# Pea galaxy

Galaxy Zoo Green Peas
Three HST pictures of Starburst Peas

A Pea galaxy, also referred to as a Pea or Green Pea, might be a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation.[1] Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey (SDSS).

Pea Galaxies were first discovered in 2007 by the volunteer users within the forum section of the online astronomy project Galaxy Zoo (GZ).[2]

## Description

The Pea galaxies are compact oxygen-rich emission line galaxies (ELG) that were discovered at redshifts between z = 0.112 and 0.360, due to emission lines from oxygen. These low-mass galaxies have an upper size limit generally no bigger than 16,300 light-years (5,000 pc)[1] across, and typically they reside in environments less than two-thirds the density of normal galaxy environments. An average starburst Pea has a redshift of z = 0.258, a mass of around 3,160 million solar masses, a star formation rate of 13 solar masses a year, an [O III] equivalent width of 69.4 nm and a low metallicity.[1][3] They have a strong emission line at the OIII wavelength of 500.7 nm. OIII, O++ or doubly ionized oxygen, is a forbidden line of the visual spectrum and is only possible at very low densities. Pea galaxies are among the most active star-forming galaxies ever found.[4]

Comparing a Pea galaxy to the Milky Way can be useful when trying to visualize these star-forming rates. The Milky Way is a spiral galaxy and has a stellar mass of 580,000 million M.[5][6] Research by the European Space Agency and NASA has shown the Milky Way converts around 4 M/yr worth of interstellar gas into stars.[7] An average starburst Pea galaxy has a mass of around 3,162 million M.[1] So, approximately, the Milky Way has the mass of 175 Peas. An average Pea converts around 13 M/yr of interstellar gas into stars, which is 3.25 times the rate of the Milky Way.[1] If the relative masses of Peas and the Milky Way is taken into account, the average Pea converts interstellar gas 568 times more efficiently.

Pea galaxies exist at a time when the Universe was three-quarters of its present age and so are clues as to how galaxy formation took place in the earlier Universe.[8][9] With the publication of Amorin's GTC paper in February 2012, it is now thought that Green Peas are old galaxies having formed most of their stellar mass several billion years ago. The presence of old stars has been spectroscopically confirmed in one of the three galaxies in that GTC study.[10]

"These galaxies would have been normal in the early Universe, but we just don’t see such active galaxies today", said Schawinski. "Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve".[4]

## History of discovery

Galaxy Zoo (GZ) is a project online since July 2007 which seeks to classify up to one million galaxies. In July 2007, a few days after the start of GZ, a discussion was started on GZ's Internet forum by Hanny Van Arkel called "Give peas a chance" in which various green objects were posted. This thread started humorously, as the name is a play on words 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, emission 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.[1][11] Enthusiasts, calling themselves the "Peas Corps" (another humorous play on the Peace Corps), collected over a hundred of these Peas, which were eventually placed together into a dedicated 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 Green Peas.

In July 2009, a paper titled "Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies" (Cardamone et al. 2009) was published in MNRAS.[1] (e.g.[12][13][14][15]) Within the Galaxy Zoo Green Peas paper, 10 GZ 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". Citations for 2009MNRAS.399.1191C from the ADS Databases (Cardamone 2009).

In April 2010 in a paper which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez explore issues concerning the metallicity of the 80 starburst Peas.[3]

In December 2010, Yuri Izotov, Natalia Guseva and Trinh Thuan published a paper in the Astrophysical Journal in which they argue that Peas are not a rare class of galaxies, but rather a subset of a class known as Luminous Compact Galaxies (LCG).[16]

In May 2011, R. Amorin, R. Perez-Montero and J.Vilchez published a 'Conference proceeding' on their work on Pea galaxies.[17] In this publication, they announce that they have conducted a set of observations using the OSIRIS imager and spectrograph at the GTC, and that there is a forthcoming paper about their research. These observations "will provide new insights on the evolutionary state of the GPs. In particular, we will be able to see whether the GPs show an extended, old stellar population underlying the young [star]bursts, like those typically dominant in terms of stellar mass in most Blue Compact Galaxies".[17]

In October 2011, Sayan Chakraborti, Naveen Yadav, Carolin Cardamone and Alak Ray released a paper titled 'Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies'. In this paper, magnetic studies using new data from the Giant Metrewave Radio Telescope GMRT describe various observations based around the Green Peas. 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.[18] In 2012, new data from the Very Large Array (VLA) will become available. This will concentrate on the three Peas' polarity and completing their spectral index.

