After two rounds of comments and questions from the journal referee, the first paper discussing the detailed results of the Hubble observations of the giant ionized clouds we’ve come to call Voorwerpjes has been accepted for publication in the Astronomical Journal. (In the meantime, and freely accessible, the final accepted version is available at http://arxiv.org/abs/1408.5159 ) We pretty much always complain about the refereeing process, but this time the referee did prod us into putting a couple of broad statements on much more quantitively supported bases. Trying to be complete on the properties of the host galaxies of these nuclei and on the origin of the ionized gas, the paper runs to about 35 pages, so I’ll just hit some main points here.
These are all in interacting galaxies, including merger remnants. This holds as well for possibly all the “parent” sample including AGN which are clearly powerful enough to light up the surrounding gas. Signs include tidal tails of star as well as gas, and dust lanes which are chaotic and twisted. These twists can be modeled one the assumption that they started in the orbital plane of a former (now assimilated) companion galaxy, which gives merger ages around 1.5 billion years for the two galaxies where there are large enough dust lanes to use this approach. In 6 of 8 galaxies we studied, the central bulge is dominant – one is an S0 with large bulge, and only one is a mostly normal barred spiral (with a tidal tail).<?p>
Incorporating spectroscopic information on both internal Doppler shifts and chemical makeup of the gas we can start to distinguish smaller areas affected by outflow from the active nuclei and the larger surrounding regions where the gas is in orderly orbits around the galaxies (as in tidal tails). We have especially powerful synergy by adding complete velocity maps made by Alexei Moiseev using the 6-meter Russian telescope (BTA). In undisturbed tidal tails, the abundances of heavy elements are typically half or less of what we see in the Sun, while in material transported outward from the nuclei, these fractions may be above what the solar reference level. There is a broad match between disturbed motions indicating outward flows and heavy-element fractions. (By “transported” above, I meant “blasted outwards at hundreds of kilometers per second”). Seeing only a minor role for these outflows puts our sample in contrast to the extended gas around some quasars with strong radio sources, which is dominated by gas blasted out at thousands of kilometers per second. We’re seeing either a different process or a different stage in its development (one which we pretty much didn’t know about before following up this set of Galaxy Zoo finds.) We looked for evidence of recent star formation in these galaxies, using both the emission-line data to look for H-alpha emission from such regions and seeking bright star clusters. Unlike Hanny’s Voorwerp, we see only the most marginal evidence that these galaxies in general trigger starbirth with their outflows. Sometimes the Universe plays tricks. One detail we learned from our new spectra and the mid-infared data from NASA’s WISE survey satellite is that giant Voorwerpje UGC 7342 has been photobombed. A galaxy that originally looked as if it night be an interacting companion is in fact a background starburst galaxy, whose infrared emission was blended with that from the AGN in longer-wavelength IR data. So that means the “real” second galaxy has already merged, and the AGN luminosity has dropped more than we first thought. (The background galaxy has in the meantime also been observed by SDSS, and can be found in DR12).
Now we’re on to polishing the next paper analyzing this rich data set, moving on to what some colleagues find more interesting – what the gas properties are telling us about the last 100,000 years of history of these nuclei, and how their radiation correlates (or indeed anti-correlates) with material being blasted outward into the galaxy from the nucleus. Once again, stay tuned!
We’re in the middle of an observing run at the Lick 3m Shane telescope, with the first part devoted to polarization measurements of the Voorwerpje clouds (which is to say, giant clouds of ionized gas around active galactic nuclei found in the Galaxy Zoo serendipitous and targeted searches), and just now switching to measure spectra to examine a few new candidate Voorwerpjes, and further AGN/companion systems that may shed light on similar issues of how long AGN episodes last.
Polarization measurements can be pretty abstruse, but can also provide unique information. In particular, when light is scattered, its spectra lines are preserved with high fidelity, but light whose direction of polarization (direction of oscillation of its electric field when considered as a wave) is perpendicular to the angle it makes during this operation is more likely to reach us instead of being absorbed. This is why polarized sunglasses are so useful – glare from such scattering light can be reduced by appropriate orientation of the polarizing filter.
In our context, polarization measurements tell us something about how much of the light we see is secondhand emission from the AGN rather than produced on the spot in the clouds (admittedly as a side effect of the intense UV radiation from the nucleus), and will show us whether we’re fortunate enough that there might be a dust cloud reflecting so much light that we could look there to measure the spectrum of the nucleus when it was a full-fledged quasar. (This trick has worked for supernovae in our galaxy, which is how we know just what kind of supernova was seen in 1572 despite not having spectrographs yet).
Polarization wizard Sebastian Hoenig (now at the Dark Cosmology Center in Copenhagen) has already produced preliminary calibrations and maps from these new data. Here are some visualizations. In each case, the lines show the direction of polarization. Their length and color show the fraction of light which is polarized at points where there is enough to measure. This fractional polarization tells about the mix of light arising on the spot (even if secondhand due to UV radiation ionizing the gas) and that reflected from dust particles. There is a telltale annular or bull’s-eye pattern when the scattered light originates in a central source, which we see over and over (as if we hadn’t figured out to blame the galaxy nuclei anyway).
First up is a personal favorite, UGC 7342 (the last one to have its Hubble images obtained, and among the largest and brightest of the Galaxy Zoo sample).
