Could Ghost-Like Object Found by Chandra Be Another ‘Voorwerp’?

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The Chandra X-ray Observatory has found a cosmic “ghost” lurking around a distant supermassive black hole. Astronomers think this high-energy apparition is evidence of a huge eruption produced by the black hole. But this blue blob looks eerily similar to another cosmic blob of gas found by Galaxy Zoo member Hanny Van Arkel, the famous object called Hanny’s Voorwerp. Could the two objects be similar?

Astronomers say the “ghost” found by Chandra is the remains of a diffuse X-ray source, lingering after other radiation from the black hole’s outburst died away. The object, HDF 130 is over 10 billion light years away and existed at a time 3 billion years after the Big Bang, when galaxies and black holes were forming at a high rate.

Hanny’s Voorwerp has been a mystery ever since it was found in 2007 as part of the Galaxy Zoo project. Recent research on the object reveals that the Voorwerp is also likely to be a remnant from a black hole outburst. In the original Sloan Digital Sky Survey images of Hanny’s Voorwerp, the object showed up as blue, however further spectral analysis showed it is actually green. The Voorwerp was studied by the Swift gamma-ray satellite, which also can pick up ultraviolet and X-ray emissions, but the satellite didn’t come up with anything conclusive. However, the Westerbork Synthesis Radio Telescope (WSRT) took a look at Hanny’s Voorwerp and determined that indeed, black hole jets were allowing beams of intense optical and ultraviolet emissions from the black hole to heat and illuminate a small part of a large gas cloud that partially surrounds the nearby galaxy, IC 2497.

But Galaxy Zoo astronomers suspect X-rays might play a role in the Voorwerp, too. It was recently imaged by the Suzaku X-ray telescopes to see if is visible in that part of the spectrum, as well as to probe the current activity of the supermassive black hole. The results of that observation are still being analyzed. Yale astronomer Kevin Schawinski recently wrote in the Galaxy Zoo Blog that detecting hard X-ray photons would provide evidence of an active supermassive black hole in IC 2497, which would be illuminating the Voorwerp. “If on the other hand we don’t pick up anything, then we can be sure that the black hole has stopped feeding, i.e. it has genuinely shut down,” Schawinski wrote.

So are the two objects, the “ghost” of HDF 130 and Hanny’s Voorwerp similar? Yes – and no – said Chandra scientist Dr. Peter Edmonds.

“There are indeed some basic similarities between these two objects, in that both were generated by eruptions from a supermassive black hole, either in the form of bright radiation or jets, Edmonds told Universe Today.”Also, in both cases the eruption from the black hole seems to have died down.”

The details of the two objects, however, are very different, Edmonds said. “Hanny’s Voorwerp involves a light echo while the X-ray ghost was thought to form by an interaction between the comic background radiation and particles in a jet. They’re obviously seen at very different wavelengths. Also, the ghost is found in the early Universe at much greater distances than Hanny’s Voorwerp and is physically much larger.”

Additionally, the Chandra team suspects a very powerful and large eruption was responsible for the formation of the ghost, much more powerful than the one for Hanny’s Voorwerp.

Andy Fabian of the Cambridge University in the United Kingdom, lead author on the paper on the ghost of HDF 130, thinks the object’s X-ray glow is evidence of an outburst equivalent to about a billion supernovas, which blasted particles at almost the speed of light. When the eruption was ongoing, it produced prodigious amounts of radio and X-radiation, but after several million years, the radio signal faded from view as the electrons radiated away their energy.

This is the first X-ray ghost ever seen after the demise of radio-bright jets. Astronomers have observed extensive X-ray emission with a similar origin, but only from galaxies with radio emission on large scales, signifying continued eruptions. In HDF 130, only a point source is detected in radio images, coinciding with the massive elliptical galaxy seen in its optical image.

This radio source indicates that HDF 130’s supermassive black hole may be growing.

With Hanny’s Voorwerp, however, astronomers are still searching for any sign of activity from the black hole.

Another argument that the two objects are different is their shape. The linear shape of the HDF 130’s X-ray source is consistent with the shape of radio jets and not with that of a galaxy cluster, which is expected to be circular. The energy distribution of the X-rays is also consistent with the interpretation of an X-ray ghost.

Hanny’s Voorwerp has all the hallmarks of an interacting system. “The gas probably arises from a tidal interaction between IC 2497 and another galaxy, which occurred several hundred million years ago,” said Dr. Tom Oosterloo, part of the team that studied the Voorwerp with WSRT.

There are more differences between the two objects, primarily that ghosts like the one from HDF 130 may be prevalent in the universe, while the Voorwerp might just be a one-time occurance. “The stream of gas ends three hundred thousand light years westwards of IC2497, and all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident,” said Oosterloo.

Chandra astronomer Caitlin Casey, also of Cambridge said, “This result hints that the X-ray sky should be littered with such ghosts, especially if black hole eruptions are as common as we think they are in the early Universe.”

So now that astronomers know where and now to look for X-ray objects like the one by HDF 130, we’re likely to hear about more cosmic X-ray ghosts in the future. But Hanny’s Voorwerp appears to be unique.

Sources: Chandra, previous UT article, email exchange with Dr. Peter Edmonds, Galaxy Zoo