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Galaxy Zoo Peas: Extremely Star-Forming Galaxies are a completely new class of galaxy that have been discovered by users of the online astronomy project Galaxy Zoo. These Peas are so-called as they have a green appearance in the pictures taken by the Sloan Digital Sky Survey that are used in Galaxy Zoo. Green Peas are low mass galaxies generally no bigger than 5 or 6 kiloparsecs (say 20,000 light years) across, and they generally reside in universal environments less dense than normal galaxies.

Picture of typical Pea taken from SDSS Explorer database: 587729777446945029

The Story So Far...

Richard Nowell/Galaxy Zoo Peas: Extremely Star-Forming Galaxies
Type of site	Volunteer Scientific Project
Available in	English, Polish
Owner	Galaxy Zoo Team
Created by	Galaxy Zoo Team
URL	http://www.galaxyzoo.org
Commercial	No
Registration	Yes
Launched	12 July 2007

When first encountered by the Galaxy Zoo users, Peas were thought of humourously; hence the name that they were given by Hanny Van Arkel on the Galaxy Zoo Forum. Indeed, as written about by the Forum's Global Moderator Alice Sheppard, no-one thought much about them as distinct, interesting astronomical objects until December 2007. To quote Alice from her amazing and comprehensive summary on the Galaxy Zoo Blog: "We had great but uneventful fun until mid-December 2007. The breakthrough took place on a different thread from the Peas- which 'Nightwatch' called What is this green coloured thingy?". From this thread sprung the information volunteered by Galaxy Zoo users that this 'green coloured thingy' was a small galaxy that had a spectrum that showed an enormous amount of ionised Oxygen present- this is usually a sign of star-formation, as Gemma Coughlin rightly posted. Click for a picture of the spectrum of this crucial first Pea. For more information on spectra in SDSS, click here or here.

The first list of 39 galaxies that were green, oxygen-rich ELGs was put together by Richard Nowell on the 14th December 2007. To quote Zookeeper Dr. Kevin Schawinski, co-founder of Galaxy Zoo: "I wonder if the 'pea selection' yields a sample of interesting objects...". From hereon in, the main task became to collect as many of these green peas as the 'peahunters' could find- the original list soon grew to 50-odd and then to 80-odd, until it eventually evolved to 251. Around mid-January 2008 another important breakthrough came when the hunt for Peas that were NOT green commenced as a result of a dialogue between 'Arralen' and 'StarryNight' (Dan Goldstein). To quote Starry: "...what really got me hooked was finding a red object that Arralen said could also be a Pea". This search for non-green Peas identified galaxies that were at different distances then the original Peas, but still had the same tell-tale spectra, only redshifted.

Then searching for Peas became a case of finding spectra that had this huge OIII spike, but little else within the continuum. Colour was no longer a barrier and it became clear that Pea-like objects were found at redshifts up to the limit of SDSS's resolution. The next major step forward was the automation of Pea searching. Starry asked if it was possible to put a search together using a program to search the SDSS database using a range of figure's that had been gleaned from the Peas already found. Around this time, 'Fermat's Brother' (Brian Legg) posted some very useful information on the SDSS filters and a table showing where varous relevant wavelengths were found. At around this time, a programmer from Eastern Germany Christian Manteuffel ('Laihro') joined the quest: this was to have a lasting effect on the Pea Hunt as time progressed.

In the second week in March, Brian Legg put together an SQL program, which he then entered into an SDSS search program. This automated the search for Peas and produced exciting results. Gone were the days of manual hunting; a new dawn had arrived! Here were some of the furthest Peas (or cranberries) yet found. Around this time, two more of the 'Peas Corps' became involved: Elisabeth Baeten ('ElisabethB') from Brussels and Mark McCallum ('Galaxy Hunters Inc.') from Sydney, Australia. The hunt continued and broadened, with various related threads, such as The Most Weird OIII objects or OIII objects, both of which made use of Christian's SQL search. he came up with a list of more than 7000 'positive emitters', and it became clear that the hunt now was for OIII objects (Peas), as well as the original Green Peas. Confused? The last posting in this chapter was by Elisabeth here.

