Almost 15 years ago, what first attracted me to be involved with Galaxy Zoo was the ability of participants to pick out rare galaxy types, especially silhouetted or overlapping galaxy systems. These highlight the effects of dust in the foreground galaxy on passing light, and offer ways to study the dust which are complementary to, for example, observations in the deep infrared where the dust itself shines, giving off the energy it absorbs from starlight. Visible-light measurements of backlit galaxies show us the dust no matter how cold it might be, where it can hide from IR detection, and at the high resolution available to optical telescopes (including the Hubble Space Telescope) rather than the more modest, wavelength-limited resolution we can achieve at longer wavelength. Better measurements of dust in galaxies affect our understanding of their energy output, stellar content, and even our view of the more distant Universe. Galaxy Zoo volunteers contributed to a catalog of nearly 2000 suitable galaxy pairs from the first iteration of the project, since expanded from Galaxy Zoo 2, GZ Hubble, and the most recent examinations using the Legacy Survey data. We have used this list for number of followup studies – although, truth be told, I have also been distracted by other rare systems found by volunteers (cough, Hanny’s Voorwerp and the Voorwerpjes, for example).
The backlit-galaxy system VV191 was first reported in the Galaxy Zoo forum as a possible galaxy merger, by user Goniners on November 2, 2007. Despite its near-perfect geometry for study of foreground dust, VV191 had eluded our earlier searches because the inner regions, where one can see that this is a superposition of undisturbed galaxies rather a merging galaxy pair, are saturated in prints of the Palomar Sky Survey, which was the best visible-light survey before the Sloan Digital Sky Survey. At the time VV191 was selected for further study, catalogs showed a substantial redshift difference between the two galaxies, which is desirable so the two galaxies are unlikely to be physically interacting with each other, and light from the background galaxy scattered by the dust becomes much fainter. That has been revised by later data which put the redshifts closer; we can’t win them all, though the two galaxies are very symmetric and undisturbed in all our later data.
(Hubble red-light image of VV191, showing silhouetted dust in the foreground spiral arms)
We got a closer look with the STARSMOG project led by colleague Benne Holwerda, which was a Hubble snapshot program – one where short exposures are inserted into gaps in the telescope schedule, much like the Zoo Gems gap-filler project. STARSMOG drew promising overlapping-galaxy pairs from Galaxy Zoo forum posts and the GAMA (Galaxy And Mass Assembly) project. Over several years, it acquired images of 55 galaxy pairs of interest. Among those was VV191, generating a very detailed map of the dust silhouette of the spiral galaxy. This was one of the galaxy pairs analyzed in a project based on the master’s thesis work by Sarah Bradford at the University of Alabama which went into a poster presentation at the January 2017 meeting of the American Astronomical Society in Texas. In fact, I used a low-contrast version of the VV191 image as the poster background. (The poster should still just be legible in this compressed PNG version):
The data quality for VV191 stood out, because the background elliptical galaxy has its brightest region right behind the edge of the dust in the spiral. We then had a 2-dimensional map of how much light gets through the dust in the spiral at the wavelengths included in that single observation. The poster was viewed by my longtime collaborator Rogier Windhorst, who is one of the interdisciplinary scientists with the James Webb Space Telescope (JWST) project. In this capacity, he had an allocation of so-called GTO (guaranteed-time) observations, asked what we could do with JWST. Rogier was struck by these images, and wondered what we could add to the science output with a little bit of JWST observing time.
This led to a plan of tracking the dust signature from ultraviolet to infrared in a single galaxy with a single technique. First Hubble had to do its part with more data, using not only its high resolution but UV sensitivity. We got Hubble images in filters around 2250 and 3360 Angstroms (0.22 and 0.34 microns) , with the short end limited mostly by the elliptical galaxy being so faint in the deeper UV that we couldn’t detect its light well enough in reasonable exposure times. These data have been processed, so we are ready for the next step – JWST. Its near-infrared camera (NIRCAM) will observe this system in four filters from 0.9-4.0 microns wavelength (two at a time since the camera can use short- and long-wavelength channels simultaneously). The wavelengths are chosen to trace the way the dust effects fall off toward longer wavelengths, which is affected both by the sizes of the interstellar dust grains and how strongly they are clumped together. One filter matches one of the wavelengths at which small grains (or indeed large molecules, so-called PAH particles) emit, so we might be able to tell how they correlate with the larger particles blocking most of the light.
