Hubble: Milky Way Raids Intergalactic 'Bank Accounts'

[bystander]

Milky Way Raids Intergalactic 'Bank Accounts'
NASA | GSFC | STScI | HubbleSite | 2019 Oct 10

This illustration envisions the Milky Way galaxy's gas recycling above and below its stellar disk. Hubble observes the invisible gas clouds rising and falling with its sensitive Cosmic Origins Spectrograph (COS) instrument. The spectroscopic signature of the light from background quasars shining through the clouds gives information about their motion. Quasar light is redshifted in clouds shooting up and away from the galactic plane, while quasar light passing through gas falling back down appears blueshifted. This differentiation allows Hubble to conduct an accurate audit of the outflowing and inflowing gas in the Milky Way's busy halo — revealing an unexpected and so-far unexplained surplus of inflowing gas. Illustration: NASA, ESA and D. Player (STScI)

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Our Milky Way is a frugal galaxy. Supernovas and violent stellar winds blow gas out of the galactic disk, but that gas falls back onto the galaxy to form new generations of stars. In an ambitious effort to conduct a full accounting of this recycling process, astronomers were surprised to find a surplus of incoming gas.

"We expected to find the Milky Way's books balanced, with an equilibrium of gas inflow and outflow, but 10 years of Hubble ultraviolet data has shown there is more coming in than going out," said astronomer Andrew Fox of the Space Telescope Science Institute, Baltimore, Maryland, lead author of the study to be published in The Astrophysical Journal.

Fox said that, for now, the source of the excess inflowing gas remains a mystery.

One possible explanation is that new gas could be coming from the intergalactic medium. But Fox suspects the Milky Way is also raiding the gas "bank accounts" of its small satellite galaxies, using its considerably greater gravitational pull to siphon away their resources. Additionally, this survey, while galaxy-wide, looked only at cool gas, and hotter gas could play a role, too.

The new study reports the best measurements yet for how fast gas flows in and out of the Milky Way. Prior to this study, astronomers knew that the galactic gas reserves are replenished by inflow and depleted by outflow, but they did not know the relative amounts of gas coming in compared to going out. The balance between these two processes is important because it regulates the formation of new generations of stars and planets. ...

The Mass Inflow and Outflow Rates of the Milky Way ~ Andrew J. Fox et al

• arXiv.org > astro-ph > arXiv:1909.05561 > 12 Sep 2019