Earth, Solar System, Milky Way. Are they Getting More or Less Massive Over Time?

According to the most widely-accepted cosmological models, the first galaxies began to form between 13 and 14 billion years ago. Over the course of the next billion years, the cosmic structures we’ve all come to know emerged. These include things like galaxy clusters, superclusters, and filaments, but also galactic features like globular clusters, galactic bulges, and Supermassive Black Holes (SMBHs).

However, like living organisms, galaxies have continued to evolve ever since. In fact, over the course of their lifetimes, galaxies accrete and eject mass all the time. In a recent study, an international team of astronomers calculated the rate of inflow and outflow of material for the Milky Way. Then the good folks at astrobites gave it a good breakdown and showed just how relevant it is to our understanding of galactic formation and evolution.

The study was led by ESA astronomer Dr. Andrew J. Fox and included members from the Space Telescope Science Institute‘s (STScI) The Milky Way Halo Research Group, the ESA’s Association of Universities for Research in Astronomy (AURA), and multiple universities. Based on previous studies, they examined the rate at which gas flows in and out of the Milky Way from surrounding high-velocity clouds (HVC).

Since the availability of material is key to star formation in a galaxy, knowing the rate at which it is added and lost is important to understanding how galaxies evolve over time. And as Michael Foley of astrobitessummarized, characterizing the rates at which material is added to galaxies is crucial to understanding the details of this “galactic fountain” model.

In accordance with this model, the most massive stars in a galaxy produce stellar winds that drive material out of the galaxy disk. When they go supernova near the end of their lifespans, they similarly drive most of their material out. This material then infalls back into the disk over time, providing material for new stars to form.

“These processes are collectively known as ‘stellar feedback’, and they are responsible for pushing gas back out of the Milky Way,” said Foley. “In other words, the Milky Way is not an isolated lake of material; it is a reservoir that is constantly gaining and losing gas due to gravity and stellar feedback.”

In addition, recent studies have shown that star formation may be closely related to the size of the Supermassive Black Hole (SMBH) at a galaxy’s core. Basically, SMBHs put out a tremendous amount of energy that can heat gas and dust surrounding the core, which prevents it from clumping effectively and undergoing gravitational collapse to form new stars.

As such, the rate at which material flows in and out of a galaxy is key to determining the rate of star formation. To calculate the rate at which this happens for the Milky Way, Dr. Fox and his colleagues consulted data from multiple sources. As Dr. Fox told Universe Today via email:

“We mined the archive. NASA and ESA maintain well-curated archives of all Hubble Space Telescope data, and we went through all the observations of background quasars taken with the Cosmic Origins Spectrograph (COS), a sensitive spectrograph on Hubble that can be used to analyze the ultraviolet light from distant sources. We found 270 such quasars. First, we used these observations to make a catalog of fast-moving gas clouds known as high-velocity clouds (HVCs). Then we devised a method for splitting the HVCs into inflowing and outflowing populations, by making use of the Doppler shift.”

In addition, a recent study showed that the Milky Way experienced a dormant period roughly 7 billion years ago – which lasted for about 2 billion years. This was the result of shock waves that caused interstellar gas clouds to become heated, which temporarily caused the flow of cold gas into our galaxy to stop. Over time, the gas cooled and began flowing in again, triggering a second round of star formation.

After looking at all the data, Fox and his colleagues were able to place constraints on the rate of inflow and outflow for this galaxy of ours:

“After comparing the rates of inflowing and outflowing gas, we found an excess of inflow, which is good news for future star formation in our Galaxy, since there is plenty of gas that can be converted into stars and planets. We measured about 0.5 solar masses per year of inflow and 0.16 solar masses per year of outflow, so there’s a net inflow.”

However, as Foley indicated, HVCs are believed to live for periods of only about 100 million years or so. As a result, this net inflow cannot be expected to last indefinitely. “Finally, they ignore HVCs that are known to reside in structures (such as the Fermi Bubbles) that don’t trace the inflowing or outflowing gas,” he adds.

Since 2010, astronomers have been aware of the mysterious structures emerging from the center of our galaxy known as Fermi Bubbles. These bubble-like structures extend for thousands of light-years and are thought to be the result of SMBH’s consuming interstellar gas and belching out gamma rays.

However, in the meantime, the results provide new insight into how galaxies form and evolve. It also bolsters the new case made for “cold flow accretion”, a theory originally proposed by Prof. Avishai Dekel and colleagues from The Hebrew University of Jerusalem’s Racah Institute of Physics to explain how galaxies accrete gas from surrounding space during their formation.

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