From The University of California-Riverside And The University of California-Irvine: “Why are dark matter halos of ultra-diffuse galaxies so … odd?”

UC Riverside bloc

From The University of California-Riverside


UC Irvine bloc

The University of California-Irvine

Iqbal Pittalwala | The University of California-Riverside

A study co-led by physicists at UC Riverside and UC Irvine has found that dark matter halos of ultra-diffuse galaxies are very odd, raising questions about physicists’ understanding of galaxy formation and the structure of the universe.

Ultra-diffuse galaxies are so called because of their extremely low luminosity. The distribution of baryons — gas and stars — is much more spread out in ultra-diffuse galaxies compared to “normal” galaxies with similar masses.

In the following Q&A, Hai-Bo Yu, an associate professor of physics and astronomy at UCR, shares his thoughts on the findings he and UCI’s Manoj Kaplinghat, a long-term collaborator of Yu’s, have published in The Astrophysical Journal [below] about newly discovered ultra-diffuse galaxies and their dark matter halos.

Yu and Kaplinghat were joined in the research by Demao Kong of Tufts University, and Filippo Fraternali and Pavel E. Mancera Piña of the University of Groningen in the Netherlands. First author Kong will join UCR this fall.

The research was supported by grants from the National Science Foundation, Department of Energy, John Templeton Foundation, National Aeronautics and Space Administration, Netherlands Research School for Astronomy, and ASTRON, the Netherlands Institute for Radio Astronomy.

Q. What is a dark matter halo?

A dark matter halo is the halo of invisible matter that permeates and surrounds a galaxy or a cluster of galaxies. Although dark matter has never been detected in laboratories, physicists are confident dark matter, which makes up 85% of the universe’s matter, exists.

Q. You’ve found dark matter halos of the ultra-diffuse galaxies are very odd. What is odd about them and what are you comparing them to?

The ultra-diffuse galaxies we studied are much less massive compared to, say, the Milky Way. They contain a lot of gas, however, and they have much higher gas mass than total stellar mass, which is opposite to what we see in the Milky Way. The ultra-diffuse galaxies also have large sizes.

The distribution of dark matter in these galaxies can be inferred from the motion of gas particles. What really surprises us is that the presence of baryonic matter itself, predominantly in the form of gas, is nearly sufficient to explain the measured velocity of gas particles and leaves little room for dark matter in the inner regions, where most of the stars and gas are located.

This is very surprising because in the case of normal galaxies, whose masses are similar to those of the ultra-diffuse galaxies, it’s the opposite: dark matter dominates over baryonic matter. To accommodate this result, we conclude that these dark matter halos must have much lower “concentrations.” That is, they contain much less mass in their inner regions, compared to those of normal galaxies. In this sense, dark matter halos of the ultra-diffuse galaxies are “odd.”

At first glance, one would expect that such low-concentration halos are so rare that the ultra-diffuse galaxies would not even exist. After looking into the data from state-of-the-art numerical simulations of cosmic structure formation, however, we found the population of low-concentration halos is higher than the expectation.

Q. What was involved in doing the study?

This is a collaborative work. Filippo Fraternali and his student Pavel E. Mancera Piña are experts on gas dynamics of galaxies. They discovered that the ultra-diffuse galaxies rotate more slowly than normal galaxies with similar masses. We worked together to interpret measurement data of the gas motion of these galaxies and infer their dark matter distribution. Furthermore, we analyzed data from simulations of cosmic structure formation and identified dark matter halos that have similar properties as those inferred from the ultra-diffuse galaxies.

Q. Your findings raise questions about our understanding of galaxy formation/structure formation of the universe. How?

We have many questions regarding the formation and evolution of these newly discovered galaxies. For example, the ultra-diffuse galaxies contain a lot of gas and we do not know how this gas is retained during galaxy formation. Further, our results indicate that these galaxies may be younger than normal galaxies. The formation of the ultra-diffuse galaxies is not well understood, and more work is needed.

Q. What makes ultra-diffuse galaxies so interesting?

These are amazing objects to study because of their surprising properties, as discussed in our work. The newly discovered ultra-diffuse galaxies provide a new window for further testing our understanding of galaxy formation, probably even the nature of dark matter.

Science paper:
The Astrophysical Journal

See the full article here .


