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Open research questions in Galaxies: Formation, Evolution, Phenomena

76 unresolved questions extracted from the limitations and future-work sections of 779 Galaxies: Formation, Evolution, Phenomena papers in our library. Each links back to the study that raised it.

What the literature leaves open

  • However, it remains to be seen if this holds for cosmologies with a much steeper VSF. However, further investigation into the impact of a fixed merging parameter across simulations with alternative cosmologies and varied galaxy- halo prescriptions is required.

    VERSUS: An excursion-set-inspired void-finder for the Stage-IV era · 2026 · DOI

  • While stellar population synthesis (SPS) models have been widely used for spectral analysis in optical wavelengths, their characteristics remain uncertain in the near-infrared (NIR) due to a relative lack of observed NIR spectra.

    Comparing the Near-infrared Spectral Energy Distributions from Different Stellar Population Synthesis Models with SPHEREx Observations · 2026

  • Although galaxy interactions are thought to trigger episodes of rapid SMBH growth, direct observations of simultaneous accretion onto multiple SMBHs in compact, intensely star-forming galaxies are lacking.

    Two Peas in a Pod: The First Confirmed Dual Active Galactic Nucleus within a Green Pea Galaxy System · 2026

  • (K. Rubinur et al. 2017; Y.-W. Zhang et al. 2021), conceivably due to challenges such as dust obscuration, AGN duty cycle, short the dual-AGN phase, and limitations imposed by the required resolution of microarcseconds. In the recent Big Multi-AGN Catalog (Big MAC; R. W. Pfeifle et al. 2025), a total of 156 confirmed dual-AGN systems have been identified using techniques including optical spectroscopy, radio imaging, and mid-infrared (MIR) imaging; a further 4180 dual-AGN candidates have been cataloged from the literature. In contrast, due to the requirement for parsec-scale resolution, only one confirmed binary AGN has been established (see below) (C. Rodriguez et al. 2006), while approximately 1368 candidates are listed in Big MAC. Since direct detection methods such as high-resolution imaging in the X-ray (M. Hou et al. 2019), optical (M. Imanishi & Y. Saito 2014; X. Liu et al. 2018; A. Bhattacharya et al. 2023), and radio bands (K. Rubinur et al. 2019; E. Schwartzman et al. 2024) and integral field spectroscopy (IFS; M. Perna et al. 2023; M. Scialpi et al. 2024) have a limited potential for this purpose, indirect methods like double-peaked emission lines (L. Blecha et al. 2013; A. Doan et al. 2020), periodicities in (E. Bon et al. 2012; optical and radio light curves M. J. Graham et al. 2015; S. O’Neill et al. 2022), and crosssymmetric (X, S, or Z) radio sources (X.-G. Zhang et al. 2007; P. Kharb et al. 2017; X. Yang et al. 2022) have been employed to short-list dual-AGN candidates. the NLR gas, In spectroscopic surveys using relatively large fibers (e.g., the 3″-diameter fibers used in the Sloan Digital Sky light from dual AGNs gets integrated/ Survey (SDSS)), aggregated if the two nuclei are encompassed within the fiber. In that situation, a sufficiently high resolution optical spectrum may reveal “double-peaked” narrow emission lines (DPNELs), each peak corresponding to the narrow-line region (NLR) of one member of the AGN pair. In the past decades, this method has resulted in spectroscopic selection of hundreds of dual- AGN candidates (J.-M. Wang et al. 2009; X. Liu et al. 2010; K. L. Smith et al. 2010; R. C. McGurk et al. 2015). However, such double-peaked emission lines could also arise from other processes, such as biconical outflows of warm gas, bulk rotation of and jet–cloud interactions (F. Müller-Sánchez et al. 2015; R. Nevin et al. 2016; K. Rubinur et al. 2019). In order to further investigate the origin of DPNELs, follow-up observations including high spatial resolution imaging combined with spatially resolved spectroscopy are required. In such efforts, only 5% of the known DPNEL galaxies at z � 0.15 have been followed up with subarcsecond spatial resolution imaging in optical and NIR bands (H. Fu et al. 2011; J. Shangguan et al. 2016). Among them, only about a sixth of such systems having two photometric counterparts within 3” (less than 10 kpc) have been confirmed with spatially resolved spectroscopy (see P.