The next one, Markarian 78, is less familiar, oddly because it makes perfect sense (so it has not figured much in the followup observations). In this case, we see a bright and obvious active nucleus, one which is powerful enough to light up the giant gas clouds without having changed over the past 60,000 years or so.
For comparison, here is a polarization map of IC 2497 and Hanny’s Voorwerp itself, from data obtained last year (the first time the weather let us get useful results). Sometimes we can hear the Universe laughing – a quick simulation shows that the reflected light from the nucleus, when it was a quasar, is just a bit too faint for us to have seen its signature broad emission lines in any of the Voorwerp spectra.
As we switch into measuring spectra for the next few nights, the aim changes to a combination of looking at a few new Voorwerpje candidates from the Galaxy Zoo forom, and a set of newly-identified AGN/galaxy pairs which may let us study the same issues of AGN lifetimes. We can sort of settle into a routine – Anna Pancoast does calibrations and setup during the day and hands over to Vardha Bennert to finish observations during the night. I typically get to work before Vardha finishes the last galaxy observation (thanks to the time-zone difference) and transfer data to start analysis, so we can change the next night’s priorities if something interesting shows up. It takes a (global) vllage, but then if there’s been any single meta-lesson from Galaxy Zoo, that would be it.
As usual when the American Astronomical Society meets, this has been an intense week of research results, comparing notes, and laying plans. Galaxy Zoo has once again been well represented. Here’s Kevin discussing the Green Valley in galaxy colors, making the case that it consists of two completely different populations when Galaxy Zoo morphologies are factored in:
Today we’re presenting first results of the Hubble imaging of Voorwerpje systems. This is what our poster looks like:
(or you can get the full-size 2.8 Mbyte PDF). We didn’t have room to lay out all the features we first had in mind, but these are the main points we make:
They show a wild variety of forms, often with filaments of gas stretching thousands of light-years. These include loops, helical patterns, and less describable forms.
The ionization, traced by the line ratio [O III]/Hα, often shows a two-sided pattern similar to the ionization cones around many AGN. This
fits with illumination by radiation escaping past a crudely torus-like structure. However, there is still less highly-ionized gas outside this whose energy source is not clear.
As in IC 2497, the parent galaxy of Hanny’s Voorwerp, many of these galaxies show loops of ionized gas up to 300 light-years across emerging from the nuclei, a pattern which may suggest that whatever makes the nucleus fade so much in radiation accompanies an increase in the kinetic energy driving outflows from its vicinity.
At the bottom of the poster we illustrate with new clarity a point we knew about in the original paper – for the two Voorwerpje systems with giant double radio sources, they completely break the usual pattern of alignment between the radio and emission-line axis. Mkn 1498 and NGC 5972 are aligned almost perpendicular, which can’t be fixed by changing our viewing angle. We’re speculating among ourselves as to how this could happen; maybe interaction of two massive black holes is twisting an accretion disk. But don’t quote me on that just yet.
The color images here show only the ionized gas, with [O III] in green and Hα in red. Starlight from the galaxies has been subtracted based on filters which don’t show the gas, so we can isolate the gas properties. The false-color insets show the [O III]/Hα ratio. The blank regions are areas whose signal is too low for a useful measurement. Red indicates the highest ionization, fading to deep blue for the lowest.
We were able to feature some new data that came in too late to be printed in the poster (by tacking up a smaller printed panel) – the long-awaited images of UGC 7342, among the largest and most complex clouds we’ve found (or more correctly, so many Galaxy Zoo participants found). Hubble observed it Monday afternoon, and after some frantic file-shuffling and processing, I got the data in the same shape as the others. And here it is:
Click on this one to see it larger. We barely know where to begin. The actual AGN may lie behind a dust lane, and there is a large region of very low-ionization as near it. Another loop near the nucleus, and fantastically twisted filaments winding their way 75,000 light-years each way.
There is still more to come – with Vardha Bennert and Drew Chojnowski, we planned the strategy for several upcoming observing runs at Lick Observatory (one starting only next week). These should include getting data on some of the most promising AGN/companion systems to look for the AGN ionizing gas in companion galaxies, and observation of regions in the Voorwerpjes that we only now see a context for. Additional X-ray and radio observations could fill in some of the blanks in our understanding. And by all means, stay tuned!
We just submitted the journal paper describing the Hubble results on Hanny’s Voorwerp, to the Astronomical Journal. It’s not on arxiv.org yet – you can get a PDF here. Here’s a “brief” summary.
Data: In this paper we look at a whole collection of new data, obtained since the original discovery paper. These include:
Hubble, of course:
– WFC3 (Wide-Field camera 3) images in the near-ultraviolet, deep red, and near-infrared. These filters were designed to exclude most of the light from the gas, so we could look for star clusters, especially in Voorwerp and its surroundings, and with the IR image, look deeper into the dust around the nucleus of IC 2497.
– ACS (Advanced Camera for Surveys) images tuned to the wavelengths of [O III] and Hα emission. These were intended for fine structure in the gas and its ionization. As it turned out, we saw streamers, fine details, embedded star formation, and local interaction with a jet from IC 2497.
– STIS (Space Telescope Imaging Spectrograph) red and blue spectra across the galaxy nucleus. This let us isolate structure near the nucleus much better than from the ground, so we would look for any gas that has a line of sight to a brighter nucleus than we see directly (perhaps because of foreground dust).
