Pea galaxy
Encyclopedia
A Pea galaxy, also referred to as a Pea or Green Pea, is a type of Luminous Blue Compact Galaxy which 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
(SDSS).
Pea Galaxies were first discovered in 2007 by the volunteer users within the forum section of the online astronomy
project Galaxy Zoo
(GZ). One of GZ's founders, Kevin Schawinski, said that "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".
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
. 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.
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 solar mass
(M☉) of 580,000 million M☉. Research by the European Space Agency
and NASA
has shown the Milky Way converts around 4 M☉/yr worth of interstellar gas into stars. An average starburst Pea galaxy has a mass
of around 3,162 million M☉. 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. 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.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 Milky Way). However, the ages of the Peas are uncertain, as it depends upon the Mathematical model
with which you estimate the ages. Also, there are differing definitions of age - from when the stars within them formed to when the current structure that we're looking at formed.
"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".
(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, 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. Enthusiasts, calling themselves the "Peas 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
. (e.g.) 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".
In April 2010 in a paper which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez from the IAA-CISC explore issues concerning the metallicity of the 80 starburst Peas.
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).
In May 2011, R. Amorin, R. Perez-Montero and J.Vilchez from the IAA published a 'Conference proceeding' on their work on Pea galaxies. 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".
In October 2011, Sayan Chakraborti, Tata Institute of Fundamental Research, Mumbai, India (TIFR), Naveen Yadav (TIFR), Carolin Cardamone, Brown University
, USA and Alak Ray (TIFR) released a paper titled 'Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies'. In this paper, magnetic studies using data from the Very Large Array
(VLA) and new data from the Giant Metrewave Radio Telescope GMRT describe various observations based around the Green Peas. They show that these very young starburst galaxies 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.
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.
(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 merger
s. These three HST images were imaged as part of a study of local ultraviolet
(UV)-luminous galaxies in 2005. 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. In this graph, the Peas are compared to the 3003 mergers from the Galaxy Zoo Merger Sample. 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.
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. 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). The dashed line is: Kauffmann et al. (2003) separating purely star-forming objects from AGN (labelled Ka03).
Pea galaxies have a strong emission line when compared to the rest of their spectral continuum. On an SDSS spectrum, this shows up as a large peak with [OIII] at the top. 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). 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.
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. 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. The Green Peas are also similar to UV luminous high redshift galaxies such as Lyman-break Galaxies and Lyman-alpha emitters. 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.
When compiling the paper, spectral classification was made using Gas And Absorption Line Fitting (GANDALF). This sophisticated software was programmed by Marc Sarzi, who helped analyze the SDSS spectra. Also, a classic emission line diagnostic by Baldwin, Phillips and Terlevich was used to separate starbursts from AGN.
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. 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.
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. 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. These metallicities are common in low mass galaxies such as Peas.
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. In a paper, which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez from the IAA-CISC, 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. 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. 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. This further suggests that Peas are likely very short-lived as the intense star formation in them would quickly enrich the gas.
As well as the optical images from the SDSS, measurements from the GALEX
survey were used to determine the ultraviolet values. 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). For the 56 of the 80 star-forming Peas with GALEX detections, the median luminosity is 30,000 million
.
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.
Its conclusions (shortened) are:
. This paper is also a modified report of a presentation at the Joint European and National Astronomy Meeting (JENAM) 2010. 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. The metallicities that Amorin et al. use agree with Izotov's findings, or vice-versa, rather than Cardamone's.
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.
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.
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, respectively. Diamonds are values for the same Wolf-Rayet galaxies as in Fig. 4.
USA, have made observations which have produced some unexpected results which raise puzzling questions about the origin and evolution of magnetic fields in young galaxies. 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). 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 VLA Very Large Array
and the Giant Metrewave Radio Tescope GMRT, Chakraborti et al. produce 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.
Chakraborti et al. find that the three Green Peas observed by the GMRT have a magnetic field of B~39μG microGauss
, 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. 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. 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.
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). 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. 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.
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
. Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
(SDSS).
Pea Galaxies were first discovered in 2007 by the volunteer users within the forum section of the online astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
project Galaxy Zoo
Galaxy Zoo
Galaxy Zoo is an online astronomy project which invites members of the public to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research. An improved version—Galaxy...
(GZ). One of GZ's founders, Kevin Schawinski, said that "this is a genuine citizen science
Citizen science
Citizen science is a term used for the systematic collection and analysis of data; development of technology; testing of natural phenomena; and the dissemination of these activities by researchers on a primarily avocational basis...
project. It's a great example of how a new way of doing Science produced a result that wouldn't have been possible otherwise".
Description
Pea galaxies are essentially compact oxygen-rich emission line galaxies (ELG) that are at redshifts between z = 0.112 and 0.360. These low-mass galaxies have an upper size limit generally no bigger than 16300 ly across, and typically they reside in environments less than two-thirds the density of normal galaxy environments. An average starburstStarburst galaxy
A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies...
