The story so far…

This started with Bill’s timeline compiled as of 4 May 2010, updated with G. Josza’s radio events in June. Various threads in the investigation are tagged with color codes: optical observations, radio observations, X-ray and ultraviolet observations, archive analysis.

1908 – Stephane Javelle publishes the third list of nebulae he discovered with the 75-cm refracting telescope of the Observatoire de Nice, with positions measured using a micrometer at the telescope focus. Number 1118 in his list is incorporated by J.L.E. Dreyer into his Second Index Catalog of Nebulae and Clusters of Stars, also published in 1908, under the designation IC 2497. Its abbreviated description is listed as vF,vS,R,r, which means that its appearance at the eyepiece was very faint, very small, round, resolvable (i.e. mottled). For decades, it seems merely one of thousands of similarly small, faint galaxy images dotting the sky. (Scans of Dreyer’s publications are available from the NGC/IC project).

25 July 2007 Galaxy Zoo opens

5 August 2007 Brian May mentions Galaxy Zoo on his web site. Hanny van Arkel signs up for the Zoo.

13 August 2007 Hanny asks on the forum, “What’s the blue stuff below”??

Same day – Redisplay SDSS images for more insight (Edd). Archive analysis.

23 August 2007 object identified in FIRST survey as radio source (Kevin). Archive analysis.

4 December 2007 – Bill suggests (wrongly, as it happened) that the strong emission might be Lyman alpha at high redshift

6 December 2007 – reanalysis of SDSS data to get better fluxes/colors not compromised by its non-galaxy-like structure (Bill). Archive analysis.

12 December 2007 Zooite Gumbosea uses RGB values in SDSS JPEGS (!) to find potentially similar candidate objects, including Mkn 266, the brightest of the “voorwerpjes” later sampled as smaller analogs. Archive analysis.

(sometime around here) Chris checks colors, finds nothing else in SDSS comparable (although the conclusion is redshift-dependent). Archive analysis.

31 December 2007 First followup data: SARA images (color version) confirm the nature of spectrum, with strong emission lines, narrow its wavelength (Bill). Why? The telescope is easy and routine to access, and a different filter set could show where the strong emission peak is by noting which filters of different kinds include and exclude it. We learned: The object is certainly real, and has a strong spectral peak at a wavelength between 500-550 nanometers. Several Zookeepers start looking at telescope schedules to see when people they know might use appropriate equipment to get an informative spectrum of the Voorwerp. Optical observations.

Early January 2008: Chris, Bill first meet in person at Austin meeting of American Astronomical Society. At very end of meeting (Jan 10), they get an emailed image of  a spectrum obtained with the 4.2m William Herschel Telescope on La Palma by Matt Jarvis and colleagues, who had been contacted by Kevin. This shows clearly a matching redshift that associates it with the adjacent galaxy IC 2497. The spectral lines from gaseous species are produced only by hard ultraviolet and X-ray illumination. These data also show that there is not enough excitement at the core of IC 2497 for it to be responsible for what we see in the Voorwerp. (Blog entry at the time). Why: An optical spectrum was the single most desirable measurement to make progress here – it would tell the object’s distance, ad reveal the physical conditions making it shine. We learned: indeed, Hanny’s Voorwerp is at the same distance as IC 2497. It contains gas so highly ionized that it must be illuminated by the powerful UV and X-ray radiation of a quasar, but there is no quasar in evidence in IC 2497. Optical observations.

18 January 2008 – Bill mentions a possible HST proposal on the forum

28 January 2008 – Jarvis and company get images at the 2.5m Isaac Newton Telescope on La Palma in gri and a filter which happens to get He II, a unique opportunity. Why: changes in the ionization level across the coud could tell us where and how far form it the energy source lies. We learned: the high-ionization gas is all over the enormous cloud, and the energy source is thus located well outside it. Optical observations.

31 January 2008 – VLA radio proposal submitted (not approved) – Kevin led

31 January 2008 (or close) – A Swift target-of-opportunity proposal is submitted for ultraviolet images and X-ray measurements (Bill). It is approved by February 7. Bill knew of this program only from sitting on a NASA mission review panel, a rare payoff.

