Archive by Author | billkeel

Reflections on Voorwerpjes

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.

Lick 3m Shane telescope in 2010

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.

UGC 7342 and the Hubble Voorwerpje roundup – can they get any weirder?

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:

Kevin makes his case

Kevin makes his case

Today we’re presenting first results of the Hubble imaging of Voorwerpje systems. This is what our poster looks like:

AAS poster paper on Hubble Voorwerpje data

AAS poster paper on Hubble Voorwerpje data

(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:

UGC 7342 Hubble image

UGC 7342 Hubble image

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!

Overlaps and backlights and silhouettes – oh, my!

After a winding path, the first overlap paper from the Galaxy Zoo search has been accepted for publication. The title and abstract pretty much tell the story (the title links to the complete preprint):

Galaxy Zoo: A Catalog of Overlapping Galaxy Pairs for Dust Studies

William C. Keel, Anna Manning, Benne Holwerda, Massimo Mezzoprete, Chris Lintott, Kevin Schawinski, Pamela Gay, and Karen L. Masters

(PASP, likely January 2013 issue)

Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light. We present a catalog of 1990 such galaxy pairs selected from the Sloan Digital Sky Survey (SDSS) by volunteers of the Galaxy Zoo project. We highlight subsamples which are particularly useful for retrieving such properties of the dust distribution as UV extinction, the extent perpendicular to the disk plane, and extinction in the inner parts of disks. The sample spans wide ranges of morphology and surface brightness, opening up the possibility of using this technique to address systematic changes in dust extinction or distribution with galaxy type. This sample will form the basis for forthcoming work on the ranges of dust distributions in local disk galaxies, both for their astrophysical implications and as the low-redshift part of a study of the evolution of dust properties. Separate lists and figures show deep overlaps, where the inner regions of the foreground galaxy are backlit, and the relatively small number of previously-known overlapping pairs outside the SDSS DR7 sky coverage.

This was the project that first drew me in to Galaxy Zoo, way back in August of 2007. Zooite’s contributions to this, perhaps the first science project organized on the Forum, exceeded my wildest hopes. As the paper shows, the previously-known set of backlit spiral galaxies in the local Universe contained only about 20, severely limiting what we could learn about the galaxies’ dust content. By the opening of Hubble Zoo, when we froze this particular list for publication, the combined catalog reached nearly 2000. Further use of this catalog is well along – we’ve had several observing runs at Kitt Peak 2.1 and 3.5m telescopes to do more detailed images, and colleague Benne Holwerda is headed to the 4.2m William Herschel Telescope atop the island of La Palma next month for more. With such a large starting sample, we can address questions we couldn’t before. How much variation in dust content and distribution do we see among apparently similar galaxies? How many dwarfish galaxies show the kind of unusual dust concentrations in their outskirts seen in one particular case? Do we see significant dust that is so cold that it eludes even far-infrared detection? On another keyboard, I’m working now to finish a paper on the ultraviolet absorption properties of dust in galaxies, combining a target list of spiral/spiral pairs from the GZ catalog with GALEX satellite data and our ground-based images. Not only is this interesting in knowing how clumpiness of dust affects its absorption properties, but is a key stepping stone toward another project – using backlit galaxies from Hubble Zoo to probe the history of dust in galaxies over cosmic time. We see some at such high redshift that the Hubble data sample light that started out well down toward the ultraviolet, so knowing how to compare that to our place and time is the basic starting point.

To show off the richness of this collection, this two-part figure from the paper shows how we divided them up into broad categories so that subsets useful for different things can be easily retrieved. This is one thing that lets us address different questions – we now have many examples of a broad range of geometry and combinations of galaxy types.


ImageNearly 600 Zooites contributed candidate pairs (we list the thread participants on the data page for seo services). A few deserve to be singled out. Half65, of course – not only did he find a remarkable fraction of these pairs, but he did a lot of work collecting their SDSS information. You’ll notice he’s a coauthor – fair is fair! Also, c_cld continues to display his remarkable SQL skills – he saved me a lot of time in revision by finding all the redshifts that were new when SDSS DR8 was released. Jean Tate corrected some typos in the data table (how does she do that?), and helped prod me to take the time to organize the pair members’ data more systematically by magnitude.

Regular Forum readers will recall that the first version of this paper was submitted last year. The MNRAS referees liked the analysis, but felt that the extensive catalog itself was better suited to another journal. As a result, we split that paper in two, sending the catalog and its documentation to the Publications of the Astronomical Society of the Pacific, and have been able to expand the analysis of dust distributions, so that paper will wind up even more substantial than we anticipated.

Once again – thank you to all who have taken part, and keep looking in the background as the Zoo takes in more of the sky from more telescopes!

It’s a bat! It’s a dragon! It’s UGC 11185!

It’s a bat! It’s a dragon! It’s a galaxy-sized set of crab claws! It’s a complex abstract design blending elements of the Mandelbrot set with classical Persian carpet design!