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
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
] equivalent width
Equivalent width
The equivalent width of a spectral line is a measure of the area of the line on a plot of intensity versus wavelength. It is found by forming a rectangle with a height equal to that of continuum emission, and finding the width such that the area of the rectangle is equal to the area in the spectral...
of 69.4 nm and a low metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. They have a strong emission line at the OIII wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 500.7 nm. OIII, O++ or doubly ionized oxygen
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
, 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.
Comparing a Pea galaxy to the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
can be useful when trying to visualize these star-forming rates. The Milky Way is a spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
and has a solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
(M☉) of 580,000 million M☉. Research by the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
has shown the Milky Way converts around 4 M☉/yr worth of interstellar gas into stars. An average starburst Pea galaxy has a mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of around 3,162 million M☉. 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. 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
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
was three-quarters of its present age and so are clues as to how galaxy formation took place in the earlier Universe.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 Milky Way). However, the ages of the Peas are uncertain, as it depends upon the Mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
with which you estimate the ages. Also, there are differing definitions of age - from when the stars within them formed to when the current structure that we're looking at formed.
"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".
History of discovery
Galaxy ZooGalaxy Zoo
Galaxy Zoo is an online astronomy project which invites members of the public to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research. An improved version—Galaxy...
(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
Internet forum
An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages. They differ from chat rooms in that messages are at least temporarily archived...
by Hanny Van Arkel
Hanny's Voorwerp
' , Dutch for Hanny's object, is an astronomical object of unknown nature. It was discovered in 2007 by Dutch school teacher Hanny van Arkel, while she was participating as an amateur volunteer in the Galaxy Zoo project...
called "Give peas a chance" in which various green objects were posted. This thread started humorously, 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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, which in SDSS color composites increases the luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
, 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 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
Monthly Notices of the Royal Astronomical Society
Monthly Notices of the Royal Astronomical Society is one of the world's leading scientific journals in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes peer-reviewed letters and papers reporting original research in relevant fields...
. (e.g.) 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".
In April 2010 in a paper which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez from the IAA-CISC explore issues concerning the metallicity of the 80 starburst Peas.
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).
In May 2011, R. Amorin, R. Perez-Montero and J.Vilchez from the IAA published a 'Conference proceeding' on their work on Pea galaxies. In this publication, they announce that they have conducted a set of observations using the OSIRIS imager and spectrograph at the GTC
Gran Telescopio Canarias
The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
, 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".
In October 2011, Sayan Chakraborti, Tata Institute of Fundamental Research, Mumbai, India (TIFR), Naveen Yadav (TIFR), Carolin Cardamone, Brown University
Brown University
Brown University is a private, Ivy League university located in Providence, Rhode Island, United States. Founded in 1764 prior to American independence from the British Empire as the College in the English Colony of Rhode Island and Providence Plantations early in the reign of King George III ,...
, USA and Alak Ray (TIFR) released a paper titled 'Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies'. In this paper, magnetic studies using data from the Very Large Array
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
(VLA) and new data from the Giant Metrewave Radio Telescope GMRT describe various observations based around the Green Peas. They show that these very young starburst galaxies 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.
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
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...
. 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.
Physics
To date only five Pea galaxies have been imaged by the Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
(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 merger
Galaxy merger
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...
s. These three HST images were imaged as part of a study of local ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV)-luminous galaxies in 2005. Major mergers are frequently sites of active star-formation and to the right a graph
Plot (graphics)
A plot is a graphical technique for representing a data set, usually as a graph showing the relationship between two or more variables. The plot can be drawn by hand or by a mechanical or electronic plotter. Graphs are a visual representation of the relationship between variables, very useful for...
is shown that plots specific star formation rate (SFR / Galaxy Mass) against galaxy mass. In this graph, the Peas are compared to the 3003 mergers from the Galaxy Zoo Merger Sample. 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.
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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR), were analyzed further and 80 were found to be starburst galaxies. 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). The dashed line is: Kauffmann et al. (2003) separating purely star-forming objects from AGN (labelled Ka03).
Pea galaxies have a strong emission line when compared to the rest of their spectral continuum. On an SDSS spectrum, this shows up as a large peak with [OIII] at the top. The wavelength of [OIII] (500.7 nm) was chosen to determine the luminosities of the Peas using Equivalent Width (Eq.Wth.). The histogram
Histogram
In statistics, a histogram is a graphical representation showing a visual impression of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson...
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). 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.
