We posted briefly about the 227th meeting of the American Astronomical Society, which several members of the Galaxy Zoo science team attended last week. I wanted to share a little bit more about the research that we presented, and the experiences we had at the meeting.

Asst. Prof Lucy Fortson, PhD student Melanie Galloway, and postdoc Kyle Willett (Univ. of Minnesota) at an evening poster session at the 227th AAS meeting.

Kyle Willett (Postdoc, University of Minnesota)

I presented a poster on the data release of the Galaxy Zoo: Hubble project. While it’s been a couple of years since we formally finished the classifications for GZH, we’ve been working hard in the meantime on aggregating and calibrating the data; this includes two supplementary data sets that users helped classify in the newest interface. One of those sets were the images of galaxies processed with the FERENGI code in order to mimic their appearances at higher redshifts; these have been absolutely critical for the calibration/debiasing procedure we’re applying to the real Hubble galaxies. The second set of new images were the deeper images of galaxies in the GOODS north and south fields, completed early last year. We’re using these to analyze the effect of depth on morphologies, and seeing how the disk/clumpy fractions change with improved imaging.

AAS went well for me, and I got to talk about GZH with a number of new colleagues. I particularly liked having my poster between Melanie Galloway’s and Brooke Simmons’ — I got to point out GZH science results on one side and the next generation of even higher redshift galaxies on the other, which made for a very nice story to tell.

Kyle Willett’s poster at the 227th AAS meeting. Click to download the full PDF.

Melanie Galloway (PhD student, University of Minnesota)

Galaxy Zoo users probably know that there are two main types of galaxies: disks and ellipticals. There is a cool relationship between these shapes and their color: disks tend to be blue (which is an indicator of young stars), while almost all ellipticals appear red (which indicates the stars are old; “red and dead” is a term commonly used.) Astronomers believe that this relationship between color and shape implies that galaxies tend to be created as disks, and over their lifetimes, transform from young disks to old ellipticals. Data from Galaxy Zoo revealed that there are also red disk galaxies in the Universe, and right now it is not known how they fit into our current perception of galaxy evolution.

Mel used data from Galaxy Zoo: Hubble to tackle this question by analyzing how the fraction of red disks changed between now and 6 billion years ago. She found that this fraction was actually much higher in the past! This result probably means that red disks don’t tend to stay red disks, and may instead be a common phase of a typical galaxy’s evolution from blue disk to red elliptical.

Melanie Galloway’s poster at the 227th AAS meeting. Click to download the full PDF.

Melanie Beck (PhD student, University of Minnesota)

At this year’s AAS conference, I presented work which focused on the relationship between galaxies’ masses and their sizes. In general, more massive galaxies are also physically larger (but not always!). However, the mass-size relationship is different for galaxies in the distant universe compared to those in the nearby universe. It’s also different between disk galaxies compared to ellipticals in that the sizes of elliptical galaxies grow much, much faster than those of disk galaxies but don’t seem to grow much in mass. To explain this behavior, models predict that disk galaxies must evolve into elliptical galaxies at a rate that mimics the growth rate of the ellipticals. These models predict that there should be many more elliptical galaxies of a particular size and mass in the nearby universe compared to disks. To test this, we need to keep track of the number of galaxies as a function of mass, size, and type (elliptical or disk) over a large period of time.

My initial work utilizes classifications from Galaxy Zoo 2 separated by Smooth or Features/Disk. All the galaxies in this catalog are considered to be in the local universe. Using sophisticated statistical techniques, I’m able to robustly determine the number of galaxies as a function of mass, size and type. Next I’m applying the same techniques to classifications from Galaxy Zoo: Hubble and Galaxy Zoo: CANDELS as these catalogs contain galaxies from the more and more distant universe. Once we have the analysis from all three we can compare the numbers of galaxies at each time and finally test those models!

Melanie Beck’s poster at the 227th AAS meeting. Click to download the full PDF.

Melanie Beck discussing her poster with Prof. Chris Conselice (Univ. of Nottingham).

Brooke Simmons (Postdoc and Einstein Fellow, UC San Diego)

AAS is always a hectic science bonanza, and presenting a poster is a way of slowing things down a bit: unlike a talk, which is over in 15 minutes or less, you get to have your results up all day. My poster was an introduction to the upcoming release of classifications for the high-redshift CANDELS galaxies, so it shows a basic overview of how the classifications work and an early science result about featureless disks at high redshift.

It was great to present 2 data releases side-by-side, with Kyle’s poster to my left, and it was even better to get to present the result of the volunteers’ efforts. Between these posters and the previous data releases for Galaxy Zoo, we’ve measured the shapes of hundreds of thousands of galaxies (actually, I think it’s over a million!) spanning the last 12 billion years of cosmic time.

Brooke Simmons’ poster at the 227th AAS meeting. Click to download the full PDF.