Or, none of the above. This is the first-cut overlay of two Hubble images of the voorwerpje-hosting AGN in UGC 11185. As usual, [O III] is in green and Hα in red; neither has yet had the contribution of starlight in the galaxy taken out. The inner set of clouds could, with a little imagination, be seen as the ind of two-sided ionization cone seen around many active galactic nuclei. But the outer cloud, the one that selected this object for our sample by extending more than 10 kiloparsecs from the core – this cloud shows pillars, loops, and gaps. (I’d like to acknowledge here the help of STScI program scientist Linda Dressel, in making suggestions for subtle changes to our pointing strategies to reduce problems from such effects as reflections from that bright star on the edge of this image).

[O III] and H-alpha images of UGC 11185

Later the same day we first saw the Hubble images, we received additional information on UGC 11185. Colleague Alexei Moiseev (who first showed up on the GZ forum when his team was making sophisticated use of the GZ1 database to seek polar rings) has been getting data on some of these objects with the 6-meter telescope (БТА, Большой Телескоп Азимутальный or BTA) of the Russian Academy of Sciences. In the right redshift ranges, they have a Fabry-Perot device which can map the Doppler shifts of a single emission line across the whole galaxy. As supporting data, he also got this multicolor wide-field image, which shows the complicated tidal disturbances in this galaxy pair. The giant loops of stars may indicate that both are partial ring galaxies. UGC 11185 is the upper left one – you can pick out its core the bright star above it, and the glowing gas clouds around the core and to its left.

UGC 11185 from 6m BTA

Alexei also forwarded this image of a familiar field. We’ve been re-examining the faint outer parts of IC 2497 and its companion galaxy, to see what we can learn about the history that pulled gas out where it could be ionized to form Hanny’s Voorwerp. There is a faint tidal tail to the east (left) seen in Hubble images, and now this wider-field and long-exposure BTA image show how far it stretches. Watch this space to see what we can all learn from this.

IC 2497 image from 6m BTA

An anniversary Voorwerpje

Following on the heels of the 5th anniversary of Galaxy Zoo itself, this week marks five years since Hanny pointed out the Voorwerp.

To help celebrate the occasion, we have new Hubble data on another of
its smallar relatives. This time the telescope pointed toward
SDSS J151004.01+074037.1 (SDSS 1510 for short). This has a type 2 (narrow-emission-line) AGN at z=0.0458. This was (as far as I can tell) first posted on the forum by Zooite Blackprojects, and also identified in the systematic hunt. Here it is in the SDSS:

SDSS 1510+07 from SDSS

SDSS 1510+07 from SDSS

Aa usual, these are minimally processed Hubble data, and in particular using filters where we can’t get the color right for both clouds and starlight at the same time without more work. As usual, green comes from [O III] and red from Hα , so green is more highly ionized gas. This one is cool enough already – I think of a Martian flamenco dancer with some cobwebs, but your view may vary:

SDSS 1510+07 from Hubble

SDSS 1510+07 from Hubble

Looking slightly ahead, next week, Alexei Moiseev, known on the Zoo forum from his wrk on a catalog of polar-ring galaxies based on a clever use of Zoo-1 click data, will be working with us next week, obtaining radial-velocity maps of three of these galaxies using the 6-meter Russian telescope in the Caucasus (the BTA, Bolshoi Teleskop Azimutalnyi or Large Altazimuth Telescope).

And on that note, I turn back to a new book on black-hole astrophysics,
which has me peeking ahead to page 749 for a table of timescales for
accretion phenomena. That, and wish everyone a happy, highly-ionized and just slightly late 5th Voorwerpendag!

Something rich and strange – Hubble eyes NGC 5972

We just got the processed Hubble images for NGC 5972. This is a galaxy with active nucleus, large double radio source, and the most extensive ionized gas we turned up in the Voorwerpje project. We knew from ground-based data that the gas is so extensive that some would fall outside the Hubble field (especially in the [O III] emission lines – for technical reasons that filter has a smaller field of view). We expected from those data that it would be spectacular. Now we have it, and the Universe once again didn’t disappoint. Another nucleus with a loop of ionized gas pushing outward (this time lined up with the giant radio source), twisted braids of gas like a 30,000-light-year double helix, and dramatically twisted filaments of dust suggesting that the galaxy still hasn’t settled down from a strong disturbance.

Here’s a combination of the Hα image (red) and [O III] (green) data, with the caution that neither has been corrected for the contribution of starlight yet. The image is about 40 arcseconds across, which translates to 75,000 light-years at the distance of NGC 5972. This gives the team plenty to mull over – for now I’ll just leave you all with this view. (Click to enlarge – you really want to.)

NGC 5972 from HST in [O III] and H-alpha

NGC 5972 from HST in [O III] and H-alpha

Hubble spies the Teacup, and I spy Hubble

Our Hubble image of Voorwerpje galaxies continue to come in, and it seems each one is stranger than the last. Overnight we got our data on the Teacup system (SDSS J143029.88+133912.0). This one attracted attention through a giant emission-line loop over 16,000 light-years in diameter to one side of the nucleus.