Because of the enormous sensitivity of JWST and NIRCAM, each filter is exposed for only 15 minutes to get very high measurement accuracy. (The telescope will probably take longer than that to point to VV191, depending on what it’s doing beforehand). Based on when JWST can view this part of the sky, these observations are most likely to be made between December 2022-March 2023, or May-July of 2023 (we should know more in a couple of weeks when the first year’s observation schedule is released). Watch this space…
Since mid-2018, the Hubble Space Telescope has taken occasional short-exposure images, filling what would otherwise be gaps in its schedule, of galaxies in the list from “Gems of the Galaxy Zoos” (otherwise known as Zoo Gems). The Zoo Gems project just passed a milestone, with acceptance of a journal paper describing the project, including how votes from Galaxy Zoo and Radio Galaxy Zoo participants were used to select some of the targeted galaxies, and acting as a sort of theatrical “teaser trailer” for the variety of science results coming from these data. (The preprint of the accepted version is here; once it is in “print”, the Astronomical Journal itself is now open-access as of last month). The journal reviewer really liked the whole project: “The use of the Galaxy Zoo project’s unique ability to spot outliers in galaxy morphology and use this input list for a HST gap filler program is a great use of both the citizen science project and the Hubble Space Telescope” and “I think it is a wonderful program with a clever, useful, and engaging use of both SDSS and Hubble.” (We seldom read statements that glowing in journal reviews).
Zoo Gems got its start in late 2017, when the Space Telescope Science Institute (STScI) asked for potential “gap-filler” projects. Even with what are known as snapshot projects, there remained gaps in Hubble’s schedule long enough to set up and take 10-15 minutes’ worth of high-quality data. We put together a shockingly brief proposal (STScI wanted 2 pages, originally to gauge interest) and were very pleased to find it one of 3 selected (the other two also deal with galaxies. Makes sense to me). We had long thought that the ideal proposal for further observations of some of the rare objects identified in Galaxy Zoo ran along the lines of “Our volunteers have found all these weird galaxies. We need a closer look”. That was essentially what the gap-filler project offered.
We estimated that we could identify 1100 particularly interesting galaxies (where short-exposure Hubble images would teach us something we could foresee) from Galaxy Zoo and Radio Galaxy Zoo. We were allocated 300 by STScI, so some decisions had to be made. A key feature of our project was the wide range of galaxy science goals it could address, so we wanted to keep a broad mix of object types. Some types were rare and had fewer than 10 examples even from Galaxy Zoo, so we started by keeping those. When there were many to choose from, we did what Galaxy Zoo history (and STScI reviewers) suggested – asked for people to vote on which merging galaxies, overlapping galaxies, and so on should go into the final list. This happened in parallel for Galaxy Zoo and Radio Galaxy Zoo objects (the latter largely managed by the late Jean Tate, not the last time we are sadly missing Jean’s contributions as one of the most assiduous volunteers). Even being on that observing list was no guarantee – gap-filler observations are selected more or less at random, taking whichever one (from whichever project’s list) fits in a gap in time and location in the sky. The STScI pilot project suggested that we could eventually expect close to half to be observed; we are now quite close to that, with 146 observations of 299 (one became unworkable due to a change in how guide stars are selected by Hubble). These include a fascinating range of galaxies. From Galaxy Zoo, the list includes Green Pea starburst galaxies, blue elliptical and red spiral galaxies, ongoing mergers, backlit spiral galaxies, galaxies with unusual central bars or rings, galaxy mergers with evidence for the spiral disks surviving the merger or reappearing shortly thereafter, and even a few gravitational lenses. From Radio Galaxy Zoo, we selected sets of emission-line galaxies (“RGZ Green”) and possibly spiral host galaxies of double radio sources (SDRAGNs, in the jargon, and so rare that we’ve more than doubled the known set already). Both kinds of RGZ selection were largely managed by Jean Tate, who we are missing once again. By now, of 300 possible objects, 146 have been successfully observed. One can no longer be observed due to changes in Hubble’s guide-star requirements, and two failed for onboard technical reasons (it was during one of those, a few months ago, that a computer failure sent the telescope into “safe mode”; I have been assured that it was not our fault).
Zoo Gems images show that every blue elliptical galaxy observed shows a tightly wound spiral pattern near the core, so small that it was blurred together in the Sloan Survey images used by Galaxy Zoo, and broadly fitting with the idea that these galaxies result from at least minor mergers bringing gas and dust into a formerly quiet elliptical system.
