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Title: Ly-alpha and UV Sizes of Green Pea Galaxies
Author: Huan Yang, Sangeeta Malhotra, James E. Rhoads, Claus Leitherer, Aida Wofford, Tianxing Jiang, Junxian Wang

Green Peas are nearby analogs of high-redshift Ly-alpha-emitting galaxies. To probe their Ly-alpha escape, we study the spatial profiles of Ly-alpha and UV continuum emission of 24 Green Pea galaxies using the Cosmic Origins Spectrograph (COS) on Hubble Space Telescope (HST). We extract the spatial profiles of Ly-alpha emission from their 2D COS spectra, and of UV continuum from both the 2D spectra and NUV images. The Ly-alpha emission shows more extended spatial profiles than the UV continuum in most Green Peas. The deconvolved Full Width Half Maximum (FWHM) of the Ly-alpha spatial profile is about 2 to 4 times that of the UV continuum in most cases. The Ly-alpha light shows significant offsets from the UV continuum in four galaxies and central absorption in one galaxy. We also compare the spatial profiles of Ly-alpha photons at blueshifted and redshifted velocities in eight Green Peas with sufficient data quality, and find the blue wing of the Ly-alpha line has a larger spatial extent than the red wing in four Green Peas with comparatively weak blue Ly-alpha line wings. These results together indicate that most Ly-alpha photons escape out of Ly-alpha-emitting galaxies through many resonant scatterings in the HI gas. Five Lyman continuum (LyC) leakers in this sample have similar Ly-alpha to UV continuum size ratios (~1.4 - 4.3) to the other Green Peas, indicating their LyC emission escape through ionized holes in the interstellar medium.

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Title: Green Pea Galaxies Reveal Secrets of Ly alpha Escape
Author: Huan Yang, Sangeeta Malhotra, Max Gronke, James E. Rhoads, Anne Jaskot, Zhenya Zheng, Mark Dijkstra

Star-formation in galaxies generates a lot of Ly alpha photons. Understanding the escape of Ly alpha photons from galaxies is a key issue in studying high redshift galaxies and probing cosmic reionisation with Ly alpha. To understand Ly alpha escape, it is valuable to study analogs of high redshift Ly alpha emitters in nearby universe. However, most nearby analogs have too small a Ly alpha equivalent width and escape fraction compared to high redshift Ly alpha emitters. One different group of nearby analogs are "Green Pea" galaxies, selected by their high equivalent width optical emission lines. Here we show that Green Pea galaxies have strong Ly alpha emission lines and high Ly alpha escape fraction (see also Henry et al. 2015), providing an opportunity to solve Ly alpha escape problem. Green Peas have a Ly alpha equivalent width distribution similar to high redshift Ly alpha emitters. The Ly alpha escape fraction correlates with many quantities of Ly alpha profile, especially the ratio of Ly alpha blue peak velocity to H line width. Comparing Ly alpha profiles with expanding-shell radiative transfer models suggest these correlations are probably caused by column density and kinematics of neutral gas. The Ly alpha escape fraction also correlates with galactic metallicity and dust extinction. Studying Ly alpha in Green Peas can reveal how these various factors combine to make Ly alpha escape.

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Title: The Origin and Optical Depth of Ionizing Photons in the Green Pea Galaxies
Author: A. E. Jaskot, M. S. Oey

Our understanding of radiative feedback and star formation in galaxies at high redshift is hindered by the rarity of similar systems at low redshift. However, the recently identified Green Pea (GP) galaxies are similar to high-redshift galaxies in their morphologies and star formation rates and are vital tools for probing the generation and transmission of ionising photons. The GPs contain massive star clusters that emit copious amounts of high-energy radiation, as indicated by intense [OIII] 5007 emission and HeII 4686 emission. We focus on six GP galaxies with high ratios of [O III] 5007,4959/[O II] 3727 ~10 or more. Such high ratios indicate gas with a high ionisation parameter or a low optical depth. The GP line ratios and ages point to chemically homogeneous massive stars, Wolf-Rayet stars, or shock ionisation as the most likely sources of the He II emission. Models including shock ionisation suggest that the GPs may have low optical depths, consistent with a scenario in which ionising photons escape along passageways created by recent supernovae. The GPs and similar galaxies can shed new light on cosmic reionisation by revealing how ionising photons propagate from massive star clusters to the intergalactic medium.

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Green Pea galaxies could help astronomers understand early universe

The rare Green Pea galaxies discovered by the general public in 2007 could help confirm astronomers' understanding of reionisation, a pivotal stage in the evolution of the early universe, say University of Michigan researchers.
Reionisation occurred a few hundred million years after the Big Bang as the first stars were turning on and forming the first galaxies. During this period, the space between the galaxies changed from an opaque, neutral fog to a transparent charged plasma, as it is today. Plasma is gas that's electrically charged.

