Pea galaxy

Years 2007 to 2009

Galaxy Zoo (GZ) is a project online since July 2007 that seeks to classify up to one million galaxies.^[19][20] 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.^[6] A discussion, or thread, was started on this forum by Hanny Van Arkel (cf. Hanny's Voorwerp) on the 12th of August 2007 called "Give peas a chance" in which various green objects were posted.^[6] 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.^[1][21] 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 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 (GPs).

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".^[1] 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 (DR7), 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".^[22] 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 (KISS).^[23] 3 of these 15 when viewed as objects in SDSS are green (KISSR 1516, KISSR 2042 and KISSRx 467). 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."^[22]

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".^[7] 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".^[7] 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".^[24] 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' (LCGs), of which there are 803.^[24] 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 (WISE)'.^[25] 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 GPs and more than double the number of known galaxies with these characteristics.^[25]

In January 2012, authors R. Amorin et al. published a 'Conference proceeding' titled "Unveiling the Nature of the "Green Pea" galaxies".^[26] In this publication, they announce that they have conducted a set of observations using the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) 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".^[26] 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".^[27] In it they compare the oxygen and nitrogen abundances derived from global emission-line SDSS spectra of galaxies using (i) the electron temperature method and (ii) two recent strong line O/N and N/S calibrations. Three sets of objects were compared: i) Composite hydrogen-rich nebula, ii) 281 SDSS galaxies and iii) A sample of GPs with detectable [OIII]-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 (such as metallicity). However, for the hottest Green Pea galaxies, which appear to be dwarf galaxies, this explanation does not seem to be plausible."^[27]

In January 2012, author S. Hawley published a paper in the PASP titled "Abundances in "Green Pea" Star-forming Galaxies".^[28] 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."^[28]

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'.^[29] 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.^[29] 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".^[11] 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.^[11] 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".^[30] 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 (ELs) 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 (or clumps) moving at speeds of over 500 km/s (five hundred km/s) relative to each other.^[30]

2013 - 2015

In January 2013, authors S. Parnovsky et al. published a paper in A&SS titled "H alpha and UV luminosities and star formation rates in a large sample of luminous compact galaxies".^[31] In it, they present a statistical study of the star formation rates (SFR) derived from the GALEX observations in the Ultraviolet continuum and in the H alpha emission line for a sample of ~800 luminous compact galaxies (LCGs). Within the larger set of LCGs, including the GPs, SFR of up to ~110 M_☉/yr (~110 solar masses a year) are found, as well as estimates of the ages of the starbursts.^[31]

In April 2013, authors A. Jaskot and M. Oey published a paper in the ApJ titled "The Origin and Optical Depth of Ionizing Radiation in the "Green Pea" Galaxies".^[32] Six "extreme" GPs are studied. Using these, the authors endeavour to narrow down the list of possibilities about what is producing the radiation and the substantial amounts of high-energy photon that might be escaping from the GPs.^[32] Following on from this paper, observations on the Hubble Space Telescope, totalling 24 orbits, were taken in December 2013.^[33] The Cosmic Origins Spectrograph and the Advanced Camera for Surveys were used on four of the "extreme" GPs. For more details see: Two papers by Jaskot and Oey

In January 2014, authors Y. Izotov et al. published a paper in A&A entitled "Multi-wavelength study of 14000 star-forming galaxies from the Sloan Digital Sky Survey".^[34] In it they use a variety of sources to demonstrate "that the emission emerging from young star-forming regions is the dominant dust-heating source for temperatures to several hundred degrees in the sample star-forming galaxies". The first source of data is SDSS from which 14,610 spectra with strong emission lines are selected. Those spectra were then cross-identified with sources from photometric sky surveys in other wavelength ranges, which are: i) GALEX for the ultraviolet, ii) The 2MASS survey for the near-infrared, iii) The WISE All-Sky Source Catalog for infrared at differing wavelengths, iv) The IRAS survey for the far-infrared and the v) NVSS Survey at radio-wavelengths. Only a small fraction of the SDSS objects were detected in the last two surveys. Among the results is a list of 20 galaxies with the highest magnitudes which have hot dust of several hundred degrees. Of these 20, all could be classified as GPs and/or LCGs. Also among the results, the luminosity is obtained in the sample galaxies in a wide wavelength range. At the highest luminosities, the sample galaxies had luminosites approaching those of high-redshift Lyman-break galaxy.^[34]

In January 2014, authors A. Jaskot et al. gave a presentation titled "Neutral Gas and Low-Redshift Starbursts: From Infall to Ionization" to the AAS at their meeting #223.^[35] The presentation included data from The Arecibo Observatory Legacy Fast ALFA Survey (ALFALFA). The authors analyzed the optical spectra of the GPs and concluded "While the ALFALFA survey demonstrates the role of external processes in triggering starbursts, the Green Peas show that starbursts' radiation can escape to affect their external environment", finding "that the Peas are likely optically thin to Lyman continuum (LyC) radiation."^[35]

In June 2014, authors A. Jaskot and M. Oey published a conference report titled "The Origin and Optical Depth of Ionizing Photons in the Green Pea Galaxies".^[36] This appears in "Massive Young Star Clusters Near and Far: From the Milky Way to Reionization", based on the 2013 Guillermo Haro Conference. For more details see: Two papers by Jaskot and Oey.

