More on the Voorwerp

Kevin asked whether I could provide an entry on our efforts to figure out just what Hanny’s Voorwerp might be. This is definitely a guest blog – I am not a ZooKeeper, but they have been gracious enough to let me feed some of the less delicate animals on occasion.

As a reminder, Hanny posted this object on galaxyzooforum.org back on August 13 (I can’t believe it was that long ago now, but apparently the topic scrolled way down in the forum until early December). It showed up on the SDSS color rendition as a deep blue, irregular cloud, just south of the spiral galaxy IC 2497. Pulling out the brightness measurements from the Sloan data in all five filters gave a very unusual result. This thing looked so blue in the color images (made from the gri images) because it puts out almost ten times as much light coming into the g filter as any of the others, and isn’t even detected in the very-far-red z band. That suggests that there is a very strong emission line somewhere in the wavelength range of that filter, about 4200-5500 Angstroms. The SDSS images do show a small object at the north tip of the blob with a more continuous distribution of light; the location is suspicious, but we don’t have direct evidence yet whether it belongs to the Voorwerp. The blob does show structure in the g image, like shells or loops.

Archive searches turned up a radio source in IC 2497, and nothing else helpful (the object just appears on the old Palomar Sky Survey blue-light photographs). A single emission line in that wavelength range could be almost anything, although it was a bit odd that no other emission line was bright enough to produce much light in the other filters. It might be some kind of small nebula in our own galaxy (really small so its dust didn’t block our view of IC 2497 just to the north), some kind of ionized gas cloud associated with IC 2497 itself (redshift z=0.05), or something like the “Lyman α blobs”, gigantic glowing gas clouds seen only in the early Universe (z=3 or so). A spectrum would tell which (if any) of these was correct. So I started emailing friends who use appropriate telescopes pretty regularly, and mostly ended up grumbling about the shortage of spectrographs on 1-3 meter telescopes these days. Meanwhile, I was able to do some measurements with the SARA 0.9-meter telescope, which our university operates remotely as part of a consortium. (In fact, I did these measurements sitting at home, assisted by one of our cats who finds a logbook in front of a monitor the most comfortable place in the house). It takes a pretty long time for a telescope that size to surpass the quick-look Sloan image, but these data were able to narrow down where that strong emission line could be. I used a different set of filters, the classic BVRI set which were designed to be optimized for certain measurements of stars (rather than galaxies), but are helpful here because they’re different. The bright peak made it into the V filter but not the others. The V band runs more or less from 5000-5900 Angstroms, so the wavelength we seek is in the overlap between v and [i]g[/i] between 5000-5500 Angstroms. Alas, that didn’t help us much, since the strong [O III] emission line at 5007 Angstroms would land in that range for something very nearby or at the redshift of IC 2497.

Finally, some of the UK zookeepers were able to find a colleague working at the 4.2-m William Herschel Telescope on the island of La Palma who was able to get a spectrum, while some of us were at the big meeting of the American Astronomical Society in Texas (just last week). The WHT is very well equipped for spectroscopy, and La Palma is a superb site (from which I’ve seen the sharpest images from any telescope I could put my hands on). We’ve got a quick-look screensnap of the spectrum, and it answers a couple of questions right away. The Voorwerp is at almost exactly the same redshift as IC 2497, and almost certainly associated with it. The strong and narrow emission lines are what one would see from a star-forming region. But there are some things about it that are strange, and need more work.

I’ve labelled some of the emission lines in the spectrum here. The spectrograph slit was oriented roughly north-south, running through IC 2497 as well, and is shown left-to-right. Wavelength increases from bottom to top; this is a slice of the violet-to-green region, from about 3400-5100 Angstroms in the reference frame of the object itself. We see the hydrogen series (labelled as H+Greek letters), produced when electrons join with free protons to make hydrogen atoms. There are also lines from heavier elements; the brackets denote so-called forbidden lines, radiation which arises from decay of energy levels excited by collisions between ions and electrons. Looking at what we can tell so far about the relative strengths of these features, there is funny business afoot. First, the gas is hot (even by the standards of ionized nebula). The ratio of [O III] lines between 4363 and 4959+5007 is sensitive to temperature (for those who really want to know why, here is an online lecture including details, with abundant thanks to the late Don Osterbrock for pounding this stuff into my thick head). To have the 4363 line even detectable, the gas has to be unusually hot, more like 15-20,000 K (exact numbers are pending getting the final calibrated spectrum from the observers). Even odder are some of the other lines. He II is produced when an electron joins a bare helium nucleus, and requires high enough temperature or radiation with enough energy to tear both electrons from helium (four times harder than for hydrogen). We don’t see this in star forming regions. The only stars hot enough to produce He II in surrounding nebulae are the central stars of planetary nebulae (which are the hottest stars known, but only for a few thousand years) and a handful of X-ray-bright stars usually associated with accretion onto black holes or neutron stars. On top of that, at the blue end of the spectrum is [Ne V]. If it’s hard to rip two electrons from helium, it’s that much harder to pull four from neon lights. This requires 97 electron volts (eV), compared to 54 to make He II and 13.6 to ionize hydrogen. [Ne V] does sometimes show up in planetary nebulae, but even there calculations suggest that it’s not the UV starlight that’s responsible, but that high-speed shock waves may be the culprit. This line is also common in the spectra of active galaxies – Seyfert nuclei and their kin, where we know that there are abundant X-rays interacting with the gas.

So the spectrum tells us where the Voorwerp is, and leaves us with a fascinating conundrum. (To quote an email from a ZooKeeper, “Hmm..that doesn’t make any sense! Excellent…” Not only do we see these high-ionization lines, but we can already see that they come from the whole cloud, not some small bright region. Are we dealing with shocks, or perhaps with radiation from an active nucleus in IC 2497 which is obscured from our point of view but shining full force toward the blob? Or something we haven’t thought of? All good questions. We’ll know more when we have the calibrated spectrum so we can do detailed numerical comparisons.

There are obviously a lot more observations we’d like to have. The gas is shining so brightly that it’s hard to tell what the stars are doing. We’re putting together a request to have the Swift orbiting observatory take a look with its UV camera and perhaps in X-rays as well. Swift was designed to follow up gamma-ray bursts, but they also take requests for where to point while sitting there waiting for a random burst to go off. And not too long from now, it will be the season to propose for Hubble imaging and spectroscopy with the Gemini telescope’s integral-field unit (which gives the spectrum not just along a line, but at every point within a small area of sky).

Whatever this is, it’s rare. After I mentioned wanting to improve my SQL fu to check for more things in the SDSS with its odd colors, Chris Lintott did just that. There are no more things in the survey database which are not imaging artifacts and have colors within 15% of what we see here. There’s more work ahead to make sure that we include the possibility of, say, having H-alpha not fall between the r and i filters as it did here, but there can’t be many more of these. Rare objects suggest rare events, just the kind of thing that it takes a deep sky survey and careful winnowing to find. Dank U wel, Hanny!