In November 2011, Y.I. Izotov, N.G. Guseva, K.J. Fricke and C. Henkel published a paper in the journal of Astronomy and Astrophysics titled 'Star-forming galaxies with hot dust emission in the SDSS discovered by the Wide-field Infrared Survey Explorer (WISE)'Wide-field Infrared Survey Explorer. In this paper, they find four galaxies that have very red colours in the wavelength range 3.4 micrometres (W1) and 4.6 micrometres (W2). This implies that the dust in these galaxies is at temperatures up to 1000K. These four galaxies are Green Peas and more than double the number of known galaxies with these characteristics.[19]

In January 2012, L.S. Pilyugin, J.M. Vilchez, L.Mattsson and T.X. Thuan published a paper in the MNRAS titled: "Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios".[20] In it they compare the oxygen and nitrogen abundances derived from global emission-line SDSS spectra of galaxies using (1) the electron temperature method and (2) two recent strong line calibrations: the O/N and N/S calibrations.[20] Three sets of objects were compared: composite hydrogen-rich nebula, 281 SDSS galaxies and a sample of Cardamone et al.'s Green Peas with detectable [OIII]-4363 auroral lines.[20] Among the questions surrounding the Peas are how much nebulae influence the spectra, and therefore results, of the GPs.[16] 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 (such as metallicity). However, for the hottest Green Pea galaxies, which appear to be dwarf galaxies, this explanation does not seem to be plausible. It would work only if the HII regions in these galaxies have a dispersion of abundances much larger than that typically found in dwarf galaxies."[20]

In January 2012, S.A. Hawley published a paper in the Publications of the Astronomical Society of the Pacific titled: "Abundances in "Green Pea" Star-forming Galaxies".[21] In this paper, former NASA astronaut Steve Hawley compares the results from previous Green Pea 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."[21]

In February 2012, R. Amorin, E. Perez-Montero, J.M. Vilchez and P. Papaderos published a paper entitled: "The Star Formation History and Metal Content of the 'Green Peas'. New Detailed GTC-OSIRIS spectrophotometry of Three Galaxies".[10] They give the results for the deep broad-band imaging and long-slit spectroscopy for the three target galaxies that had been observed using the OSIRIS instrument, mounted on the 10.4m GTC at the Observatory Roque de los Muchachos (Gran Canaria).[10] (Please see section on this for more detail further down this article.)

In July 2012, authors R. Amorín, J. M. Vílchez, G. Hägele, V. Firpo, E. Pérez-Montero and P. Papaderos posted a new paper to astro-ph entitled "Complex gas kinematics in compact, rapidly assembling star-forming galaxies".[22] Using the Gran Telescopio Canarias they publish results of the high-quality spectra that they took of six galaxies, five of which are Green Peas. After studying the Hydrogen alpha emission lines in the spectra of all six, it is shown that these H alpha emission lines are made up of different lines. The multiple lines mean that the Peas have several chunks of gas and stars moving at large velocities relative to each other. Indeed, these lines also show that Peas are effectively a 'turbulent mess', with parts (or clumps) moving at speeds of over 500 km/s (five hundred kilometres per second) relative to each other.

