Eight Years & the 8th Paper: Green Peas – Living Fossils of Galaxy Evolution
As we approach the 8th anniversary of the Galaxy Zoo project, it is a great opportunity to look back at one of the most fascinating discoveries of citizen science in Galaxy Zoo – the “Green Pea” galaxies. Volunteers on the forum first noted these galaxies due to their peculiar bright green color and small size. Their discovery was published in our 8th paper: ‘Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies’ and is noted on the blog here. But the story doesn’t end with their discovery.
Top Row: Green Peas in the original imaging are compact & bright green.
Bottom Row: Green Pea galaxies as imaged by the Hubble Space Telescope show patches of starformation.
In the years since the publication of their discovery paper by the Galaxy Zoo Science Team, the Green Peas are beginning to fulfill their promise as a living fossil of galaxy evolution. Because they aren’t too far away, they provide a unique local laboratory in which we can investigate processes key to the formation and evolution of galaxies in the early universe. They are living ‘fossils,’ undergoing extraordinary, intense starbursts unlike any other galaxies known in the local universe. Their color is due to a large amount of emission in an oxygen line [OIII]/5007A that made their appearance green in the images.
Follow-up studies of the Green Peas have looked in great detail at their abundances of various elements, something that cannot be done in their high redshift analogs. The results of these studies show that they have energetic outflows of gas and lower oxygen abundances than other typical local galaxies with similar masses. They also suggest what might be responsible for ionizing the gas in the galaxies and producing those bright emission lines (e.g., Wolf-Rayet stars). Their clumpy morphologies (or shapes) have been confirmed and suggest that star formation in the peas occurs in several separate knots throughout the galaxy. Their radio emission implies they have strong magnetic fields, larger than that of the Milky Way. All of these results paint a picture of galaxies very similar to those that formed in the early Universe.
This image shows radio emission detected from a combination (stack) of 32 different Green Pea Galaxies.
Results from studies of these galaxies can provide challenges to commonly accepted models. For example, the strong magnetic fields challenge models that suggest magnetic fields grow slowly over time and observations of the variation in Lyman alpha emission line profiles and strengths challenge models of the dependence of the emission line shape on gas properties in the galaxy. The Green Peas have held up their promise of lending new insights into galaxy evolution by characterizing an active mode of star formation, which contrasts with the typical more passive evolution dominating the local galaxy population. Studies of the Peas have suggested that a galaxy’s evolutionary pathway may depend on stochastic initial conditions, leading insights into our understandings of how galaxies throughout the Universe form.
How to Navigate the Astro Literature, Part 1
So you want to learn about current astrophysics research? You’re in luck! Not only are there many excellent blogs, pretty much all of the peer reviewed literature is out there accessible for free. In many areas of science, the actual papers are behind paywalls and very expensive to access. Astrophysics, like a few other areas of physics and mathematics, puts most papers on the arxiv.org preprint server where they are all available for download form anywhere. In addition, we have a very powerful search tool in the form of the NASA Astrophysics Data System which allows you to perform complex searches and queries across the literature.
Suppose you wanted to learn more about the green peas, one of our citizen science-led discoveries. Your first stop could be the ADS:
ADS, like any search engine, will now scour the literature for papers with the words “green peas”, “green” and “peas” in it, and return the results:
As you can see, the discovery paper of the peas, “Cardamone et al. (2009)” is not the first hit. That’s because in the meantime there has been another paper with “green peas” in the title. You can click on Cardamone et al. and find out more about the paper:
This is just the top of the page but it already contains a ton of information. Most importantly, the page has a link to the arxiv (or astro-ph) e-print (highlighted). Clicking there will get you to the arxiv page of the paper where you can get the full paper PDF.
Also there is a list of paper which are referencing Cardamone et all, at the moment 23 papers do so. By clicking on this link you can get a list of these papers. Similarly, just below, you can get a list of paper that Cardamone et al. is referencing.
Lower still are links to NED and SIMBAD, two databases of astronomy data. The numbers in the brackets indicate that SIMBAD knows 90 objects mentioned in the paper, and NED knows 88. By clicking on them, you can go find out what those databases know about the objects in Cardamone et al. (i.e. the peas).
Obviously there’s a lot more, but just with the arxiv and NASA ADS you can search and scour the astrophysics literature with pretty much no limits. Happy resarching!
Radio Peas on astro-ph
Today on astro-ph the Peas radio paper has come out! I discussed the details of the radio observations in July, after the paper had been submitted. The refereeing process can take several months, from the original submission until the paper is accepted.
