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Hanny’s Voorwerp: An Anomaly Explained?

It hasn’t been all that long since Hanny van Arkel, a Dutch school teacher, lent her name to the anomalous object since known as ‘Hanny’s Voorwerp.’ Working with data from the Galaxy Zoo project, van Arkel was scanning galaxy images when she ran across what seemed to be a green blob of extremely hot gas with a hole in its center. That hole turned out to be 16000 light years across, its cause unknown, and the object itself seemed to be lit by an unseen source. Theories abounded, including a ‘light echo’ from a defunct quasar in a nearby galaxy.

And then there was the fact that the remarkably hot object (15000 degrees Celsius or more) was not only enormous but also empty of stars. Baffling astronomers for the past year, Hanny’s Voorwerp may now be swimming into sharper focus. An international team has been observing both the object and the nearby galaxy IC2497 using the Westerbork Synthesis Radio Telescope, with results that indicate the presence of a jet coming from a massive black hole at the center of the galaxy. Mike Garrett (ASTRON/Leiden) comments:

“It looks as though the jet emanating from the black hole clears a path through the dense interstellar medium of IC2497 towards Hanny’s Voorwerp. This cleared channel permits
the beam of intense optical and ultraviolet emission associated with the black hole, to illuminate a small part of a large gas cloud that partially surrounds the galaxy. The optical and ultraviolet emission heats and ionises the gas cloud, thus creating the phenomena known as Hanny’s Voorwerp.”

Meanwhile, the new data indicate that the huge hydrogen cloud, extending hundreds of thousands of light years and boasting a total mass amounting to five billion times that of the Sun, may well be the result of tidal interactions between IC2497 and another galaxy hundreds of millions of years ago. Hanny’s Voorwerp just might be the signpost of a cosmic mishap as these galaxies passed in the night, although even that theory leaves us with what Garrett calls ‘…a few more secrets to reveal.’ More radio telescope observations are planned in the near future.

Image (click to enlarge): WSRT observations reveal a radio jet (white contours) emanating from the center of the nearby galaxy IC2497, headed straight in the direction of Hanny’s Voorwerp (green). The observations also reveal a huge reservoir of hydrogen gas (colored orange) that probably arose from a previous encounter between IC2497 and another galaxy. The presence of strong neutral hydrogen absorption (top right plot) argues that the central regions of IC2497 are highly obscured. Credit: Main image left and top right hand corner (ASTRON). Hanny’s Voorwerp (bottom right) Dan Herbert, Isaac Newton Telescope.

Comments on this entry are closed.

  • James M. Essig December 3, 2008, 17:42

    Hi Paul;

    My hope and belief is that Hanny’s Voorwerp is just one of a large number of simmilarly exotic if not more exotic classes of objects yet to be discovered.

    The discovery of the cloud of hydrogen gas with a mass as great as 5 billion solar masses associated with Hanny’s Voorwerp gives me hope that vast quantities of interstellar, and intergalactic gas might be available in high enough concentrations for stellar scale if not galactic scale engineering projects and space faring infrastructure.

    I am also artistically intrigued by Hanny’s Voorwerp as its green color reminds me of the green cloud like ghost called Slimer within the popular Ghost Busters movie series.



  • David December 4, 2008, 9:26

    The green color probably is from oxygen atoms. Oxygen is relatively abundant compared to other atoms and shows that color when the atoms areexcited by light. For that reason you will see a similar green color in photos of interstellar gas.
    This assumes that the green color was not put in to simply enhance the visibility of the feature.

  • ljk July 1, 2009, 15:02

    Galaxy Zoo : ‘Hanny’s Voorwerp’, a quasar light echo?

    Authors: Chris Lintott, Kevin Schawinski, William Keel, Hanny van Arkel, Nicola Bennert, Edward Edmondson, Daniel Thomas, Daniel Smith, Peter Herbert, Matt Jarvis, Shanil Virani, Dan Andreescu, Steven Bamford, Kate Land, Phil Murray, Robert Nichol, Jordan Raddick, Anze Slosar, Alex Szalay, Jan Vandenberg

    (Submitted on 29 Jun 2009)

    Abstract: We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny’s Voorwerp, is bright in the SDSS g band due to unusually strong OIII 4959-5007 emission lines.

    We present the results of the first targeted observations of the object in the optical, UV and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous AGN, the source of this ionization is not apparent. The emission-line properties, and lack of x-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy’s own circumnuclear gas, or, as we argue, the first detection of a quasar light echo.

    In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10^5 years. This object may thus represent the first direct probe of quasar history on these timescales.

    Comments: 13 pages, 10 figures, Accepted by MNRAS

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Cite as: arXiv:0906.5304v1 [astro-ph.CO]

    Submission history

    From: Chris Lintott [view email]

    [v1] Mon, 29 Jun 2009 15:49:46 GMT (851kb)


  • ljk September 17, 2009, 10:06

    Is an obscured AGN at the centre of the disk galaxy IC 2497 responsible for Hanny’s Voorwerp?

    Authors: H. Rampadarath, M.A. Garrett, T. Muxlow, G. I. G. Józsa, T. A. Oosterloo, Z. Paragi

    (Submitted on 16 Sep 2009)

    Abstract: We present the results of VLBI and MERLIN observations of the massive disk galaxy IC 2497. Optical observations of IC 2497 revealed the existence of a giant emission nebula “Hanny’s Voorwerp” in the proximity of the galaxy.

    Earlier short-track 18 cm observations with e-VLBI at 18 cm, detected a compact radio component (C1) at the centre of IC 2497. The brightness temperature of C1 was measured to be greater than 4E5 K. Deeper, long-track e-VLBI observations presented here, re-confirm the existence of C1 but also reveal the existence of a second compact component (C2) located about 230 milliarcseconds to the North-East of C1.

    The brightness temperature of C2 is measured to be greater than 1.4E5 K, suggesting that both components may be related to AGN activity (e.g. a radio core and jet hotspot). Lower resolution 18cm MERLIN observations show both components. C1 is shown to be compact with a slight elongation along the direction of Hanny’s Voorwerp, and C2 shows a lot of extended emission in an almost perpendicular direction to the direction of the Voorwerp.

    Our results continue to support the hypothesis that IC 2497 contains an Active Galactic Nucleus (AGN), and that a jet associated with this AGN clears a path that permits ionising radiation from the AGN to directly illuminate the emission nebula.

    Comments: Presented at The 8th International e-VLBI Workshop: the Science and Technology of Long Baseline Real-Time Interferometry, EXPReS09, June 22-26 2009 Madrid, Spain. 5 pages, 5 articles

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Cite as: arXiv:0909.3038v1 [astro-ph.CO]

    Submission history

    From: Hayden Rampadarath [view email]

    [v1] Wed, 16 Sep 2009 15:14:26 GMT (293kb,D)