Research
Understanding Galaxy Evolution Through Post-Starburst, E+A Systems
Galaxy spectrum with SDSS u, g, r, i, z bandpasses overlaid and a wavelength–color map. The rainbow beneath the spectrum encodes the observed colour at each wavelength (Å), while the semi‑transparent white lobes show the SDSS filter response and the ranges each band covers. UV, visible, and IR domains are indicated across the top. The normalized single‑fiber spectrum for 7443‑12701 exhibits a blue‑sloped continuum with strong Balmer absorption, yielding higher flux shortward of 5000 Å than in the red. For an E+A galaxy dominated by A‑type stars but lacking ongoing star formation, the integrated colour is expected to fall in the green–gold portion of the rainbow (the green‑valley regime), rather than the deep blue of the star‑forming cloud or the red of the passive sequence. Bandpass ranges are drawn from astroML datasets.
What are E+A Galaxies?
Post-starburst E+A galaxies represent one of the most dramatic transformation events in the universe: systems that recently shut down all star formation after an intense burst of stellar birth. These galaxies are evolutionary snapshots—caught in the act of transitioning from vibrant, star-forming spirals into quiescent, "red and dead" ellipticals.
The name "E+A" describes exactly what we see in their spectra: strong Balmer absorption lines characteristic of A-type stars (young, massive stars from the recent starburst) superimposed on an elliptical-like continuum with no ongoing star formation. By the time we identify these systems, all O and B-type stars have already died (lifetimes of only 10-100 million years), leaving behind a population dominated by A-type stars with lifespans of ~1 billion years. This gives E+A galaxies their signature golden color and places them in the green valley—that transitional region between the blue cloud of star-forming galaxies and the red sequence of passive systems.
What makes them extraordinary: Something catastrophic happened to these galaxies approximately 500 million to 1 billion years ago. A galaxy-galaxy merger, ram-pressure stripping, or AGN feedback triggered an intense starburst, then abruptly cut off the gas supply, quenching all star formation almost overnight (in cosmic terms). Understanding this quenching mechanism is one of the fundamental questions in galaxy evolution.
Why they matter: E+A galaxies are rare—comprising less than 1% of the local galaxy population—yet they may represent a critical evolutionary phase that all galaxies pass through during major transformation events.
Refining the E+A Galaxy
I developed a refined visual classification system specifically designed for spatially-resolved integral field spectroscopy (IFS) data from SDSS-IV MaNGA. Traditional single-fiber surveys capture only the central ~3" of a galaxy, but MaNGA's IFS cubes provide thousands of spectra across each system—revealing spatial variations that single-fiber studies miss entirely.
Example of a prototypical E+A single-fiber spectrum from SDSS.
Plate-IFU: 7443-12701
MaNGA ID: 12-98126
Single-fiber spectrum of prototypical E+A galaxy 7443-12701 showing the characteristic blue-sloped continuum with strong Balmer absorption lines (HU+3b4, HU+3b3, HU+3b2, HU+3b1; marked by pink arrows) and absent HU+3b1 emission. The 4000Å break (blue dashed line) indicates a stellar population not dominated by very young stars, while weak [OII] emission (light blue region, left) confirms ended star formation. The colored wavelength ranges correspond to analysis windows used in the MEWS automated pipeline, optimized for different spectral regions.
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The Greene Method requires four key spectroscopic signatures:
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Blue-sloped continuum – Higher flux in the blue than the red, indicating a young-to-intermediate age stellar population
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Strong Balmer absorption lines (Hδ, Hγ, Hβ, Hα) – Deep absorption features from A-type stars produced in the recent starburst
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Dn4000 > 1.5 – The 4000 Å break strength confirms an older population not dominated by very young stars
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Absent Hα emission & weak [OII] λ3727 – Proves star formation has ended (no ionizing photons from young stars)
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Crucially, candidates must meet these criteria across the entire spatially-resolved system—not just in the central fiber. Many galaxies classified as E+A by single-fiber surveys actually have ongoing star formation in their outskirts or AGN signatures in their cores that traditional methods miss.