9 February 2008 – Swift observations are done. Why: even if a quasar so close were hidden behind vast amounts of dust and gas, X-rays should still escape that we could detect. We learned: there is no X-ray detection at a limit that indicates that any still active quasar in IC 2497 that is powerful enough to drive the ionization in the Voorwerp must be strongly obscured by foreground gas to an unprecedented degree. The UV images tell us about possible scattering of quasar spectrum from dust grains in the Voorwerp (with a contribution from gas emisson as well). X-ray/UV observations.

9 February 2008 – at almost the same time, Vardha (Nicola) Bennert gets a higher-dispersion spectrum of IC 2497 and the Voorwerp with the 3m Shane telescope at Lick Observatory in California. Why: the Lick spectrograph was set up  for finer resolution in wavelength, so we could distinguish close pairs of spectral lines that would tell us (among other things) the density of the gas (and the power of the needed quasar light, which would be related to the density of the gas in the Voorwerp). We learned: the required power of the quasar, and got our best spectrum of the core of IC 2497 and how weak the observed activity around its nucleus is. Vardha becomes a valuable team member, having done her dissertation work on directly relevant aspects of the spectra of gas near AGN cores. Optical observations.

16 February 2008 – The GALEX UV satellite telescope starts observations of Hanny’s Voorwerp, set up by Galaxy Zoo and GALEX team member (and one-time Hungarian rock star) Alex Szalay. Why: there are strong spectral lines from some elements in the UV which don’t have signatures in visible light, which would help us further narrow down conditions in the gas of the Voorwerp. Also, any reflected quasar light would be strongest in the UV. We learned: when analyzed a year later, these data show characteristic spectral features of the ionized gas, and strengthen understanding of how much ultraviolet light might be from embedded stars or reflected light from the unseen quasar (in turn giving more confidence that we know what the Hubble UV data show). X-ray/UV observations.

7 March 2008 – submitted Hubble proposal by Keel and company.

20 March 2008 - team goes public with the light-echo hypothesis (Chris, Bill) on the GZ blog

20 March 2008 – Chandra proposal submitted (led by Bill). Not approved (which may be OK; from what we learned later, Chandra might not have seen anything)

26 March 20082.1m telescope at Kitt Peak used to get Hα image of Voorwerp in narrow filter. Bill does this while instructing a graduate student from the University of Alabama Huntsville campus (whose background was in X-ray work) in ground-based astronomy. Thank you, Mihoko Yukita! Why: A filter was available that could isolate the radiation from ionized hydrogen, giving us a good comparison (all across the object) with the oxygen and helium data already taken. What we learned: Shows Hα blob in galaxy disk (possibly a bunch of star-forming regions, which would be misdirection to the Voorwerp question). Goes into later paper to map ratio of lines and thus ionization level across Voorwerp, showingh changes that might reflect the fading behavior of the former quasar. (Of course, Hubble data do it better much later).Optical observations.

April 2008 - Bill gives talk to staff at Kitt Peak National Observatory looking for alternative suggestions. Discussion of looking for reflected light form embedded dust come up. Former grad-school classmate who’s delighted in telling Bill he doesn’t have things quite right for the last 20 years has no other hypothesis to propose.

14 April 2008 – WHT proposal to map velocity field in Voorwerp using OASIS instrument approved for queue scheduling (led by Chris). First data obtained 2 June 2008.

May 2008 Michael Garrett (“Mike”), director of the Netherlands Foundation for Radio Astronomy is listening to the Jodcast driving to a very boring meeting in Utrecht. Stuart Lowe is interviewing Chris Lintott about galaxy Zoo and the discovery of the Voorwerp. Garrett’s interest is roused especially as the discover is a Dutch School teacher, Hanny van Arkel. The meeting is so boring, he forgets all about the Voorwerp until a few months later when he comes across it on the web again. He takes a look at the VLA FIRST survey and notices that a nearby galaxy is detected in the radio, confirmed also by Bill Keel’s pages who also suggests some faint emission at the location of the Voorwerp itself. Radio Observations

30 May 2008 Proposal team notified that the Hubble proposal was successful. This is also, coincidentally, Hanny’s 25th birthday.

