Bars feed supermassive black holes at center of galaxies analogous to Milky Way

Bars are huge elongated clusters of stars resulting from gravitational instabilities. The study provides novel insights into the process of star formation and galactic evolution.

By José Tadeu Arantes  |  Agência FAPESP – Our galaxy, the Milky Way, is about 13 billion years old. The 200 billion-odd stars in the galaxy add up to around 60 billion solar masses or around 4% of the total. At its center is a supermassive black hole corresponding to some 4 million solar masses. Observers on Earth and the center of the galaxy are in the same plane – the galactic plane – so that optical access to the black hole is hindered by huge gas and dust clouds surrounding it and blocking visible light. However, observations have been made at certain wavelengths that can penetrate matter, such as X-ray, radio or infrared. A complementary approach consists of studying galaxies similar to the Milky Way in order to understand the role of central structures in the feeding of supermassive black holes and in star formation.

This was the focus of a study conducted by astronomer Patrícia da Silva at the Paris Observatory in France, with FAPESP’s support via a scholarship for a research internship abroad. An article on the study is published in the journal Astronomy & Astrophysics.

Da Silva is a postdoctoral researcher at the University of São Paulo’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP) in Brazil. The article is signed by her and by her supervisor abroad, Françoise Combes, an astrophysicist at the Paris Observatory and a professor at Collège de France.

“We investigated how ‘bars’ and ‘spirals’ in galaxies influence the movement of gas toward the center, where it can feed supermassive black holes and trigger starbursts [rapid star formation processes],” Da Silva said, adding that “bars” and “spirals” are huge structures made up of stars and gas. Bar formation is a highly complex process that results from gravitational instabilities in the galaxy’s disk, leading stars and gas to spread out into an elongated structure. Bars are found in roughly two-thirds of all spiral galaxies in the local universe, including the Milky Way.

“This study derived from a larger project, which I pursued for my postdoctoral research into the nuclei of galaxies similar to the Milky Way, with or without bars. This parent project used data from a major survey known as DIVING 3D, which stands for Deep Integral Field Spectrograph View of Nuclei of Galaxies. This was a study of the central regions of all galaxies in the southern hemisphere, within specified criteria for brightness and coordinates. The sample for the survey was 170 galaxies, 15 of which are Milky Way morphological twins [MWMTs]: barred intermediate galaxies with SABbc- and SBbc-type morphologies. Eight are Sbcs, galaxies analogous to ours but without bars. These 23 are my objects of interest. To study gas dynamics in this context, in addition to DIVING 3D, I used data from the Atacama Large Millimeter/Submillimeter Array (ALMA), the Hubble Space Telescope, and the Legacy Survey. In light of the available data, I had to narrow the focus down to ten galaxies: eight MWMTs and two Sbcs,” Da Silva explained.

The parent project aims, among things, to compare MWMTs and Sbcs in order to find out whether bars influence emission in the nuclear and circumnuclear region, she said: “When we analyzed nuclear and circumnuclear emission in the MWMTs, we observed a wide array of structures, such as circumnuclear rings and nuclear spirals connecting the galaxy with its nucleus. This led to the idea of analyzing the influence of bars in the context of gas being driven to the nuclear region. To what extent are bars responsible for forming the structures we observed? Is the bar also responsible for feeding gas directly to the nucleus and making the supermassive black hole active, i.e. creating an AGN [active galactic nucleus]? This was what gave birth to the child project, which I conducted for my research abroad.”

The researchers set out to quantify the bars’ efficiency in driving gas on different scales, from hundreds to thousands of parsecs. A parsec (pc) is about 3.26 light-years, or almost 31 trillion km.

“Bars are elongated strips of stars that create regions of gravitational resonance where rotating gas is driven toward the nucleus or toward the edges of the galaxy, depending on the gravitational torque exerted on the gas by the bar. In SABbc- and SBbc-type barred galaxies, the gravitational torque is predominantly negative, resulting in the loss of the gas’s angular momentum and hence of its motion toward the center of the galaxy. The gas flows can fuel the supermassive black hole and trigger star formation activities,” Da Silva said.

In the barred galaxies, negative torques were found to predominate in the region located between the bar and the circumnuclear rings (around 300 parsecs), facilitating the transport of gas to this region and contributing to the formation of new stars. However, torques are known to tend to invert to positive inside the circumnuclear rings, interrupting the flow of gas to the nucleus. The presence of an AGN can create new resonances, producing more structures that lead up to the supermassive black hole, connecting it to other large-scale structures in the galaxy, and continuously driving gas into the nucleus.

“While barred galaxies display a clear pattern of driving gas to the center, non-barred galaxies [Sbcs] have different dynamics. In these cases, gas is driven toward the edges of the galaxy owing to predominantly positive torques, suggesting that in non-barred galaxies other mechanisms such as gravitational interaction of spiral arms must play a role in driving the flow of gas to the nucleus, if any such flow exists,” Da Silva said.

One of the study’s most significant findings is the impact of gravitational torque on the formation of circumnuclear structures, such as nuclear rings and spirals. These structures are fundamental to the understanding of how gas is redistributed on a smaller scale inside the galaxy. Circumnuclear rings are sites of intense star formation, fueled by compression of gas as it accumulates in regions of gravitational resonance. Nuclear spirals (in some cases connected to circumnuclear rings) can play an essential role in the continuous driving of gas to the nucleus, feeding the supermassive black hole and maintaining AGNs.

“In non-barred galaxies, the gravitational torques are less efficient in driving gas toward the center. These galaxies are being analyzed right now in the DIVING 3D Survey. We’re investigating the key characteristics of their nuclear regions in order to compare them with MWMTs and relate them to the absence of bars in these scenarios,” Da Silva said.

A key factor in the evolution of galaxies is the flow of gas to fuel the central black hole and central star formation. Supermassive black holes are present in almost all galaxies. They are extremely dense and regulate the amount of gas available in the center. The data analyzed in the study showed that bars drive gas to the central region. “However, as the gas approaches the nucleus, other mechanisms must come into play in order for it to fall toward the supermassive black hole. High-resolution observations have revealed that on a scale of approximately 10 parsecs, smaller nuclear bars can play this role, channeling gas directly to the center,” Da Silva said.

Although the study achieved significant advances in understanding gravitational torques and their influence on gas transport, many questions remain open. The researchers faced limitations such as the low spatial resolution of the available data, which prevented a more detailed analysis for smaller scales, such as 10 parsecs. Da Silva noted that future studies using higher-resolution images are necessary to understand fully how gas is channeled to the supermassive black hole and how this process affects the long-term evolution of galaxies. In addition, the study suggests bar strength may not be long-lasting but may vary during cosmological evolution (i.e. over billions of years). Factors such as the presence of gas in the galactic disk and gravitational interactions may affect bar longevity, making gas transport a complex and dynamic process.

“A fuller understanding of how gas is redistributed in Milky Way twins will enable us to obtain novel insights into the evolution of our own galaxy and the role played in this process by the supermassive black hole at its center. Our study offers a sound basis for future investigations,” Da Silva said.

The article “Multiple-scale gas infall through gravity torques on Milky Way twins” is at: www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202450500.