There is much more to come as harvesting the knowledge from these data continues. Already, a project led by Leonardo Clarke at the University of Minnesota used Zoo Gems images to demonstrate that Green Peas are embedded in redder surroundings, possibly the older stars in the galaxies that host these starbursts. Beyond these, these data can be used to examine the histories of poststarburst galaxies, dynamics and star-formation properties of 3-armed spirals, and nuclear disks and bars – some of these show galaxies-within-galaxies patterns where the central region nearly echoes the structure of the whole galaxy.
While going through some of the Zoo Gems images to see which should go in various montages in this paper, I considered the multilayer overlapping galaxy system including UGC 12281. It didn’t go into the paper, but the visual sense of deep space in this image is so profound that it became the 2nd most-retweeted thing I’ve sent out in more than 10 years.
In presenting these data, we wanted to make the case for the value of wide-ranging, even short, programs such as this. These gap-filler projects are continuing with Hubble, until STScI starts to have trouble filling the gaps and needs to call for more projects. Premature as it seems, I can’t help musing that someone may eventually work out a low-impact way for the James Webb Space Telescope to make brief stopovers as it slews between long-exposure targets – we have suggestions…
Data from the Zoo Gems project (like the other gap-filler programs, Julianne Dalcanton’s program on Arp peculiar galaxies and the one on SWIFT active galaxies led by Aaron Barth) are immediately public, accessible in the MAST archive under HST program number 15445 (the others are 15444 and 15446). Claude Cornen maintains image galleries for the Zoo Gems, Arp and SWIFT projects in Zoo Gems Talk. Our thanks go to everyone who helped draw attention to these galaxies, or voted in the Zoo Gems object selection.
I recently received word from his wife of the death of Jean Tate on November 6. Jean had been a very active participant in several astronomical Zooniverse projects for a decade, beginning with Galaxy Zoo. It does no disservice to other participants to note that he was one of the people who could be called super-volunteers, carrying his participation in both organized programs and personal research to the level associated with professional scientists. He identified a set of supergiant spiral galaxies, in work which was, while in progress, only partially scooped by a professional team elsewhere, and was a noted participant in the Andromeda project census of star clusters in that galaxy. In Radio Galaxy Zoo, he was a major factor in the identification of galaxies with strong emission lines and likely giant ionized clouds (“RGZ Green”), and took the lead in finding and characterizing the very rare active galactic nuclei with giant double radio sources from a spiral galaxy (“SDRAGNs”). He did a third of the work collecting public input and selecting targets to be observed in the Gems of the Galaxy Zoos Hubble program. Several of us hope to make sure that as much as possible of his research results from these programs are published in full.
Jean consistently pushed the science team to do our best and most rigorous work. He taught himself to use some of the software tools normally employed by professional astronomers, and was a full colleague in some of the Galaxy Zoo research projects. His interests had been honed by over two decades of participation in online forum discussions in the Bad Astronomy Bulletin Board (later BAUT, then Cosmoquest forum), where his clarity of logic and range of knowledge were the bane of posters defending poorly conceived ideas.
Perhaps as a result of previous experiences as a forum moderator, Jean was unusually dedicated to as much privacy as one can preserve while being active in online fora and projects (to the point that many colleagues were unaware of his gender until now). This led to subterfuges such as being listed in NASA proposals as part of the Oxford astronomy department, on the theory that it was the nominal home of Galaxy Zoo. Jean was married for 27 years, and had family scattered in both hemispheres with whom he enjoyed fairly recent visits. Mentions in email over the years had made me aware that he had a protracted struggle with cancer, to the extent that someday his case may be eventually identifiable in medical research. He tracked his mental processes, knowing how to time research tasks in the chemotherapy cycle to use his best days for various kinds of thinking.
This last month, emails had gone unanswered long enough that some of us were beginning to worry, and the worst was eventually confirmed. I felt this again two days ago, which was the first time I did not forward notice of an upcoming Zoo Gems observation by Hubble to Jean to be sure our records matched.
Ad astra, Jean.