The paper is very exciting to all of us that worked on the original Peas paper, because it is a great example on how these exciting young galaxies (not too far away) are giving us insights into the way galaxies form and evolve. In the case of the Radio Peas, the observed radio emission suggests that perhaps galaxies start out with very strong magnetic fields.
The Peas – Now detected in Radio!
Last September I blogged about a proposal that had just been accepted at the Giant Metrewave Radio Telescope (GMRT) to follow up on the Peas with radio observations. Now the observations are in, and we have successfully detected the Peas at radio wavelengths!
The Peas, which have very high star formation rates, are expected to host a large number of supernova, which are created when the most massive stars die. These supernova create shocks that accelerate electrons in galaxy to relativistic energies. These relativistic electrons emit a type of emission, synchrotron radiation, that is visible in radio wavelengths. Therefore, the radio emission can tell us about the stars that live (or lived) in the galaxy.
Three of the Peas from our paper (Cardamone et al. 2009), were followed up with deep observations using the GMRT. It turns out that the Peas have comparable, but systematically lower flux when compared to local starbursts.
Using the observed radio emission, the magnetic field of the galaxy can be derived. These new observations suggest a magnetic field in the peas similar to that of the Milky Way. Because galaxies are thought to build up their magnetic fields over time, it is surprising to see such a large magnetic field in such a young galaxy. (Estimates of the age of the stars in the Peas are roughly 1/100th that of the age of the stars in the Milky Way).
One of the reasons that the Peas are so fascinating is their similarities to vigorously star forming galaxies found in the early universe (known as Lyman Break Galaxies). These Lyman Break Galaxies are so far away, they haven’t yet been directly detected in radio emission. However, estimates of their radio flux (from a technique called ‘stacking’) also suggest consistent radio fluxes with those observed for the Peas.
These observations suggest that galaxies like the Peas (and the Lyman Break Galaxies), may start out early in their life with very large magnetic fields. These observations challenge the assumption that galaxies build up their magnetic fields slowly over time and it is another piece of the puzzel in understanding of how galaxies are formed.
The article will be coming soon to astro-ph and I will post it here to let you all know.
Radio Peas
Working with scientists in India, we have been awarded time on the Giant Metrewave Radio Telescope (GMRT) to study the radio properties of the Green Pea galaxies discovered by Galaxy Zoo users. We hope to use this telescope to detect the first signs of radio emission from the Peas, establishing them as a new class of radio sources.
Why do we want to search for radio signals from the Peas? The radio emission comes from remnant supernovae which can accelerate relativistic electrons that emit synchrotron radiation. So when we are detecting star forming galaxies in radio emission, we are finding signatures from these supernovae, which tell us about the stars that live (or lived) in the galaxy. Therefore, using the radio emission we can trace recent star formation activity in the galaxy.
We are particularly interested in these Green Peas, because they are the closest analogues to a class of vigorously star forming galaxies found in the early universe (known as Lyman Break Galaxies). These galaxies behaved very differently from star forming galaxies in the present day universe, and can help us to understand how galaxies formed in the early universe. Because Lyman Break Galaxies are so far away, Astronomers have not yet been able to detect radio emission from any of these galaxies individually. In contrast, the Peas are much closer and we have a good chance of being able to directly detect them in radio emission. Detecting this radio emission, and determining whether or not the radio emission from the Peas is like that in nearby star forming galaxies will help us to understand the nature of star formation in the youngest galaxies.
The latest on the peas – do they lack metals?
It’s sometimes difficult to know which papers will excite other scientists and get them to follow-up what you’ve done. Our peas paper already has seven references to it, so I wasn’t entirely surprised to find a whole paper discussing the peas on astro-ph today. Astro-ph is required reading for all astrophysicists and contains pre-prints of papers that are updated every day. Some papers are posted when they’re submitted to a journal, others only once they’ve been accepted. A wonderful thing about the field of astronomy is the free access to data and the wide sharing of ideas through forums such as astro-ph. This creates new and exciting scientific results at an amazing pace.
This paper, written by Ricardo O. Amorín, E. Pérez-Montero and J.M. Vílchez (all at the IAA-CISC), follows up on one of the aspects of the peas: the metallicity (amount of elements other than hydrogen and helium) that are polluting the gas in the peas. These elements (or metals, as astronomers confusingly say) are generated in supernovae, so the metallicity,and the ratios of specific elements, can give astronomers some idea of how “evolved” a galaxy is. The more metals, the more supernovae must have gone off and polluted the gas.
