Thursday, March 20, 2008

Tales from the Zoo: More news on the Hanny's Voorwerp

Mar 20

Voorwerp fever

Ever since it was first identified, Hanny's Voorwerp has grabbed the attention of the Zookeepers and everyone else who comes across it. One reason we've been successful in getting such a wide range of observations over just a few months (and therefore why posts on here have been delayed!) has been that colleagues seem to find it equally compelling. So what is it? Our current best guess goes something like this:

A hundred thousand years ago, a quasar blazed behind the stars which would have already looked recognizably like the constellation Leo Minor. Barely 700 million light-years away, it would have been the nearest bright quasar, shining (had anyone had a telescope to look) around 13th magnitude, several times brighter than the light of the surrounding galaxy. This galaxy, much later cataloged as IC 2497, is a massive spiral galaxy which was in the process of tidally shredding a dwarf galaxy rich in gas - gas which absorbed the intense ultraviolet and X-ray output of the quasar and in turn glowed as it cooled. But something happened to the quasar. Whether it turned off, dropped to a barely simmering level of activity as its massive black hole became starved for gas to feed its accretion, or it was quickly shrouded in gas and dust, we don't see it anymore.

But we see its echo. How could we come to such startling conclusions? An earlier blog entry showed some of our earliest data, when we already knew that the gas in Hanny's Voorwerp was ionized in such a way that it must experience a radiation field of higher energy than normal stars can produce. In fact, it looks just like the pattern of emission given off by gas around the center of Seyfert galaxies, and on the outskirts of quasars and radio galaxies. This makes sense, except for the minor detail that we don't see the active nucleus that should be there to light up the gas.

However, we could start from calculations done by astronomers trying to understand these objects, which could tell us how much radiation it would take to light up the Voorwerp. This wound up telling us how many ionizing photons there are per atom in the gas (known as the ionization parameter). That meant that we could find out how bright the missing core had to be if we could learn how dense the gas is.

Spectra are wonderful things - there is a pair of emission features from ionized sulfur atoms out in the red whose ratio depends on how often the atoms undergo collisions, and therefore on the density where they float. We had been contacting colleagues all over the map to see who might be doing spectroscopy in the red, and were fortunate to be put in touch with Nicola Bennert, who is a postdoctoral researcher at the University of California campus in Riverside. She was about to work for several nights with Lick Observatory's 3-meter Shane telescope and a double spectrograph optimized to observe blue and red parts of the spectrum at once, and was intrigued by what we already knew of the Voorwerp.

She got a useful data set, in particular a very nice observation of the spectrum in red light. From this, we now know that the typical density of gas (for the pickier readers, that's the RMS density) is no greater than about 15 particles per cubic centimeter - which means that the UV and X-ray luminosities of the object were somewhat less than a hundred billion times the Sun's total energy output, in the range of quasars. (It was a nice extra feature that Nicola did her dissertation work on analysis involving measuring ionization parameters of gas in Seyfert galaxies, and she's enthusiastically joined in the project).

From the features of sulfur and nitrogen, we also have good evidence that these elements are not very abundant in the gas - maybe 10% of the fraction seen in our part of the Milky Way, more like what we find in dwarf galaxies such as the Small Magellanic Cloud. So the gas looks more like something from a low-mass dwarf rather than something ejected from the center of a luminous galaxy like IC 2497.


Meanwhile, we had asked for a quick look with instruments on the Swift satellite. Swift is designed to detect gamma-ray bursts and follow them up quickly with X-ray and ultraviolet or visible-light observations, to localize them as fast as possible ("Swift - catching gamma-ray bursts on the fly" is their motto). Thus, Swift spends a lot of its time staring at the sky, especially parts of the sky that are easy to see from ground-based telescopes, waiting for something to happen. From being on one of too many NASA committees, Bil recalled that the Swift science team had realized that, since it didn't matter exactly where they looked waiting for something to happen, they have a program to take requests. Usually these requests are for transient, time-sensitive events, but principal investigator Neil Gehrels agreed that our request would be appropriate.