One of the most enduring serendipitous finds of the original Galaxy Zoo was a category of giant gas clouds shining from the energy input of active galactic nuclei (AGN) which have since faded (being a little cavalier here with time and verb tenses, since we can’t get news faster than light travels). The most famous of the is of course Hanny’s Voorwerp, whose discovery led to subprojects which turned up many more (“Voorwerpjes”). We have new results now on a related project going back to the Galaxy Zoo Forum, where we searched for gas in companions to active galaxies which is ionized by the AGN, and therefore gives us one more way to learn about how bright the AGN was tens of thousands of years before our direct view. Read More…
It’s away! The final observation plan for the Gems of the Galaxy Zoos Hubble program was submitted earlier this week (24 hours before our deadline, I want you all to know).
We collected votes for over 2 weeks, separately for Galaxy Zoo and Radio Galaxy Zoo objects since they needed distinct image layouts. About 18,000 votes were cast. The Talk interfaces turned out to be very useful for immediate practical matters – some users seeing images twice, some cases of the wrong coordinates being used for images that had been uploaded and needed to be replaced, and finding duplicates both within the candidates and versus older Hubble observations. (A major “thank you” to the volunteers who contributed in these ways). I was strongly impressed at the level of discussions on the Talk sites that went into some of these decisions.
Galaxy Zoo and Radio Galaxy Zoo participants have an unusual opportunity to help shape a list of galaxies to be observed by the Hubble Space Telescope, as part of the “Gems of the Galaxy Zoos” project.
The project came about when the Space Telescope Science Institute circulated a message in August of 2017, seeking proposals for a new category of observation – gap-fillers. These projects will provide lists of target objects around the sky for brief observations when high-priority projects leave gaps in the telescope schedule, allowing 10-12 minutes of observation at intermediate places in the sky. Read More…
Among the results being presented at this week’s meeting of the American Astronomical Society in Texas (near Dallas) is this poster presentation on the status of the STARSMOG project. This program, a “snapshot” survey using the Hubble Space Telescope, selected targets from a list of overlapping galaxy pairs with spiral members and very different redshifts, so they are not interacting with each there and likely to be more symmetric. The source list includes pairs from Galaxy Zoo (about 60%) and the GAMA (Galaxy And Mass Assembly) survey. These data will allow very extensive analysis; this presentation reads more like a movie trailer in comparison, highlighting only a few results (primarily from the master’s thesis work by Sarah Bradford).
Among the highlights are:
Sharp outer edges to the location of dust lanes in spiral disks.
Distinct dust lanes disappearing for galaxies “late” in the Hubble sequence (Scd-Sd-Sdm-Sm, for those keeping track), maybe happening earlier in the sequence when there is a bar.
The dust web – in the outer disks of some spirals, we see not only dust lanes following the spiral pattern, but additional lanes cutting almost perpendicular to them. This is not completely new, but we can measure the dust more accurately with backlighting where the galaxy’s own light does not dilute its effects.
A first look at the fraction of area in the backlit regions with various levels of transmitted light. This goes beyond our earlier arm/interam distinction to provide a more rigorous description of the dust distributions.
Bars and rings sweeping adjacent disk regions nearly free of dust (didn’t have room for a separate image on that, although the whole sample is shown in tiny versions across the bottom)
Here is a PNG of the poster. It doesn’t do the images justice, but the text is (just) legible.
Just in time to brighten our holiday season, we got word that the Astrophysical Journal has accepted out next paper on the Voorwerpje clouds around fading active galactic nuclei (AGN). The full paper is now linked on the arXiv preprint server.
This time, we concentrated on the clouds and what they can tell us about the history of these AGN. To do this, we worked pixel-by-pixel with the Hubble images of the clouds in the H-alpha and [O III] emission lines, augmented by a new (and very rich) set of integral-field spectroscopy measurements from the 8-meter Gemini North telescope, velocity maps from the Russian 6-meter telescope, and long-slit spectra from the 3-meter Shane telescope at Lick Observatory.
To examine the history of each AGN, our approach was that the AGN had to be at least bright enough to ionize the hydrogen we see glowing at each point at the time the light reaching that point was given off. Certainly we can’t expect each piece of the cloud to absorb all the deep-UV radiation, so this is a lower limit. Two external checks, on quasars unlikely to have faded greatly and on the Teacup AGN which has had detailed modeling done from spectra, suggests that the very brightest pixels at each radius absorb comparable fractions of the ionizing radiation. This gives confidence that we can track at least the behavior of a single object, underestimating its brightness by a single factor, if we look at the upper envelope of all pixels in the H-alpha images. We hoped this would be feasible all the way back to the original Hubble proposal to look at Hanny’s Voorwerp. Here is a graphic from the new paper comparing our AGN in this way. The distance in light-years at each point corresponds to the time delay between the AGN and cloud, and the curve labelled “Projection” shows how much one of these points would change if we view that location not perpendicular to the light but at angles up to 30 degrees each way. To be conservative, the plot shows the data corresponding to the bottom of this curve (minimum AGN luminosity at each point).
