
From The University of North Carolina-Chapel Hill
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Science Alert
28 MAY 2022
CAROLYN COLLINS PETERSEN
We all know that a humongous black hole exists at the center of our galaxy. It’s called Sagittarius A* (Sgr A* for short) and it has the mass of 4 million suns. We got to see a radio image of it a few weeks back, showing its accretion disk.
So, we know it’s there. Astronomers can chart its actions as it gobbles up matter occasionally and they can see how it affects nearby stars.
What astronomers are still trying to understand is how Sgr A* formed.
The answer looks like it involves smaller black holes, especially ones from so-called dwarf galaxies. According to a paper published this past week in The Astrophysical Journal by astronomers at the University of North Carolina at Chapel Hill, there’s a whole treasury of them out there.
These things are sitting inside many dwarfs and may provide a missing link to the growth of supermassive black holes in larger galaxies.
Massive (and Supermassive) Black Holes and their Lairs
So, let’s dig into this a bit more, starting with supermassive black holes.
They lurk in the hearts of many, many galaxies. These monsters have millions or billions of solar masses. How did they get to be so big?
The answer involves a topic that we see across astronomy and planetary science: hierarchical models. That’s a fancy way of saying that big things are created from smaller things.
For example, planets get started as dust grains that stick together to make rocks that slam together to make asteroids that collide to create planetesimals that glom onto each other to make planets.
Galaxy formation has its own hierarchical model, too. What creates one of those stellar cities? Galaxies like the Milky Way started out as a collection of gas in the early Universe.
That gas formed stars, which evolved, died, and spread their materials out to help create new generations of stars (and their planets).
In many senses, dwarf galaxies are more like the primordial galaxies than they are the evolved spirals and ellipticals.
Okay, so we simplified things here to give a look at a complex topic that takes up entire textbooks. And, that’s even before we get to galaxy mergers.
Growing a Big Galaxy from Little Ones
Let’s look at the Milky Way’s past more closely. It has an extensive merger history, going back billions of years. It started as an infant (maybe it was a dwarf) some 14 billion years ago. Other little ones merged with it.
Eventually, we got the home galaxy we all know and love today. (And let’s not forget that it will, in fact, merge with the Andromeda Galaxy in a few billion years.)

Andromeda Galaxy (Messier 31). Credit: Adam Evans.
So, those little guys that merged to make the current Milky Way; chances are good some were dwarfs. They’re the little cousins of the big spirals and ellipticals. A typical one has maybe a thousand to a billion stars and sports an irregular shape.
Their stars are what astronomers call “metal-poor” (meaning they’re mostly hydrogen and helium). And, these weird little galaxies swarm around some larger ones like fireflies. Sometimes they even get caught and swallowed up.
The Milky Way has about 20 or so of them orbiting around it. One – the Sagittarius Dwarf – is getting interacting and getting cannibalized as you read this. It’s made the trip through our galaxy numerous times.
It seems that dwarf galaxies like this one could have what’s called “growing black holes” as part of their structures. How do we know this? Astronomers found ways to survey the nearby Universe to look for candidate dwarf galaxies with such growing black holes.
Finding Black Holes in All the Small Places
The North Carolina team actually found a number of such dwarfs. It all began when they posed the question: where do supermassive black holes come from?
The answer seems to be they grow by collisions with other black holes. That makes sense in a hierarchical model way.
Small stellar-mass black holes could collide, particularly in crowded environments (like a dwarf galaxy or a thickly settled cluster). Eventually, they form more-massive ones.
Such “growing black holes” are seen in big, bright galaxies, but what about the dwarfs? Could they have them? If they do, how abundant are they in such small galaxies? And, could they be key to understanding the growth of supermassive black holes?
To get answers to all those questions, a team led by UNC-Chapel Hill faculty members Sheila Kannappan and Mugdha Polimera got to work.
They analyzed galaxy data from several surveys to hunt for evidence of growing black holes. The team looked for bright emissions like those you’d see indicating star formation or around black hole accretion disks.