    Origin of the Double-peaked Narrow Emission Lines in the Optical Spectra of X-shaped Radio Galaxies · 2026 · DOI

  • 2024), the role of AGN feedback in driving the rapid quenching of star formation remains debated. In contrast, other works argue that AGN outflows are insufficient to globally deplete or heat the cold-gas reservoirs (H.

    Poststarburst Galaxies with Active Galactic Nuclei: Properties and Evolutionary Sequences · 2026 · DOI

  • The cosmic star formation rate density (CSFRD) has declined sharply toward the present day, but the roles of the atomic and molecular gas reservoirs remain uncertain.

    Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years · 2026

  • While classical strong-lenses are well understood, much remains to be explored for hyperbolic-umbilic (HU) exotic lenses, which produce unique telescopic effects and uncommon images with potentially very high magnifications.

    Predisposition of galaxy clusters to producing exotic hyperbolic umbilic lensing configurations · 2026

  • While the exact identity of these progenitors remains uncertain, several early- Universe channels associated with extremely massive, metal- poor stars may contribute, including PISNe and related pair- instability outcomes, such as pulsational pair-instability events and near-threshold massive star endpoints (Heger & Woosley 2002, 2010; Woosley et al.

    Chemical hints of Population III stars from silicon abundances in massive galaxies · 2026 · DOI

  • Both of these studies have found conflicting results on the relationship between the integrated luminosity of the Lyα nebula and the total molecular gas mass of the host galaxy. With the clear difference found in our Lyα SB profile and the apparent weak trend in LLyα versus S870, further investigations into the host galaxy of known Lyα nebulae of QSOs are warranted.

    Mapping the Extended Ly <i>α</i> Emission within the Circumgalactic Medium of Quasars Hosted by Dusty Starbursts with <b> <tt>CubeCarve</tt> </b> · 2026 · DOI

  • The physical origin of the narrow-line Seyfert 1 (NLS1) and broad-line Seyfert 1 (BLS1) dichotomy remains debated, with competing scenarios invoking host-galaxy evolution or intrinsic accretion physics.

    The Narrow-Line Seyfert 1 Phenomenon: Accretion State Versus Host Galaxy Properties · 2026

  • While X-ray-selected active glactic nuclei AGNs preferentially reside in star-forming galaxies, the role of host-galaxy morphology in regulating both star-formation enhancement and AGN triggering across the star-forming main sequence remains unclear.

    The incidence and star formation properties of X-ray active galactic nuclei across the star-forming main sequence in DESI–eRASS1: The role of host-galaxy morphology · 2026 · DOI

  • While recently proposed BH-star/envelope models have made progress in explaining their optical features, their rest-UV emission, which is considered to originate from host galaxies, remains poorly investigated.

    The UV Side of Little Red Dots: Red, Compact, and Iron-Enhanced Rest-UV Emission with a Strong Downturn around Ly$α$ · 2026

  • The properties of the CL-AGNs thus found suggest that they may represent AGNs at a pivotal state, which likely occur CL transitions due to enhanced accretion activity, while the cause of the accretion activity, determined to have a time scale of several years, remains to be investigated.

    Extreme color-magnitude variability: connection to changing-look AGNs · 2026

  • Abstract Our knowledge of the space velocity of Messier 87, which is the dominant galaxy in the Virgo cluster, has been limited to the radial velocity component.

    The Peculiar Velocity of Messier 87 from Microarcsecond Geodetic Very Long Baseline Interferometry Astrometry · 2026 · DOI

  • Abstract The origin of low surface brightness (LSB) galaxies remains a key open question in galaxy formation, reflecting the balance between internal mechanisms and environmental influence.

