Hunting Black Holes at the South Pole

Each of the telescopes that the astronomers of the Event Horizon Telescope (EHT) are currently working to bring into their black-hole-observing, planet-size array is a special case. Mexico’s Large Millimeter Telescope, for example, is an enormous single dish on top of an exceptionally high mountain, not to mention the biggest science project of any kind in its country. The Atacama Large Millimeter Array (ALMA) is a billion-dollar class instrument, the world’s most powerful radio telescope.

The South Pole Telescope is a special case in several ways. First, the obvious: it’s at the South Pole. That makes it incredibly hard to get to even in good weather, and completely inaccessible during the austral winter. But there are less obvious distinguishing characteristics as well. For one, the South Pole Telescope was designed for the very specific task of studying the cosmic microwave background–something completely different than what the astronomers of the EHT want it to do.

Which is why last December, University of Arizona-Tucson astronomer and EHT collaborator Dan Marrone flew, along with several colleagues, to the South Pole. Their job was to install the gear the South Pole Telescope would need to join the EHT in observing black holes.

Normally, the South Pole Telescope’s 10-meter dish funnels extremely faint radiation from the cosmic microwave background into a camera called a bolometer. “That camera effectively just measures the heat from the sky in a given direction by sensing how much the accumulated light heats each detector,” Marrone explains. To do Very Long Baseline Interferometry (VLBI), the technique used by the Event Horizon Telescope, the SPT needs a different kind of camera–a single-pixel instrument that records the waveform of microwaves (specifically those with a frequency of 230 GHz) hitting the telescope. With that single, highly sensitive pixel, Marrone explains, “we can actually record a ‘movie’ of the electric field that the radiation from our targets is creating on the surface of the telescope.”

Eventually, during a full Event Horizon Telescope observing run, telescopes around the world will record such a “movie” on banks of 8-terabyte hard drives, then ship them back to MIT’s Haystack Observatory near Boston, where scientists will combine the data collected at all sites using a supercomputer called a correlator.

By the time Marrone left Tucson on December 1, his team had already shipped 13 crates of equipment to the Pole. When Marrone arrived on December 9, only two of those crates were waiting for him. “It turns out never to be easy to do anything at the South Pole,” Marrone says.

For the first couple of weeks, as cargo trickled in, Marrone and his colleagues did what work they could. Space was tight, and the building was under construction. “For the first month and a half we were there, every day there was someone soldering with an acetylene torch in our ear, filling the air with weird acrid smoke,” Marrone says. When last of the straggling cargo arrived in late December, “we could really fly, and so we did,” Marrone says. “They were very long days. We’d stagger out at 8 or 8:30 am, come back for lunch or dinner and work until midnight. Christmas, New Year’s, it didn’t matter.”

In mid January, Marrone and crew finished their installation and pointed the modified telescope at the sky. They obtained first light with the South Pole Telescope VLBI receiver early on January 16, local time, making images by scanning their single pixel across the sky and, in Marrone’s words, “making maps of the pixel value recorded for each sky position.”

The official first light image, signed by the South Pole installation crew, is a map of carbon monoxide near the center of the Milky Way.

Another image depicts the moon at 230 gigahertz. “Instead of seeing reflected light, you see the heat escaping from the moon’s surface,” Marrone says. “Notably the crescent is wider at 1mm than in the optical, because the parts of the moon that have just lost the sunlight are still cooling. You can also see real signatures of the dark and light patches that you’re used to seeing with your eye.”

These are just test images, but they prove that the equipment that will allow the South Pole Telescope to join the Event Horizon Telescope works. The next step will be to link the South Pole Telescope up with another, faraway telescope. The SPT and the Atacama Pathfinder Experiment (APEX) telescope in Chile observed together not long after the SPT recorded the images above, and the resulting data is currently being analyzed at Haystack Observatory. If “fringes” emerge, it will be a big step toward getting the full EHT array ready to take a picture of the black hole at the center of the Milky Way.