In September 2012, S. Parnovsky, I. Izotova and Y. Izotov published a paper in the journal 'Astrophysics and Space Science' entitled "H alpha and UV luminosities and star formation rates in a large sample of luminous compact galaxies".[23] In it, they present 'a statistical study of the star formation rates (SFR) derived from the Galaxy Evolution Explorer (GALEX) observations in the Ultraviolet continuum and in the H alpha emission line for a sample of ~800 luminous compact galaxies (LCGs). Green Peas are considered as a subset of these LCGs (See Izotov et al. 2011 below). Within the larger set of LCGs, including the Green Peas, SFR of up to ~110 solar masses a year are found.

In December 2012, a discovery of a new type of galaxy named 'Green Beans' was announced. Although these Green Beans have Active Galactic Nuclei, and are not starburst galaxies, the superficial similarities between Peas and Beans -they are green- has been noted in the paper. Beans are ultra-luminous galaxies with AGN that have 'turned off', leaving a 'light echo', much like Hanny's Voorwerp (Hanny's_Voorwerp), yet galaxy-wide.[24]

In January 2013, A.E. Jaskot and M.S. Oey submitted a paper to the Astrophysical Journal entitled: "The Origin and Optical Depth of Ionizing Radiation in the "Green Pea" Galaxies".[25] In this they endeavour to narrow down the list of possibilities about what is producing the radiation and the substantial amounts of high-energy photons that might be escaping from the G.Ps. Through trying to observe these photons in nearby galaxies such as the G.Ps., our understanding of how galaxies behaved in the early Universe might well be revolutionised. Six "extreme" G.Ps. are studied: SDSS 587738947196944678, 587735349111947338, 587738410863493299, 587739828742389914, 587741392649781464 and 587727179006148758.

Six Extreme Peas

## Physics from the Cardamone 2009 paper

Graph showing specific star formation rate plotted against galaxy mass, with the Peas (purple diamonds) and the Galaxy Zoo Merger Sample (black points)

To date only five Pea galaxies have been imaged by the Hubble Space Telescope (HST). Three of these images reveal Peas to be made up of bright clumps of star formation and low surface density features indicative of recent or ongoing galaxy mergers.[1] These three HST images were imaged as part of a study of local ultraviolet (UV)-luminous galaxies in 2005.[26] Major mergers are frequently sites of active star-formation and to the right a graph is shown that plots specific star formation rate (SFR / Galaxy Mass) against galaxy mass.[27] In this graph, the Peas are compared to the 3003 mergers from the Galaxy Zoo Merger Sample.[13] It shows that the Peas have low masses typical of dwarf galaxies and much higher star-forming rates compared to the GZ mergers. The black, dashed line shows a constant SFR of 10 M/yr. Most Peas have SFR between 3 and 30 M/yr.

Graph showing 103 Peas plotted as Starburst galaxies (red stars), transition objects (green crosses) or A.G.N. (blue diamonds)

Pea galaxies are rare. Of the one million objects that make up GZ's image bank, only 251 Green Peas were found. After having to discard 148 of these 251 because of atmospheric contamination of their spectra, the 103 that were left, with the highest signal-to-noise ratio (SNR), were analyzed further and 80 were found to be starburst galaxies.[1] The graph classifies 103 narrow-line Peas (all with SNR ≥ 3 in the emission lines) as 10 Active Galactic Nuclei (AGN) (blue diamonds), 13 transition objects (green crosses) and 80 starbursts (red stars). The solid line is: Kewley et al. (2001) maximal starburst contribution (labelled Ke01).[28][29] The dashed line is: Kauffmann et al. (2003) separating purely star-forming objects from AGN (labelled Ka03).[30]

Histogram showing [OIII] Eq.Wth. of 10,000 comparison galaxies (red); 215 UV-luminous Galaxies (blue); Peas (green)