I was worried to get email this morning that there had been a failure to lock on to one of the two needed guide stars so that the telescope might have rolled enough during the observations to compromise data quality. Inspecting the data, it looks like we’re OK. We’re OK and the galaxy is strange. This is a composite of [O III] (green) and Hα (red), right out of the software pipeline without any additional processing:

The Teacup AGN in raw emission-line Hubble images

Another giant hole whose origin is obscure. The loop doesn’t even show much sign of being connected to the galaxy. The strongest [O III] does seem to trace out ionization cones, as in showing from structures near the nucleus, but that seems independent of the distribution of the gas. There are filaments in the gas that are nearly parallel, sort of like waves. Well have our work cut out for us to understand more of what’s going on here. I can hardly wait for the next one!

There was an extra treat for me with these observations. Last night, I interrupted a session with my summer class at the campus observatory to look south with binoculars and catch Hubble passing far to the southeast, no more than 13 degrees from our horizon. This was during the Hα exposure, so I saw it while it was doing these observations (it was pointed just about up in my frame of reference, as it happens). I got a picture through a 125mm telescope, showing the telescope streaking by just north of the star k Lupi. At the time, Hubble was 1600 km away over Cuba. Hubble was watching the Teacup, I was watching Hubble, and a couple of slightly puzzled students were watching me.

Hubble trails across the sky north of the star k Lupi in this telescopic view.

Curiouser and curiouser – Hubble and Mkn 1498

Fresh off the telescope, here’s a first view of the “Voorwerpje” gas clouds around the Seyfert galaxy Markarian 1498. Its nucleus, shown in our Lick and Kitt Peak spectra, is a type 1 Seyfert, meaning that we see the broad-line region of gas very close to the central black hole, moving at high velocity. Those data showed highly-ionized gas to a radius of at least 20 kiloparsecs (65,000 light-years). Its nucleus is too dim to account for the ionization of the extended gas clouds, which landed it a spot in our list of seven objects for the Hubble proposal. Getting these data now was an unexpected treat – they were originally scheduled to be taken next November. As another bonus, the good people at the Space Telescope Science Institute just last week implemented the software to deal with charge-transfer problems in the Advanced Camera CCDs, right in the pipeline, improving the image quality a lot (it took months to get to this point with the Hanny’s Voorwerp data). And here it is, Markarian 1498 in a combination of [O III] emission (green) and Hα (red):

This is… interesting. From the few of these galaxies where we have data so far, loops of ionized gas near the nucleus may be a recurring theme. I could add speculations on what we’re seeing in Mkn 1498 – but for now, I’ll just let everyone enjoy the spectacle.

A first Hubble look at UGC 7342

Overnight, Hubble got our first data on perhaps the most spectacular Voorwerpje host galaxy, the merging system UGC 7342. We have to wait until almost the end of the year for what we really wanted to see, the ionized gas. The telescope has particular time pressure in some parts of the sky (as if it doesn’t have extreme time pressure on everything people want to do with it), so we split the two sets of images to fit the schedule better. This time, we got data in WFC3 for two medium-width filters in the orange and deep red, selected to be essentially blind to emission from the gas. These will be used to subtract the contribution of starlight from the gas images, so we can analyze the gas properties cleanly. The emission-line images use the older ACS camera, which has a set of tunable filters which can isolate any optical wavelength we need. They come at year’s end, because we have to specify a particular range of orientation angle of the telescope to fit all the gas in their 40×80-arcsecond filter field. That, plus the requirement that the solar arrays can face the Sun directly, gives us a restricted time window.

As a reminder, here’s UGC 7342 from the SDSS data.

And here is a first look at the Hubble images, warts and all. With only the two filters in orange and deep red, the color information we get is pretty muted. Here’s the whole galaxy, shrunk 4 times from the original pixel scale to fit:

This show the companion and tidal streamers of stars. UGC 7342 itself shows shells of stars, which can be formed when a lower-mass disk galaxies merges with a more massive elliptical. With some additional velocity information, those might be able to give a time since the merger (with some tailored simulations). The elliptical reflections are from bright foreground stars outside this trimmed view; the emission-line images will at least have these in different places, using a different camera and different telescope orientation.

Zooming in 4 times to the nucleus shows that UGC 7342 has complex dust lanes crossing in front of the core. These are perpendicular to the directions where we see that distant gas is ionized by radiation from the nucleus, which is pretty common. The fact that the dust (and almost certainly associated gas) wraps at right angles to most of the structure in the galaxy is another indication that a merger took place recently enough that the situation hasn’t settled down into a long-lasting remnant.

Next up? The scheduling windows for SDSS 1510+07, NGC 5972, and the Teacup AGN all happen in overlapping spans from June to August. Bring on the bits!

Kitt Peak wrapup – for now

The end of this observing run made up for the unnecessarily interesting variety of weather early on. Calm, clear skies, ran through an object list just as fast as we said in the proposal. Here’s a montage of the red-light images from 9 of the target galaxy pairs done on night 4.

Overlapping galaxy pairs from KPNO 2.1m telescope

That’s it for this run. But we’ll be back there late next month to really clean up the target list.