    Beyond Mass and Multiscale Environments: What Shapes Low Surface Brightness Galaxies? Evidence from MaNGA · 2026 · DOI

  • The idea that galaxies are "island universes," although proposed centuries ago by philosophers such as Wright and Kant, was not fully explored by science until 1929, when Hubble’s revolutionary study established that galaxies are extra-Galactic.

    Exploring Galaxy Formation: A Multi-Criteria Evaluation of Theoretical Models · 2026 · DOI

  • Physically, this point corresponds to the transition between cosmology–dominated and feedback–regulated regimes: at low metallicities (Z⋆ < Z⋆,k), enrichment is limited by the global matter density, while at high metallicities (Z⋆ > Z⋆,k), stellar and AGN feedback saturate the enrichment efficiency, weakening the dependence on Ωm.

    Cosmology with One Galaxy: An Analytic Formula Relating Ω <sub> <i>m</i> </sub> with Galaxy Properties · 2026 · DOI

  • Flux calibration in HETDEX is performed independently within each IFU. While this approach works well for detecting the low-S/N emission lines that are the primary targets of HETDEX, it can lead to inaccurate flux measurements near bright or extended sources such as large galaxies, bright stars, or the occasional planetary nebula that would benefit from larger-scale modeling of the sky. To address this, we also generate a full-focal-plane sky estimate using the entire VIRUS array for each observation. In practice, however, the current implementation of the full-frame sky model introduces low-level spectral features that degrade the default flux calibration. For this reason, fiber spectra calibrated with the full-frame sky model are not included in this release, although this may be revisited in future data products. Despite these limitations, M. Lujan Niemeyer et al. (2026) successfully used full-frame sky-subtracted data to perform Lyα intensity mapping by statistically removing contaminating components. Because of these issues, we caution users against perform- ing intensity-mapping analyses (e.g., M. Lujan Niemeyer et al. 2026) or absolute sky-brightness measurements (L. H. Weiss et al. 2025) directly from the public PDR1 data cubes. The local sky subtraction provided here is optimized for source detection but is not suitable for analyses requiring a globally uniform sky model. Users studying very extended sources should also note that the local sky subtraction can suppress low surface-brightness emission on spatial scales comparable to the IFU footprint. 5.5. Stacking spectral continuum signal One of the biggest strengths of HETDEX is its potential for studying the properties of faint targets via spectral stacking. By combining thousands to hundreds of thousands of spectra, the faint is boosted revealing details about its stellar population (D. Davis et al. 2023b) and signatures of neutral hydrogen in the circumga- lactic medium (L. H. Weiss et al. 2024; M. Mirza Khanlari et al. 2025). from LAEs When stacking HETDEX spectra, we account for faint sky residuals by constructing a correction spectrum from stacks of 200 randomly selected empty-sky apertures per field (D. Davis et al. 2023b). This residual stack, typically <1% of the sky level in individual spectra, is subtracted from source stacks to remove small calibration offsets that only become significant when combining large numbers of spectra. This measured offset, which is applied in published HETDEX stacking analyses, such as D. Davis et al. (2023b), L. H. Weiss et al. (2024), and M. Mirza Khanlari et al. (2025), is not included in the HETDEX data cubes. 29 ..0.5 The Astrophysical Journal Supplement Series, 284:67 (36pp), 2026 June Mentuch Cooper et al.