In May 2015, authors A. Henry, C et al. published a paper in the ApJ entitled, "Lyα Emission from Green Peas: The Role of Circumgalactic Gas Density, Covering, and Kinematics".^[37] In this paper, ten Green Peas were studied in the ultraviolet, using high-resolution spectroscopy with the Hubble Space Telescope using the Cosmic Origins Spectrograph. This study showed, for the first time, that Green Peas have strong Lyα emission much like distant, high-redshift galaxies observed in a younger universe. Henry et al. explored the physical mechanisms that determine how Lyα escapes from the Green Peas, and concluded that variations in the neutral hydrogen column density were the most important factor.^[37] For more details see: Lyman Alpha Emission from Green Peas.

2016 - 2017

In May 2016, author Miranda C. P. Straub published a research paper in the open access journal Citizen Science: Theory and Practice called 'Giving Citizen Scientists a Chance: A Study of Volunteer-led Scientific Discovery'.^[5] The abstract states: "The discovery of a class of galaxies called Green Peas provides an example of scientific work done by volunteers. This unique situation arose out of a science crowdsourcing website called Galaxy Zoo."^[5]

In April 2016, Yang et al. published "Green Pea Galaxies Reveal Secrets of Lyα Escape."^[38] Archival Lyman-alpha spectra of 12 GPs that have been observed with the HST/COS were analysed and modelled with radiative transfer models. The dependence of Lyman-alpha (LyA) escape fractions on various properties were explored. All 12 GPs show LyA lines in emission, with a LyA equivalent width distribution similar to high-redshift emitters. Among the findings are that the LyA escape fraction depends strongly on metallicity and moderately on dust extinction. The papers results suggest that low H1 column density and low metallicity are essential for LyA escape. "In conclusion, GPs provide an unmatched opportunity to study LyA escape in LyA Emitters."^[38]

In a presentation to the AAS Meeting #229 in January 2017, Matt Brorby and Philip Kaaret describe the observations of two GPs and their x-ray emission.^[39] Using both space telescope programs Chandra GO: 16400764 and Hubble GO: 13940, they examine Luminous Compact Galaxies, both GPs, J0842+1150 and SHOC 486. They conclude: i) These are the first x-ray observations of GPs. ii) The two GPs studied are the first test of Lx-SFR-Z planar relation and that they are consistent with this. iii) Low-metallicity galaxies exhibit enhanced x-ray emission relative to normal metallicity starforming galaxies. iv) GPs are useful for predictions of X-ray output in the early universe.^[39]

In March 2017, Yang et al. published a paper in the ApJ called: "Lyα and UV Sizes of Green Pea Galaxies".^[40] The authors studied the Lyman-alpha (LyA) escape in a statistical sample of 43 GPs with HST/COS LyA spectra, taken from 6 HST programs. Their conclusions include: i) Using GPs that cover the whole ranges of dust extinction and metallicity, they find about two-thirds are strong LyA emitters. This confirms that GPs generally are "the best analogs of high-z (redshift) Lyman-alpha Emitters (LAEs) in the nearby universe." ii) The authors find many correlations regarding the dependence of LyA escape on galactic properties, such as dust extinction and metallicity. iii) The single shell radiative transfer model can reproduce most LyA profiles of GPs. iv) An empirical linear relation between the LyA escape fraction, dust extinction and the LyA red peak velocity.^[40]

In August 2017, Yang et al. published a study in the ApJ called: "Lyα profile, dust, and prediction of Lyα escape fraction in Green Pea Galaxies".^[41] The authors state that GPs are nearby analogues of high-redshift Lyman-alpha (LyA)- emitting galaxies. Using spectral data from the HST-COS MAST archive, 24 GPs were studied for their LyA escape and the spatial profiles of LyA and UV continuum emissions. Results include: i) Having compared LyA and UV sizes from the 2D spectra and 1D spatial profiles, it is found that most GPs show more extended LyA emission than the UV continuum. ii) 8 GPs had their spatial profiles of LyA photons at blueshifted and redshifted velocities compared. iii) The LyA escape fraction was compared with the size ration of LyA to UV. It was found that GPs that have LyA escape fractions greater than 10% "tend to have more compact LyA morphology".^[41]

In October 2017, Lofthouse et al. published a study in MNRAS named:^[42] The authors used integral field spectroscopy, from the SWIFT and Palm 3K instruments, to perform a spatially-resolved spectroscopic analysis of four GPs, numbered 1,2,4 and 5. Among the results are that GPs 1 & 2 are rotationally-supported (they have a rotating centre), while GPs 4 & 5 are dispersion-dominated systems. GPs 1 & 2 show morphologies indicative of ongoing or mergers. However, GPs 4 & 5 show no signs of recent interactions and have similar star-forming rates. This indicates mergers are not "a necessary requirement for driving the high star formation in these types of galaxies".^[42]

