Hanny's Voorwerp / /, (Dutch for Hanny's object) is a very rare type of astronomical object called a quasar ionization echo. It was discovered in 2007 by Dutch school teacher Hanny van Arkel, while she was participating as an amateur volunteer in the Galaxy Zoo project. Photographically, it appears as a bright blob close to spiral galaxy IC 2497 in the constellation Leo Minor.
Hanny's Voorwerp (HsV) is about the size of the Milky Way galaxy and has a central hole over 16,000 light years across. The original Sloan Digital Sky Survey (SDSS) image that led to the discovery is to the left and shows HsV as blue. In the HST image on the right, HsV is colored green, a standard false color that is used to represent the presence of several luminous emission lines of glowing oxygen. HsV has been shown to be at the same distance from Earth as the adjacent galaxy IC 2497, which is about 650 million light-years away.
Star birth is occurring in the region of HsV that faces IC 2497. Radio observations indicate that this is due to an outflow of gas arising from the IC 2497's core which is interacting with a small region of HsV to collapse and form stars. The youngest stars are several million years old.
A picture of HsV is presented on the NASA Astronomy Picture of the Day website with data taken by Dan Smith (Liverpool John Moores University), Peter Herbert (University of Hertfordshire) and Chris Lintott (University of Oxford) on the 2.5m Isaac Newton Telescope. HsV is the subject of a video made by Dr Meghan Grey and Brady Haran on the University of Nottingham's Sixty Symbols YouTube Channel.
One hypothesis suggests that HsV consists of remnants of a small galaxy showing the impact of radiation from a bright quasar event that occurred in the center of IC 2497 about 100,000 years before how it is observed today. The quasar event is thought to have stimulated the bright emission that characterizes HsV. The quasar might have switched off in the last 200,000 years and is not visible in the available images. This might well be due to a process known as AGN feedback.
One possible explanation for the missing light-source is that illumination from the assumed quasar was a transient phenomenon. In this case, its effects on HsV would be still visible because of the distance of several tens of thousands of light years between HsV and the quasar in the nearby galaxy: HsV would show a "light echo" or "ghost image" of events that are older than those currently seen in the galaxy.
On 17 June 2010, a group of researchers at the European VLBI Network (EVN) and the UK’s Multi-Element Radio Linked Interferometer Network (MERLIN), proposed another related explanation. They hypothesized that the light comes from two sources: (1) a supermassive black hole at the center of IC 2497, and (2) light produced by an interaction of an energetic jet from the black hole and the gas surrounding IC 2497.
In February 2012, authors W.C. Keel, S. Drew Chojnowski, Vardha N. Bennert, K. Schawinski and 7 others published a paper in the MNRAS titled "The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black hole accretion events" . As a result of the interest in similar ionized clouds for the study of both the history and obscuration of Active Galactic Nucleus (AGN), participants in the Galaxy Zoo (GZ) project carried out a wide search for such clouds using data from the Sloan Digital Sky Survey (SDSS). This search yielded a list of 19 galaxies with AGN-photoionized clouds detected to beyond 10 kiloparsecs from the nuclei. These were nicknamed 'Voorwerpjes' from the Dutch for 'small objects'.
In August 2013, authors F. Schweizer, P. Seitzer, D. Kelson, E. Villanueva, and G. Walth published a paper in the Astrophysical Journal titled "The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint Ionization Echo". This reports the finding of a Voorwerpje on the outskirts of the well-studied NGC 7252. Quoting from the abstract: "We present images and spectra of a ~10 kiloparsec-sized emission-line nebulosity discovered in the prototypical merger remnant NGC 7252 and dubbed the '[OIII] nebula' because of its dominant [OIII]_5007 line."
HsV and the neighboring galaxy are the object of active astrophysical research. Observations of IC 2497 with the XMM-Newton and Suzaku X-ray space telescopes have been arranged that will probe the current activity of the supermassive black hole.
In June 2009, authors G.I.G. Józsa, M.A. Garrett, T.A. Oosterloo, H. Rampadarath, Z. Paragi, H. van Arkel, C.J. Lintott, W.C. Keel, K. Schawinski and E. Edmondson published a paper in Astronomy and Astrophysics titled "Revealing Hanny's Voorwerp: radio observations of IC 2497".
In November 2010, authors K. Schawinski, D.A. Evans, S. Virani, C.M. Urry, W.C. Keel, C.J. Lintott H. van Arkel and 9 others published a paper in the Astrophysical Journal Letters titled "The Sudden Death of the Nearest Quasar".
In August 2012, authors W.C. Keel, C.J. Lintott, K. Schawinski, Vardha N. Bennert, D.Thomas, A. Manning, S. Drew Chojnowski, H. van Arkel and S. Lynn published a paper in the Astrophysical Journal titled "The History and Environment of a Faded Quasar: Hubble Space Telescope Observations of Hanny's Voorwerp and IC 2497".
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