school proposal in 2006. Upon its completion in 2013, it was the first new medical school built in California in 40 years.


As a campus of the University of California(US) system, UC-Riverside is governed by a Board of Regents and administered by a president. UC-Riverside’s academic policies are set by its Academic Senate, a legislative body composed of all UC-Riverside faculty members.

UC-Riverside is organized into three academic colleges, two professional schools, and two graduate schools. UC-Riverside’s liberal arts college, the College of Humanities, Arts and Social Sciences, was founded in 1954, and began accepting graduate students in 1960. The College of Natural and Agricultural Sciences, founded in 1960, incorporated the CES as part of the first research-oriented institution at UC-Riverside; it eventually also incorporated the natural science departments formerly associated with the liberal arts college to form its present structure in 1974. UC-Riverside’s newest academic unit, the Bourns College of Engineering, was founded in 1989. Comprising the professional schools are the Graduate School of Education, founded in 1968, and the UC-Riverside School of Business, founded in 1970. These units collectively provide 81 majors and 52 minors, 48 master’s degree programs, and 42 Doctor of Philosophy (PhD) programs. UC-Riverside is the only UC campus to offer undergraduate degrees in creative writing and public policy and one of three UCs (along with University of California-Berkeley (US) and University of California-Irvine (US)) to offer an undergraduate degree in business administration. Through its Division of Biomedical Sciences, founded in 1974, UC-Riverside offers the Thomas Haider medical degree program in collaboration with University of California-Los Angeles(US). UC-Riverside’s doctoral program in the emerging field of dance theory, founded in 1992, was the first program of its kind in the United States, and UC-Riverside’s minor in lesbian, gay and bisexual studies, established in 1996, was the first undergraduate program of its kind in the University of California system. A new BA program in bagpipes was inaugurated in 2007.

Research and economic impact

UC-Riverside operated under a $727 million budget in fiscal year 2014–15. The state government provided $214 million, student fees accounted for $224 million and $100 million came from contracts and grants. Private support and other sources accounted for the remaining $189 million. Overall, monies spent at UC-Riverside have an economic impact of nearly $1 billion in California. UC-Riverside research expenditure in FY 2018 totaled $167.8 million. Total research expenditures at UC-Riverside are significantly concentrated in agricultural science, accounting for 53% of total research expenditures spent by the university in 2002. Top research centers by expenditure, as measured in 2002, include the Agricultural Experiment Station; the Center for Environmental Research and Technology; the Center for Bibliographical Studies; the Air Pollution Research Center; and the Institute of Geophysics and Planetary Physics.

Throughout UC-Riverside’s history, researchers have developed more than 40 new citrus varieties and invented new techniques to help the $960 million-a-year California citrus industry fight pests and diseases. In 1927, entomologists at the CES introduced two wasps from Australia as natural enemies of a major citrus pest, the citrophilus mealybug, saving growers in Orange County $1 million in annual losses. This event was pivotal in establishing biological control as a practical means of reducing pest populations. In 1963, plant physiologist Charles Coggins proved that application of gibberellic acid allows fruit to remain on citrus trees for extended periods. The ultimate result of his work, which continued through the 1980s, was the extension of the citrus-growing season in California from four to nine months. In 1980, UC-Riverside released the Oroblanco grapefruit, its first patented citrus variety. Since then, the citrus breeding program has released other varieties such as the Melogold grapefruit, the Gold Nugget mandarin (or tangerine), and others that have yet to be given trademark names.

To assist entrepreneurs in developing new products, UC-Riverside is a primary partner in the Riverside Regional Technology Park, which includes the City of Riverside and the County of Riverside. It also administers six reserves of the University of California Natural Reserve System. UC-Riverside recently announced a partnership with China Agricultural University[中国农业大学](CN) to launch a new center in Beijing, which will study ways to respond to the country’s growing environmental issues. UC-Riverside can also boast the birthplace of two name reactions in organic chemistry, the Castro-Stephens coupling and the Midland Alpine Borane Reduction.