GALEX (the Galaxy Evolution Explorer satellite): wide-field ultraviolet spectra and images, which help us put the Hubble UV data in context.
Kitt Peak 2.1m telescope – redshift of the companion galaxy, and the nature of star-forming regions just southwest of the nucleus of IC 2497
3.5m WIYN telescope BVI images – the best non-HST data, which we used to confirm techniques for reducing effects of cosmic-ray impacts in ACS images.
In no particular order, our major science results are:
1 – We found no evidence that the quasar in IC 2497 is still bright. The Hubble spectra show no highly-ionized gas near the center, as we might expect if it were bright but blocked by dust clouds from our point of view. In fact, with HST data we can refine our estimates of the radiation intensity seen by gas near the core and out in the Voorwerp, and these estimates only widen the shortfall. That is, we can constrain the quasar’s (former) brightness to have been brighter than our earlier lower limits from the ground-based data (because of the sharper Hubble images, we can tell better how close gas is to the nucleus of IC 2497 and how bright the brightest peaks are in the Voorwerp). To quote a famous 1960s television character, “It’s dead, Jim.”
2 – As we told everyone at the AAS, we found regions of star formation in the Voorwerp. This is part of a broader picture of a directed flow of gas out from IC 2497 in a fairly narrow jet or cone. The small radio jet seen with VLBI radio techniques (the Rampadarath et al. paper, using data from the UK MERLIN network at the European VBI network) points within about 10 degrees of the direction where we see a “small” area of star formation in Hanny’s Voorwerp (no more than 5000 light-years across), and this is precisely aligned with the one area where we see tendrils of gas pointed away from IC 2497 (the area I once called Kermit’s Fingers). We see these areas of star formation in two ways – in the images filtered to minimize the gas contribution, we see the light from young star clusters themselves, even into the ultraviolet. And in the images which isolate the ionized gas, we see its ionization state (and emission-line ratios) shift in local regions around the star clusters, to the ratios that we see when the gas is ionized by young stars and not AGN. In the color images, that shows up a a shift from green (where [O III] is much stronger) to red (where Hα is the stronger line). However, compared to some other active galaxies whose jets impact surrounding gas, the effects are modest in Hanny’s Voorwerp; the jet or outflow has compressed gas and triggered star formation, but at only a quarter the rate of the similarly-sized Minkowski’s Object, which sits right in the path of a more powerful jet from the radio galaxy NGC 541. The balance tells us something about the amount of material that can be in the outflow, in order not to have pulled any more gas out into filaments , and form any more stars, than we see. This also suggests something to look for in the future – star clusters in the middle of nowhere that were formed by outflows from now-faded active galactic nuclei.
The outflow of gas we see toward the Voorwerp may roughly match, in age, another find from the Hubble data – an expanding ring of gas, 1500 light-years in size, heading out from the core in the opposite direction. This is another sign that the AGN has begun to affect its environment through mass motions rather than radiation alone. This was a serendipitous find – only because the spectrograph slit happened to cut across it could we spot this region only a half arcsecond from the core where the Doppler shifts and emission-like properties were quite distinct. With some extra processing, it turned up in the emission-line images as well, which is how we know it forms a loop. We suspect that putting it all together may show that the black hole’s accretion in IC 2497 hasn’t completely shut down, but has shifted from producing radiation to pumping more energy into motions of surrounding gas (as it’s called in the jargon, switched from quasar mode to radio mode). The speed of such a switch would inform theoretical understanding of these accretion disks, happening not on the periods of days that we see for black holes in our neighborhood with a few times the Sun’s mass, to a million years (or now maybe rather less) in a galactic nucleus.
3 – As we could sort of see from the SDSS images, IC 2497 is disturbed. Its spiral arms are twisted out of a flat plane, with dust lanes cutting in front of the central region. This fits with the idea that a tidal collision pulled out the massive tail of neutral hydrogen. On the other hand, we now see that the companion galaxy just to its east is a beautifully symmetric, undisturbed spiral (which we now know to have a precisely matching redshift, so they are almost certainly close together). One picture that would fit these data would be that IC 2497 is the product of a merger something like a billion years ago (more precisely, before the time when we see it), a merger which was either quite unequal in galaxy masses or unusual in leaving the disk of the galaxy in place although warped. There is a suggestion that the patch of star-forming areas just to the southwest of the center of IC 2497 might be all that remains of the other galaxy.
4 – This result may be a bit of an acquired taste, delving into emission-line physics. From the lack of a correlation between level of ionization and intensity of Hα emission, we can tell that, despite the amazing level of detail of blobs and strings we can see in the Hubble images, that there is fine structure on still smaller scales. The areas that are brighter are not, by and large, any denser than average (which would be the most natural way to have brighter H-alpha emission), they have more small blobs and filaments with about the same density. This could be general – if the outflow from IC 2497 has not reshaped the gas in the Voorwerp, most of the neutral hydrogen that’s not ionized by the galaxy nucleus would have the same kind of structure. That in turn would suggest that the common giant hydrogen tails around interacting galaxies are composed of masses of narrow threads of gas (maybe held together by magnetic fields), which is not the first thing we would guess from the limited-resolution radio data that are the only way we can see these tails unless they are ionized by a nearby AGN.
Several of these are results we will also look for in the Hubble images of selected Voorwerpjes – do we see star formation indicating there is an outflow from the AGN, and do we see the same evidence for fine structure in the gas?