From: Amorin et al. (2010), arXiv:1004.4910. Horizontal axis: galaxy mass; Vertical axis: “metallicity”
What they find is different from our paper. Using a different method to measure the metallicity of the peas, they include the abundance of Nitrogen. This turns out to be anomalous in the peas, and suggests that the peas are less metal-enriched than we concluded. They then look at whether the peas have the amount of metals that other galaxies of similar mass have, and conclude that the peas are off the “mass-metallicity relation” (see plot above – green points are the peas,which are below the grey shaded area representing normal star forming galaxies). This is definitely different from what we concluded – we deduced that the peas are actually on the mass-metallicity relation.
They discuss what this means – if they are right, this makes the peas even more exceptional, since they don’t fit in with normal galaxies in our old, evolved Universe, and underscores their role as “living fossils” since the peas are more like primordial galaxies than evolved ones. The differences in this nitrogen abundance tells us something about the way the peas convert gas into stars that is quite different from what occurs in galaxies like our own Milky Way. Amorin et al. further suggest that the “pea” phase is likely short-lived as the intense star formation in the peas will quickly enrich the gas to make them appear more like their normal cousins. The differences in this nitrogen abundance can imply
So who is right? We don’t know yet. The Amorin et al. paper is appearing in the Astrophysical Journal as a Letter and hopefully starts off a debate on the topic. Stay tuned!
Kevin & Carie
Leaving La Silla
Our pea hunting observing run is over and, as you will have gathered from previous posts, it has gone very well. We travelled back from La Silla to Santiago last night, for another brief stay at the ESO Guesthouse. Now I’m about to get on the plane back to the UK.
We are thrilled with how well the run has gone. I’ve done a rough analysis of the data already and we’ve already started drafting the paper presenting the results! I’ll do a more careful analysis once I get back to the office, but early indications are that we’ve got a nice collection of higher redshift objects that are very similar to the fascinating SDSS Peas that were discovered on the Galaxy Zoo forum, and an almost perfect technique for finding more! I’ll keep you posted as our work progresses.
Technical problems
The Chilean Pea hunt continues…
You may have noticed the Galaxy Zoo blog was down over the weekend. Well, it wasn’t the only one to be experiencing technical difficulties. On Sunday night we unfortunately lost four hours of observing time to technical gremlins. First we tried to use a new filter, which resulted in a nice 10 minute exposure of nothing. After a trip to the telescope to look around inside the instrument, the support staff worked out the problem: the filter was mislabelled on the computer. With that figured out, we changed to the correct filter and carried on – only to be stopped in our tracks again a couple of hours later by the whole telescope control system crashing! This time it took three hours of methodical troubleshooting to fix the problem, apparently some problem with a power lead. By then the night was almost over.
Peas please me
Well, at the start of the night we were a bit nervous that none of our objects would turn out to be Peas. Fortunately, by the end of the night our quick looks at the data indicate that four of the seven i-Pea candidates that we managed to observe are emission line objects at the redshift we expected our selection to give, i.e. they are Peas! That’s about as good a success rate as we dared hope for.
The night hasn’t been without its difficulties. The seeing (how blurry the atmosphere makes our images) wasn’t great for most of the night, and it has turned out to be hard to actually find our targets because they are so faint. Those issues combined meant that we didn’t get through as many candidates as we hoped for, but we are still happy with the collection we got.
Right now, time for a quick breakfast then some sleep before trying to net some more Peas tomorrow night.
Here we go…
The continuing adventures of the Pea hunting trip to Chile…
We’ve spent the afternoon finalising our target selection, preparing our observing strategy, and taking calibrations. The sun has now set and we’re already pointing in the right direction for our first target. In a few minutes time we will start our observations. We begin with a standard star, which will be used to calibrate the spectra of our main targets. The we are on to our first Pea candidate.
The SDSS Peas that Carrie studied were green in the SDSS colour images because they were bright in the r-band, actually the red region of the visible spectrum. On this occasion we are looking for similar object that are a bit further away, so we expect them to be very bright in the i-band (further into the red) ar the z-band (on the border between red and the near infrared). Tonight we are observing the i-bright objects, and hopefully we will get through about a dozen of them. The weather looks good, so fingers crossed.
Anyway, better concentrate on the observing.
Galaxy Zoo 