The common feature is the rapid brightness drop in the last 20,000 years for each (measured from the light now reaching us from the nuclei). Before that, most of them would not have stood out as having enough of an energy shortfall to enter our sample. Because of smearing due the large size of the clouds, and the long time it takes for electrons to recombine with protons at such low densities, we would not necessarily see the signature of similar low states more than about 40,000 years back.
We could also improve another measure of the AGN history – the WISE satellite’s mid-infrared sky survey gave us more accurate measure of these objects’ infrared output. That way, we can tell whether it is at least possible for the AGN to be bright enough to light up the gas, but so dust-blocked in our direction that we underestimate their brightness. The answer in most cases is “not at all”.
New data brought additional surprises (these objects have been gifts that just keep on giving). The Gemini data were taken with fiber-optic arrays giving us a spectrum for each tiny area 0.2 arcseconds on a side (although limited to 3.5×5 arc second fields), taken under extraordinarily steady atmospheric conditions so we can resolve structures as small as 0.5 arc second. We use these results to see how the gas is ionized and moves; some loops of gas that earlier looked as if they were being blown out from the nuclei are mostly rotating instead. Unlike some well-studied, powerful AGN with giant emission clouds, the Voorwerpje clouds are mostly just orbiting the galaxies (generally as part of tidal tails), being ionized by the AGN radiation but not shoved around by AGN winds. This montage shows the core of NGC 5972 seen by these various instruments, hinting at the level of mapping allowed by the Gemini spectra (and helping explain why it took so long to work finish the latest paper).
Work on the Voorwerpjes continues in many ways. Galaxy Zoo participants still find possible clouds (and the moderators have been excellent about making sure we see them). There is more to be learned from the Gemini data, while X-ray observatories are gradually bringing the current status of the AGN into sharper focus. A narrowband imaging survey from the ground can pick out fainter (and sometimes older) clouds. Colleagues with expertise in radio interferometry are addressing questions posed by the unexpected misalignments of optical and radio structures in some of our galaxies. Finally, the new DECaLS and Pan-STARRS survey data will eventually bring nearly the whole sky into our examination (for a huge range of projects, not just AGN history).
Once again, thanks to all who have helped us find and unravel these fascinating objects!
It’s a week until the 8th anniversary of the launch of Galaxy Zoo.
The Hubble Space Telescope observations of giant ionized Voorwerpje clouds near galaxies with active nuclei, many found for the first time though the effort of Galaxy Zoo participants gives us another 8 – one at the end of a long road of numbers. 16,000 galaxies with known or possible active nuclei, 200 highly-ranked cloud candidates based on input from 185 participants, 50 spectroscopic observations, 19 giant ionized clouds, among which we found 8 with evidence that the nucleus has faded dramatically (and then observed by one Hubble Space Telescope). (You wondered where the numeral 8 would come in by now… and there is another one hidden below.) The first batch of scientific results from analysis of these images was described here, and the NASA/ESA press release with beautiful visualizations of the multi-filter image data can be seen here. As a visual summary, here are the images, with starlight and emission from [O III] and H-alpha shown in roughly true visual color.
This project was an outgrowth of the discovery of Hanny’s Voorwerp, which remains probably the signature discovery of Galaxy Zoo. In astronomy, one is a pet rock, ten is a statistically valid sample – so we wanted to know more about how common such clouds might be, and what they could tell us about quasars more generally. Zoo participants answered this challenge magnificently.
The scientific interest in these objects and their history remains intense, and observations continue. I’ve recently finished processing integral-field spectra from the 8-meter Gemini-North telescope, where we have spectra at every point in a small field of view near the nucleus, and just recently we learned that our proposal for spectra in a few key areas at the high resolution of the Hubble telescope has been approved for the coming year.
Even (or especially) for kinds of objects behind its original statistical goals, Galaxy Zoo has provided an amazing ride these last 8 years. Stay with us – and if you see weirdly colored clouds around galaxies, feel free to flag them in Talk!
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!