From the spectrum, we have a pretty good idea what the chemical mix of elements is – by mass, around 77% hydrogen, 23% helium, and 0.25% of everything else (what astronomers like to call “metals”, although that mostly means carbon, nitrogen, and oxygen). This tells us a bit about where the gas didn’t come from – it was not blown out from deep inside IC 2497, because gas near the centers of big galaxies gets progressively enriched in heavy elements produced inside massive stars (then blown out in supernova explosions or less violent planetary nebulae). If the gas in the 21-cm hydrogen tail was pulled out from the outer regions of IC 2497 during an interaction with another galaxy, this would fit, since gas far out in spirals has been less affected by material produced in stars.
This is the first technique that has been able to look across times of many thousands of years, rather than the few decades that astronomers have been able to watch AGN. Getting a better handle on this was a big part of the search for voorwerpjes (so we get a better since of how unusual IC 2497 and the Voorwerp might be). From our initial sample of 19, we could make a crude estimate that AGN stay bright for roughly 20,000-200,000 years at a time. This comes from comparing the numbers of galaxies with clouds whose AGN are bright enough to account for them with the number where the AGN is too faint to light up the clouds (where iC 2497 is the strongest example). I have a project slowly getting started to look for even fainter examples (too dim to be picked up by the SDSS) around bright galaxies, so we might be able to look back even longer (up to a million years if very lucky). Geeky it might be, but I couldn’t resist calling this the TELPERION survey. As an acronym it’s forced, but Middle-Earth aficionados will see how appropriate the connotation is.
All in all, quite an adventure beginning with “What’s the blue stuff?”
Yesterday marked a milestone in the Galaxy Zoo study of AGN-ionized gas clouds (“voorwerpjes”), when we received notice that the paper reporting the GZ survey and our spectroscopic study of the most interesting galaxies
has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. We’ve now posted the preprint online – at http://arxiv.org/abs/1110.6921 on the preprint server, or, until publication, I have a PDF with full-resolution graphics. Here’s the front matter:
The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion events
As Bill has spotted on the forum, the video of the press conference at the January AAS meeting where we presented the Hubble Space Telescope of Hanny’s Voorwerp is now available from the AAS. We shared the event with two other cool results, so you can hear about them as well. The video (warning, large file!) is available here: video of the January press event
Some of us continue to exult over the approval of our Hubble proposal to look at some of the “voorwerpjes” found through the GZ forum. These are clouds of gas ionized by an active galactic nucleus, similar to Hanny’s Voorwerp except for being smaller and dimmer (hence the Dutch diminutive form of the word). This has been a very fruitful project, going back to the first few possibilities posted for discussion in different contexts by Zooites. From these, I realized that such clouds could be spotted based on their unusual colors from the SDSS images, and with contributions from Waveney and laihro setting up the web interface and one of our source lists, the hunt was on! The results staggered us – within 6 weeks each of the 18,000 candidate galaxies had been examined by at least 10 Zooites. Seven of you looked at them all! Then we could examine the highest-scoring galaxies, in three sessions from Kitt Peak and Lick observatories, measuring spectra to see which ones really show gas ionized by an active nucleus. Once again, Drew, alias sdrew123, has done a lot of the data reduction and Python action in this part of the project. Our sample of giant AGN clouds now includes 19, each showing gas more than 10 kiloparsecs (32,000 light-years) from the galaxy core, so we get information on its history over at least that many years.
Why do we want to find these? From what we’ve learned about Hanny’s Voorwerp, we have the possibility of tracing the history of active galactic nuclei – how fast they can fade, how long they stay on at once, and maybe how they influence their surroundings. It’s hard to generalize from a single instance (though astronomers are notorious for trying), so we want enough of a sample for statistically defensible conclusions.
Which of our objects should we ask to look at, balancing the demand for Hubble time against the fact that very often more data are better? From our current point of view, investigating whether active nuclei shut down very often within time spans roughly 100,000 years, we want to concentrate on the ones that show evidence for a deficit of energy from the core compared to that needed to light up the gas we see. This gives us a set of 7 (plus NGC 5252, which has been known for many years and already has archival Hubble images). They are:
SDSS J143029.88+133912.0, the Teacup (Kevin started calling it that in honor of the handle-like loop of gas extending more than 15,000 light-years to the side). This is the most distant galaxy in the sample, at z=0.085.
UGC 7342 (also known in some Zoo threads as the Crab galaxy), with its enormous filaments of gas stretching more than 100,000 light-years on each side (fully half as large as Hanny’s Voorwerp, and at about the same distance).
Another new SDSS AGN, and new Zoo find for its gas clouds, is
SDSS J220141.64+115124.4 (which I tend to abbreviate to SDSS 2201+11 for my own sanity).
SDSS J151004.01+074037.1, with symmetric clouds around a type 2 Seyfert nucleus, is another SDSS//GZ discovery.
NGC 5972 is pretty close to us at z=0.0297. This galaxy attracted some brief notice in a 1995 paper by Mira Veron-Cetty and Phillippe Veron, who established that what look like spiral arms are purely ionized gas features, and noted that this AGN is surrounded by a double radio source.
UGC 11185 is part of a very disturbed interacting pair, with a bright spray of ionized gas seen to the east of the AGN. Note to self: we need to take care in where we center the Hubble images to avoid problems with scattered light from the annoyingly bright star.