Disaster struck the Galaxy Zoo forum in mid-May when a fire damaged the servers, as reported here by Edward Edmondson from Portsmouth University. This was a catastrophe indeed, and it was not clear what would happen- all that information had gone up in smoke! Arghhh! Pea hunting continued in a smaller way on a temporary forum, which is no longer available for view, but it was only when original Forum had been started again with the use of a backup that all that info became available again. Those were a nervous couple of months for everyone concerned, as not all the lists had been backed up. This perhaps was GZ's lowest point...

A backup had been made of the Forum, minus a couple of weeks, and that was put back online at the beginning of July 2008. The first posting in this new Galaxy Zoo chapter was from Elisabeth on July 7th. July 8th a thread was started by Carolin Cardamone The Peas Project. Carie is a postgraduate Astrophysicist from Yale University, and the task of assimilating all this information and producing a scientific paper was given to her by Zookeeper Kevin. No easy task! After some initial confusion about what was actually wanted, Richard Nowell produced his 'Celebratory list'. A definitive list had to be gathered, which involved searchs high-and-low using queries concocted by Christian. Every nook-and-cranny of the SDSS database was searched, but as Zookeeper Kevin commented, a final total was proving to be elusive. As Richard Nowell commented, there were a lot of lists and we needed to look through all of them to say what was what.

The solution was agreed between Christian and Richard Proctor to use Richard Proctor's ('Waveney') remarkable PERL program. This program had first been put together voluntarily to help decide whether objects from SDSS were mergers, which eventually led to over 3000 mergers being identified. With Peas the task was simpler: decide whether this is "green, not green or greenish". Richard Nowell's comment summed it up for experienced Peahunters! A more refined selection was organised, and on July 17th, Carie announced the final, definitive listing was ready, and it then became a case of patiently waiting for news from Yale. Meanwhile Zookeeper Kevin had found a Hubble Space Telescope image of a Pea.

Updates were given occasionally, as well as applications for time with the HST. Through Autumn and Winter 2008/09, Carie worked very hard on getting all the data together. "I must have computed all those figures a hundred times each!" to paraphrase her. Finally, in April 2009, the paper was submitted to the MNRAS and was finally accepted in July 2009. Carie wrote about the process and the results on the Galaxy Zoo Blog here. The paper was titled: "Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies". This paper gave rise to intense interest in the Media (see links), as volunteers had helped discover 'the most efficient star-forming galaxies in the local Universe', to quote Galaxy Zoo co-founder Zookeeper Dr. Chris Lintott. For the 'Peas Corps', that grouping of people from GZ, it was an intensely proud moment, and one many of us will remember for a very long tme yet, not least as we gained acknowledgements in the paper itself (twice!)...It was a great priviledge to meet up in August 2009 in Greenwich, London, with Carie and most of the rest of the team, and started to bring home just what had been accomplished. A video has been made, which we (nervously) await.

It could be said that the story of the Peas is only just beginning. As a result of Carie's paper, Zookeeper Dr. Steven Bamford of Nottingham University headed a team which was given five nights at the European Southern Observatory in Chile in November to look for Pea-like objects at high redshifts using data garnered by colleague Seb Foucaud from the UKIDSS survey. To quote Steven writing at the end of those five nights: "Our Pea hunting observing run...has gone very well. We are thrilled...". Peahunters now await the next chapter in this remarkable series of events...

So What Are Peas?

Peas, or more specifically Green Peas, are essentially Oxygen-rich Emission Line Galaxies (ELGs: CADIS ELG) that are at distances between approximately 2 billion to 4.7 billion light years away. As seen from their spectra, they have an enormous 'spike' at the wavelength of 5007 Angstroms (not redshifted), which is where OIII is found. OIII, or Doubly_ionized_oxygen, is a very rare Oxygen ion, found only where there is great heat, but little mass (see also Forbidden_line).