In December 2017, authors Jaskot et al. published a paper in the ApJ Letters titled:"Kinematics and Optical Depth in the Green Peas: Suppressed Superwinds in Candidate LyC Emitters".^[43] Within the paper, they say that current thinking describes how superwinds clear neutral gas away from young starburst galaxies, which in turn regulates the escape of Lyman Continuum photons from star-forming galaxies. Models predict however that in the most extreme compact starbursts, those superwinds may not launch. The authors explore the role of outflows in generating low optical depth in GPs, using observations from the Hubble Space Telescope. They compare the kinematics of ultraviolet absorption and with Lyman alpha escape fraction, Lyman alpha peak separation or low-ionisation absorption. The most extreme GPs show the slowest velocities, which "are consistent with models for suppressed superwinds, which suggests that outflows may not be the only cause of LyC escape from galaxies."^[43]

2021 - 2024

In this study using images of Peas taken as part of the Zoogems project, Leonardo Clarke et al. examine PG content to find out about the different ages of the stars and find that, while the central star-forming clusters were up to 500 million years old, there are stars, possibly the host galaxy stars, which are older and are thought to be more than 1 billion years old.^[44] Peas have been intensively studied as they are the only population that has hydrogen-ionizing radiation escaping in large amounts and so are substitutes for the earliest galaxies.^[44] Yet Clarke et al. argue the substantial presence of old stars would not have been possible at the earliest stages of the first galaxies. The mix of old and new stars within Pea galaxies could create different gravitational conditions which might influence galactic winds and element retention. Their conclusions imply that Pea galaxies are not real analogs of the galaxies responsible for the Epoch of Reionisation.^[44]

This study from January 2023 uses Early Release Observations from the James Webb Space Telescope to analyse the Near Infrared Spectrograph of three galaxies at a redshift of z~8 to determine their metallicities, gas temperatures and ionisation.^[3] Using robust measurement procedures, the scientists compare the abundances and emission-line ratios to a nearby sample of Green Pea galaxies. JWST data shows further similarities between these GPs and the three high-redshift galaxies. These three galaxies show a compact morphology typical of emission-line-dominated galaxies at all redshifts and based on similarities with GPs, "it is likely that these are the first rest-optical spectra of galaxies that are actively driving cosmological reionization".^[3]

The JWST image used by Rhoads et al. is called SMACS 0723 and within it three galaxies that looked particularly far away were followed up using spectroscopic observations.^[45] This caught the attention of Rhoads et al. as the spectra of the three galaxies resembled GPs. Astronomer Trinh Thuan from the University of Virginia says he was amazed to see the similarity between the distant trio and GPs. Before JWST, the furthest GPs were measured at about 10 billion years after the Big Bang. Daniel Schaerer, an astronomer at the University of Geneva said that GPs can now be measured from only 700 million years after the Big Bang – "It's completely mind-boggling". As reported in Nature back in 2016, GPs are strong sources of ionizing radiation that are thought to be able to free the early universe from its 'dark ages'. Thuan said: "Now I really do think that these star-forming dwarf galaxies are the agent of reionization".^[45]

A study published in the ApJ by Bhat et al. in January 2024 investigates the influence of jets on GPs and Green Beans.^[46] Using 12 subjects selected from the SDSS and Radio Sky at 20cm survey, the team use the Large Binocular Telescope–Multi-Object Double Spectrograph long-slit spectroscopy at two position angles for each galaxy: one aligned with the jet direction and another perpendicular to it.^[46] By tracing the [OIII] emission along these slits, the team aimed to assess the extension of the jets, which revealed that there was no preferred direction on the EELRs.^[46] When comparing the extension of [OIII] emission with that [OII], it was found that [OII] emission extended along a greater extension along the galactic plane, suggesting a stronger association of [OII] with stellar processes.^[46]

In the 2024 study "Ubiquitous broad-line emission and the relation between ionized gas outflows and Lyman continuum escape in Green Pea galaxies" R. Amorin et al. report evidence obtained by observations of ionized gas kinematics in a sample of 20 LyC emitters.^[47] These were made up of GPs from 3 different studies i) 14 from the Low-z Lyman Continuum (Flury et al. 2022a,b & Saldana-Lopez et al. 2022.) ii) 5 from Izotov et al. (2016a,b & 2018a) iii) 1 from Wang et al. (2021). A subsample of 13 spectra were obtained with the X-shooter instrument at the Very Large Telescope, while a subsample of 7 used spectra from the William Herschel Telescope's ISIS. Presenting new high-resolution optical spectra of 6 strong, 11 weak and 3 insignificant LyC leakers, they performed a first kinematic analysis of resolved emission-line profiles using multicomponent Gaussian fitting. They find a significant correlation between the intrinsic velocity dispersion and maximum line-width velocities of galaxies and their LyC escape fraction. Their results strongly suggest "that the physical mechanisms driving the observed kinematic complexity play a significant role in the escape of ionizing photons in galaxies."^[47]