‘Bars’ nourish supermassive black hole – 10/02/2024 – Science

With an estimated age of 13 billion years, and a total mass equivalent to around 60 billion solar masses, our galaxy, the Milky Way, has in its bulge a supermassive black hole, of the order of 4 million solar masses. As the terrestrial observer and the center of the Milky Way are on the same plane — the galactic plane —, optical access to the bulge is made impossible by huge clouds of gas and dust that get in the way and block visible light. But observations at certain wavelengths (such as X-rays, radio or infrared) capable of crossing the matter barrier have been carried out. A complementary path is to study galaxies similar to the Milky Way to understand the role of central structures in feeding supermassive black holes and star formation.

This was the focus of the study carried out by astronomer Patrícia da Silva at the Observatoire de Paris, in France, supported by a Research Internship Abroad Grant from Fapesp. The results were published in the journal Astronomy & Astrophysics.

Da Silva is a postdoctoral fellow at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP). The article in question was signed by her and her supervisor abroad, Françoise Combes, from the Observatoire de Paris and the Collège de France.

“We investigated how the ‘bars’ and ‘spirals’ in galaxies influence the movement of gas towards the center, where it can fuel supermassive black holes and trigger bursts of star formation,” says Da Silva. She explains that “bars” and “spirals” are huge structures made up of stars and gas. The bar formation process is significantly complex and results from gravitational instabilities in the galaxy’s disk, causing stars and gas to be distributed in an elongated structure. Bars are found in about two-thirds of all spiral galaxies in the local universe, including the Milky Way.

“This project abroad was generated from a larger work, the subject of my post-doctorate, which seeks to study the nuclei of galaxies similar to the Milky Way, with or without bars. This parent project, in turn, used data of a large survey, DIVING 3D, Deep IFS View of Nuclei of Galaxies, whose objective is to study the central regions of all galaxies in the Southern Hemisphere, in a certain range of brightness and coordinates. The survey sample is 170 galaxies. Of these, 15 are morphological twins of the Milky Way (Milky Way Morphological Twins – MWMTs), intermediate barred galaxies that have the morphological types SABbc and SBbc and eight are Sbcs, galaxies similar to ours, but without bars. 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. close the focus on ten galaxies: eight MWMTs and two Sbcs”, says Da Silva.

According to the researcher, one of the several objectives of the parent project is to compare the MWMTs and Sbcs samples to discover whether there is any influence of the bar on the emission from the nuclear and circum-nuclear region. “When studying nuclear and circum-nuclear emission from MWMTs, we noticed that there is a wide variety of structures, such as circum-nuclear rings and nuclear spirals, that connect the galaxy with the nucleus. From this, the idea of ​​analyzing the influence of the bar in the context of gas transport to the nuclear region. At what level is the bar responsible for the formation of the structures we observed? Would the bar also be responsible for the direct supply of gas to the nucleus, making the supermassive black hole that resides there active? , that is, generating an AGN [do inglês Active Galactic Nucleus ou Núcleo Ativo de Galáxia]? This is how the daughter project was born, which I conducted in my research abroad”, he informs.

One of the main objectives was to quantify the efficiency of the bars in transporting gas at different scales, from hundreds to thousands of parsecs. It is worth remembering that each parsec (whose symbol is pc) is worth around 3.26 light years, that is, almost 31 trillion kilometers.

“The bars are elongated structures of stars that create regions of gravitational resonance, where the rotating gas is transported towards the nucleus or towards the edges of the galaxy, depending on the gravitational torque exerted on it. In barred galaxies of the types SABbc and SBbc , these torques are predominantly negative, which results in the loss of angular momentum of the gas and therefore its movement towards the galactic center. These gas flows can feed the supermassive black hole and trigger star formation activities,” describes Da. Silva.

In the barred galaxies in the sample, it was observed that negative torques dominate in the region located between the bar and the circum-nuclear rings (around 300 parsecs), facilitating the transport of gas to these regions and contributing to the formation of new stars . However, it is known that, within circumnuclear rings, torques tend to invert to positive, interrupting the flow of gas to the nucleus. The presence of active supermassive black holes (AGNs) can generate new resonances, producing more structures up to the supermassive black hole, connecting it with other large-scale structures in the galaxy and allowing the continuous supply of gas to the galactic nucleus.

“While barred galaxies demonstrate a clear pattern of gas transport towards the center, barred galaxies [Sbcs] present different dynamics. In these cases, the gas is transported towards the edges of the galaxy due to predominant positive torques. This suggests that, in barless galaxies, other mechanisms, such as the gravitational interaction of the spiral arms, must play a role in the transport of gas to the nucleus, if any”, reports Da Silva.

One of the most relevant aspects of the study was the impact that gravitational torques have on the formation of circum-nuclear structures, such as nuclear rings and spirals. These structures are fundamental to understanding how gas is redistributed on smaller scales within the galaxy. Circumnuclear rings are sites of intense star formation, fueled by the compression of gas as it accumulates in regions of gravitational resonance. Nuclear spirals (in some cases connected to these circum-nuclear rings) can play an essential role in the continuous transport of gas to the nucleus, feeding the supermassive black hole and maintaining active galaxy nuclei (AGNs).

Da Silva adds: “In non-barred galaxies, gravitational torques are less efficient in transporting gas towards the center. These galaxies are currently being analyzed in Survey DIVING 3D. We are investigating the main characteristics of their nuclear regions to compare with MWMTs and relate the absence of the bar in these scenarios”.

A fundamental ingredient in the evolution of galaxies is the conduction of gas to galactic centers and the feeding of supermassive black holes. These extremely dense bodies, present in almost all galaxies, regulate the amount of gas available in the central regions and influence surrounding star formation. The data analyzed in the study shows that the bars facilitate transport to the central region. “However, as the gas approaches the nucleus, new mechanisms must come into play for it to continue falling toward the supermassive black hole. High-resolution observations have revealed that, on scales of approximately 10 parsecs, smaller nuclear bars can play this role, channeling gas directly to the center”, informs Da Silva.

Although the study has made significant advances in understanding gravitational torques and their influence on gas transport, there are still many open questions. One of the limitations faced by the researchers was the spatial resolution available in the data, which prevented a more detailed analysis on smaller scales, such as 10 parsecs. Da Silva emphasizes that future studies with higher resolution images are needed to fully understand how gas is funneled into the supermassive black hole and how this process affects the long-term evolution of galaxies. Furthermore, the study suggests that the strength of the bars is not a permanent characteristic, varying over billions of years. Factors such as the presence of gas in the galactic disk and gravitational interactions can influence the longevity of the bars, making gas transport a dynamic and complex process.

“By better understanding how gas is redistributed in the Milky Way’s twin galaxies, we can gain new insights into the evolution of our own galaxy and the role that the central supermassive black hole plays in that process. Our research provides a solid foundation for future investigations” , concludes Da Silva.