19 June 2008 Alex Szalay speaks at Pittsburgh meeting on digital libraries. He mentions, in passing, the discovery of Hanny’s Voorwerp as an example of discoveries enabled by access to massive digital resources. Janet Raloff picks up the story and discusses online for Science News.

25 June 2008 Hanny’s Voorwerp featured as Astronomy Picture of the Day.

30 July 2008 – The story is all over the web. We get slashdotted. For seven minutes, Hanny’s Voorwerp is the top story on the US edition of cnn.com. Bill saves a screenshot.

26 August 2008 – Michael Garrett, director of the Netherlands Foundation for Radio Astronomy, thinks about getting some continuum and HI spectral line data using the Westerbork Radio Synthesis Telescope (WSRT). Since he is pretty busy running the institute, as he has no clue how to analyse data any longer, especially spectral line data, he asks a bright young researchers, Gyula Józsa (“Josh”) if he is interested to help – luckily he says yes! Together with other members of the team, including Tom Oosterloo another HI expert, they write a WSRT observing proposal to image Hanny’s Voorwerp in both the radio continuum and in neutral hydrogen. They are lucky, the project gets time! Michael, asks Hanny to catch the train up to Westerbork, and puts together a proposal to use the Westerbork radio array to examine IC 2497 and the Voorwerp. Why: Even if behind an enormous screen of dust and gas, such powerful nuclear activity should be seen at radio wavelengths. Further, some activity in galactic nuclei shows up only in the radio whether it’s obscured or not in visible light. What we learned: These data uncover the weak radio source at the galaxy core and the 5-billion-solar-mass arc of neutral hydrogen gas of which the Voorwerp is the ionized section. Radio observations.

17 September 2008 Around the same time, Mike proposes a short Very Long Baseline Interferometry (VLBI) radio observation with the EVN (European VLBI Network) to look for radio signatures of an AGN at the highest possible resolution. The time is granted. Radio observations.

28 September 2008 – 12 October 2008 The first radio observations with the WSRT are being carried out, using a quite complex multi-frequency mode to get the most out of the instrument. Because of their low priority, the observations are conducted incoherently and in chunks. Some observations fail. Radio observations

30 September 2008 The EVN radio observations of the centre of IC 2497 are being carried out. Why: This continent-spanning network of radio telescopes can show the structure of radio-emitting objects in very fine detail, beyond what even Hubble can deliver in visible light, and often revealing important structures and een motions. We learned: a very small source is detected, possibly even unresolved with a resolution of 40 milliarcseconds. It is so luminous in the radio regime that there is quite little doubt that it indicates the presence of an AGN, either a jet hitting the interstellar matter or the core of an AGN itself. So there seems to be an AGN at the centre of IC 2497. The question is if it might still be very luminous in other wavelength regimes. The observations do not answer that question. Radio observations

7 November 2009 After a laborious data reduction, stitching together the single pieces, the results of the radio observations with the WSRT turn out to be overwhelming. We learned: in the 21cm continuum maps, there is an weak, elongated component visible, pointing towards- and reaching the Voorwerp: a possible (but very weak) large-scale radio jet, a signature of an AGN. In the HI (neutral hydrogen line) maps, the results are even more fascinating. There seems to be a huge gas reservoir around IC 2497. Hanny’s Voorwerp is literally embedded in a huge cloud of gas. But this is still not all. One can see HI in absorption, a shadow of the neutral gas against the central radio source. This is an indication that there is a lot of interstellar material (gas and dust) in front of the central radio continuum source. Is this really enough to hide an AGN? Especially the neutral hydrogen detections are quite noisy an not of the best quality. Hence, Mike, Josh and Tom Oosterloo submit an urgent proposal to confirm the results. The proposal gets accepted. Radio observations