Mkn 1498, as the name suggests, has a long-known AGN, but its ionized gas shows such strong radiation
reaching it that this one may have faded even to its observed brightness.
We based our observation request on what we learned from working on the Hanny’s Voorwerp data. The most valuable results for these will probably come from images in Hα and [O III] for these, the clouds sometimes extend into the galaxy, so we need red and green starlight images to distinguish stars and gas. Our filter selection depends on the galaxy redshift – the brand-new Wide-Field Camera 3 (WFC) has cleaner performance but only a limited set of narrow filters, while the decade-old Advanced Camera for Surveys (ACS) has ramp filters which can tune to any desired optical wavelength, but its CCD detectors are really showing the ill effects of damage from years in the space environment. So we use WFC where we can and ACS otherwise, emboldened by the release of Jay Anderson’s software for partially undoing the effects of that radiation damage. We end up using 3 orbits per galaxy – one for each emission line and the other for broad-band filters to map the structure and color of each galaxy’s starlight.
We know some of the things to look for in the data – regions of star formation, gas outflow, maybe shadowing effects from material near the galaxy nuclei. But the best is often in what we don’t know to look for beforehand…
Next up – detailed observation planning, at which point we get our first hints as to when each galaxy could be observed.
And here they are! The Hubble Space Telescope results on Hanny’s Voorwerp (and IC 2497) were just released during the Seattle meeting of the American Astronomical Society. You can see the press release and download several formats of the main image from STScI, download the poster presentation from the meeting, and view the abstract of the meeting presentation, but we can give you the whole story in detail here on the GZ blog. But before that, here’s the money shot:
This combines different filter sets for IC 2497 and Hanny’s Voorwerp, to give the best overall view in a single image. The Voorwerp shows up in [O III] and Hα emission, close to natural color were our eyes sensitive enough. The galaxy IC 2497 is rendered from red and near-IR filters, so it’s a bit redder than we see in other images (for example, from the SDSS or WIYN data). This version does a very attractive job of contrasting the two objects in both color and texture; for reasons you’ll read below, Zolt Levay and Lisa Frattare at STScI had a daunting task to make a single image look this nice from our disparate data sets.
Looking back, we proposed these observations early in 2008, as it was just becoming clear what an interesting object the Voorwerp is and what the right questions might be. It was our good fortune that these observations remained relevant with all we’ve leaned since then, including radio and X-ray measurements. Each data set was specified with a particular goal as to selection of filter or diffraction grating, location, and exposure time. We had images and spectra, the latter looking for material very close to the nucleus of IC 2497 so we could zero in on what’s happening there with much less confusion from its surroundings than we get from the Earth’s surface. Full details of the observation planning appeared in this blog post after our proposal was approved.
The released image product combines four filters and two cameras – the Wide Field Camera 3 (WFC3), installed during the 2009 shuttle servicing mission, and the Advanced Camera for Surveys (ACS), electronically repaired during that mission. WFC3 offers superior sensitivity and small enough pixels to exploit the telescope’s phenomenal resolution, while ACS has a set of tunable “ramp” filters that allow us to observe the wavelength of any desired spectral feature at a galaxy’s particular redshift. Our WFC3 images were designed to look at IC 2497, at wavelengths deliberately minimizing the brightness of the gas in the Voorwerp so we could also look for star clusters in its vicinity, that might tell us whether its gas came from a disrupted dwarf galaxy. (This turned out to be a good idea for a slightly different reasons). Our filter choices included one in the near-infrared, at 1.6 micrometers; one in the deep red, close to the SDSS i band, and one in the ultraviolet. Comparing these, we would be able to say something about the age of any star clusters we found. The UV image would also show whether there were any particularly bright clumps of dust, which could serve as “mirrors” so we could later measure the spectrum of the illuminating light source.
The ACS narrow-band filter data were designed to probe the Voorwerp itself, seeking fine structure that is blurred away in ground-based images. We use both the strong [O III] emission (which made it so recognizable when Hanny picked it up from the SDSS images), and the Hα line of hydrogen. Their ratio changes as the conditions ionizing the gas change, so we could watch for variations as conditions differ from place to place. This also netted us gains we hadn’t quite asked for. We also had to work for them. it was known before its repair that the CCD detectors on ACS had suffered from exposure to energetic particles in space during its decade in orbit; all such devices do to some extent. One result of this cumulative damage is that they don’t transfer charge as efficiently as they originally did, and in particular, after being struck by a charged particle (“cosmic ray”), when the image reads out, the detector registers not only a bright spot (which is easy to filter out) but a long trail, so long that it’s not at all easy to filter out without blacking out much of the image. The situation improves if you combine two images, moving the telescope slightly in between them. That lets the software reject the bright cosmic-ray spots, but even after tuning the rejection parameters, the streaks still remain down the device rows. We ended up using what we knew about the remaining trails to filter them out, at the expense of losing the dimmest, smooth regions of emission. Fortunately, these dim regions don’t have enough signal to show new features in Hubble data, so we used ground-based images to fill them in around the edges and in the fainter interior regions of the Voorwerp. This panel shows four successive stages in the process:
Now, as proud as I am of fixing this all up, you don’t really care – you want to know what we’ve actually learned from Hubble. I’m with you – here goes!
