The central region of the Milky Way is a key region of our galaxy. The processes that take place there also shape star formation in other areas of the galaxy. Now, for the first time, astronomers have mapped the inner 650 light-years of the Milky Way in high resolution and at the same time comprehensively in the radio range. Data obtained from the ALMA array in Chile show the distribution and motions of cool molecular gases in the galactic center and also reveal some unknown structures and phenomena. These include violent turbulence and shock waves, six spiral arms of excited gases, and potentially a whole new class of molecular objects.
The center of the Milky Way is one of the most star-rich areas in the universe. There are at least 20 million stars within a radius of around 26 light-years around the supermassive black hole Sagittarius A*. The entire central zone extends more than 300 light-years and includes other star-rich areas and dense clouds of gas and dust. “The central molecular zone is home to some of the most massive stars in our home galaxy, many of which have short lives and end their lives in massive supernova explosions or even hypernovae,” explains lead author Steve Longmore from Liverpool John Moores University in the UK. The particularly rapid cycle of star formation and demise there and the associated flows of matter shape the stellar population of the Milky Way far beyond the galactic core. “The bar and other large-scale stellar structures, as well as molecular clouds and star-forming complexes, develop from these flows and cycles,” the astronomers write.
Mapping molecular gases in the Milky Way center
“The central regions of galaxies are key to understanding their evolution,” said Longmore and his colleagues. “The center of the Milky Way is the only galactic nucleus for which we can trace the physics of star formation and its effects down to individual protostellar nuclei.” So far there have already been recordings and data of individual aspects of the Milky Way center. But above all, the data from radio telescopes, which are important for mapping gas and dust flows, usually had too low a resolution. Other observations showed individual gas clouds in great detail, but only a small section of the galactic core. That’s why astronomers have now used the powerful antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study the distribution of cooler molecular gases in the center of the Milky Way. The ACES survey, short for ALMA CMZ Exploration Survey, has mapped the entire central molecular zone of our galaxy, which is around 650 light years in size.
The resulting mosaic image is the largest image ever created with the ALMA array. It shows a section of the sky the size of three full moons lying next to each other. But even more important is the high resolution of this mapping: it shows gas structures on scales from dozens of light years down to small gas clouds around individual stars. “The spectrum of ACES is also designed to capture 71 different spectral features and bandwidths to record physical, chemical and kinematic conditions,” Longmore and his team explain. It is the first time that the cold gas in the entire central molecular zone has been studied in such detail.
(Video: European Southern Observatory)
Dozens of types of molecules and some surprises
The ACES survey now shows in more detail for the first time which molecules are present in the center of the Milky Way and how these gases move. The spectral data maps the distribution of dozens of different molecules, from simple inorganic compounds like silicon monoxide to complex organic molecules like methanol, acetone or ethanol. The ACES data also show that the gas flows in the Milky Way center are characterized by high-energy, dynamic processes: “The molecular gas in the central molecular zone is highly turbulent, with spectral signatures of supersonic speeds and shocked material,” the astronomers write. For example, initial analyzes identified a gas cloud that was torn apart by the hypernova of a massive star and is now racing outwards as a ring-shaped shock front. “It is a place of extremes, invisible to our eyes, but now made visible in extraordinary detail,” says co-author Ashley Barnes of the European Southern Observatory (ESO).
At larger spatial scales, the mapping also revealed several previously unrecognized structures: “One team has already identified and described a nested system of six spiral arms within the central molecular zone,” report Longmore and his colleagues. “These ‘arms’ appear as coherent, tilted speed bumps in the spectral lines of carbon monosulfide (CS) and hydrogen cyanide (HCN).” Astronomers interpret these curved arms as ring-like structures whose center lies in the area of the Milky Way’s black hole.
The initial analyzes of the ACES data revealed further surprises. Among them is a compact but bright structure that produces broadened spectral lines and evidence of excited sulfur dioxide, sulfur monoxide and carbon monosulfide. “Despite its large mass and speed distribution, this object shows no evidence of high-speed shocks. In addition, no counterpart in infrared, X-ray or centimeter wavelengths is known to date,” write the researchers. According to their assessment, the object dubbed “MUBLO” could belong to a previously unknown, novel class of molecular objects in the center of the Milky Way.
“We expected a high level of detail when planning the study, but were still surprised by the complexity and diversity that emerged in the final mosaic,” says co-author Katharina Immer from ESO. The evaluation of the ACES mapping has only just begun. The initial data are now presented in five papers that have been accepted for publication in the Monthly Notices of the Royal Astronomical Society. A sixth article is in the final stages of review.
Source: Steve Longmore (Liverpool John Moores University, Liverpool, UK) et al., Monthly Notices of the Royal Astronomical Society, accepted