13 November 2010 Second, “urgent” observing run with the WSRT. This time a simpler setup is used to observe the 21cm continuum emission and the HI (neutral hydrogen) line in IC 2497. Josh, Mike, and Tom combine the observations with the former WSRT observations. Why: the previous Westerbork data had been taken in less-than-optimal ways, sandwiched among other observations, and showed hints of interesting features in need of confirmation. We learned: now it was clear that IC 2497 is surrounded by a huge reservoir of gas, detected in neutral hydrogen, with a mass of 8.500.000.000 suns. That gas is probably the debris of an encounter of IC 2497 with another galaxy or even a whole group of galaxies. Hanny’s Voorwerp sits right in the middle of the HI cloud, and it has the same velocity. This means that it is now clear where the gas of Hanny’s Voorwerp comes from. Hanny’s Voorwerp is not a single, lonely ionisation nebula, but it is part of a huge cloud complex, which happens to become ionised at the position of The Voorwerp. The presence of the extended radio continuum structure, probably a weak radio jet is confirmed. Also the HI line absorption is confirmed. Possibly, there is enough gas and dust in front of a central, still active AGN to hide it from us at other wavelengths, the optical and the UV. Radio observations

18 November 2008 – Hanny starts to chronicle her reactions to her discovery an its implications at www.hannysvoorwerp.com.

20 November 2008 – Bill and UA grad student Anna Manning take time from a GZ overlapping-galaxies observing session to use the 3.5m WIYN telescope for the best pre-Hubble images. Why: the telescope’s rapid-guiding camera yielded extraordinarily sharp images, likely to reveal yet-unknown details.  What we learned: there is complex dust near the galaxy core, not far from where the radio sources in the galaxy lie, and further hints on detailed structures in the Voorwerp itself. This gave the team food for thought in what to be prepared for in the Hubble data. Optical observations.

25 November 2008 The ASTRON radio group around Mike brings out a press release summarising the results of the radio observations so far. Radio Observations

27 November 2008 Hanny herself travels to ASTRON and presents the Galaxy Zoo project and the Voorwerp to all of the staff at ASTRON – she also does a press interview for a local school newspaper. Hanny tells us everything about Galaxy Zoo but also how she made the discovery and the fame it has brought her over the last year. Josh also presents the new radio observations to Hanny and to the ASTRON staff – its a great day, enjoyed by one and all! Radio observations

5 December 2008 - proposal for hard X-ray observations using Japanese Suzaku satellite observatory submitted (Kevin et al.)

2-3 February 2009 On request of Mike, the centre of IC 2497 gets observed in spare time with the UK Multi-Element Radio Linked Interferometer Network (MERLIN) at a wavelength of 18cm. Radio Observations

1 February 2009 Hayden submits another radio proposal to observe the centre of IC 2497 with global VLBI at the highest possible resolution. The proposals is refused for global VLBI, but accepted for an observation with the European VLBI Network (EVN). Radio Observations

21-23 March 2009 The centre of IC 2497 gets observed in spare time with the UK Multi-Element Radio Linked Interferometer Network (MERLIN) at a wavelength of 6cm. The radio images look more complex than expected. It becomes clear that a careful data reduction is necessary. Radio observations

27 April 2009 – Suzaku X-ray data obtained (Kevin et al.). Why: Suzaku is currently the most sensitive satellite for detecting the highest-energy (“hard”) X-rays. If the quasar in IC 2497 is still “on” but hidden by intervening material, these  hard X-rays would be the most likely to escape so we can see them. X-ray/UV observations.

12 May 2009 – A letter to the editor about the WSRT radio observations and the first EVN observations of Hanny’s Voorwerp and IC 2497 compiled by Josh gets accepted by Astronomy & Astrophysics, a leading European professional journal. The letter gets published in June 2009, Astronomy & Astrophysics decide to use an illustration of the results, using optical and radio data to be put on the front cover of the magazine. Studies of Hanny’s Voorwerp are published for the first time in a professional journal, and it gets on the front page!!!! Radio Observations Meanwhile in the USA… The space shuttle Atlantis lifts off on mission STS-125, for the final service call on the Hubble Space Telescope. An electronic failure on the telescope delayed the mission 6 months to be able to repair it. Bill thought it important to watch in person, adding moral support. Only after performance  verification of the new and repaired instruments in orbit could the next year’s observations (including the Voorwerp project) be scheduled and carried out.

