Vast Rivers of Hydrogen Flow into Galaxies
Composite of NGC 6946 in optical (blue), neutral hydrogen as seen by an interferometer (orange), and the newly mapped hydrogen gas outside the galaxy in red.
The Atacama Large Millimeter/submillimeter Array (ALMA) is finally complete, after the project's final 12-meter antenna was handed over on Sept. 30, 2013. The 66th dish, shown here, is the last of 25 European-built instruments. The Joint ALMA Observatory (JAO) is a collaboration between the European Southern Observatory (ESO), the National Radio Astronomy Observatory (NRAO) and the National Astronomical Observatory of Japan (NAOJ).
All 66 millimetre/submillimetre-wave radio antennas are expected to be operational by the end of 2013, working together as one large telescope. ALMA will operate as an interferometer, spread over 16 kilometers of the Chajnantor Plateau in the Atacama Desert, Chile.
ALMA is sensitive to millimetre and submillimetre wavelengths, between infrared light and radio waves in the electromagnetic spectrum, a range that will help astronomers peel back the veil on distant objects in the Cosmos.
ALMA (ESO/NAOJ/NRAO)/Jaime Guarda
The giant antenna transporter, called Otto, delivers the final antenna to the array on Sept. 30, 2013.
The final dish was built by the European AEM Consortium, the largest of the project's contracts. North America delivered 25 12-meter antennas and East Asia delivered 16 (four 12-meter and twelve 7-meter).
"This is an important milestone for the ALMA Observatory since it enables astronomers in Europe and elsewhere to use the complete ALMA telescope, with its full sensitivity and collecting area," said Wolfgang Wild, the European ALMA Project Manager.
Clem & Adri Bacri-Normier (wingsforscience.com)/ESO
An artist's impression of the complete ALMA array in the Atacama Desert.
Sometimes it really does take the most sensitive telescopes in the world to find what was in front of us all along. That’s why the Green Bank Telescope was so valuable in the discovery of star-forming gas around the spiral galaxy NGC 6946 this week.
Galaxies continue to form stars from clouds of gas that collapse and become dense enough to start hydrogen fusion. In order to collapse in the first place, that gas has to be cold, or, more specifically, not having its atoms moving around too quickly and chaotically. Much of the gas in galaxies is too diffuse and fast-moving, or “warm,” to do so, thus galaxies only transform a small percentage of their gas into stars at any given time.
Our Galaxy and galaxies like it make new stars at about the rate of one solar mass per year. The reservoirs of cold gas that fall onto galaxies only account for 10 percent of that gas, at least until now. Where does all the star forming material come from?
D.J. Pisano from West Virginia University used the Green Bank Telescope to find the missing gas as rivers of hydrogen falling onto the spiral galaxy NGC 6946. A deep search revealed enough gas in these filaments to account for the star-formation happening in that galaxy.
This gas has not yet entered the galaxy to be stirred up, coming from the kind of intergalactic material that was only recently directly seen by the Keck telescope. There is still a chance that these streams were created in interactions with other galaxies, but follow-up looking for stars among the gas or for similar filaments around other galaxies would build the case for the new one-day-to-be-star-forming material.
The Green Bank Telescope, or GBT, was used for this search because of its size and sensitivity. Interferometers made of several dishes, such as the Very Large Array, miss the most diffuse gas as a function of their design which is geared towards mapping small features. The GBT is the largest fully steerable single dish radio telescope and has been built for extreme sensitivity. This is why it is located in the Radio Quiet Zone, an area of the United States where radio emissions are strictly controlled so as to not interfere with the sensitive observations.
Of course, this comes at an interesting time in the GBT’s short history, when the National Science Foundation may no longer have the funds to keep it running. The National Radio Astronomy Observatory is currently seeking other opportunities to keep the GBT, and thus the observatory in West Virginia, running well into the future so it can continue to be used for cutting edge science.
In any case, the 100-meter GBT is up and running and available for amazing science right now and will continue to be for some time as astronomers scramble to finish their proposals for time on the telescope this time of year. And it’s a good bet that astronomers like Pisano will continue to use it to its full potential and find more secrets hidden in plain sight.
This work is accepted for publication in Astronomical Journal; and a preprint is available at arXiv.org.
Full disclosure: The author spent a good bit of her grad school years working at the Green Bank site and has used the telescope for research.