Whilst ELGs are nothing new, Peas attracted interest as they appeared green when viewed through SDSS and its five filters. This is highly unusual for galaxies and was a sign of enormous activity. How this activity was created was the subject of research, and it was found that of the 251 that were isolated, many had enormous rates of starburst. Indeed, in the Peas paper, one of the most star-forming galaxies makes stars at a rate more than 1500 times our own galaxy. This for an object that has on average one-hundredth of the mass of our own Milky_way. But almost as importantly, these Green Peas exist at a time when our universe was three-quarters of its present age. To quote Kevin: "These galaxies would have been normal in the early Universe, but we just don't see such active galaxies today. Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve."

If we look at one example taken from SDSS Explorer. It has the reference 587729777446945029 and a picture of it is here. Obviously the deep emerald green shows well, but also the apparent lack of structure- not spiral or elliptical is obvious. When the spectrum is viewed, a very large spike is seen with OIII above it- this is the doubly-ionised Oxygen wavelength redshifted: ..5029 is at a distance of 4,048,561,158 light years.

To quote at length from the Yale University's Press release, July 27 2009:

Carolin Cardamone: "These are among the most extremely active star-forming galaxies we've ever found."

Kevin Schawinski: "They're growing at an incredible rate. these galaxies would have been normal in the early Universe, but we just don't see such active galaxies today". "This is a genuine citizen science project...It's a great example of how a new way of doing Science produced a result that wouldn't have been possible otherwise".

Some Facts & Figures

Of the one millon galaxies that make up Galaxy Zoo's image bank, the team found only 251 Green Peas. This was the original sample number. From these 251, 80 were selected through rigorous scientific procedure to take the final data from. These figures are taken from Table 4, Properties of Star-Forming Galaxies, Galaxy Zoo Green Peas. As a comparison, our own galaxy the Milky_Way is approximately 100 times the mass of an average green Pea, makes around 3 suns a year, has a mass of 5.8 x 10^11 solar masses and is over 13 billion years old. When astronomers talk about rates of star-formation, they often use our Sun as a guide to the size of stars 'being made'. The values presented are from SDSS DR7.

Most star forming = 587728906099687546 - 58 suns a year.

Most luminous (using Equivalent width) & nearest = 587738410863493299. ...3299 has an Equivalent Width of 2388 Angstroms.

Average distance = 3,539,048,359 light years. Furthest away (green peas) = 587732134315425958 4,792,174,432 light years.

Most luminous in the Ultraviolet using info. from GALEX = 587733080270569500. To quote: "The GALEX survey is well matched in depth and area, and 139 of the sampled 251 Peas are found in GR4. For the 57 of the 80 star-forming Peas with GALEX detections, the median luminosity is 3x10^10 L solar masses [which is rare]."

Most massive (number of suns) = 587725818034913419 10^10.31 solar masses

Least massive = 587741392649781464 10^8.55 solar masses

Average star-forming rate of Galaxy Zoo Green Peas is around ten a year. This means that (as a ball-park figure), Peas make stars at a rate of 300 times our own galaxy. The most star-forming Pea makes stars at a rate more than 1500 times our own. This implies a doubling time of between 100 million and 1000 million years- very fast!

The average Pea has a metallicity of log[O/H] + 12 approximately equal to 8.7 .

Peas are particularly low in dust content, unlike other local star-forming galaxies.

References

Galaxy Zoo Green Peas Abstract of 'Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies' C. Cardamone et al.
Yale University press release Yale University official press release: Galaxy Zoo Hunters Help Astronomers Discover Rare ‘Green Pea’ Galaxies.
Peas In The Universe GZ Global Moderator's Alice Sheppard definitive story of the Peas.
List of 80 Peas List of Peas in Table 4 of the Peas paper.
La Silla blog Steven Bamford's last blog post from La Silla.

External links