Cosmic Choreography: How cool gas halos prolong galactic growth

Astronomers have observed a choreographed dance between star-forming galaxies and their surrounding gas halos — a breakthrough in unravelling how galaxies grow.

A team of astronomers have discovered an amazing case of cosmic choreography among typical star-forming galaxies. The galactic waltz sees cool gas halos locked in-step with the galactic disks — both spinning in the same direction.

The researchers, led by Crystal Martin and Stephanie Ho of the University of California, Santa Barbara, used W. M. Keck Observatory to obtain the first-ever direct observational evidence showing that corotating halo gas is not only possible but common. The research — published in the most recent issue of the Astrophysical Journal — also suggests that this corotating gas halo will eventually spiral in towards the disc.

The various components of the Milky Way. The purely stellar thick disk contrasts with the narrower, gas-rich thin disk. (Swinburne University)

Martin, Professor of Physics at UC Santa Barbara and lead author of the study, says: “This is a major breakthrough in understanding how galactic disks grow.

“Galaxies are surrounded by massive reservoirs of gas that extend far beyond the visible portions of galaxies. Until now, it has remained a mystery how exactly this material is transported to galactic disks where it can fuel the next generation of star formation.”

The team gathered their results by observing 50 standard star-forming galaxies over a period of several years.

The research confirms current theoretical models that predict the angular momentum of the spinning cool halo gas partially offsets the gravitational force pulling it towards the galaxy — slowing down the gas accretion rate and, in-turn, lengthening the period of disk growth.

This means that the angular momentum of the halo gas is sufficient to slow down the infall rate but not so high as to shut down feeding the galactic disk entirely.

Finding the tempo of the cosmic waltz

In order to build their case, the astronomers collected the spectra of bright quasars behind star-forming galaxies. This allowed them to detect the usually invisible halo gas by isolating its absorption-line signature in the quasar spectra.

J165930+373527 is among the galaxies detected with corotating halo gas. This high-resolution W. M. Keck Observatory NIRC2 image (red) combined with Hubble space telescope WFC3 imaging (blue and green) resolves the galactic disk. The galactic rotation was measured from W. M. Keck Observatory and apache point observatory emission-line spectra. ( S. Ho & C. Martin, UC Santa Barbara/W. M. Keck Observatory/STSCI)

Co-author of the study, Ho, a physics graduate student at UC Santa Barbara, describes what sets this work apart from previous studies: “Our team also used the quasar as a reference ‘star’ for Keck’s laser guide star AO system.”

“This method removed the blurring caused by the atmosphere and produced the detailed images we needed to resolve the galactic disks and geometrically determine the orientation of the galactic disks in three-dimensional space.”

To obtain high-resolution images of the galaxies to be observed, the team was able to take advantage of the Keck Observatory’s laser guide star adaptive optics (LGSAO) system and the near-infrared camera (NIRC2) on the Keck II telescope, alongside Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

The team then measured the Doppler shifts of the gas clouds using the Low-Resolution Imaging Spectrometer (LRIS) at Keck Observatory, as well as obtaining spectra from Apache Point Observatory. This Doppler shift — which manifests as the widths of spectral lines — allows researchers to determine what direction the gas is spinning and how fast.

The data suggest that the gas is rotating in the same direction as the galaxy and that the angular momentum of the gas is not stronger than the force of gravity. Thus meaning the gas will spiral into the galactic disc.

Martin explains: “Just as ice skaters build up momentum and spin when they bring their arms inward, the halo gas is likely spinning today because it was once at much larger distances where it was deposited by galactic winds, stripped from satellite galaxies, or directed toward the galaxy by a cosmic filament.”

The Next Steps

Martin and her team now aim to measure the rate at which the halo gas is being pulled into the galactic disc.

They hope that comparing the inflow rate to the star formation rate will provide them with a better timeline of the evolution of normal star-forming galaxies.

In turn, this should explain how galactic disks continue to grow over very long timescales that span billions of years.