Pea galaxies have a strong emission line when compared to the rest of their spectral continuum.[31] On an SDSS spectrum, this shows up as a large peak with [OIII] at the top.[32] The wavelength of [OIII] (500.7 nm) was chosen to determine the luminosities of the Peas using Equivalent Width (Eq.Wth.). The histogram on the right shows on the horizontal scale the Eq.Wth. of a comparison of 10,000 normal galaxies (marked red), UV-luminous Galaxies (marked blue) and Peas (marked green).[1] As can be seen from the histogram, the Eq.Wth. of the Peas is much larger than normal for even prolific starburst galaxies such as UV-luminous Galaxies.[33]

Within the GZ Green Peas paper, comparisons are made with other compact galaxies, namely Blue Compact Dwarfs and UV-luminous Galaxies, at local and much higher distances.[34] The findings show that Peas form a different class of galaxies than Ultra Blue Compact Dwarfs, but may be similar to the most luminous members of the Blue Compact Dwarf Galaxy category.[35] The Green Peas are also similar to UV luminous high redshift galaxies such as Lyman-break Galaxies and Lyman-alpha emitters.[36][37][38] It is concluded that if the underlying processes occurring in the Peas are similar to that found in the UV-luminous high redshift galaxies, the Peas may be the last remnants of a mode of star formation common in the early Universe.[1][39][40]

An example spectral fit from GANDALF for 587724241767825591, showing the spectrum from a typical star-forming Pea. In black is the rest-frame observed spectrum and in red the fit from GANDALF. The SDSS filter band passes are included as blue dotted lines, shifted into the rest-frame of the Pea. In the example, the [OIII] wavelength at 500.7 nm is redshifted inside the "r" color band

When compiling the paper, spectral classification was made using Gas And Absorption Line Fitting (GANDALF).[1] This sophisticated software was programmed by Marc Sarzi, who helped analyze the SDSS spectra.[41] Also, a classic emission line diagnostic by Baldwin, Phillips and Terlevich was used to separate starbursts from AGN.[42]

Histogram showing reddening values for Peas

Pea galaxies have low interstellar reddening values, as shown in the histogram on the right, with nearly all Peas having E(B-V) ≤ 0.25. The distribution shown indicates that the line-emitting regions of star-forming Peas are not highly reddened, particularly when compared to more typical star-forming or starburst galaxies.[1] This low reddening combined with very high UV luminosity is rare in galaxies in the local Universe and is more typically found in galaxies at higher redshifts.[14]

Cardamone et al. describe Pea galaxies as having a low metallicity, but that the oxygen present is highly ionized. It should be explained that Astronomers label all elements other than hydrogen or helium as 'metals'. The average Pea has a metallicity of log[O/H]+12~8.69, which is solar or sub-solar, depending on which set of standard values is used.[1][43][44][45][46] Although the Peas are in general consistent with the mass-metallicity relation, they depart from it at the highest mass end and thus do not follow the trend. Peas have a range of masses, but a more uniform metallicity than the sample compared against.[47] These metallicities are common in low mass galaxies such as Peas.[1]

However, in April 2010, Amorin et al. dispute the metallicities calculated in the original Cardamone et al. Green Peas paper, which are found in Table 4, Column 8, page 16.[1][3] In a paper, which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez use a different methodology from Cardamone et al. to produce metallicity values more than one fifth (20%) of the previous values (about 20% solar or one fifth solar). These mean values are log[O/H]+12~8.05, which shows a clear offset of 0.65dex between the two papers' values. It should be noted that Amorin et al. use a smaller sample of 80 Peas, of which all are starburst galaxies, rather than the sample of over 200 that were used by Cardamone et al. For these 80 Peas, Amorin et al., using a direct method, rather than strong-line methods as used in Cardamone et al., calculate physical properties, as well as oxygen and nitrogen ionic abundances.[48] These metals pollute hydrogen and helium, which make up the majority of the substances present in galaxies. As these metals are produced in Supernovae, the older a galaxy is, the more metals it would have. As Peas are in the nearby, or older, Universe, they should have more metals than galaxies at an earlier time.

