If you thought that the fictional depiction of the orbiting planets around the supermassive black hole Gargantua in last year’s Hollywood science fiction epic ‘Interstellar’ was an unrealistic one, you might have to reconsider. A set of recent observations of the Milky Way’s central regions that were made with the Very Large Array radio astronomy observatory in New Mexico, have provided new evidence for the formation of stars near the supermassive black hole Sagittarius A* which lies at the heart of our galaxy.
Due to our location in the Milky Way galaxy, which is 26,000 light-years away from the galactic center, our view of the latter is heavily obscured by the large amounts of gas and dust that lie in between, making it possible to observe it in only certain parts of the electromagnetic spectrum from the ground as well as from space. A series of studies that had been undertaken during the last 20 years in X-ray, infrared and radio wavelengths, have helped to establish that the center of our galaxy harbors a supermassive black hole, called Sagittarius A* or Sgr A* for short, with a mass of approximately 4 million times that of the Sun. The surroundings of Sgr A* out to a few light years, have been found to be a quite complex place, one that features thousands of old, red giant stars as well as many other specimens of the cosmic zoo, like, supernova remnants, neuron stars and vast amounts of molecular gas, that orbit the behemoth black hole.
One of the more surprising findings in recent years, has been the discovery of a population of more than 150 young and massive O and B-type blue-white stars orbiting the galactic center within a radius of just 2 light-years away from Sgr A*, which astronomers have estimated to be no more than 7 million years-old. This young population of stars near the Milky Way’s supermassive black hole has presented a new challenge for astrophysicists, giving rise to the Paradox of Youth: current theoretical models posited that such stars would be impossible to have been formed due to the searing tidal stresses that are dominant in the vicinity of Sgr A*. The gravitational collapse of molecular gas clouds that lead to the formation of stars, should have been interrupted by the immense gravity of the supermassive black hole at the galactic center, eventually tearing the gas clouds to pieces. Furthermore, the stars near Sgr A* instead of being randomly distributed around the latter, were found to be concentrated within two distinct disks around Sgr A*, with each one revolving in opposite directions, one clock-wise and the other counter-clockwise, indicating that they share a common origin.
There hasn’t been a shortage of proposed explanations within the scientific community in order to account for these observations. The two leading scenarios regarding the existence of this young cluster of stars in the vicinity of Sgr A* have included either their in-situ formation from the gravitational collapse of a surprisingly dense and massive molecular gas cloud near the supermassive black hole, or their previous formation in another part of the Milky Way and their subsequent migration into the galactic center. A third possibility according to some astronomers is that these stars are not young in reality but they only appear to be so due to the past merger of an older generation of stars, or due to some other process of stellar evolution that we are currently unaware of, leaving scientists with a nagging mystery on their hands.
Now, a study that was recently published at the The Astrophysical Journal Letters, comes to shed more light to the enigma, by presenting new observational evidence for the formation of even younger stars in the Milky Way’s central regions. A research team led by Dr. Farhad Yusef-Zadeh, an astrophysicist at the Northwestern University’s Department of Physics and Astronomy, in Evanston, IL, observed Sgr A* with the Very Large Array (VLA) radio observatory in New Mexico in March 2014, in order to search for any stellar populations that might be younger still than those that have been already detected. The researchers’ efforts quickly paid off, when they identified a total of 44 partially resolved compact radio sources which were clustered together in two regions along the edge of an elongated molecular cloud just 2 light-years away from Sgr A*. What caught the attention of Yusef-Zadeh’s team, was the fact that these radio sources were relatively small in size (between 400 and 1,600 Astronomical Units) and exhibited a distinct bow-shock, comet-like appearance which was facing towards the direction of the young and bright O and B-type stars that are located at similar distances very near the galactic center.
The overall morphology and characteristics of these new radio sources is strikingly similar to the ones that have been observed inside various well-studied stellar nurseries across the Milky Way, like those found within the Orion Nebula, the Carina Nebula and the Trifid Nebula, where star formation is taking place. Within the latter, astronomers have detected many dozens of photo-evaporating protoplanetary disks, called proplyds, which are heavily irradiated from nearby stars. Proplyds mark the earliest stages of stellar formation and are characterised by the presence of a thin and opaque disk of gas and dust that surrounds a newborn star that is still embedded in the gaseous envelope from which it emerged. Likewise, Yusef-Zadeh’s team noted that the bow-shock appearance of the newly found radio sources near Sgr A* were externally illuminated by the nearby young, massive stars. “Like other star forming regions in the Galaxy where proplyds are detected with tear-shaped geometry or a disk silhouetted against the background radiation, the emission from [the new] radio proplyds as well as the shape and size of proplyds, are consistent with being photoevaporated from a gaseous disk by the far-ultraviolet radiation and then photoionized by the extreme ultraviolet Lyman continuum radiation from hot stars in the inner 0.4 parsecs (1.3 light-years) of the Galaxy,” write the researchers in their study. “In analogy with the proplyds found in the Orion Nebula, we interpret these radio continuum sources as proplyd candidates that are photoionized and photoevaporated by the radiation emitted by massive OB and WR stars near Sgr A*. The upper limits to the infrared flux from [these] Galactic center proplyd candidates are consistent with gaseous disks orbiting low mass stars at a projected distance of 0.6 − 0.8 parsecs [1.9 – 2.6 light-years] from Sgr A*.”
These findings by Yusef-Zadeh’s team come to complement a series of similar observations that have been conducted in recent years with various ground and space-based observatories, like NASA’s Chandra X-Ray Observatory and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the latter of which were also undertaken by a team led by Yusef-Zadeh. “People think it is very hard to form stars near a supermassive black hole,” he explains. “This is because the gravity of the black hole produces extreme tidal forces that would stretch and elongate molecular clouds, preventing them from ever accumulating enough mass to trigger star formation. But what we seem to have found are patches of dust and gas that have become so dense that they are able to overcome their inhospitable surroundings.”
The possibility of stellar formation near the vicinity of black holes naturally brings up the question of whether any planets could also form around them as well. According to Zadeh’s team, low-mass stellar objects like the proplyds that have been identified in the researchers’ new study, present a much more hospitable environment for the formation of planets, since the radiation pressure from these stars would be low enough to allow the gas and dust in the surrounding protoplanetary disk to eventually coalesce into fully fledged planets. “An additional implication of low-mass star formation near Sgr A* is that proplyd-like objects in the far-ultraviolet-dominated region have lower mass-loss rates, perhaps could retain sufficient disk mass to allow potential planetary formation,” write the researchers.
Despite these tantalising evidence however, much work still lies ahead before scientists can confirm that such processes do indeed take place in the inhospitable surroundings of black holes. The ALMA array could play a leading role in this search with its superior observing capabilities and much higher sensitivity in millimeter wavelengths, which could provide astronomers with their first detailed glimpses of the inner several light-years of the Milky Way’s center. “The presence of proplyds implies current in-situ star formation activity near Sgr A* and opens a window for the first time to study low-mass star, planetary and brown dwarf formations near a supermassive black hole,” notes the study by Zadeh’s team. “Future mid-IR and millimeter observations should conclusively test whether proplyds lie close to Sgr A* and potentially measure the disk mass of Galactic center proplyd-like objects.”
Whatever the case, the environment near our galaxy’s supermassive black hole is steadily emerging as a much more exciting and unique one than what we could have ever imagined.