Their data came from the Sloan Digital Sky Survey, plus the REsolved Spectroscopy of a Local VolumE (RESOLVE) and Environmental COntext Catalog (ECO).
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Apache Point Observatory
SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft).
Apache Point Observatory near Sunspot, New Mexico Altitude 2,788 meters (9,147 ft).
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They found evidence of growing black holes in a significant percentage of dwarf galaxies. These galaxies sometimes get “tossed out” of surveys of brighter, bigger galaxies because their emissions aren’t (or weren’t) well-understood.
It turns out, they are a treasure trove for black hole research.
Bright Emissions Reveal Black Holes
The clue was in the strong emissions the regions around those black holes give off.
Kannappan compared this black hole discovery to a familiar source of light here in some places on Earth.
“Just like fireflies, we see black holes only when they’re lit up – when they’re growing – and the lit-up ones give us a clue to how many we can’t see,” she said.
Essentially, Kannapan and the team are talking about dwarf galaxies with active black holes at their hearts (in other words, active galactic nuclei).
Of course, there are other reasons why a dwarf galaxy could have strong emissions. For example, the dwarfs could have massive spurts of star formation going on. That activity causes bright spectral emissions, too.
“We all got nervous,” Polimera said. “The first question to my mind was: Have we missed a way in which extreme star formation alone could explain these galaxies?”
Polimera spent years researching any alternative explanations for these dwarf galaxy AGNs. After excluding all the other possibilities, growing black holes fit the data the best.
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Implications for Growing Black Hole Monsters
The discovery of growing black holes in dwarf galaxies brings us back to the Milky Way and its central black hole.
Based on the implications of the North Carolina research, Sgr A* very likely grew as our galaxy did. Not only did its past mergers mingle stars, but each dwarf could also have brought along its own growing black hole.
They had to go somewhere, right? So, why wouldn’t they gravitate (excuse the pun) to each other to add to the greatness of Sgr A*?
“The black holes we’ve found are the basic building blocks of supermassive black holes like the one in our own Milky Way,” Kannappan said. “There’s so much we want to learn about them.”
See the full article here .

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The University of North Carolina at Chapel Hill is a public research university in Chapel Hill, North Carolina. The flagship of the University of North Carolina system, it is considered a Public Ivy, or a public institution which offers an academic experience similar to that of an Ivy League university. After being chartered in 1789, the university first began enrolling students in 1795, making it one of the oldest public universities in the United States. Among the claimants, the University of North Carolina at Chapel Hill is the only one to have held classes and graduated students as a public university in the eighteenth century.
The first public institution of higher education in North Carolina, the school opened its doors to students on February 12, 1795. North Carolina became coeducational under the leadership of President Kemp Plummer Battle in 1877 and began the process of desegregation under Chancellor Robert Burton House when African-American graduate students were admitted in 1951. In 1952, North Carolina opened its own hospital, University of North Carolina Health Care, for research and treatment, and has since specialized in cancer care through University of North Carolina’s Lineberger Comprehensive Cancer Center which is one of only 51 national NCI designated comprehensive centers.
The university offers degrees in over 70 courses of study and is administratively divided into 13 separate professional schools and a primary unit, the College of Arts & Sciences. Five of the schools have been named: the University of North Carolina Kenan–Flagler Business School, the University of North Carolina Hussman School of Journalism and Media, the University of North Carolina Gillings School of Global Public Health, the University of North Carolina Eshelman School of Pharmacy, and the University of North Carolina Adams School of Dentistry. All undergraduates receive a liberal arts education and have the option to pursue a major within the professional schools of the university or within the College of Arts and Sciences from the time they obtain junior status. It is classified among “R1: Doctoral Universities – Very high research activity”, and is a member of The Association of American Universities.According to the National Science Foundation, UNC spent $1.14 billion on research and development in 2018, ranking 12th in the nation.