Stars are forming in a small part of the Voorwerp. We couldn’t see this from the ground because the regions involved are dim and blend with the overall gas emission. Hubble shows them in two ways. Our filters selected to minimize emission from the gas show bright blue spots, with the size and brightness of young star clusters, in a single area only 2 arcseconds long (about 2 kpc, 6500 light-years). Because the deep-ultraviolet spectrum of starlight is quite different than from an AGN, we see a second signature – the balance between light from [O III] and Hα tips in favor of Hα , again very different from the highly ionized gas elsewhere. In the picture at the top, that makes the star-forming regions show up as the reddish spots near the top bright region of the Voorwerp. These gaseous regions are likewise so small and comparatively dim that we couldn’t see them in even our best ground-based data. The hottest stars are so bright that it’s hard to tell how long these clusters have been forming stars; we know that they include stars so hot that they can be no more than a few million years old. It seems suspicious that we see these star-forming regions only in a small area, which lies closest to IC 2497 at least in our view (we don’t have enough information to be sure it’s really the part closest to the galaxy), and roughly lined up along the direction of outflowing material seen with radio telescopes. This all suggests that the star formation has been set off by compression as gas outflowing from the galaxy encounters the gas in the Voorwerp. We don’t see such star clusters anywhere else in the enormous trail of cold hydrogen of which the Voorwerp is the illuminated part, so their occurrence is connected to the Voorwerp specifically.
This kind of event – an active galactic nucleus blowing gas outward and driving formation of new stars – is one form of feedback, connections between AGN and their surrounding galaxies that seem to be important in regulating aspects of galaxy evolution. We see this process in some other galaxies, in some cases much more intensely than in Hanny’s Voorwerp. A favorite example is Minkowski’s Object, a brilliant emission-line object near the radio galaxy NGC 541 at only a third the distance of IC 2497. In Minkowski’s Object, the central AGN is almost solely a radio object, without the ultraviolet output that ionizes Hanny’s Voorwerp. In that case, where we see ionized gas, it’s lit up by hot, newly-formed stars. This object lies right in the path of the jet of radio-emitting plasma from NGC 541, which is disrupted right as it reaches the object; this looks like a cloud of gas, maybe a dwarf galaxy, that was quietly minding its own business until it was hit by a flow of relativistic particles. Star formation is happening over regions of about the same size in both Minkowski’s Object and Hanny’s Voorwerp, but there are important differences: gas is shedding back away from Minkowski’s Object much more violently than in the Voorwerp, the radio jet in NGC 541 is much more powerful than the small one in IC 2497, and it doesn’t have the additional UV output to light up the gas which is not forming stars. The point I take from this is that the outflowing material reaches the Voorwerp, but with only enough pressure to set off star formation in the densest and closest parts, without the massive reshaping we see in some other galaxies.
The Hole This is probably the most obvious structural feature of the Voorwerp (sometimes seen as the space between the kicking frog’s legs). We wondered whether it might mark the site of some kind of titanic explosion, the space where a jet from IC 2497 drilled its way through the gas, or even the shadow cast by some dense cloud close to the galaxy nucleus. What we learned on this was mostly negative – we do not see streamers of gas blasting away from the hole, or the ionization of the gas being any higher near it, as we would expect for a jet or explosion. The idea of a shadow still makes sense, but we’d still like to know more (maybe from some Gemini-North velocity data still being analyzed).
The nucleus has faded dramatically, but it still knows how to make other kinds of fireworks.One idea we have been investigating, since we saw the first spectra, has been that the nucleus of IC 2497 might have faded in its output of ionizing radiation by hundreds of times since the light that we see reaching the Voorwerp left it. (Ignore for a moment the odd complications of verb tenses involved in light-travel time discussions). What Hubble could add is the ability to measure spectra from very small regions free of the mixing with light from the surroundings that occurs from the ground. This way we could get a good look at the nucleus of IC 2497, how much ionization its gas sees, and whether there might be shreds of highly ionized gas peeking through surrounding obscuring dust and gas which otherwise obscures our view. For this we used two spectra (blue and red) with the Space Telescope Imaging Spectrograph, newly restored to service by the astronauts of STS-125. We get a clearer measure of the gas – it sees a slightly weaker ionizing source than we thought from the ground because the gas is mostly closer to the core than we would have thought. This fits with the X-ray data in suggesting that the nucleus isn’t just hidden, it really has faded so we see its echo in the Voorwerp. (To indulge once again in a bit of geekdom, “It’s dead, Jim”.)
Or maybe not so much dead as… transformed. The spectra also show something else that we didn’t know to look for. Only a half arcsecond from the core we see a second set of emission lines, redshifted by about 300 km/second from the nucleus. At this location in the ACS images, we see a loop or bubble of Hα emission. The nucleus has been blasting out material, driving this wave into the surrounding gas. Depending on whether it has been slowed down as it snowplows into these surroundings, and on whether it lies exactly in the disk of the galaxy, this would have started less than about 700,000 years ago. This was probably well before the nucleus faded, but connects to an intriguing idea long suggested from looking at the various kids of active galaxies. There may be two modes of accretion power – one in which most of the energy comes out as radiation, and one in which it emerges as kinetic energy of expelled matter (“radio mode”). Has IC 2497 switched between them almost as we watched?
Here is that expanding loop. At left is the Hα image, showing it sticking out above the nucleus. In the middle is a broad-filter image taken to zero in the pointing to take the spectra – you don’t see the loop because it’s swamped by the starlight. At right, the spectrum near Hα showing the distinct emission-line clouds just above the nucleus.