So we crammed our whole science argument into 300 words and it was approved. Showing that "Swift" has more than one meaning, within a week we had our data. We had two questions in mind for its instruments. First, its X-ray telescope (known as the XRT) would easily see any active galactic nucleus, even a typical Seyfert galaxy. It saw - nothing. Second, we asked for ultraviolet images with the 30-cm Ultraviolet/Optical Telescope (UVOT). These were intended to tell whether the light outside of the bright gaseous emission lines came from stars or was reflected from dust particles. The distinction could be made because, as in the scattering that makes our sky blue, short-wavelength radiation scatters more effectively from interstellar dust. As an example, the blue reflected piece of the Triffid Nebula is bluer than the illuminating star - in fact bluer than any kind of star can be. And this is what we found in the Voorwerp. Filtering a slice of ultraviolet light that shouldn't be much affected by the gas, we found the object to be ten times brighter in the mid-ultraviolet than in the shortest wavelength seen by the Sloan Survey. Not only does the gas see something bright, so does the dust.

UVOT image on the left, v band on the right

So now we have a bunch of pieces of the puzzle. Highly ionized gas, ionized by nothing we can see. Dust reflecting ultraviolet light from no apparent source. No central X-ray source, which makes it very hard to hide
something behind a cloud of gas and dust that leaves it visible from the Voorwerp. This was starting to look like a giant version of a phenomenon that astronomers have had to rediscover for several generations now - the light echo. Over the years, when we see a supernova explosion, bright nova, or a star that for some other reasons flares brightly, we often see reflections from foreground dust. If we trace the geometry of what dust we see at different times after the outburst, it must fall along an ellipsoid with the star at one focus and ourselves at the other.

It's important that the echo has spectral characteristics of the exciting source. One team has used this fact to find locations of supernovae which we would have seen in the Large Magellanic Cloud centuries ago, as their reflections still come our way from larger and larger circles of foreground dust (see this very cool and very new press release). And now we are proposing that we've found the light echo from a faded quasar, which was there 50-75,000 years ago but is invisible now.

The importance of checking on this whole picture goes well beyond the admitted coolness value, or the flashiness of a proposal that we hope our colleagues who decide who gets to use big telescopes will look on with favor. We already know that quasars (and their relatives such as Seyfert galaxies) can undergo dramatic change on everything from cosmic timescales to human ones. We observe them to fluctuate in brightness, sometimes dramatically, over times as short as weeks. And at the outside, relations between quasars and mergers in some of their surrounding "host" galaxies wouldn't exist if the quasars stay bright for much more then the nearly billion-year duration of a galaxy merger. (Only in astronomy and cosmology do we get to lump "mere" and "billion years"). In fact, we know that the whole population of quasars has changed over cosmic time - there used to be many more, and they grew brighter, in an era about 10 billion years ago. For that matter, the most powerful quasars must be temporary - if one were to shine at these enormous levels for all of cosmic history, even as miserly as gravitational energy can be about producing energy wile consuming mass, the central object would have long ago eaten its entire surrounding galaxy.

Of course we want to know more. There are more observations we can make which would test this idea, and tell us more about the nature of the Voorwerp and the history of the illuminating core. Chris headed up a proposal to map the gas with the OASIS system on the 4.2-meter William Herschel Telescope, so we could measure the Voorwerp's Doppler shifts point-by-point and see whether there are correlated changes in strengths of emission lines that would show us brightening and fading of the central source (which would make rings in our view unless the gas has a very odd structure). And there was the Hubble proposal, which would take high-resolution images of the gas in two emission lines and then look in filter bands between them to see whether the Voorwerp has stars. Actually, with all the reflecting dust, we hope mostly to see star clusters, to tell whether it started life as a dwarf galaxy. And we want to take a really close look at the nucleus of IC 2497, using Hubble's exquisite resolution to isolate the light from its innermost region in search of any gas that is lit up by even a weak active nucleus. Speaking of the nucleus of IC 2497, Bill is even as we write working to complete a proposal to use Chandra to see if we can tease out any X-rays from a now-quiet AGN. We've also requested time in the radio to see if we are only seeing part of a much larger structure.

So here we have a new possibility - of watching the history of a quasar either flaring up, practically turning off, or being hidden over a time span that we've had no other way to examine. The pattern of light emitted by gas in Hanny's Vooorwerp, and the way its dust reflect the quasar light, should be able to trace the history of its decline. Never mind heading back to the future, we can go onward into the past. Once in a while, we have the opportunity to do what paleontologists can do only in the movies.

(Chris and Bill weren't sure who should blog this. So in the spirit of Galaxy Zoo, we both did.)

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