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Reference Links:
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See Greene et al., 2021, ApJ, 910, 162 for the MPL-5 sample and detailed methodology.
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See Greene, 2026 for the full Dissertation.
Why Spatially-Resolved Diagnostics Matter
Traditional galaxy surveys observe only the central ~3 arcseconds of each galaxy through a single fiber, capturing a single integrated spectrum. But galaxies are not uniform—star formation, AGN activity, and quenching can occur in different spatial regions simultaneously. This is where integral field spectroscopy (IFS) becomes essential.
WHAN Diagnostic Diagram for 8623-9102 — Left: Individual spaxels (spatial pixels) from a galaxy plotted by Hα equivalent width vs. [NII]/Hα ratio, separating star-forming (SF), AGN, and old stellar populations. This galaxy shows >90% of spaxels classified as "old stars." Right: The spatially-resolved map reveals this classification holds across the entire galaxy—uniformly quiescent with no hidden star formation. The hexagonal footprint represents MaNGA's fiber bundle coverage, with concentric circles marking effective radii.
The WHAN diagram classifies ionization sources using two easily measured quantities:
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Equivalent width of Hα - Strength of the H-alpha emission line (vertical axis)
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[NII]/Hα ratio - Ratio of nitrogen to hydrogen emission (horizontal axis)
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Spaxels fall into three categories:
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Star-forming (SF) - Strong Hα emission with low [N II]/Hα (ionization from young stars)
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AGN - Strong emission with high [N II]/Hα (ionization from active galactic nucleus)
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Old stars - Weak or absent emission (no ionization sources; passive stellar population)
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The key insight: By examining the spatially-resolved map alongside the diagnostic diagram, we can immediately see if a galaxy is uniformly quiescent or contains hidden pockets of star formation. For true E+A classification, quiescence must extend throughout the entire galaxy—spatially-resolved diagnostics ensure we're not misled by central measurements that miss ongoing activity in outer regions.
The Contamination Problem: Why Single-Fiber Surveys Fail
Many "E+A" catalogs built from single-fiber surveys suffer from severe contamination. A galaxy may appear quiescent in its central 3", but harbor vigorous star formation in outer annuli that single fibers never sample.
Figure: The danger of single-fiber classification. This galaxy appears to meet E+A criteria in its central spaxel (which a single-fiber survey would sample), yet the spatially-resolved WHAN map reveals active star formation throughout the majority of the system (blue regions). The few spaxels classified as AGN or old stars in the outer regions result from low signal-to-noise, but the galaxy is clearly dominated by ongoing star formation. This system is undergoing inside-out quenching—central bulge has shut down while the disk continues forming stars—but it is NOT a post-starburst galaxy. These contaminating systems were identified and removed from our sample through spatially-resolved diagnostics. Single-fiber surveys would have incorrectly classified this as E+A.
This example perfectly illustrates why spatially-resolved spectroscopy is non-negotiable for building clean E+A samples. The single-fiber spectrum meets all traditional criteria (strong Balmer absorption, no central H-alpha emission, appropriate colors), yet the galaxy is predominantly star-forming when viewed as a whole. Such systems are consistent with inside-out quenching—where the central bulge quenches first while the disk continues forming stars—but they are NOT post-starburst galaxies.
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Impact on sample purity: Spatially-resolved diagnostics allowed us to identify and remove contaminating systems that single-fiber methods would have missed entirely. This included not only obviously star-forming systems like the example shown, but also galaxies with subtle ongoing activity in outer regions or localized AGN emission. The careful application of spatial criteria—ensuring E+A characteristics hold throughout the entire galaxy, not just the center—was essential for maintaining sample integrity. This demonstrates why previous E+A catalogs built from single-fiber surveys may suffer from significant contamination, potentially confusing interpretations of post-starburst galaxy properties and evolution.