IC 2497 has had a troubled past. It looked a bit odd in our earlier data, but the Hubble images make clear how disturbed this galaxy is. Spiral arms are twisted and warped out of a single plane, and thick dust patches also show that it has yet to settle into a simple form after the disturbance. This looks a lot like the aftermath of a strong interaction – and since we see no other culprit nearby, probably a merger where the other victim is now part of IC 2497. The companion barred spiral just to the east (left) may be only an innocent bystander – it shares the redshift and distance of IC 2497, but is so symmetric and undisturbed that it’s hard to picture it being involved in a collision that wracked the larger galaxy (and tore loose 9 billion solar masses of hydrogen gas from somewhere). These dust patches, plus our better understanding of the Voorwerp’s continuous spectrum aided by the GALEX UV data, now suggest that instead of slightly in front of IC 2497, the Voorwerp is more likely slightly behind it, roughly over the pole of the galaxy. This means that we see the reprocessed radiation via the Voorwerp perhaps 200,000 years after it left the galaxy nucleus. Old news can still be very informative.
Putting it all together, here is the sequence of events that we think led to what we see today. Perhaps a billion years before our present view (which is itself 700 million years behind the times, and there’s nothing we can do about the speed of light), a merger led to IC 2497 forming from two progenitors, with its disk slowly settling but still warped. One product of this merger was an enormous tidal tail of gas, which came to stretch nearly a million light-years around IC 2497. During this process, material accreted into its central supermassive black hole, rapidly enough to produce the energy output of a central quasar as a byproduct, and illuminating and ionizing gas that was exposed to its radiation, to make the Voorwerp. About a million years before we see it, it started to blow material away (and this may have been when its radio jet and outflow started). Then, later still, the core faded, maybe as its energy output switched from being mostly in radiation to mostly powering the motion of material out of the galaxy. And now we see Hanny’s Voorwerp as a very lively echo of the past, as the last radiation from the fading core aces outward but has yet to complete the zigzag trip from galaxy to Voorwerp to us.
Of course, we’d like to test these ideas further. There are plans to improve the spectral mapping, and perhaps look at the history of star formation to tell when things happened to IC 2497. And we want to see whether this behavior is at all common in galactic nuclei – if so, there may have been more active galaxies lately than we thought, and it would change how we think about the relation between “normal” and active galaxies. Zooites have pushed us along greatly in finding smaller cousins – voorwerpjes. But that was a separate meeting presentation…
As anyone who’s been following the twitter feed in the last few days knows, today is a very exciting day for Galaxy Zoo as the long-awaited Hubble Space Telescope image of Hanny’s Voorwerp is released. Kevin has already shown the image in a science talk this morning, and Bill’s poster is up in the conference hall giving more details, but the official release is in just a few minutes. I’m writing live from the press conference room at the American Astronomical Society’s meeting in Seattle, sitting next to the object’s discoverer and namesake Hanny van Arkel and Zooniverse developer Rob who will be keeping twitter up to date.
There’s a separate, long post from Bill ready to go live describing the results, including some very exciting new details, but I’ll use this post to keep you up to date with what happens in the press conference itself.
Three minutes to go…
12.44 : Kevin and Bill are on stage, alongside Leo Blitz of the University of California and Amy Reines from the University of Virginia, who will also be talking about their work during the conference.
12:52 : Introductions from Rick Feinberg, AAS press officer are over, and Bill’s taking the stand.
12:54 : Bill : ‘Conclusive proof that we have seen a quasar turn off’. Giving the background to the Voorwerp, and introducing Hanny, who immediately became the focus of every camera in the room.
12.56 : The image appears on screen. It looks terrible under the press room lights. Go to the online version, everyone!
12.58 : On to Kevin – the quasar is either hidden or has shut down, in which case the Voorwerp would be an echo of light, not of sound. In the meantime, for those wanting gory details Bill’s blog post is up.
13:00 : IC 2497 does shine in x-rays, but it’s like looking for a floodlight through a bank of fog and finding only a laser pointer – totally inadequate to light up the Voorwerp.
13:04 : In fact, the difference is at least 10,000 times. The timescale on which the reduction must have happened is now believed to be 200,000 years or less (we used to think 70,000 years or less).
13:06 : Back to Bill for one of the major results from Hubble : The bright area close to IC2497 is an area of star formation – so the interaction is triggering star formation in the Voorwerp. We couldn’t have seen this without Hubble, but it fits with the Westerbork radio results that showed evidence for a jet and larger outflow of gas in this direction. We think these are compressing the gas – rather as happens in Minkowski’s object (which was mentioned in my original Voorwerp paper). Much milder in the Voorwerp, though.
13:10 : Spectrum of the main galaxy confirms x-ray results – but we also cut across another area, which seems to be a bubble being blown into the disk of the galaxy – another aspect of an outflow of gas fostered by the nucleus of IC 2497.
13:15 : Bill again : We think that one reason this object is important is because it would be a coincidence for us to get lucky only once, with the nearest quasar. The Galaxy Zoo volunteers have poured through 15,000 candidate images and found 18 related objects – which have been confirmed by follow up. Being presented in another poster, the lead author of which is a Galaxy Zoo volunteer (and undergraduate on a summer program).