    HETDEX Public Data Release 1: Source Catalog 2 and Data Cubes from ∼90 deg <sup>2</sup> of Integral-field Optical Spectroscopy · 2026 · DOI

  • From our ongoing KCWI program to search for Lyα halos around z > 6.5 quasars, we have detected significant extended Lyα emission out to ≃ 11 pkpc around the BAL quasar J0910−0414. This result demonstrates the potential of KCWI’s red channel for detecting the low surface brightness emission at redshifts that were previously inaccessible with ground-based integral-field instruments. Future papers will include analysis for the rest of our sample, extending our search range to z = 7.1. The capabilities of KCWI’s red channel provide a unique opportunity to probe the fuel supply powering these early quasars, and to push the frontier of extended Lyα searches deeper into the epoch of reionization. ACKNOWLEDGMENTS We would like to thank the anonymous referee for their comments, which have improved the quality of this manuscript. RR would like to thank the PypeIt team for their efforts to adapt the software to work on data from KCWI’s red channel. FW acknowledges support from NSF award AST- 2513040. EPF is supported by the international Gemini Observatory, a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation, on behalf of the Gemini partnership of Argentina, Brazil, Canada, Chile, the Republic of Korea, and the United States of America. The data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. Data presented herein were obtained at the W. M. Keck Observatory from telescope time allocated to the National Aeronautics and Space Administration through the agency’s scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the 9 summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. Facility: Keck:II (KCWI) Software: astropy ( Astropy Collaboration et al. 2013, 2018, 2022), Matplotlib (J. D. Hunter 2007), NumPy (C. R. Harris et al. 2020), SciPy (P. Virtanen et al. 2020), PypeIt (J. X.

    Detection of an Extended Ly <i>α</i> Halo around a <i>z</i> = 6.64 Broad Absorption Line Quasar with the Keck Cosmic Web Imager · 2026 · DOI

  • The framework is diagnostic, not dynamical. 2. The datasets used are not perfectly matched: SPARC and SLACS involve different galaxy populations. 3. Stellar mass-to-light assumptions can shift the numerical values of the ratios. 4. Lensing ratios depend on stellar population modeling, IMF assumptions, and aperture definitions. 5. The anchors e1/n are structural scales, not exact attractors that every object must hit. 6. The geometric interpretation of the anchors is supportive, not a first-principles derivation of dark matter phenomenology.

    Dimensional Stratification of Galaxy Mass Discrepancy · 2026 · DOI

  • Larger samples at high redshifts would need to be studied in order to see population-wide trends in AGN hosts compared to regular star-forming galaxies.

    SHELLQs-JWST: Revealing the Spectra of Extended Emission in 12 <i>z</i> > 6 Quasar Host Galaxies Using the JWST NIRSpec Fixed Slit · 2026 · DOI

  • The planes of satellite galaxies problem, suggested solutions, and open questions. M200/LK conversion The conversion from luminosity to halo mass in the LVG cat- alogue is limited by the amount of information contained in this survey, which forces us to assume certain general approx- imations.

    HINORA II: Testing the Existence of the Council of Giants in ΛCDM simulations · 2026 · DOI

  • Model performance at z > 3.0 remains uncertain as the training set spans only to z ≈ 3.0; validation of QUEST-generated spectra for high-redshift quasars (z > 3.5) against reionization-epoch observations and comparison with theoretical predictions of Lyman forest features is required.

    QUEST: Quasar unsupervised encoder and synthesis tool. A machine-learning framework for generating quasar spectra · 2026 · DOI

  • The Info-VAE architecture was not explicitly compared against alternative machine-learning architectures (e.g., transformer-based models, diffusion models, or flow-based generative models) for quasar spectrum synthesis; investigation of architectural improvements beyond the current VAE design for improved spectral feature fidelity is deferred.

    QUEST: Quasar unsupervised encoder and synthesis tool. A machine-learning framework for generating quasar spectra · 2026 · DOI

  • The comparison with qsogen reveals stronger Lyman-α emission in QUEST's median sampled spectrum; quantitative analysis of how averaging over thousands of model realisations and training set averaging affects emission-line width and peak wavelength predictions requires validation against high-resolution spectroscopic observations.

    QUEST: Quasar unsupervised encoder and synthesis tool. A machine-learning framework for generating quasar spectra · 2026 · DOI

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76 open questions have been extracted from the limitations and future-work passages of 779 Galaxies: Formation, Evolution, Phenomena papers in our library. Each one below links back to the study that raised it, so you can read the original claim in context.

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