Pea galaxy
A Pea galaxy, also referred to as a Pea or Green Pea, might be a type of luminous blue compact galaxy that is undergoing very high rates of star formation. Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey.
"Pea" galaxies were first discovered in 2007 by the volunteer citizen scientists within the forum section of the online astronomy project Galaxy Zoo, part of the Zooniverse web portal.
Description
The Pea galaxies, also known as Green Peas, are compact oxygen-rich emission line galaxies that were discovered at redshift between z = 0.112 and 0.360. These low-mass galaxies have an upper size limit generally no bigger than across, and typically they reside in environments less than two-thirds the density of normal galaxy environments. An average GP has a redshift of z = 0.258, a mass of ~3,200 million , a star formation rate of /yr, an Doubly ionized oxygen| equivalent width of 69.4 nm and a low metallicity. They have a strong emission line at the wavelength of 500.7 nm. , O++ or doubly ionized oxygen, is a forbidden mechanism of the visible spectrum and is only possible at very low density. When the entire photometric SDSS catalogue was searched, 40,222 objects were returned, which leads to the conclusion the GPs are rare objects.GPs are the least massive and most actively star-forming galaxies in the local universe. "These galaxies would have been normal in the early Universe, but we just don't see such active galaxies today", said astrophysicist Dr. Kevin Schawinski. "Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve".
GPs exist at a time when the universe was three-quarters of its current age and so are clues as to how galaxy formation and evolution took place in the early universe. With the publication of Amorin's GTC paper in February 2012, it is now thought that GPs might be old galaxies having formed most of their stellar mass several billion years ago. Old stars have been spectroscopically confirmed in one of the three galaxies in the study by the presence of magnesium.
File:GP J1219 NearUV HST.jpg| thumb| A Hubble Space Telescope Cosmic Origins Spectrograph Near-UV image of Pea galaxy GP_J1219
In January 2016, a study was published in the journal Nature identifying J0925+1403 as a Lyman continuum photons 'leaker' with an escape fraction of ~8%. A follow-up study using the same Hubble Space Telescope data identifies four more LyC leakers, described as GPs. In 2014–15, two separate sources identified two other GPs to be likely LyC leaking candidates, suggesting that these two GPs are also low-redshift analogs of high-redshift Lyman-alpha and LyC leakers. Finding local LyC leakers is crucial to theories about the early universe and reionization.
The image to the right shows Pea galaxy GP_J1219. This was observed in 2014 by a HST team whose principal investigator was Alaina Henry, using the Cosmic Origins Spectrograph and the Near Ultraviolet channel. The scale bar in the image shows 1 arc second, which corresponds to ~10,750 light years at the distance of 2.69 billion light years for GP_J1219. When using the COS Multi-Anode Micro-channel Array, in NUV imaging mode, the detector plate scale is ~40 pixels per arcsecond.
GPs feature significantly within the Zoogems project, which uses HST to examine images of interest from the citizen science websites Galaxy Zoo and Radio Galaxy Zoo, collected since 2007. Among the ~300 possible candidates for the Zoogems observations are 75 GPs. The original GP classifications used SDSS images, which are not as good quality as the HST examples.
History of discovery
Years 2007 to 2009
is a project online since July 2007 that seeks to classify up to one million galaxies. On July 28, 2007, two days after the start of the Galaxy Zoo Internet forum, citizen scientist 'Nightblizzard' posted two green objects thought to be galaxies. A discussion, or thread, was started on this forum by Hanny Van Arkel on the 12th of August 2007 called "Give peas a chance" in which various green objects were posted. This thread started humorously, as the name is a word play of the title of the John Lennon song "Give Peace a Chance", but by December 2007, it had become clear that some of these unusual objects were a distinct group of galaxies. These "Pea galaxies" appear in the SDSS as unresolved green images. This is because the Peas have a very bright, or powerful, spectral line in their spectra for highly-ionized oxygen, which in SDSS color composites increases the luminosity, or brightness, of the "r" color band with respect to the two other color bands "g" and "i". The "r" color band shows as green in SDSS images. Enthusiasts, calling themselves the "Peas Corps", collected over a hundred of these Peas, which were eventually placed together into a dedicated discussion thread started by Carolin Cardamone in July 2008. The collection, once refined, provided values that could be used in a systematic computer search of the GZ database of one million objects, which eventually resulted in a sample of 251 Pea galaxies, also known as Green Peas.In November 2009, authors C. Cardamone et al. published a paper in the MNRAS titled "Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies". Within this paper, 10 Galaxy Zoo volunteers are acknowledged as having made a particularly significant contribution. They are: Elisabeth Baeten, Gemma Coughlin, Dan Goldstein, Brian Legg, Mark McCallum, Christian Manteuffel, Richard Nowell, Richard Proctor, Alice Sheppard and Hanny Van Arkel. They are thanked for "giving Peas a chance". For more details see: [|Cardamone 2009 Physics]
The original 80 GPs were part of a sample from the SDSS data-release 7, but did not include galaxies from other sources which might have been classed as GPs if they were in the SDSS sample. One example of a paper that demonstrates this is: In April 2009, J. J. Salzer et al. published a paper in the Astrophysical Journal Letters titled "A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts". In this paper, "new spectroscopy and metallicity estimates for a sample of 15 star-forming galaxies with redshifts in the range 0.29 – 0.42" were presented. These objects were selected using the KPNO International Spectroscopic Survey. 3 of these 15 when viewed as objects in SDSS are green. Quoting from Salzer et al. 2009 "A New Class of Galaxy? Given the large number of studies of metal abundances in galaxies with intermediate and high redshift mentioned in the Introduction, it may seem odd that systems similar to those described here have not been recognized previously."
