What Hanny’s Voorwerp Reveals About Quasar Deaths

The green "blob" is Hanny's Voorwerp. Credit: Dan Herbert, Peter Smith, Matt Jarvis, Galaxy Zoo Team, Isaac Newton Telescope


Hanny’s Voorwerp is a popular topic of conversation due to its novel discovery by Hanny Van Arkel perusing images from the Galaxy Zoo project. The tale has become so well known, it was made into a comic book (view here as .pdf, 35MB). But another aspect of the story is how enigmatic the object is. Objects that are so green are rare and it lacked a direct power source to energize it. It was eventually realized a quasar in the neighboring galaxy, IC 2497 could supply the necessary energy. Yet images of the galaxy couldn’t confirm a sufficiently energetic quasar. A new paper discusses what may have happened to the source.

The evidence that a quasar must be involved comes from the green color of the voorwerp itself. Spectra of the object has shown that this coloration is due to a strong level of ionized oxygen, specifically the λ5007 line of O III. While other scenarios could account for this feature alone, the spectra also contained He II emission as well as Ne V and the lines were especially narrow. Should star formation or shockwaves energize the gas, the motions would cause Doppler broadening. An quasar powered Active Galactic Nucleus (AGN) was the best fit.

But when telescopes searched for this quasar in the galaxy, it proved elusive. Optical images from WIYN Observatory were unable to resolve the expected point source. Radio observations discovered an object emitting in this range, but far below the amount of energy necessary to power the luminous Voorwerp. Two solutions have been proposed:

“1) the quasar in IC 2497 features a novel geometry of obscuring material and is obscured at an unprecedented level only along our line of sight, while being virtually unobscured towards the Voorwerp; or 2) the quasar in IC 2497 has shut down within the last 70,000 years, while the Voorwerp remains lit up due to the light travel time from the nucleus.”

Recent observations from Suzaku have ruled out the first of these possibilities due to the lack of potassium absorption that would be expected if light from the galaxy were being absorbed in a significant amount. Thus, the conclusion is that the AGN has dropped in total output by at least two orders of magnitude, but more likely by four. In many ways, this is not entirely unexpected since quasars are plentiful in the distant universe where raw material on which to feed was more plentiful. In the present universe, quasars rarely have such material available and can’t maintain it indefinitely.

Analogs exist within our own galaxy. X-Ray Binaries (XRBs) are stellar mass black holes which form similar accretion disks and can shut down and excite on short timescales (~1 year). The authors of the new paper attempted to scale up a model XRB system to determine if the timescales would fit with the ~70,000 year upper limit imposed by the travel time. While they found a good agreement with the output from direct accretion itself (10,000–100,000 years) the team found a discrepancy in the disk. In XRBs, the material around the black hole is heated as well, and takes some time to cool down. In this case, the core of the galaxy should still retain a hot disc of material which isn’t present.

This oddity demonstrates that there is still a large amount of knowledge to be gained on the physics surrounding these objects. Fortunately, the relatively close proximity of IC 2497 allows for the potential for detailed followup studies.

Radio Observations Provide New Explanation for Hanny’s Voorwerp

The green "blob" is Hanny's Voorwerp. Credit: Dan Herbert, Peter Smith, Matt Jarvis, Galaxy Zoo Team, Isaac Newton Telescope


Is Hanny’s Voorwerp the result of a “light echo” of a violent event that happened long ago or perhaps is this mystifying blob of glowing gas being fueled by an ongoing, and current phenomenon? A just-released paper about the Voorwerp offers a new explanation for this perplexing, seemingly one-of-a-kind object in the constellation of Leo Minor. If you haven’t heard the remarkable story, the object was discovered in 2007 by Dutch school teacher Hanny Van Arkel while she was classifying galaxies for the Galaxy Zoo online citizen science project. Until now, the working hypothesis for the explanation of this unusual object was that we might be seeing the “light echo” of a quasar outburst event that occurred millions of years ago. But new radio observations reveal that instead, a black hole in that same nearby galaxy might be producing a radio jet, shooting a thin beam directly at this cloud of gas, causing it to light up.

Hanny’s Voorwerp (Dutch for object) consists of dust and gas – but no stars – so astronomers know it is not a galaxy, even though it is galaxy-sized. Previously, astronomers studying the object thought the gas and dust were illuminated by a quasar outburst within the nearby galaxy IC 2497. While the outburst would have faded within the last 100,000 years, the light only reached the dust and gas in time for our telescopes to see the effect. But this explanation was slightly unsatisfactory in that such an event, where an entire galaxy would flare up suddenly and briefly, is unexplained.