Amorin et al. find that the amount of metals, including the abundance of nitrogen, are different from normal values and that Peas are not consistent with the mass-metallicity relation, as concluded by Cardamone et al.[1][49] This analysis indicates that Peas can be considered as genuine metal-poor galaxies. They then argue that this oxygen under-abundance is due to a recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss driven by Supernovae winds and that this can explain their findings.[47][50] This further suggests that Peas are likely very short-lived as the intense star formation in them would quickly enrich the gas.[3]

As well as the optical images from the SDSS, measurements from the GALEX survey were used to determine the ultraviolet values.[51] This survey is well matched in depth and area, and 139 of the sampled 251 Green Peas are found in GALEX Release 4 (G.R.4).[52] For the 56 of the 80 star-forming Peas with GALEX detections, the median luminosity is 30,000 million $L_{\odot}$.

## Analysis of the Cardamone 2009 paper

The 80 starburst Peas

These figures are from Table 4, pages 16–17 of "Galaxy Zoo Green Peas" showing the 80 starburst Peas that were analyzed in the Peas paper. The long 18-digit numbers are the SDSS reference numbers, which link to the appropriate entry at the SDSS Skyserver website.

Greatest Least Average Nearest to Average
Distance z=0.348
(587732134315425958)
z=0.141
(587738947196944678)
z=0.2583 z=0.261
(587724240158589061)
Mass 1010.48 M
(588023240745943289)
108.55 M
(587741392649781464)
109.48 M 109.48 M
(587724241767825591)
Rate of star-forming 59 M/yr
(587728906099687546)
2 M/yr
(588018090541842668)
13.02 M/yr 13 M/yr
(588011122502336742)
Luminosity ([OIII] Eq.Wth.) 238.83 nm
(587738410863493299)
1.2 nm
(587741391573287017)
69.4 nm 67.4 nm
(588018090541842668)
Luminosity (UV) 36.1×1036 W
(587733080270569500)
1.9×1036 W
(588848899919446344)
12.36×1036 W 12.3×1036 W
(588018055652769997)
r-i vs g-r color-color diagram for 251 Peas (green crosses), a sample of normal galaxies (red points) and all quasars (purple points)

Color selection was by using the difference in the levels of three filters, in order to capture these color limits: u-r ≤ 2.5 (1), r-i ≤ -0.2 (2), r-z ≤ 0.5 (3), g-r ≥ r-i + 0.5 (4), u-r ≥ 2.5 (r-z) (5).[1] If the diagram on the right (one of two in the paper) is looked at, the effectiveness of this color selection can be seen. The color-color diagram shows ~100 Green Peas (green crosses), 10,000 comparison galaxies (red points) and 9,500 comparison quasars (purple stars) at similar redshifts to Peas. The black lines show how these figures are on the diagram.

One of the original ways of recognizing Pea galaxies, before SQL programming was involved, was because of a discrepancy about how the SDSS labels them within Skyserver.[53] Out of the 251 of the original sample that were identified by the SDSS spectroscopic pipeline as having galaxy spectra, only 7 were targeted by the SDSS spectral fibre allocation as galaxies i.e. 244 were not.[1][54]

## Comparison of Cardamone GPs to Luminous Compact Emission-Line Galaxies

In December 2010, Yuri Izotov, Natalia Guseva and Trinh Thuan published a paper examining the Green Peas and comparing the 80 Cardamone GPs to a larger set of 803 Luminous Compact Galaxies (LCGs). They use a different set of selection criteria from Cardamone et al. These are: a) a high extinction-corrected luminosity > 3x10^40 ergs s^-1 of the hydrogen beta emission line; b) a high equivalent width greater than 50 Angstroms (5 nm); c) a strong [OIII] wavelength at 4363 Angstroms (436.3 nm) emission line allowing accurate abundance determination; d) a compact structure on SDSS images; and e) an absence of obvious Active Galactic Nucleii spectroscopic features.[16]

Its conclusions (shortened) are:

1. The selected galaxies have redshifts between 0.02 and 0.63, a redshift range equal or greater than a factor of 2 when compared to Cardamone's z=0.112 - 0.360. They find the properties of LCGs and GPs are similar to Blue Compact Dwarf (BCD) galaxies. Explaining how the colours of emission-line galaxies change with distance using SDSS, they conclude that GPs are just subsamples within a narrow redshift range of their larger LCG sample.[16]
2. Although there were no upper limits on the Hydrogen beta luminosities, it was found that there was a 'self-regulating' mechanism which bound the LCGs to a limit of approximately 3x10^42 ergs s^-1.[16]
3. In the [OIII] wavelength 500.7 nm ratio to hydrogen beta vs [NII] wavelength 658.3 nm ratio to hydrogen alpha, LCGs occupy the region of star-forming galaxies with the highest excitation. However, some AGNs also lie in this region.[16]
4. The oxygen abundances 12 + log O/H in LCGs are in the range 7.6-8.4 with a median value of approximately 8.11, confirming Amorin et al.'s analysis of a subset of GPs.[3][16] This range of oxygen abundances is typical of nearby lower-luminosity BCDs. These results show that the original Cardamone et al. median oxygen abundance of approximately 8.7 is overestimated, as a different, empirical method was originally used, rather than the direct method by Amorin et al. and Izotov et al.[1] There is no dependence of oxygen abundance on redshift.
5. In the luminosity-metallicity diagram (fig. 8 in paper), LCGs are shifted by approximately 2 magnitudes brighter when compared to nearby emission-line galaxies. LCGs form a common luminosity-metallicity relation, as for the most actively star-forming galaxies. Some LCGs have oxygen abundances and luminosities similar to Lyman-break galaxies (LBGs), despite much lower redshifts, thus enabling the study of LBGs through LCGs.[16]

## Paper by R.Amorin, J.M.Vilchez and E.Perez-Montero

N/O vs. O/H abundance ratio
O/H vs. stellar mass

In May 2011, R.Amorin, J.M.Vilchez and E.Perez-Montero published a conference proceeding paper reviewing recent scientific results and announcing a forthcoming paper on their recent observations at the GTC.[17] This paper is also a modified report of a presentation at the Joint European and National Astronomy Meeting (JENAM) 2010.[55] They conclude that GPs are a genuine population of metal-poor, luminous and very compact starburst galaxies. Amongst the data, five graphs illustrate the findings they have made. Amorin et al. use masses calculated by Izotov, rather than by Cardamone.[3][16] The metallicities that Amorin et al. use agree with Izotov's findings, or vice-versa, rather than Cardamone's.[3][16]

The first graph (fig.1 in paper) plots the nitrogen/oxygen vs. oxygen/hydrogen abundance ratio. The 2D histogram of SDSS star forming galaxies is shown in logarithmic scale while the GPs are indicated by circles. This shows that GPs are metal-poor.

The second graph (fig.2 in paper) plots O/H vs. stellar mass. The 2D histogram of SDSS SFGs is shown in logarithmic scale and their best likelihood fit is shown by a black solid line. The subset of 62 GPs are indicated by circles and their best linear fit is shown by a dashed line. For comparison we also show the quadratic fit presented in Amorin et al. 2010 for the full sample of 80 GPs. SFGs at z ≥ 2 by Erb et al. are also shown by asterisks for comparison.[3][56]

N/O vs. stellar mass
O/H vs. B-band (rest-frame) absolute magnitude

The third graph (fig.3 in paper) plots N/O vs stellar mass. Symbols as in fig.1.

The fourth graph (fig.4 in paper) plots O/H vs. B-band (rest-frame) absolute magnitude. The meaning of symbols is indicated. Distances used in computing (extinction corrected)absolute magnitudes were, in all cases, calculated using spectroscopic redshifts and the same cosmological parameters. The dashed line indicates the fit to the HII galaxies in the MLR given by Lee et al. 2004.[57]

Gas Mass Fraction v. Metallicity

The fifth graph (fig.5 in paper) plots gas mass fraction vs. metallicity. Different lines correspond to closed-box models at different yields, as indicated in the legend. Open and filled circles are GPs which are above and below the fit to their MZR in,[3] respectively. Diamonds are values for the same Wolf-Rayet galaxies as in Fig. 4.