The University of North Carolina’s faculty and alumni include 9 Nobel Prize laureates, 23 Pulitzer Prize winners, and 51 Rhodes Scholars. Additional notable alumni include a U.S. President, a U.S. Vice President, 38 Governors of U.S. States, 98 members of the United States Congress, and nine Cabinet members as well as CEOs of Fortune 500 companies, Olympians and professional athletes.
The campus covers 729 acres (3 km^2) of Chapel Hill’s downtown area, encompassing the Morehead Planetarium and the many stores and shops located on Franklin Street. Students can participate in over 550 officially recognized student organizations. The student-run newspaper The Daily Tar Heel has won national awards for collegiate media, while the student radio station WXYC provided the world’s first internet radio broadcast. University of North Carolina Chapel Hill is one of the charter members of the Atlantic Coast Conference, which was founded on June 14, 1953. Competing athletically as the Tar Heels, UNC has achieved great success in sports, most notably in men’s basketball, women’s soccer, and women’s field hockey.
Chartered by the North Carolina General Assembly on December 11, 1789, the university’s cornerstone was laid on October 12, 1793, near the ruins of a chapel, chosen because of its central location within the state. The first public university chartered under the US Constitution, The University of North Carolina at Chapel Hill is one of three universities that claims to be the oldest public university in the United States and the only such institution to confer degrees in the eighteenth century as a public institution.
During the Civil War, North Carolina Governor David Lowry Swain persuaded Confederate President Jefferson Davis to exempt some students from the draft, so the university was one of the few in the Confederacy that managed to stay open. However, Chapel Hill suffered the loss of more of its population during the war than any village in the South, and when student numbers did not recover, the university was forced to close during Reconstruction from December 1, 1870, until September 6, 1875. Following the reopening, enrollment was slow to increase and university administrators offered free tuition for the sons of teachers and ministers, as well as loans for those who could not afford attendance.
Following the Civil War, the university began to modernize its programs and onboard faculty with prestigious degrees. The creation of a new gymnasium, funding for a new Chemistry laboratory, and organization of the Graduate Department were accomplishments touted by University of North Carolina president Francis Venable at the 1905 “University Day” celebration.
Despite initial skepticism from university President Frank Porter Graham, on March 27, 1931, legislation was passed to group the University of North Carolina with the State College of Agriculture and Engineering and Woman’s College of the University of North Carolina to form the Consolidated University of North Carolina. In 1963, the consolidated university was made fully coeducational, although most women still attended Woman’s College for their first two years, transferring to Chapel Hill as juniors, since freshmen were required to live on campus and there was only one women’s residence hall. As a result, Woman’s College was renamed the “University of North Carolina at Greensboro”, and the University of North Carolina became the “University of North Carolina at Chapel Hill.” In 1955, The University of North Carolina officially desegregated its undergraduate divisions.
During World War II, the University of North Carolina was one of 131 colleges and universities nationally that took part in the V-12 Navy College Training Program which offered students a path to a Navy commission.
During the 1960s, the campus was the location of significant political protest. Prior to the passage of the Civil Rights Act of 1964, protests about local racial segregation which began quietly in Franklin Street restaurants led to mass demonstrations and disturbance. The climate of civil unrest prompted the 1963 Speaker Ban Law prohibiting speeches by communists on state campuses in North Carolina. The law was immediately criticized by university Chancellor William Brantley Aycock and university President William Friday, but was not reviewed by the North Carolina General Assembly until 1965. Small amendments to allow “infrequent” visits failed to placate the student body, especially when the university’s board of trustees overruled new Chancellor Paul Frederick Sharp’s decision to allow speaking invitations to Marxist speaker Herbert Aptheker and civil liberties activist Frank Wilkinson; however, the two speakers came to Chapel Hill anyway. Wilkinson spoke off campus, while more than 1,500 students viewed Aptheker’s speech across a low campus wall at the edge of campus, christened “Dan Moore’s Wall” by The Daily Tar Heel for Governor Dan K. Moore. A group of the University of North Carolina-Chapel Hill students, led by Student Body President Paul Dickson, filed a lawsuit in U.S. federal court, and on February 20, 1968, the Speaker Ban Law was struck down. In 1969, campus food workers of Lenoir Hall went on strike protesting perceived racial injustices that impacted their employment, garnering the support of student groups and members of the University and Chapel Hill community.