29 June 2009 – Voorwerp discovery paper by Lintott et al. accepted by MNRAS. This took 51 weeks! Our analysis was tightened up a good bit and made more internally consistent, but none of the major conclusions changed from submission.

1 July 2009 Tom, Josh, and Mike publish an article about Hanny’s Voorwerp and the WSRT and EVN radio observations in the Dutch amateur magazine “Zenit”. After that, it becomes quite quiet around the radio group for a long time. But they are chewing over the MERLIN data and the new EVN data. Radio Observations

23 October 2009 – find out that the winner of a competition in UK schools for use of 1 hour of Gemini-North 8m time was to observe the Voorwerp. The staff asks us for advice as to the best use of the time (we go for integral-field spectroscopy to complement the HST data which we know are coming).

4 April 2010 – Hubble images using the new Wide-Field Camera 3, WFC3) look at IC 2497 in three bands from ultraviolet to near-infrared. Why: These are filters selected not to show the Voorwerp strongly, so we could look for star clusters in it that would mean its gas was once part of a dwarf galaxy (or, if they are young, that stars have been formed there recently). We also hope these will show us helpful detail near the core of IC 2497. Optical observations. UV/X-ray observations.

12 April 2010 – ACS HST images, in narrow bands at redshifted [O III] and Hα. Processing starts right away to remove instrumental artifacts and cosmic-ray effects. Optical observations.

12 April 2010 (same day!) - Hayden Rampadarath leads submission of second radio paper on IC 2497 and Hanny’s Voorwerp, using a VLBI network to resolve two radio source near the core (possibly an AGN and part of a jet pointing roughly toward the Voorwerp). Kevin and Bill start correcting raw coordinates to see where these two sources fall in the Hubble structure.

19 April 2010 – observations in soft X-rays by ESA’s XMM-Newton satellite observatory obtained (Kevin led). Includes simultaneous UV images by the observatory’s Optical Monitor telescope. X-ray/UV observations.

22 April-6 May 2010 – spectroscopic mapping of the Voorwerp is carried out using the Gemini Multi-Object Spectrometer on the 8-meter Gemini North telescope on Mauna Kea (Hawai’i). This originated in a public-outreach contest in UK high schools, where an hour of Gemini time was awarded for the best entry suggesting what to look at and why. Why: this instrument provides point-by-point mapping of spectra, so we get data on the Doppler shifts and ionization of the gas all over it (instead of just at a few points or along a line). This should be especially valuable when coupled with the Hubble images, seeking subtle changes in the conditions in the gas coupled with fine structures seen by Hubble. Optical observations.

23 May 2010 – Hubble spectra of the core of IC 2497 using the Space Telescope Imaging Spectrograph. Why: Hubble’s resolution allows us to separate the core of the galaxy from the surrounding cloud of stars much more precisely than from the ground, and lets us look for any clouds of gas that are exposed to strong radiation that might escape a heavily obscured nucleus. If we are fortunate in how the stars are distributed and where dust lies, we might be able to estimate the mass of the galaxy’s central black hole. Optical observations.

17 June 2010 Hayden leads the submission of the second radio paper on IC 2497 and Hanny’s Voorwerp, presenting the new EVN and MERLIN data sets. The new EVN maps show now two point sources, instead of one, due to the higher sensitivity. Together with the MERLIN observations, the data indicate that we are witnessing an AGN and part of a jet pointing roughly toward the Voorwerp, now on small scales. But most of the radio emission comes from a diffuse structure, indicating that IC 2497 also undergoes a central starburst with a massive star formation rate, producing 70 solar masses in stars per year. With that central star formation rate, IC 2497 becomes a typical LIRG, a luminous infrared galaxy. This type of galaxy is very rare in the local universe. It is also known that in LIRGs the central region is often highly obscured and possibly the birth place of AGN.  The paper gets accepted for publication as a letter in Astronomy & Astrophysics. Radio observations

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