## Radio detection of Green Peas

In October 2011, a team of scientists released a paper which deals with the magnetic properties of the Green Peas. Sayan Chakraborti, Naveen Yadav, Alak Ray and Carolin Cardamone have made observations which have produced some unexpected results which raise puzzling questions about the origin and evolution of magnetic fields in young galaxies.[18] The age estimate in the Radio Pea's paper is from looking at the star formation that the Peas currently have ongoing and estimating the age of the most recent starburst. Peas are very young galaxies, with models of the observed stellar populations indicating that they are around 10^8 years old (1/100th the age of the MW).[18] There is some question as to whether the Peas all started from the same starburst or if multiple starbursts went on (much older stellar populations are hidden as we can't see the light from these).

Using data from the Giant Metrewave Radio Telescope (GMRT) and archive observations from the Very Large Array (VLA), Chakraborti et al. produced a set of results which are based around the VLA FIRST detection of stacked flux from 32 Pea galaxies and three 3-hour low frequency observations from the GMRT which targeted the 3 most promising candidates which had expected fluxes at the milli-Jansky (mJy) level. In 2012, new data will become available from the VLA from which Chakraborti will examine "Green Peas: Magnetic Fields and Nonthermal Electrons in Young Galaxies".

Chakraborti et al. find that the three Green Peas observed by the GMRT have a magnetic field of B~39 μG, and more generally a figure of greater than B~30μG than for all the Green Peas. This is compared to a figure of B~5μG for the Milky Way.[18] The present understanding is of magnetic field growth based on the amplification of seed fields by dynamo theory and its action over a galaxy's lifetime.[18] The observations of Green Peas challenge that thinking.

Given the high star-forming rates of the Green Peas generally, Peas are expected to host a large number of Supernovae. Supernovae accelerate electrons to high energies, near to the speed of light, which may then emit synchrotron radiation in radio band frequencies.

## GTC-OSIRIS Spectrophotometry

In February 2012, R. Amorin, E. Perez-Montero, J. Vilchez and P. Papaderos published a paper in which they presented the findings of observations carried out using the Gran Telescopio Canarias on the island of La Palma. They presented deep broad-band imaging and long-slit spectroscopy of three Green Peas using much higher precision than previous studies which used SDSS data.[10]

The three galaxies are (using SDSS references):[10]

Their findings show that three Green Peas display relatively low Extinction (astronomy), low oxygen abundances and high nitrogen-to-oxygen ratios.[10] Also reported are the clear signatures of Wolf–Rayet stars, of which many are found (between ~800 and ~1200).[10] A combination of population and evolutionary synthesis models strongly suggest a formation history dominated by starbursts.[10] These models show that these three Green Peas currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However as these models imply, they are old galaxies having formed most of their stellar mass several billion (gigayear) ago.[10] The presence of old stars has been spectroscopically verified in one of the three galaxies by the detection of Magnesium.[10] Surface photometry using data based from the Hubble Space Telescope indicates that the three Green Peas possess an exponential low surface brightness envelope (Low surface brightness galaxy).[10] This suggests that green Peas are identifiable with major episodes in the assembly history of local BCDsBlue compact dwarf galaxy.[10]

The imaging and spectroscopy for the three Green Pea galaxies was carried out using the OSIRIS instrument mounted on the GTC 10.4m telescope. OSIRIS has an optical wavelength range of ~365.0 to 1000 nm. It consists of two 2048x4096 Marconi CCD42-82 with a 9.2 arcsec gap between them. The unvignetted instrument field of view is 7.8x7.8 arcmin with a pixel scale of 0.125 arcsec.

## References

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