13:15 : Dodgy internet connection – sorry! We’re going to try a ustream chat with Kevin, Bill and Hanny after the conference ends. I see Hanny’s mother is watching – we can confirm she’s been very well behaved.
13:22 : Leo Blitz moves us away from the Voorwerp – They believe they’ve found a missing link between galaxies that form stars and those that are dead. A major topic of astronomical interest is how galaxies loose their gas and become old, red and dead. NGC 1266 is an unremarkable galaxy, slightly less massive and smaller than the Milky Way. It has an active galactic nucleus, but to all appearances is otherwise dead. Observations of the molecular gas from which stars mind form show that it’s concentrated in the nucleus of the galaxy. Excitingly, it appears to be fueling a wind that’s been blowing for more than 2.5 million years, and moving fast enough to escape the galaxy. About 13 Sun’s worth of gas escapes each year, and so in about 85 million years’ time, at the current rate, the gas will have been exhausted and the galaxy will have completed its transition to an old galaxy.
13:26 : Our final speaker, Amy Reines, is introducing us to Heinze 2-10. It’s a small galaxy, and yet it hosts a super-massive black hole. In fact, the black hole is the same size as that in the Milky Way but the galaxy is closer to the Milky Way’s satellite galaxies, the Magellenic clouds.
13:30 : Possibly explanation is that black holes form before the bulges at the center of the galaxies do – which would answer an age-old chicken and egg problem! Now on to questions – my laptop is dying, but I’ll hang on as long as I can.
13:36 : Good question on Twitter from Ann Finkbeiner – ‘if the galaxy is 650 million LY away and the quasar turned off 200,000 years ago, how come we’re now seeing it with no quasar?’. We mean that we’re now seeing light that was emitted no more than 200,000 years after the quasar switched off.
An especially nice side project of Galaxy Zoo has been uncovering giant gas clouds ionized by active galactic nuclei. Of course, the most striking of these has been Hanny’s Voorwerp, whose study has been fruitful enough! In addition, Zooites proved to be good at finding smaller, dimmer versions (“voorwerpjes”, using the Dutch diminutive form). We harvested these both from Forum postings, and from a targeted hunt of galaxies with known AGN (arranged by Waveney). To be sure what we’re dealing with, we need spectra of these clouds. We had observing runs to do this last summer from Kitt Peak and Lick Observatories, confirming many new cases (some of which may have a similar history of faded glory to what we infer for Hanny’s Voorwerp). Kevin led a further proposal to look at these in X-rays, so we can tell whether the suspiciously dim nuclei are really dim or actually hidden by foreground gas and dust.
We have another observing session this week with the 3-meter Shane telescope of Lick Observatory, a facility of the University of California. The proposal was led by Vardha Nicola Bennert from Santa Barbara, observing as before with UCSB students Anna Pancoast and Chelsea Harris. Here they are standing in front of the telescope’s mirror cell and instrument cluster.
I was also able to arrange going to Mount Hamilton for these four nights. This will be sort of a homecoming for me – as a graduate student at UC Santa Cruz, Lick was where I cut my observational teeth. Checking some old logbooks, I have entries slightly over thirty years ago (and a collection of photos which should shortly be updated!). For my thesis, on the spectra of gas in normal galaxies and whether they contain weak AGN, I used over 70 nights on the then new 1m telescope. So not only was UCSC where I studied interpretation of spectra, but Lick was where I got my first (few hundred) galaxy spectra. Back then, the 3m was the primary telescope for UC astronomers, so graduate students worked with it only while assisting faculty members. I did manage to spend a few hours at the prime focus of the 3m while we were working on what might have been the observatory’s first CCD spectrograph (constructed by painful use of a hacksaw on one originally built for Margaret Burbidge to use image tubes with). We looked at a planetary nebula for wavelength calibration, giving me a lasting memory of just how green the [O III] emission lines appear. (I also managed to see the radio galaxy Cygnus A through the eyepiece while lining up the spectrograph slit). Another student and I managed to get allocated a single otherwise unused night of 3m time to split – and it snowed. Here’s documentation – that young-looking guy is in the cage sitting in the middle of the telescope at the top of the tube. This was such a stopgap setup that the only way to refill the CCD’s liquid nitrogen involved running a long tube between circuit boards of the control electronics. I categorically deny ever having dumped liquid nitrogen on my advisor. Almost.
Times have changed. Everything is remotely operated, and not only do CCDs rule, but the Kast double spectrograph uses a dichroic beamsplitter to separate blue and red light so that each goes into a separate spectrograph and CCD optimized for that part of the spectrum. We’re ready with a list of target galaxies winnowed down by reanalysis of the SDSS images after selection by Zooites. If last summer’s set is a guide, at least half of these will prove to have huge, galaxy-sized clouds illuminated by seen or unseen nuclei. I hope to get the results processed quickly; at the Seattle meeting of the American Astronomical Society in January, there will be a display presentation from last summer’s work led by Drew Chojnowski, and it would be great to double our sample size.
Of course, as with any ground-based observations, we will be at the mercy of the weather. You can keep track using the especially crisp webcam views provided by Lick (one of which looks across the brilliant city lights of the whole San Francisco Bay area, which have been kept slightly under control by extensive light-pollution lobbying but made Lick astronomers some of the first to use digital sky subtraction for spectra). And of course we’ll keep the Zoo updated on our data.