The naturally weighted 18 cm MERLIN radio map of IC 2497 (black contours), showing both C1 & C2, embedded within a region of smooth extended emission, overlaid over the same map with the point sources subtracted. Credit: Rampadarath, et al.

But radio observations with the European Very Long Baseline Interferometry (VLBI) Network at 18 cm, and the Multi-Element Radio Linked Interferometer Network (MERLIN) at 18 cm and 6 cm show evidence of black hole, or active galactic nuclei (AGN) activity and a nuclear starburst in the central regions of IC 2497.

This event is hard to see from our vantage point on Earth because another cloud of dust and gas sits between us on Earth and IC 2497, preventing us from directly seeing the black hole.

“The new data shows that the nucleus continues to produce a radio jet, in about the direction of Hanny’s Voorwerp,” said Bill Keel from the University of Alabama, one of the astronomers who has been studying the object intently ever since its discovery, and was part of the new observations. “The core is still too weak in the radio to be able to conclude that it puts off enough UV and X-rays to light up the gas, however. There may well be interaction between outflowing material connected with the jet and the gas outside the galaxy, helping to shape the Voorwerp, but the spectra in the discovery paper already made it clear that the gas is ionized not by shocks from such an interaction, but by radiation. ”

Keel said, though, there is still remaining uncertainty — and different astronomers have varying estimates of this likelihood – of whether the radiation from the quasar core remains strong or whether it shoots in fits and starts.

“Some active galaxies put out a lot of energy in jets and outflows compared to radiation, and we are considering the possibility that this one has switched to such a “radio mode” in the recent past,” he said. “If so, the Voorwerp would be an ionization echo, or light echo, since the re-radiation from ionized gas is not instantaneous, as scattering is.”

The Voorwerp has captured enough attention and curiosity that astronomers have trained numerous telescopes on the object in an effort to sort out the mystery. But Keel said this approach is essential in eventually figuring this out.

“Each wavelength range gives us a different, and usually complementary, piece of the story,” he said. “The earlier radio data tell us something about where all that gas came from, and we got another connection from recent data putting an apparent companion spiral galaxy at the same distance as IC 2497. Even the early X-ray data showed us that there was an interesting puzzle as to why we didn’t see the core AGN. The GALEX UV spectrum is informing our interpretation of the Hubble UV image.”

Yes, Hubble recently looked at the Voorwerp in a couple of different wavelengths, (read our article about the Hubble observations here) and while Keel couldn’t comment directly about data from the iconic telescope, (everything is still being analyzed) he did say it holds some interesting surprises.

“One of the first things we started checking with Hubble data was whether we have a clear view in at least the infrared to the nucleus, starting from the location of the radio source,” he said. “Also, these results give us particular reason to look at the structural details of the gas in Hanny’s Voorwerp, for signs that it may be affected by an outflow from the nucleus. I can mention that there are some interesting surprises from the HST data, which is what we always hope for!”

Keel said he also has been observing at Kitt Peak, looking at other candidate “voorwerpjes” – similar “ionized clouds on a somewhat smaller scale around AGN, where the same lifetime-versus-obscuration issues apply but we can usually see the AGN responsible,” he said.

And look for some upcoming public outreach projects on the Voorwerp based on the Hubble data, as well, including one in Bloomington, Minnesota on July 1-4 at the CONvergence, where writers and scienctists will be writing a graphic novel based on the discovery of Hanny’s Voorwerp. Check out this website for more information.

Read the team’s paper: Hanny’s Voorwerp: Evidence Of AGN Activity And A Nuclear Starburst In The Central Regions Of IC 2497.