From the late 1990s and onward, the University of North Carolina-Chapel Hill expanded rapidly with a 15% increase in total student population to more than 28,000 by 2007. This is accompanied by the construction of new facilities, funded in part by the “Carolina First” fundraising campaign and an endowment that increased fourfold to more than $2 billion within ten years. Professor Oliver Smithies was awarded the Nobel Prize in Medicine in 2007 for his work in genetics. Additionally, Professor Aziz Sancar was awarded the Nobel Prize in Chemistry in 2015 for his work in understanding the molecular repair mechanisms of DNA.
In 2011, the first of several investigations found fraud and academic dishonesty at the university related to its athletic program. Following a lesser scandal that began in 2010 involving academic fraud and improper benefits with the university’s football program, two hundred questionable classes offered by the university’s African and Afro-American Studies department (commonly known as AFAM) came to light. As a result, the university was placed on probation by its accrediting agency in 2015. It was removed from probation in 2016.
That same year, the public universities in North Carolina had to share a budget cut of $414 million, of which the Chapel Hill campus lost more than $100 million in 2011. This followed state budget cuts that trimmed university spending by $231 million since 2007; Provost Bruce Carney said more than 130 faculty members have left the University of North Carolina since 2009, with poor staff retention. The Board of Trustees for the University of North Carolina-Chapel Hill recommended a 15.6 percent increase in tuition, a historically large increase. The budget cuts in 2011 greatly affected the university and set this increased tuition plan in motion and the University of North Carolina students protested. On February 10, 2012, the University of North Carolina Board of Governors approved tuition and fee increases of 8.8 percent for in-state undergraduates across all 16 campuses.
In June 2018, the Department of Education found that the University of North Carolina at Chapel Hill had violated Title IX in handling reports of sexual assault, five years after four students and an administrator filed complaints. The university was also featured in The Hunting Ground, a 2015 documentary about sexual assault on college campuses. Annie E. Clark and Andrea Pino, two students featured in the film, helped to establish the survivor advocacy organization End Rape on Campus.
In August 2018, the university came to national attention after the toppling of Silent Sam, a Confederate monument which had been erected on campus in 1913 by the United Daughters of the Confederacy. The statue had been dogged by controversy at various points since the 1960s, with critics claiming that the monument invokes memories of racism and slavery. Many critics cited the explicitly racist views espoused in the dedication speech that local industrialist and the University of North Carolina Trustee Julian Carr gave at the statue’s unveiling on June 2, 1913, and the approval with which they had been met by the crowd at the dedication. Shortly before the beginning of the 2018–2019 school year, the Silent Sam was toppled by protestors and damaged, and has been absent from campus ever since. In July 2020, the University’s Carr Hall, which was named after Julian Carr, was renamed the “Student Affairs Building.” Carr had supported white supremacy and also the Ku Klux Klan.
After reopening its campus in August 2020, the University of North Carolina-Chapel Hill reported 135 new COVID-19 cases and four infection clusters within a week of having started in-person classes for the Fall 2020 semester. On 10 August, faculty and staff from several of the University of North Carolina’s constituent institutions filed a complaint against its board of governors, asking the system to default to online-only instruction for the fall. On 17 August, the University of North Carolina’s management announced that the university would be moving all undergraduate classes online from 19 August, becoming the first university to send students home after having reopened.
