Gems of the Galaxy Zoos – help pick Hubble observations!

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.  These are similar to the “snapshot” projects STScI has supported for many years (including two linked to Galaxy Zoo), but are aimed at still shorter blocks of time. We combined Galaxy Zoo and Radio Galaxy Zoo science goals in a proposal submitted for the deadline in September (they wanted 2-page proposals, astonishingly short for Hubble work). In mid-December, we were informed that ours was one of 3 gap-filler projects selected (of 52 submitted). The other two also deal with galaxies – one led by Aaron Barth, dealing with X-ray selected active galaxies, and one led by Julianne Dalcanton, on peculiar galaxies from the Arp and Arp-Madore atlases. (We’ve been in contact to avoid duplicate targets; Julianne’s project has a few galaxies we would have proposed, which is fine because the data all become public right away).

This kind of project is a good fit for many Galaxy Zoo followup ideas, involving only a  few galaxies with special properties or where we need more data to tell which galaxies have certain properties. We were able to put together a list of more than 1100 candidate galaxies for the proposal. We need to pare this down to a list of 300 for the actual project (and do so in time for a February 28, 2018 deadline to submit the full observation specifications). Of this list of 300, perhaps half will be observed over the 2-year span of the project – the exact number depends on details of everything else Hubble is doing over this time. Each observation consists of about 11 minutes’ worth of exposures with the telescope’s Advanced Camera for Surveys (ACS), a sensitive workhorse instrument first taken into space and installed in 2002 and then, following an electronics failure, repaired by the astronauts of the shuttle Atlantis in 2009. We can specify which filter to use for each object, and where to place it in the field of view. Because these observation fit into gaps between previously-scheduled ones, we cannot specify the orientation of the camera on the sky (which depends on the time of year and celestial coordinates).

Vote for favorites!

Our initial object lists were taken mostly from comments in Galaxy Zoo Talk (and its predecessor the forum). To maintain the spirit of Galaxy Zoo, we invite participants to help us trim down the list to fit in our 300-target cap. (The Space Telescope Science Institute also mentioned, when approving the proposal, that they would like us to do this). There were a number of science categories mentioned in the proposal, or brought up from classic Galaxy Zoo discoveries. A few of these categories have only 5-10 examples, so we’ll plan to submit all of those. However, there are others – polar and collisional ring galaxies, backlit galaxies for dust studies, post-starburst galaxies, unusual spiral patterns – where we can include only subsets in the final list. So we set up a Zooniverse project to collect participant votes in the priorities of these objects, which will let us out the favorite ones in the final Hubble list.


Our science categories from Galaxy Zoo include:

Backlit galaxies – this is a longtime personal favorite of mine. The Galaxy Zoo catalog of overlapping-galaxy pairs provides many more than we could observe even if that were all we did, so we have selected some that look particularly informative. Help pick which ones to put forward.

Polar and collisional ring galaxies – there rare galaxy types result from particular kinds of galaxy collisions, and in some cases the Hubble image will show which ones fall in these categories. Some of the fainter candidates may turn out to be gravitationally lensed  background galaxies.

Unusual spiral patterns – some of the have been Galaxy Zoo specialties. One-armed spirals, three-armed spirals, galaxies with three arms and a bar, wonderfully symmetric four-armed spirals… One- and three-armed spirals are particularly interesting for their internal dynamics, and they say different things about the galaxies’ past environment.

Regrowing disks – most studies of the effects of mergers on the galaxy population use the usual outcome of simulations, that a violent merger of two spirals eventually yields an elliptical. But simulations also indicate that if they merge from just the right directions with the right coming speeds, the merger remnant could end up with a new disk and become a large spiral. There are a handful of candidates for this process, with two galaxy cores surrounded by rings with at least hints of spiral structure. Knowing how often this happens would sharpen our understanding of what galaxy mergers do and don’t do.

Poststarburst galaxies – the question of when and how galaxies quench – shutting down their star formation – is a big one in understanding galaxy history. Combining their spectra with Galaxy Zoo data shows that galaxies look more like disks before quenching and more like bulges or ellipticals afterward. So this subset concentrates on galaxies which have virtually shut down star formation in the last few hundred million years, to see where they fall regarding disks and bulges.

Green Peas – a Galaxy Zoo specialty, after a group of participants identified them and made the case to the science team that this was an interesting and coherent group of galaxies. That they proved to be, hosting some of the most extreme levels of star formation in today’s Universe and featuring in at least 16 published research projects by astronomers outside the Galaxy Zoo team (and playing important roles in two doctoral dissertations). Some of these are going into the Zoo Gems list, to study their older populations of stars and the galaxies’ environments.

Blue ellipticals and red spirals – more Galaxy Zoo specialty, since the Galaxy Zoo classic data were among the first to show these populations going against the usual color patterns. Hubble data on blue ellipticals (with and without active nuclei) could show where the unusual star formation occurs, and whether it comes in knotty regions as in other galaxy types. For red spirals, we would trace the residual star-forming regions, and see whether even small-scale bars occur within the spiral patterns. In both cases, we start by including galaxies with additional data (radio measurements of cold hydrogen clouds, MANGA spectroscopic mapping), and ask you to help decide which ones to put on the list.

Emission-line clouds – these galaxies may have the kinds of giant clouds of ionized gas made famous by Hanny’s Voorwerp and its smaller analogs. The Hubble images would distinguish them more clearly from the central galaxy, and provide better estimates of the last history of the ionizing nucleus that we can get from ground based data.

Compact star-forming knots – in some galaxies, knots of star formation appear unusually bright or small. These attract attention because they are somewhat like Green Peas but embedded in a big galaxy, or as possible analogs to the clumpy galaxies that become more and more common as we look back in cosmic time (and were a major part of the Galaxy Zoo: Hubble project).

Outrageous bulges – some spirals have central bulges that are huge in comparison to the disks, appear to be prolate and elongated perpendicular to the disks, or stand out in other ways. Hubble images would show dust features and other structural details to help unravel why these look so different.

Extreme mergers – these include very long tidal tails, strongly warped disks after deep perturbations, and other unusual aspects. Especially combined with additional data, these images would help understand can happen when galaxies are most strongly disturbed.

The idea for most of these is to observe galaxies where we can make progress with only a  handful of objects observed. It’s OK for some not to pan out, that’s why we take the  data.

You can take part right away here, voting for the priority of objects in eight of these categories (the ones where we need to trim lists). The images are SDSS cutouts, even if the Talk discussion involved DECaLS or other data. You can select which category to view on the start page; for each galaxy category, if you click the “help” box, a link comes up where you can see a montage of all galaxies of that type, for comparison so you can know “vote for this as opposed to what?” Voting will be open through February 16, so we have time to prepare the full observation specifications on the whole list by month’s end. (Many browsers will need you to scroll down from the initial view to see the buttons which select a particular galaxy type.)

There are obviously no data from Gems of the Galaxy Zoos yet – when there are, you can tell what has been observed via the Hubble archive link or the Hubble Legacy Archive interface here.


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