Hidden Complexity in "Quiescent" Systems
Using spatially-resolved diagnostics, we discovered that many galaxies appearing completely quiescent in traditional WHAN analysis actually harbor ionization signatures when examined with multiple BPT diagnostics:
BPT (Baldwin, Phillips & Terlevich) Diagnostics
Galaxy 8315-3703 demonstrates the power of multi-diagnostic analysis. The WHAN diagnostic (top-right panel) classifies this galaxy as entirely "old stars" with no detected star formation, meeting traditional E+A criteria. However, the four BPT diagnostic panels (lower right), modeled after Kewley et., al., 2006, reveal substantial AGN signatures: 89% AGN classification in the [N II] diagram and significant LINER emission in [O I] and [S II] diagrams. The visual image (top-left) shows the galaxy's optical appearance with MaNGA hexagon overlay, while the SDSS single-fiber spectrum (center-left) displays the integrated light. The diagnostic table (bottom-left) quantifies spaxel percentages across all five systems.
The spatially-resolved maps show where this ionization exists across the galaxy—information completely invisible to single-fiber surveys or WHAN-only analysis.
Many systems classified as entirely old stellar populations by WHAN reveal detectable LINER or weak AGN emission across multiple BPT diagrams. This finding has implications for:
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AGN duty cycles in post-starburst systems (residual low-level activity after main quenching event)
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Residual ionization from post-AGB stars, shocks, or low-luminosity AGN
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Classification methods that rely on single diagnostic systems (WHAN alone is insufficient)
The complexity revealed by spatially-resolved, multi-diagnostic analysis demonstrates that post-starburst galaxies are not the simple, uniformly quiescent systems that single-fiber studies suggested. They contain spatial structure, residual ionization, and evolutionary signatures that require IFS data to properly characterize.
The Complete Catalog of E+A Galaxies:
183 Spatially-Resolved Systems from MaNGA MPL-11
​This work has created a uniformly-selected, by-eye catalog of post-starburst E+A galaxies using spatially-resolved spectroscopy. Working with MaNGA's final data release (DR17, MPL-11)—which contains over 10,000 galaxies—I led a citizen science initiative to identify and characterize these rare systems.
Our 183 E+A galaxy sample, colored-coded by morphological type (gathered from the MVM-VAC). Note how this sample of E+A galaxies spans across morphological types but lies primarily in the approximated green valley region (Wild et al., 2009), between the blue cloud of star-forming galaxies and the red sequence of quiescent systems. Two empirically-derived green valley boundaries (green lines) define this transitional region. Background contours show 50,000 random galaxies selected from SDSS. ​
The Citizen Science Approach
​To visually classify 10,000+ galaxy candidates, I recruited, trained, and managed 27 citizen scientists across three countries. Together, we developed:
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Training protocols explaining E+A spectroscopic signatures and diagnostic maps
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Quality control systems requiring multiple independent classifications per galaxy
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Custom visualization tools generating spatially-resolved diagnostic maps (WHAN, BPT) for each candidate
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Project management workflows coordinating international volunteers across time zones
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The team classified galaxies using spatially-resolved ionization maps that reveal where star formation, AGN activity, and old stellar populations exist within each system. This approach dramatically improved classification accuracy compared to single-fiber methods.
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From ~600 candidates to 183 validated systems: The rigorous spatial validation culled contaminating galaxies that would have polluted single-fiber samples. Some candidates showed clear ongoing star formation throughout their disks despite quiescent centers. Others revealed localized AGN emission or residual ionization incompatible with true post-starburst status. The careful application of spatial criteria—ensuring E+A characteristics throughout the entire galaxy structure—was essential for building a clean, reliable sample.
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Final catalog statistics:
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183 confirmed E+A galaxies meeting the Greene Method criteria across their entire IFS footprints
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Complete measurements: stellar masses (10^8.8 to 10^11.5 M☉), colors, effective radii, morphologies
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Spatially-resolved diagnostics: WHAN and BPT classification percentages for every galaxy
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Equivalent width measurements: 10 key spectral lines per galaxy using automated pipeline
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The catalog is being prepared for publication—independent of my dissertation—and will be publicly available with all measurements, diagnostic images, and analysis tools through GitHub.