Notable leaders of the university include the 26th Governor of North Carolina, David Lowry Swain (president 1835–1868); and Edwin Anderson Alderman (1896–1900), who was also president of Tulane University and the University of Virginia. On December 13, 2019 the University of North Carolina System Board of Governors unanimously voted to name Kevin Guskiewicz the university’s 12th chancellor.
the University of North Carolina-Chapel Hill offers 71 bachelor’s, 107 master’s and 74 doctoral degree programs. The university enrolls more than 28,000 students from all 100 North Carolina counties, the other 49 states, and 47 other countries. It is the third largest university in North Carolina, just behind North Carolina State University and the University of North Carolina at Charlotte in enrollment. State law requires that the percentage of students from North Carolina in each freshman class meet or exceed 82%. The student body consists of 17,981 undergraduate students and 10,935 graduate and professional students (as of Fall 2009). Racial and ethnic minorities comprise 30.8% of the University of North Carolina-Chapel Hill’s undergraduate population as of 2010 and applications from international students have more than doubled in the last five years (from 702 in 2004 to 1,629 in 2009). Eighty-nine percent of enrolling first year students in 2009 reported a GPA of 4.0 or higher on a weighted 4.0 scale. The University of North Carolina-Chapel Hill students are strong competitors for national and international scholarships. The most popular majors at the University of North Carolina-Chapel Hill are biology, business administration, psychology, media and journalism, and political science. The University of North Carolina-Chapel Hill also offers 300 study abroad programs in 70 countries.
At the undergraduate level, all students must fulfill a number of general education requirements as part of the Making Connections curriculum, which was introduced in 2006. English, social science, history, foreign language, mathematics, and natural science courses are required of all students, ensuring that they receive a broad liberal arts education. The university also offers a wide range of first year seminars for incoming freshmen. After their second year, students move on to the College of Arts and Sciences, or choose an undergraduate professional school program within the schools of medicine, nursing, business, education, pharmacy, information and library science, public health, or media and journalism. Undergraduates are held to an eight-semester limit of study.
For 2021, U.S. News & World Report ranks For 2021, U.S. News & World Report ranks UNC-Chapel Hill 5th among the public universities and tied for 28th nationally among both public and private universities in the United States. The Wall Street Journal ranked The University of North Carolina-Chapel Hill 3rd best public university behind The University of Michigan and The University of California-Los Angeles.
The university was named a Public Ivy by Richard Moll in his 1985 book The Public Ivies: A Guide to America’s Best Public Undergraduate Colleges and Universities, and in later guides by Howard and Matthew Greene. Many of The University of North Carolina-Chapel Hill’s professional schools have achieved high rankings in publications such as Forbes magazine, as well as annual U.S. News & World Report surveys. In 2020, US News & World Report ranked the School of Medicine #1 in primary care and #23 in research. In 2016, U.S. News & World Report ranked UNC-Chapel Hill business school’s MBA program as the 16th best in the nation. In the 2019 edition, U.S. News & World Report ranked the UNC Gillings School of Global Public Health as the second best school of public health in the United States (behind Johns Hopkins and tied with Harvard). The UNC Eshelman School of Pharmacy was ranked #1 among pharmacy schools in the United States in 2020 by U.S. News & World Report. In 2005, Business Week ranked The University of North Carolina-Chapel Hill business school’s Executive MBA program as the 5th best in the United States. The University of North Carolina also offers an online MBA program, MBA@UNC, that is ranked #1 in the country in 2019 for Best Online MBA Programs (tied with the Kelley School of Business at Indiana University). Other highly ranked schools include journalism and mass communication, law, library and information science, medicine, dentistry, and city and regional planning. Nationally, The University of North Carolina-Chapel Hill is in the top ten public universities for research. Internationally, the 2016 QS World University Rankings ranked North Carolina 78th in the world (in 2010 Times Higher Education World University Rankings and QS World University Rankings parted ways to produce separate rankings). The University of North Carolina-Chapel Hill 5th among the public universities and tied for 28th nationally among both public and private universities in the United States. The Wall Street Journal ranked UNC-Chapel Hill 3rd best public university behind The University of Michigan and The University of California-Los Angeles.
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