Unveiling Hanny's Voorwerp – one step at a time
Among the new data we have now is a set of fabulous images taken late last year from the 3.5m WIYN telescope at Kitt Peak, Arizona. We were using a special rapid-guiding CCD camera, which tracks rapid motion due to the atmosphere or wind shaking the telescope, delivering even sharper images than the telescope normally would on long exposures At the end of a 3-night session observing overlapping galaxies to measure their dust content, I couldn’t help noticing that IC 2497 passed nearly overhead just at the start of morning twilight. The atmosphere had been unusually steady so our images were very good. Steady atmosphere, target straight overhead, fast-guiding camera – there was no way we’d pass up this opportunity. The combination paid off; our images in two of the filters were the sharpest ground-based images I’ve ever gotten from anywhere. I knew the telescope could reach that image quality, but never before had it done so for me. In some parts of the image, close to reference stars, the atmospheric blurring amounts to only four tenths of an arcsecond. (When I was in school, it was the received wisdom that the atmosphere would never allow images this sharp for more than a fraction of a second).
Just look at all the new structures we can see! The spiral arms of IC 2497 show a warp, suggesting that it has been disturbed by a close encounter with some other galaxy within the last few hundred million years. This fits with the radio data showing an enormous cloud of hydrogen forming an arc outside of the galaxy and stretching almost halfway around it. Close to the center, more evidence of disturbance comes from the intricate network of dust lanes that we can see in the new images. Especially so close to the core, these should settle down into a flat plane (like the disk of the galaxy itself) quickly on cosmic timescales; more evidence of an exciting history.
The dust will also interfere with what we can see of the enigmatic nucleus of the galaxy. The radio observations, especially the measurements from the continent-spanning European VLBI network, show us exactly where the nucleus must be. By matching stars on the SDSS image, we can transfer its coordinate information and see just where the radio core lies on our visible-light images:
The core lies just behind one of the dust lanes, so we don’t see it directly by visible light. Using the pieces of the surrounding bulge light that we do see, we can estimate that more than half of the bulge light has been absorbed by the dust. That central bulge is at least twice as bright as we would measure from the SDSS images, which don’t show us enough detail to trace the dust. Since the typical properties of the supermassive black holes in galactic nuclei vary with the bulge of stars around them, this increases the plausible mass of a central black hole which has been powering all the fireworks.
And we see new detail in Hanny’s Voorwerp as well. Knots, clumps, filaments. There is a hint that its north end breaks up into little curves looking like bow shocks, such as we would see if there is unseen material flowing into it from the north side. The radio observations do show something that might do the job; higher resolution may tell the tale. If that is true, it might be a gigantic version of the famous “Pillars of Creation” in the Eagle Nebula. There are more green shreds of ionized gas scattered around the bright part as well.
Over and over throughout astronomical history, we see not only new things, but new kinds of things, when we can see them more deeply, or sharply, or over a wider wavelength range. Hanny’s Voorwerp and IC 2497 are no exceptions. But as informative as these data are, they only whet our appetites. Depending on which wavelength they use, our planned Hubble images will be from three to seven times sharper than this. And even before the Hubble data become available, X-ray observations (from the Japanese Suzaku and European XMM-Newton orbiting observatories) promise to go a long way toward distinguishing between the two major ideas for what we see – whether the quasar in the core of IC 2497 has virtually shut down in the recent past (well, tens of thousands of years is very recent by astronomers’ standards), or it’s still there but so totally shrouded by dust and gas that we cannot yet see it in any of our observations. Stay tuned…