Hydrogen gas falling into the galaxy has a possible extraterrestrial origin: Nagoya University group overturns established theory

A research group led by Researcher Takahiro Hayakawa and Professor Emeritus Yasuo Fukui of the Graduate School of Science at Nagoya University announced that they have succeeded in creating a precise map of the distribution of heavy elements in gas clouds (high- and intermediate-velocity clouds) that fall on our Galaxy, and revealed the possibility that hydrogen gas from outside our system is being brought in. New developments in the understanding of the growth and evolution of our Galaxy are expected. The results were published in the Monthly Notices of the Royal Astronomical Society on February 28.

Most of the neutral hydrogen gas observed from the solar system follows the rotational motion of the Galaxy; however, some clouds exhibit line-of-sight velocities far out of the rotational motion. Clouds with particularly high velocities of about 100 km/s or more are called high-velocity clouds, and those with smaller velocities are called intermediate-velocity clouds. Intermediate-velocity clouds are thought to be thousands of light-years away, while high-velocity clouds tens of thousands of light-years away.

Excluding dark matter, galaxies are composed of stars and large amounts of gas, and most of this gas is composed of hydrogen. Observations of radio waves (wavelength = 21 cm) led to a theory in the 1960s that a large amount of hydrogen gas exists not only in the disk of the Galaxy but also outside of it, with most of it being pulled in by the gravity of the Galaxy and falling onto it. Alternatively, the "galactic-fountain," a model in which gas in the galactic disk is blown away by the explosion of a massive star and falls back into the Galaxy, was proposed as a theory to explain high-velocity and intermediate-velocity clouds. Trace amounts of heavy elements (other than hydrogen and helium) in the gas are synthesized only through nuclear fusion reactions in a star and supernova explosions. They are more abundant in gases inside the Galaxy and less abundant in gases coming from outside the system. In discussing which theory is correct, the amount of heavy elements in the gas (heavy element/hydrogen ratio) has been considered an important factor in determining the correct theory.

In the period between 1990-2000, atomic absorption line spectroscopy measurements revealed that intermediate-velocity clouds are responsible for fountain-model material cycles with heavy-element abundances comparable to those of the surrounding gas in the solar system, while high-velocity clouds are gas falling from outside the Galaxy with about one-tenth of that amount of heavy elements. In principle, this measurement uses galaxies and stars that are in principle bright as light sources. The limit is a few dozen locations where there is a light source, and for the rest, the answer was unknown and unclear.

Since around 2015, Fukui and his team have been a part of the Planck satellite international collaborative team. As part of their research, they developed a method for the precise determination of neutral hydrogen atoms in dust and interstellar gas. In this study, Hayakawa used two types of high-resolution all-sky maps, one based on radio wave observations at a wavelength of 21 cm and the other based on dust from the Planck satellite. He introduced advanced mathematical methods to derive heavy element/hydrogen ratios for high-velocity and low-velocity clouds as well as gases around the solar system serving as a basis for comparison, and he succeeded in creating a novel all-sky map.

Statistical analysis of the obtained information reveals that most of the intermediate-velocity clouds have low heavy element content, while the high-velocity clouds have even lower content. Intermediate-velocity clouds contained many components that were less than one-third of that found around the sun. This indicates that the intermediate-velocity clouds may be primordial gas of extra-systemic origin. If the intermediate-velocity and high-velocity clouds are coming from outside the system, they are still bringing primordial gas to the Galaxy from outside. This had made it clear that the established theory stating "the amount of heavy elements in intermediate-velocity clouds is about the same as that of gases around the solar system" is incorrect.

Journal Information
Publication: Monthly Notices of the Royal Astronomical Society
Title: Dust-to-neutral gas ratio of the intermediate- and high-velocity H I clouds derived based on the sub-mm dust emission for the whole sky
DOI: 10.1093/mnras/stae302