Astronomers Begin Observing Hanny’s Voorwerp with the Hubble Space Telescope

The green "blob" is Hanny's Voorwerp. Credit: Dan Herbert, Peter Smith, Matt Jarvis, Galaxy Zoo Team, Isaac Newton Telescope


The green “blob” is Hanny’s Voorwerp. Credit: Dan Herbert, Peter Smith, Matt Jarvis, Galaxy Zoo Team, Isaac Newton Telescope

A storybook astronomy mystery is now part of the most famous telescope in history. A team of astronomers secured time on the Hubble Space Telescope to observe Hanny’s Voorwerp, the unusual object found by Dutch teacher Hanny Van Arkel while she was scanning through images for the Galaxy Zoo project. Hubble will be trained on the Voorwerp during three separate observing sessions, the first of which occurred on April 4, 2010. “The WFC3 (Wide Field Camera 3) images were obtained (Sunday),” said Principal Investigator Bill Keel from the University of Alabama in an email to Universe Today “and I was able to pull the calibrated files over last night for a quick look. Combining pairs of offset images to reject cosmic rays optimally will take some further work, but we’re happy to start working with the data and see what emerges at each step.”

The Voorwerp (also known by the much less endearing name of SDSS J094103.80+344334.2) created a sensation among amateur, armchair and professional astronomers alike, almost immediately after Van Arkel saw the object in 2007 and posted a question on the Galaxy Zoo forum, asking “What is this?” All this took place just a month after the Galaxy Zoo project opened up their online citizen science shop, and the rest is history. But in case you haven’t heard the story yet, a quick rundown is that ‘voorwerp’ means ‘object’ in Dutch – and as of yet, no one has determined exactly what Hanny’s Voorwerp is.

The working hypothesis, according to the Galaxy Zoo team, is that Hanny’s Voorwerp might be a “light echo” of an event that occurred millions of years ago. The object itself consists of dust and gas which perhaps was illuminated by a quasar outburst within the nearby galaxy IC 2497 (see the images). The outburst has faded within the last 100,000 years but the light reached the dust and gas in time for our telescopes to see the effect.

Hanny's Voorwerp. Credit: Matt Jarvis, William Herschel Telescope.

The Galaxy Zoo images come from observations done by the Sloan Digital Sky Survey. In evidence of the interest in this object, since 2007 Hanny’s Voorwerp has also been imaged by the Swift gamma-ray satellite, the Suzaku X-ray telescope, the Westerbork Synthesis Radio Telescope (WSRT), the Issac Newton Telescope and the William Herschel Telescope, to name a few.

But now, the most famous telescope of all – with its new and updated instruments – will take a gander to see if the mysteries of the Voorwerp can be solved.

The team – which includes Keel, and fellow Galaxy “Zookeepers” Chris Lintott, Kevin Schawinski, Vardha Nicola Bennert, Daniel Thomas, and Hanny Van Arkel herself – submitted a proposal to the Space Telescope Science Institute back in 2008 and were among the proud and few from close to 1000 proposals submitted to be granted observing time on Hubble.

During the three observing sessions, three different Hubble instruments will be used.

“The observations use three instruments and would naturally be broken into three target visits,” said Keel, “some constrained to be at different times because of the required orientations on the sky –for example, to have both Hanny’s Voorwerp and IC 2497 in the narrow field of view of ACS (Advanced Camera for Surveys) with the monochromatic ramp filters.”

“The next observations will probably be the most visually striking,” Keel continued. “Two orbits’ worth of ACS images in narrow bands including [O III] an H-alpha emission, and are scheduled for April 12. The final visit in the program has 2 orbits of STIS (Space Telescope Imaging Spectrograph) spectroscopy around the nucleus of IC2497, and should be coming up by mid-June.”

The April 4 observations included three orbits of data from the WFC3.

So, even though the first images have now been seen, the team won’t be able to share their findings until all the observations have occurred and the data has been analyzed.

Hanny Van Arkel. Image courtesy of Hanny.

“I indeed can’t say much more than that we got the first data in our mailboxes,” Van Arkel said in an email to Universe Today. “The team is still working on it and until they’ve worked it out, I won’t even understand enough of it myself to explain anything on the matter. It is exciting however that the investigations have started and it’s nice to see how many curious people are sending me messages about it and ‘retweeting’ my quotes on Twitter. After almost two years, I’m very much looking forward to the outcome of all of this!”

Van Arkel isn’t the only one excited.

“Through a combination of geometry and weather,” Keel shared,”I saw HST sail by to our south less than two orbits after it finished this first data set. So I waved in what was probably a most unprofessional manner.”

And the rest of us will be waiting – and waving – until Hubble can tell us more about Hanny’s Voorwerp.

For more information:

Hanny Van Arkel’s website

A post on the Galaxy Zoo blog by Bill Keel explaining in greater detail the Hubble observing sessions.

Galaxy Zoo

For other citizen science projects, visit the Zooniverse