2010 - 2012
In June 2010, authors R. Amorin et al. published a paper in ApJ Letters titled "On the oxygen and nitrogen chemical abundances and the evolution of the "green pea" galaxies". In it they explore issues concerning the metallicity of 79 GPs, disputing the original findings in Cardamone et al. They conclude, "arguing that recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss drive by supernova winds may explain our findings and the known galaxy properties". For more details see: [|Two papers by Amorin]In February 2011, authors Y. Izotov et al. published a paper in the ApJ titled "Green Pea Galaxies and Cohorts: Luminous Compact Emission-line Galaxies in the Sloan Digital Sky Survey". They find that the 80 GPs are not a rare class of galaxies on their own, but rather a subset of a class known as 'Luminous Compact Galaxies', of which there are 803. For more details see: [|Luminous Compact Galaxies]
In November 2011, authors Y. Izotov et al. published a paper in A&A titled 'Star-forming galaxies with hot dust emission in the SDSS discovered by the Wide-field Infrared Survey Explorer '. In this paper, they find four galaxies that have very red colours in the wavelength range 3.4 micrometres and 4.6 micrometres. This implies that the dust in these galaxies is at temperatures up to 1000K. These four galaxies are GPs and more than double the number of known galaxies with these characteristics.
In January 2012, authors R. Amorin et al. published a 'Conference proceeding' titled "Unveiling the Nature of the "Green Pea" galaxies". In this publication, they announce that they have conducted a set of observations using the Optical System for Imaging and low Resolution Integrated Spectroscopy at the Gran Telescopio Canarias, and that there is a forthcoming paper about their research. These observations "will provide new insights on the evolutionary state of the Green Peas. In particular, we will be able to see whether the Green Peas show an extended, old stellar population underlying the young starbursts, like those typically dominant in terms of stellar mass in most Blue Compact Galaxies". For more details see: Two papers by Amorin
In January 2012, authors L. Pilyugin et al. published a paper in the MNRAS titled: "Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios". In it they compare the oxygen and nitrogen abundances derived from global emission-line SDSS spectra of galaxies using the electron temperature method and Composite hydrogen-rich nebula, ii) 281 SDSS galaxies and iii) A sample of GPs with detectable -4363 auroral lines. Among the questions surrounding the GPs is how much nebulae influence their spectra and results. Through comparisons of the three objects using proven methodology and analysis of metallicity, they conclude that "the high nitrogen-to-oxygen ratios derived in some Green Pea galaxies may be caused by the fact that their SDSS spectra are spectra of composite nebulae made up of several components with different physical properties. However, for the hottest Green Pea galaxies, which appear to be dwarf galaxies, this explanation does not seem to be plausible."
In January 2012, author S. Hawley published a paper in the PASP titled "Abundances in "Green Pea" Star-forming Galaxies". In this paper, former NASA astronaut Steven Hawley compares the results from previous GP papers regarding their metallicities. Hawley compares different ways of calibrating and interpreting the various results, mainly from Cardamone et al. and Amorin et al. but some from Izotov et al., and suggests why the various discrepancies between these papers' findings might be. He also considers such details as the contribution of Wolf–Rayet stars to the gas ionization, and which sets of emission lines give the most accurate results for these galaxies. He ends by writing: "The calibrations derived from the Green Peas differ from those commonly utilized and would be useful if star-forming galaxies like the Green Peas with extremely hot ionizing sources are found to be more common."
In February 2012, authors S. Chakraborti et al. published a paper in The ApJ Letters titled 'Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies'. In this paper, magnetism studies using new data from the Giant Metrewave Radio Telescope describe various observations based around the GPs. They show that the three "very young" starburst galaxies that were studied have magnetic fields larger than the Milky Way. This is at odds with the current understanding that galaxies build up their magnetic properties over time. For more details see: [|Radio detection]
In April 2012, authors R. Amorin et al. published a paper in the ApJ titled "The Star Formation History and Metal Content of the 'Green Peas'. New Detailed [|GTC-OSIRIS] spectrophotometry of Three Galaxies". They give the results for the deep broad-band imaging and long-slit spectroscopy for 3 GPs that had been observed using the OSIRIS instrument, mounted on the 10.4m Gran Telescopio Canarias at the Roque de los Muchachos Observatory. For more details see: GTC-OSIRIS
In August 2012, authors R. Amorín et al. published a paper in the ApJ Letters titled "Complex gas kinematics in compact, rapidly assembling star-forming galaxies". Using the ISIS spectrograph on the William Herschel Telescope, they publish results of the high-quality spectra that they took of six galaxies, five of which are GPs. After studying the hydrogen alpha emission lines in the spectra of all six, it is shown that these ELs are made up of multiple lines, meaning that the GPs have several chunks of gas and stars moving at large velocities relative to each other. These ELs also show that the GPs are effectively a 'turbulent mess', with parts moving at speeds of over 500 km/s relative to each other.