Gems of the Galaxy Zoos – coming soon to a space telescope near your planet!

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.


This montage (at various scales to show the galaxies’ structures) shows the top 5 vote-getters in four large categories from SDSS images. A few have been brightened to show the important features more clearly.

These galaxies span a wide variety of scientific topics:

Unusual mergers, with very long tidal tails, one-sided distortions, or multiple
galaxies involved.

Overlapping galaxy pairs – these let us study the attenuation of galaxy light by dust in a way free of limitations set by dust temperature for IR investigation, and independent of how the galaxies’ stars are distributed a needed for some optical studies. (Full disclosure, this topic has been a personal favorite since I first saw a photograph of NGC 3314 circa 1977).

Polar and collisional ring galaxies. There are some very obvious ones (like entry number 1 in the standard Whitmore et al. catalog of polar rings) that have yet to be observed with Hubble’s resolution. And people voted highly for the “Heart” double ring galaxy. This category also includes some faint structures that may turn out to be gravitationally lensed arcs, distorted images of much more distant galaxies.

Regrowing disks. Simulations indicate that many galaxy mergers lead to elliptical (or at least elliptical-like) galaxies, although which ones do and don’t depends on details of the merger history. We know of a handful of merging galaxies that really seem be forming new disks even before the two cores have melded together. Hubble observations would show, for example, whether the new spiral patterns really wrap around both nuclei, and are likely to become giant spiral galaxies rather than ellipticals.

Active galactic nuclei with extended emission-line regions (EELRs, some of which are large enough for us to label them voorwerpjes). Another Galaxy Zoo speciality, we will try to observe some found recently. The same applies to a large class of galaxies from  Radio Galaxy Zoo, the RGZ Green galaxies. In these, redshifted [O III] gaseous emission shows green in the Sloan composite images. Their clouds may include those lit up by direct interaction with radio jets earthing out from the core, or energetic photons released near the central black hole as in the Galaxy Zoo voorwerpjes (and the original, Hanny’s Voorwerp).

Compact star-forming knots in galaxies, including close red/blue pairs of objects found in DECaLS. Here, we are interested in how small, and thus now intense, the regions are. These are related to another Galaxy Zoo greatest-hit subject, the compact and extreme star-forming galaxies known as Peas (color depending on their redshift). For these, at certain redshifts we can use the camera’s filters to isolate the starlight from the surrounding gas and get a clearer look at them as galaxies. In 3 redshift ranges, we will also examine their immediate surroundings to clues as to their occurrence – are they parts of larger but faint galaxies, or do they have dim companions more often than expected by chance?

And following the theme of star formation, our list includes post-starburst galaxies. We removed some stars that sneaked into a spectroscopically selected list, based on recent SDSS pipeline analysis and close inspection. These data will help show how these burst fit into the general picture of galaxy evolution with quenching of star formation as a key process. So do observations of the blue elliptical and red spiral systems that were early galaxy Zoo findings.

We also probe the edge cases of galaxy structures – outrageous galaxy bulges (large  bulges, prolate ones, X-shaped bulges) and unusual spiral patterns (3-armed spirals, very asymmetric or extremely symmetric patterns.) Close inspection has also turned up spirals with unusual large, distinct disks of stars around the nuclei, and bars which seem to be undergoing instability as we watch.

Speaking of unusual galaxy structures – an especially important category of target for the Gems program consists of radio galaxies which seem to be spiral, rather than the ubiquitous elliptical or lenticular systems. The have come to be known as SDRAGNs (Spiral Double Radio Active Galactic Nuclei). A few of these have been well-documented, but too few to show what makes them special. Environment? Black-hole mass? A larger, more systematic sample might tell. This sample, like the Radio Galaxy Zoo voting site, has been largely compiled by longtime Galaxy Zoo participant Jean Tate.

A few galaxies migrated between Galaxy Zoo and Radio Galaxy Zoo programs as we  learned more, but it’s all one big Zooniverse.

In the short time available (about 11 minutes for observation on each object) except for a few very bright and compact Green Peas, we can use only a single filter, so that had to be selected to be as informative as possible given what else we already knew. This was made more challenging bytes short exposures, so for some kinds of galaxy, we had to compromise between seeing exactly what we most wanted to see, and having enough signal coming through the filter to pull it above the readout noise (and more subtle effects) in the CCD detectors. Some examples: for Green Peas in a narrow redshift range around z=0.34, the ACS filter most like Sloan i rejects all the strong gaseous emission lies, so we can see the starlight by itself. Or for AGN-ionized clouds, we want to include [O III], so for a broad range of low redshifts, we use the Sloan g equivalent. For SDRAGNs, we use a blue filter to emphasize the features which trace spiral arms most clearly (bluer stars and dark dust lanes).

The program status for Gems (proposal 15545) can always be found here. You can see a formatted description of the observing plan object-by-object in this 314-page PDF. Longtime Zoo participant c_cld collected the coordinates and posted them in a format suitable to paste into the SDSS image-cutout tool here.

As images are obtained, you can see links to the archived files at the bottom of this status page. The gap-filler lists should be available to insert in the telescope schedule soon now.


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