The “green pea” galaxies were one of the first discoveries of the Galaxy Zoo; they were first noticed by several of our early volunteers, and appeared in a paper led by Carie Cardamone in 2009 (with over 100 citations so far!). They’ve been the subject of a great deal of follow-up research since then, much of which we’ve tried to follow on this blog.

Examples of the green pea galaxies originally discovered in Galaxy Zoo. Top Row: Sloan Digital Sky Survey images. Bottom Row: Hubble Space Telescope images.

A new paper on the Green Peas has just appeared in Nature, one of the most prestigious and widely-read journals in science. A truly international team of researchers (working in Ukraine, Czech Republic, Switzerland, France, Germany, and the United States) made observations of one green pea galaxy, known as J0925+1403, using an ultraviolet spectrograph on the Hubble Space Telescope. They were able to measure emission from what astronomers call “Lyman continuum” photons; this is light produced by massive stars that are solidly in the ultraviolet wavelengths.

The reason this is so important and interesting relates to one of the most fundamental steps in the history of the Universe that astronomers know of. The majority of matter in the Universe is hydrogen (formed shortly after the Big Bang), and much of it exists in diffuse clouds between galaxies, which is called the intergalactic medium. We know from observations that almost all of that hydrogen is currently ionized – that means instead of consisting of a neutral atom with one proton and one electron orbiting it, the average hydrogen atom between galaxies has had its electron stripped away from the proton. This is a big difference because neutral atoms interact with light differently than ionized atoms. If the hydrogen between galaxies were neutral, it would absorb much of the light coming from individual stars and galaxies, making a huge difference in our ability to observe distant objects.

It’s been known for years the Universe is currently ionized; however, about 700 million years after the Big Bang, we know that the Universe used to be neutral. That’s pretty well-established — however, there’s a great deal of debate about what caused the sudden reionization. Something must have produced large numbers of photons that traveled into the intergalactic medium and ionized all of the hydrogen fairly quickly. There have been lots of papers proposing different possible sources for this, including dwarf galaxies, active galactic nuclei, quasars, very early and massive stars, etc.

Diagram illustrating how individual galaxies may have re-ionized the neutral hydrogen in the Universe shortly after the Big Bang. From Erb (2016).

This new paper proposes that green pea galaxies could be responsible for re-ionizing the early Universe. The measurements from this paper show that at least one green pea galaxy is actively emitting photons with sufficient energy to ionize neutral hydrogen. Lots of galaxies can create such radiation, but one unique aspect of the peas is that the photons are escaping the galaxy where they’re being formed. Usually they’re absorbed by dust or gas clouds within the galaxy before they can affect the rest of the Universe. This is the first time that it’s been demonstrated to occur for a green pea galaxy.

The paper (Izotov et al. 2016) is available online. Nature has also published a nice summary at a slightly less technical level to accompany the article that I’d recommend – you can read that here. Please post if you have any questions or want to discuss more about what this means. We’re extremely excited that your discoveries are still yielding new and interesting science!

We’re really excited to report that, with your help, the first batch of galaxy images from the Illustris simulation was finished last week! While we still have plenty of images still available to be classified (both from Illustris and DECaLS), I wanted to explain again how the images are being sorted in Galaxy Zoo and show some of the very early results we’re getting from your classifications.

The galaxies we selected from Illustris were a really big set – after eliminating galaxies which were likely to be too small or dim for accurate visual identification (we did this by filtering on the mass of the galaxies), we had over 110,000 images. In designing this phase of Galaxy Zoo, though, we wanted to try and prioritize the order of the images being shown so that we could do some early science projects along the way, rather than waiting many months for the entire data set to be finished before we started our analysis. This first set of Illustris data included 10,832 images, which were classified a total of more than 430,000 times by Galaxy Zoo volunteers.

One of the main questions we wanted to answer was: “How is the apparent morphology of a galaxy affected by the angle at which it’s viewed?”  This is an important one – for observations of the real Universe, we can’t change the position of our telescope relative to the objects we’re looking at. If a galaxy is edge-on, for example, we’re really limited in being able to determine if there’s a bar, how many spiral arms there are, etc. In Illustris, though, we can change the viewing angle in the simulation! As a result, we might hypothesize that all edge-on disks should be identifiable as spiral or S0 galaxies at all the other viewpoints.

Here’s a quick test I’ve run of that. Using the set of collated classifications that just finished from Illustris, I looked for unique galaxies that were classified as “edge-on disks” from at least 1 of the 4 viewing angles that we have data for. Then I looked at the GZ classifications for the other viewing angles to see if they were still edge-on. Results:

Very close to what we expect! Only about 10% of galaxies had any edge-on classifications; of those, almost all of them are classified as face-on at every other angle (the big bump at N=1 in this plot). The exceptions are where the disk is aligned with two of the virtual cameras — then, we see it as edge-on twice and face-on twice. Since the cameras are oriented like they’re at points of a pyramid with the galaxy at the center, geometry tells us we should expect a typical disk to be edge-on for 0 cameras most of the time, 1 sometimes, 2 very rarely, and never 3 or 4. Just what we see!

