The Mystery of the Voorwerp Deepens!
A couple weeks ago, I talked about the Voorwerp (“object”), the strange blue object that Hanny posted to the Galaxy Zoo forum. She asked if anyone knew what it was, and we sure didn’t. Part of the problem was that we didn’t have a spectrum for it, so it could have been literally anywhere from right next door in our galaxy to the edge of the universe. Our colleague Bill Keel took a spectrum, which he posted about here in the blog, and found that the Voorwerp is associated with the galaxy above it. We’ve since been looking around for other colleagues that can help us figure out what the Voorwerp is.
Thanks to Matt Jarvis, who was observing at the 4.2m William Herschel Telescope in La Palma, we’ve been able to get some better images. The William Herschel Telescope is bigger than the telescope that gathers images for the Sloan Digital Sky Survey (SDSS is 2.4 m; WHT is 4.2 m), and the images that Matt took are longer exposures, so we can see fainter features in them. The conditions were also quite good (good “seeing” in astronomer’s lingo) and so the image has very good resolution (it’s “sharper”) as the atmosphere didn’t blur things too badly.
So what kind of data did we get? We got three images in filters very similar to the SDSS ones. We got a g, r and i-band image. Those correspond roughly to green, red and infra-red for human eyes. Just to make things confusing though, we colour g in blue, r in green and i in red to stay consistent with the SDSS/GZ images. Without further ado, here are the original SDSS and new WHT images:
Original SDSS image
New WHT image
The WHT image is rotated with respect to the SDSS image; look at the orientation of the galaxy and the Voorwerp to see how they compare. Once you mentally rotate the images so they match, you can see clearly that the Voorwerp is quite a lot bigger than we initially thought, because so much of it was too faint to be visible in the SDSS image. This immediately makes us want to get an even deeper g-band (blue colour) image to see just how much bigger it is! For that, we will probably go to the world’s largest telescopes such as ESO’s Very Large Telescopes, Gemini or Keck.
To give you an idea just how big the Voorwerp is by now, look at the spiral galaxy next to it. This galaxy is a very massive spiral galaxy, likely as big or bigger than our own Milky Way! That’s really, really big!
If you look at the new WHT image of the Voorwerp, you can also see a huge, gaping hole. From the SDSS images, it wasn’t really clear whether the fuzzy structure there was anything real, but the WHT image makes it clear that this is a genuine hole. Again, just to put it into proportion, that hole has a diameter of something like 10 000 light-years. We have no good idea of what could punch such a large hole. One possibility is that a massive burst of star formation occurred there, causing a string of powerful supernova explosions, causing an expanding bubble. Such holes presumably caused by supernovae have been seen in other galaxies, but as far as we know, nothing anywhere near this size.
In his last post, Bill mentioned that the spectrum of the Voorwerp showed some very odd emission lines, in particular Helium II (HeII) and Neon V. HeII only really appears in spectra when there is something really hot around to excite the gas – something hotter than the hottest star. This could be an active galactic nucleus(i.e. gas falling into a supermassive black hole, and heating up as it falls), or perhaps some high velocity shocks. We’re busy analysing the spectrum to understand better what’s going on here.
By a luck coincidence, the Voorwerp turned out to be at a redshift where the HeII line “redshifted” into a common narrow-band filter. Such a filter blocks all light except in a very narrow wavelength range, and so lets us take an image focusing only on those areas which are emitting light in that wavelength range. Below is the image of the Voorwerp in the wavelength range of the HeII line:
The Voorwerp in HeII
The HeII emission clearly comes from a good chunk of the whole Voorwerp (again,a deeper image might show even more), so whatever is exciting the gas in the Voorwerp seems to do it over quite a large volume.
What’s next? We really still have no idea of what the Voorwerp really is. The more data we take on it, the stranger it gets. Many of us are busy trying to convince friends of ours on observing runs to take observations of the Voorwerp so we can figure out what it is.
That’s how an observational science like astrophysics works: you find something new, you don’t know what it is, so you take more data to try and understand it better and form some hypothesis about what’s actually going on and then you confirm it with more data. But we’re still at the very start of this process. The mystery deepens… *cue scary music.