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Since 1965, the University of California-Irvine has combined the strengths of a major research university with the bounty of an incomparable Southern California location. UCI’s unyielding commitment to rigorous academics, cutting-edge research, and leadership and character development makes the campus a driving force for innovation and discovery that serves our local, national and global communities in many ways.

With more than 29,000 undergraduate and graduate students, 1,100 faculty and 9,400 staff, UCI is among the most dynamic campuses in the University of California system. Increasingly a first-choice campus for students, UCI ranks among the top 10 U.S. universities in the number of undergraduate applications and continues to admit freshmen with highly competitive academic profiles.

UCI fosters the rigorous expansion and creation of knowledge through quality education. Graduates are equipped with the tools of analysis, expression and cultural understanding necessary for leadership in today’s world.

Consistently ranked among the nation’s best universities – public and private – UCI excels in a broad range of fields, garnering national recognition for many schools, departments and programs. Times Higher Education ranked UCI No. 1 among universities in the U.S. under 50 years old. Three UCI researchers have won Nobel Prizes – two in chemistry and one in physics.

The university is noted for its top-rated research and graduate programs, extensive commitment to undergraduate education, and growing number of professional schools and programs of academic and social significance. Recent additions include highly successful programs in public health, pharmaceutical sciences and nursing science; an expanding education school; and a law school already ranked among the nation’s top 10 for its scholarly impact.

University of California-Riverside Campus

The University of California-Riverside is a public land-grant research university in Riverside, California. It is one of the 10 campuses of The University of California 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 The University of California-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.

The University of California-Riverside ‘s undergraduate College of Letters and Science opened in 1954. The Regents of the University of California declared The University of California-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 The University of California-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.

The University of California-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 The University of California-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 The University of California-Riverside 2nd in the United States in terms of social mobility, research and community service, while U.S. News ranks The University of California-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 The University of California-Riverside students graduate within six years without regard to economic disparity. The University of California-Riverside ‘s extensive outreach and retention programs have contributed to its reputation as a “university of choice” for minority students. In 2005, The University of California-Riverside became the first public university campus in the nation to offer a gender-neutral housing option. The University of California-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 University of California-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 University of California 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 alumni, lobbied aggressively for a University of California -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 The 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.”

The University of California-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. The University of California-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. The University of California-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 University of California-Riverside to keep the campus open.

In the 1990s, The University of California-Riverside experienced a new surge of enrollment applications, now known as “Tidal Wave II”. The Regents targeted The University of California-Riverside for an annual growth rate of 6.3%, the fastest in The University of California system, and anticipated 19,900 students at The University of California-Riverside by 2010. By 1995, African American, American Indian, and Latino student enrollments accounted for 30% of The University of California-Riverside student body, the highest proportion of any University of California 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 The University of California-Riverside.

With The University of California-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 The University of California-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 The University of California-Riverside, with The University of California-Riverside School of Medicine and the School of Public Policy becoming reality in 2012. In June 2006, The University of California-Riverside received its largest gift, 15.5 million from two local couples, in trust towards building its medical school. The Regents formally approved The University of California-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 system, The University of California-Riverside is governed by a Board of Regents and administered by a president University of California-Riverside ‘s academic policies are set by its Academic Senate, a legislative body composed of all UC-Riverside faculty members.

The University of California-Riverside is organized into three academic colleges, two professional schools, and two graduate schools. The University of California-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 The University of California-Riverside; it eventually also incorporated the natural science departments formerly associated with the liberal arts college to form its present structure in 1974. The University of California-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 University of California-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. The University of California-Riverside is the only UC campus to offer undergraduate degrees in creative writing and public policy and one of three UCs (along with The University of California-Berkeley and The University of California-Irvine) to offer an undergraduate degree in business administration. Through its Division of Biomedical Sciences, founded in 1974, The University of California-Riverside offers the Thomas Haider medical degree program in collaboration with The University of California-Los Angeles. The University of California-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 The University of California-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

The University of California-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 The University of California-Riverside have an economic impact of nearly $1 billion in California. The University of California-Riverside research expenditure in FY 2018 totaled $167.8 million. Total research expenditures at The University of California-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 The University of California-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, The University of California-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, The University of California-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. University of California-Riverside can also boast the birthplace of two-name reactions in organic chemistry, the Castro-Stephens coupling and the Midland Alpine Borane Reduction.