Example of a spiral galaxy in Illustris seen from four different viewing angles (“cameras”) in the Galaxy Zoo images. This is a rare case where the galaxy is seen twice as an edge-on disk (upper right, lower left) and twice as a face-on disk (upper left, lower right).

We’re excited to be starting on the analysis phase and are, as always, extremely thankful for your help.

Just a quick reference to a piece that came out in Currents, the newsletter of the National Optical Astronomy Observatory (NOAO) in the US. They included a short piece on the classifications from the new images in DECaLS, a survey which is being co-led by NOAO staff and carried out at their southern observing site in Chile. This followed a longer piece in their September newsletter on the first data release for DECaLS, which includes more details on the tremendous capabilities of the new survey. Good reads if you have a moment!

http://www.noao.edu/currents/201512.html#decals

A quick update: Galaxy Zoo volunteers have already provided more than 750,000 classifications of DECaLS images. We’re completely done with about a quarter of the first data release, and all the images have enough early classifications that we’re starting on preliminary analysis soon. As always, thanks to everyone for your interest and help!

I think the most common question/comment we’ve been seeing for classifiers of the simulated Illustris galaxies is along the lines of: “What’s the blue stuff?”

Image of a synthetic galaxy (AGZ00089n5) from the Illustris simulation, being classified in Galaxy Zoo. Blue-ish emission can be seen extending from the lower left to upper right of the center galaxy.

It’s a great question. Let’s talk about it in more detail.

The short answer is that the blue regions are the simulations’ method of reproducing the light emitted by young stars. A star’s lifetime generally scales as a function of its mass – the more massive the star is when it’s first formed, the hotter it is and the faster it burns fuel. Emission from hotter objects will tend to be bluer (ie, produce more photons at shorter wavelengths) compared to less massive stars. These are trends we see in optical images of stars in galaxies, including naked-eye views and composite color images. The exact color depends on the filters being used as well as processing of the images – that’s the difference between images you may have seen of star-forming regions being pink in some images and blue in others, such as those in Illustris.

A couple more specific questions that we’ve received:

What’s causing the blue colors in the galaxies? Are they caused by individual atomic or molecular lines that we can see in the spectra?

Volunteers who worked on the original GZ green peas project might be familiar with the term “nebular emission” – individual, narrow lines caused by ionized or hot gas surrounding stars, or whether they’re the result of the broadband colors of the stars themselves. The GZ-Illustris images use a stellar population model that only computes the broadband colors, due to some issues with unrealistic green images caused by the interaction of the codes that deal with both the emission lines and effects of dust. The model we’re using – based on work by Bruzual & Charlot (2003) – omits the emission lines for that reason. However, we’ve made extensive comparisons of the two sets of images and find that they agree very well for our scientific goals, including the morphology classifications.

A plot of the synthetic spectra for galaxies in the Illustris simulation; each thin horizontal line is the spectrum of an individual galaxy. The most massive galaxies are at the top, while the lowest mass galaxies are at the bottom. Wavelength increases from left to right, or going from bluer to redder colors. The lack of sharp features in this plot (which uses the BC03 model adopted by the Galaxy Zoo images) are a result of excluding the nebular line emission. Figure courtesy P. Torrey (MIT/Caltech).

How should visual morphology classifiers deal with the star-forming regions? Ignore them and look at the underlying stellar populations? Treat them as part of the galaxy? Something else?

This is a tough one.  Many galaxies have the “blobby” star-forming regions but others have nicer looking disk or spiral distributions.  Our analysis suggests is that this is a pretty tight function of the total star formation rate (higher SFR = more realistic looking features).  We suggest that users treat them as part of the galaxy; it might lead to some odd results in lower mass galaxies, but we expect they should trace each other very well for the more massive galaxies. If you see geometry that’s distinctly different from a well-formed spiral disk or elliptical, don’t be hesitant to click the “Anything Odd” or “Other” buttons – that’s one of the simplest ways in which we can measure the unusual effects of the blue regions, given the constraints of our classification scheme.

One option for measuring the effect of the blue blobs is to select “Other” under the “Anything Odd” question.

The distribution of the blue blobs is often disconnected and/or in unusual shapes compared to Sloan. What determines the spatial distribution of the star forming regions?

This results from the extremely discrete sampling of the density of stars in the images.  Stars can only form in “chunks” of about 1 million solar masses, instead of the more typical small clusters and regions that we know exist in the real Universe.  Moreover, these chunks have their light spread over a significant fraction of ~1 kpc (which is pretty big, compared to a typical galaxy radius of ~20 kpc), and so they often won’t look much like real star-forming regions.  This, coupled with the lack of dust, leads to what you see in the GZ images.

Thanks as always to everyone for your help. Please post here or on Talk if you have more questions!

This post was written with the help of researchers Gregory Snyder (Space Telescope Science Institute) and Paul Torrey (MIT/Caltech), who worked extensively on the development of Illustris and the generation of the mock images for Galaxy Zoo.