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Title: The Origin and Optical Depth of Ionising Radiation in the "Green Pea" Galaxies
Authors: A. E. Jaskot, M. S. Oey (University of Michigan)

Although Lyman continuum (LyC) radiation from star-forming galaxies likely drove the reionisation of the Universe, observations of star-forming galaxies at low redshift generally indicate low LyC escape fractions. However, the extreme [O III]/[O II] ratios of the z=0.1-0.3 Green Pea galaxies may be due to high escape fractions of ionising radiation. To analyse the LyC optical depths and ionising sources of these rare, compact starbursts, we compare nebular photoionisation and stellar population models with observed emission lines in the Peas' SDSS spectra. We focus on the six most extreme Green Peas, the galaxies with the highest [O III]/[O II] ratios and the best candidates for escaping ionising radiation. The Balmer line equivalent widths and He I {\lambda}3819 emission in the extreme Peas support young ages of 3-5 Myr, and He II {\lambda}4686 emission in five extreme Peas signals the presence of hard ionising sources. Ionisation by active galactic nuclei or high-mass X-ray binaries is inconsistent with the Peas' line ratios and ages. Although stacked spectra reveal no Wolf-Rayet (WR) features, we tentatively detect WR features in the SDSS spectra of three extreme Peas. Based on the Peas' ages and line ratios, we find that WR stars, chemically homogeneous O stars, or shocks could produce the observed He II emission. If hot stars are responsible, the Peas' optical depths are ambiguous. However, accounting for emission from shocks lowers the inferred optical depth and suggests that the Peas may be optically thin. The Peas' ages likely optimise the escape of Lyman-continuum radiation; they are old enough for supernovae and stellar winds to reshape the interstellar medium, but young enough to possess large numbers of UV-luminous O or WR stars.

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Title: The star formation history and metal content of the "Green Peas". New detailed GTC-OSIRIS spectrophotometry of three galaxies
Authors: Ricardo Amorín, E. Pérez-Montero, J. M. Vílchez, P. Papaderos

We present deep broad-band imaging and long-slit spectroscopy of three compact, low-mass starburst galaxies at redshift z~0.2-0.3, also referred to as Green Peas (GP). We measure physical properties of the ionised gas and derive abundances for several species with high precision. We find that the three GPs display relatively low extinction, low oxygen abundances, and remarkably high N/O ratios We also report on the detection of clear signatures of Wolf-Rayet (WR) stars in these galaxies. We carry out a pilot spectral synthesis study using a combination of both population and evolutionary synthesis models. Their outputs are in qualitative agreement, strongly suggesting a formation history dominated by starbursts. In agreement with the presence of WR stars, these models show that these GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as models imply, they are old galaxies having had formed most of their stellar mass several Gyr ago. The presence of old stars has been spectroscopically verified in one of the galaxies by the detection of Mg I 5167, 5173 absorption line. Additionally, we perform a surface photometry study based on HST data, that indicates that the three galaxies posses an exponential low-surface brightness envelope. If due to stellar emission, the latter is structurally compatible to the evolved hosts of luminous BCD/HII galaxies, suggesting that GPs are identifiable with major episodes in the assembly history of local BCDs. These conclusions highlight the importance of these objects as laboratories for studying galaxy evolution at late cosmic epochs.

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Title: Unveiling the nature of the "Green Pea" galaxies
Authors: Ricardo Amorín, José M. Vílchez, Enrique Pérez-Montero (IAA-CSIC. Granada, Spain)

We review recent results on the oxygen and nitrogen chemical abundances in extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3 recently named to as "Green Pea" galaxies. These galaxies are genuine metal-poor galaxies (~ one fifth solar) with N/O ratios unusually high for galaxies of the same metallicity. In combination with their known general properties, i.e., size, stellar mass and star-formation rate, these findings suggest that these objects could be experiencing a short and extreme phase in their evolution. The possible action of both recent and massive inflow of gas, as well as stellar feedback mechanisms are discussed here as main drivers of the starburst activity and their oxygen and nitrogen abundances.

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Title: Green Pea Galaxies and cohorts: Luminous Compact Emission-Line Galaxies in the Sloan Digital Sky Survey
Authors: Y.I.Izotov (1), N.G.Guseva (1), T.X.Thuan (2) ((1) Main Astronomical Observatory, Kyiv, Ukraine, (2) University of Virginia, Charlottesville, USA)

We present a large sample of 803 star-forming luminous compact galaxies (LCGs) in the redshift range z = 0.02-0.63, selected from Data Release 7 of the Sloan Digital Sky Survey (SDSS). The global properties of these galaxies are similar to those of the so-called "green pea" star-forming galaxies, in the redshift range z=0.112-0.360 and selected from the SDSS on the basis of their green colour and compact structure. In contrast to green pea galaxies, our LCGs are selected on the basis of both their spectroscopic and photometric properties, resulting in a ~10 times larger sample, with galaxies spanning a redshift range >2 times larger. We find that the oxygen abundances and the heavy element abundance ratios in LCGs do not differ from those of nearby low-metallicity blue compact dwarf (BCD) galaxies. The median stellar mass of LCGs is ~1e9 Msun. However, for galaxies with high EW(Hbeta), >100A, it is only ~7x1e8 Msun. The star formation rate in LCGs varies in the large range of 0.7-60 Msun yr^{-1}, with a median value of ~4 Msun yr^{-1}, a factor of ~3 lower than in high-redshift star-forming galaxies at z>3. The specific star formation rates in LCGs are extremely high and vary in the range ~1e-9 - 1e-7 yr^{-1}, comparable to those derived in high-redshift galaxies.

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A team of astronomers has discovered a group of rare galaxies called the "Green Peas", which could lend unique insights into how galaxies form stars in the early universe.
The galaxies were discovered with the help of citizen scientists working through an online project called Galaxy Zoo.


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Green Peas
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A team of astronomers has discovered a group of rare galaxies called the "Green Peas" with the help of citizen scientists working through an online project called Galaxy Zoo. The finding could lend unique insights into how galaxies form stars in the early universe.
The Galaxy Zoo users, who volunteer their spare time to help classify galaxies in an online image bank, came across a number of objects that stuck out because of their small size and bright green colour. They dubbed them the Green Peas.

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