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The Power of Citizen Science​
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The citizen science approach proved remarkably effective for large-scale visual classification:
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27 volunteers classified 10,000+ candidates in ~18 months
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Quality control through multiple independent classifications caught edge cases
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International collaboration (3 countries) brought diverse perspectives
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Reproducible methodology documented for future surveys (DESI, Rubin Observatory)
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This demonstrates that rigorous science can be conducted through well-designed citizen science programs, democratizing access to cutting-edge astronomical research while maintaining professional-quality results.
Color-Magnitude Diagrams
Left: (g−r) color vs. absolute r-band magnitude. Right: (u−g) color vs. absolute g-band magnitude. E+A galaxies (colored by stellar mass) span a range of luminosities but consistently occupy intermediate-color space, demonstrating their transitional evolutionary state across multiple wavelength baselines.
Open-Access Analysis Tools
@InfinitelyCurious
To make this research reproducible and accessible beyond large institutions, I developed three Python pipelines, all publicly available on GitHub, with comprehensive documentation:
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1. MEWS: Measuring Equivalent Width in Spectra
An automated pipeline that replaced obsolete PyRAF tools for spectral line measurements. MEWS features:
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Four continuum fitting algorithms with automatic best-fit selection
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Smart profile fitting (Gaussian vs. Voigt) optimized for each line type
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Adaptive wavelength windows across UV through IR
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Visual diagnostic plots for quality validation at every measurement stage
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Critical role: MEWS processed ~600 E+A candidates and enabled curation of the final 183-galaxy sample through comprehensive visual diagnostics. Generated 1,830+ equivalent width measurements (183 galaxies × 10 lines) with quality-control plots for each.
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2. Color-Mass Analysis Toolkit
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Tools for positioning galaxies in color-mass-magnitude space, including:
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Empirically-derived green valley boundaries (explicitly defined, not vaguely referenced)
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Uniform dust corrections using modern extinction coefficients
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Morphology integration with MaNGA Visual Morphologies catalog
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Publication-quality visualization tools
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Most papers reference "the green valley" without defining its boundaries—this toolkit provides rigorous, reproducible definitions.
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3. SED Multi-Survey Unit Converter (In Development)
Python tools for converting SDSS, 2MASS, WISE, and GALEX photometry into uniform units (mJy) for multi-wavelength spectral energy distribution analysis. Handles survey-specific magnitude systems (AB, Vega, asinh) and preserves error propagation. Essential for UV-through-IR galaxy characterization.
Publications & Resources​
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Greene, O. A., et al. (2026). "A Complete Catalog of Post-starburst, E+A Galaxies in SDSS-IV MaNGA (MPL-11): A Citizen Science Approach to Spectrophotometric Classification & the Automation of Equivalent Width Measurements." (In Prep for ApJ)
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Open-Access Code:​
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SED Multi-Survey Converter (coming soon)
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Complete Catalog of Spatially Resolved, Post Starburst E+A Galaxies in SDSS-MaNGA (MPL-11) (coming soon)​​​​
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Dissertation:​
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Greene, O. A. (2026). "Seeing What Is, What Was, What Could Be, What Must Not: Refining, Cataloging, and Investigating A Complete, Spatially Resolved Spectrophotometric Sample of Nearby Post-Starburst E+A Galaxies in SDSS-IV MaNGA." Vanderbilt University. 339 pages. [Will soon be available on arXiv]
This research demonstrates that transitional galaxies like E+A systems—once thought to be simple, uniform objects—contain remarkable internal complexity that only spatially-resolved spectroscopy can reveal. By making the tools, data, and methodology publicly available, I hope to enable the next generation of studies into these fascinating evolutionary laboratories and advance our understanding of how galaxies transform across cosmic time.
