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World’s Largest Radio Dish Spots a Galaxy Capturing Gas

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dark dust lanes spiral around the glowing central bulge of M106
This stunning view of Messier 106 is composed of Hubble Space Telescope and ground-based images taken by professional and amateur astronomers. [NASA, ESA, the Hubble Heritage Team (STScI/AURA) and R. Gendler (for the Hubble Heritage Team); Acknowledgment: J. GaBany]

How do galaxies form stars for billions of years without running out of gas? Theory suggests that galaxies siphon material from the tenuous intergalactic medium or nearby galaxies, but it’s challenging to detect the cold gas trails that would bolster this theory. A new article details observations that may explain how a Milky Way-like galaxy gets its gas.

Sustaining Star Formation

FAST radio telescope

The 500-m FAST radio dish, built into a natural basin in southwest China. [Xinhua/Ou Dongqu]

Across the universe, galaxies are making stars long after their star-forming days should have ended, and it’s not always clear why. When a galaxy in a dense cluster environment consumes another galaxy, the influx of gas can spur a new rush of star formation — but are collisions the only way for a galaxy to gain star-forming material?

Another possibility — and one that applies to more isolated galaxies as well as their cluster counterparts — is that galaxies can steal gas from small satellite galaxies, even from a great distance, which would create long trails of cold, low-density hydrogen gas. A team of astronomers led by Ming Zhu (National Astronomical Observatories, Chinese Academy of Sciences) searched for streams of neutral hydrogen gas using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world’s largest single-dish radio telescope.

hydrogen density map with density contours connecting M106 to NGC 4288. several other satellite galaxies are present, including NGC 4248 and S5

FAST hydrogen density map of the M106 environment with density contours (blue lines). The green lines are density contours from the Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey. [Adapted from Zhu et al. 2021]

A Northern Sky Survey

Zhu and collaborators scanned the northern sky on a hunt for the tell-tale gas trails that can signal a galaxy accreting material. In a new publication, the team reported on their discovery of a trail of hydrogen gas connected to spiral galaxy Messier 106. Messier 106, also known as M106, has a similar luminosity and structure to the Milky Way, but somewhat more massive satellite galaxies.

Twelve hours of data — the deepest radio observations of this system yet — revealed a filamentary trail of gas stretching at least 424,000 light-years between M106 and a likely satellite galaxy, NGC 4288. This gas filament could be a sought-after sign that M106 is accreting gas from a satellite over a great distance, but it could also be evidence of a tidal tail caused by a galaxy pulling gas away from M106 as it swings by. Which scenario is more likely?

Tidal Tail or Accretion Stream?

ultraviolet images show two lines of star clusters off the southern edge of one of M106's spiral arms

Left: Neutral hydrogen density contours overlaid on a near-ultraviolet image of M106. Right: Zoomed-in image of the area contained in the black rectangle with far-ultraviolet data in blue and near-ultraviolet data in yellow. Click to enlarge. [Zhu et al. 2021]

Zhu and collaborators point out a few key factors that support the accretion stream hypothesis: the gas filament doesn’t appear to contain any stars, it doesn’t sweep outward along a spiral arm, and it lacks the characteristic density gradient of a tidal tail. One of the strongest pieces of evidence that M106 is actively accreting gas is the presence of young star clusters where the filament attaches to the galaxy — evidence that star formation has already been sparked by infalling gas.

Follow-up observations will be needed to confirm the accretion-stream hypothesis, including verification of NGC 4288’s satellite galaxy status. One thing is certain: the team’s results highlight the importance of using deep radio observations to study interactions between galaxies, since optical images gave little to no sign that M106 and NGC 4288 were engaged in a gaseous tug-of-war.

Bonus

Check out this video from the authors’ article, which shows a rotating three-dimensional view of their FAST data cube. East and north indicate the right ascension and declination of the target, respectively, while the z direction shows the velocity of the gas.

Citation

“FAST Discovery of a Long HI Accretion Stream toward M106,” Ming Zhu et al 2021 ApJL 922 L21. doi:10.3847/2041-8213/ac350a