Within the GZ Green Peas paper, comparisons are made with other compact galaxies, namely Blue Compact Dwarfs
Blue compact dwarf galaxy
In astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, hot, massive stars. These stars cause the galaxy to appear blue in color. Since they exist mostly in clusters, blue compact dwarf galaxies don't have an exact uniform shape...
and UV-luminous Galaxies, at local and much higher distances. 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. The Green Peas are also similar to UV luminous high redshift galaxies such as Lyman-break Galaxies and Lyman-alpha emitters. 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.
When compiling the paper, spectral classification was made using Gas And Absorption Line Fitting (GANDALF). This sophisticated software was programmed by Marc Sarzi, who helped analyze the SDSS spectra. Also, a classic emission line diagnostic by Baldwin, Phillips and Terlevich was used to separate starbursts from AGN.
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. 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.
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. 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. These metallicities are common in low mass galaxies such as Peas.
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. In a paper, which appears as a letter to The Astrophysical Journal, R. Amorin, E. Perez-Montero and J. Vilchez from the IAA-CISC, 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. 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. 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. This further suggests that Peas are likely very short-lived as the intense star formation in them would quickly enrich the gas.
As well as the optical images from the SDSS, measurements from the GALEX
GALEX
The Galaxy Evolution Explorer is an orbiting ultraviolet space telescope launched on April 28, 2003. A Pegasus rocket placed the craft into a nearly circular orbit at an altitude of and an inclination to the Earth's equator of 29 degrees....
survey were used to determine the ultraviolet values. 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). For the 56 of the 80 star-forming Peas with GALEX detections, the median luminosity is 30,000 million
.Comparison of Cardamone GPs to Luminous Compact Emission-Line Galaxies
In December 2010, Yuri Izotov, Natalia Guseva from the National Academy of Sciences of Ukraine and Trinh Thuan from the University of VirginiaUniversity of Virginia
The University of Virginia is a public research university located in Charlottesville, Virginia, United States, founded by Thomas Jefferson...
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.
Its conclusions (shortened) are:
- 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.
- 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.
- 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.
- 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. 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. There is no dependence of oxygen abundance on redshift.
- 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.
Paper by R.Amorin, J.M.Vilchez and E.Perez-Montero
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 GTCGran Telescopio Canarias
The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
. This paper is also a modified report of a presentation at the Joint European and National Astronomy Meeting (JENAM) 2010. 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. The metallicities that Amorin et al. use agree with Izotov's findings, or vice-versa, rather than Cardamone's.
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.
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.
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, 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. From the Tata Institute of Fundamental Research, Mumbai, India, TIFR, Sayan Chakraborti, Naveen Yadav and Alak Ray and also Carolin Cardamone from Brown UniversityBrown University
Brown University is a private, Ivy League university located in Providence, Rhode Island, United States. Founded in 1764 prior to American independence from the British Empire as the College in the English Colony of Rhode Island and Providence Plantations early in the reign of King George III ,...
USA, have made observations which have produced some unexpected results which raise puzzling questions about the origin and evolution of magnetic fields in young galaxies. 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). 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 VLA Very Large Array
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
and the Giant Metrewave Radio Tescope GMRT, Chakraborti et al. produce a set of results which are based around the VLA FIRST detection of stacked flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
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
Jansky
The flux unit or jansky is a non-SI unit of spectral flux density equivalent to 10−26 watts per square metre per hertz...
(mJy) level.
Chakraborti et al. find that the three Green Peas observed by the GMRT have a magnetic field of B~39μG microGauss
Gauss
Gauss may refer to:*Carl Friedrich Gauss, German mathematician and physicist*Gauss , a unit of magnetic flux density or magnetic induction*GAUSS , a software package*Gauss , a crater on the moon...
, 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. The present understanding is of Magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
growth based on the amplification of seed fields by Dynamo theory
Dynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
and its action over a galaxy's lifetime. 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
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
in Radio band frequencies.
Analysis of the Cardamone et al. paper
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 Watt The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:... (587733080270569500) |
1.9×1036 W (588848899919446344) |
12.36×1036 W | 12.3×1036 W (588018055652769997) |
Color selection was by using the difference in the levels of three filters
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
, 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). 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
Color-color diagram
In astronomy, color–color diagrams are a means of comparing the apparent magnitudes of stars at different wavelengths. Astronomers typically observe at narrow bands around certain wavelengths, and objects observed will have different brightnesses in each band. The difference the brightness in two...
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
SQL
SQL is a programming language designed for managing data in relational database management systems ....
programming was involved, was because of a discrepancy about how the SDSS labels them within Skyserver. 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.
See also
- Blue compact dwarf galaxyBlue compact dwarf galaxyIn astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, hot, massive stars. These stars cause the galaxy to appear blue in color. Since they exist mostly in clusters, blue compact dwarf galaxies don't have an exact uniform shape...
- Dwarf galaxyDwarf galaxyA dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
- Galaxy formation and evolutionGalaxy formation and evolutionThe study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
- Ultraviolet astronomy