Platypus galaxy
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A platypus galaxies may represent a type of astronomical object at high redshift, discovered by the James Webb Space Telescope between 2025 and 2026. They are characterized by point-like optical morphology and narrow resolved emission lines, and they exhibit extremely high star formation rates with low interstellar medium velocity dispersion.[1][2][3][4][5]
The main representative of platypus galaxies is the galaxy CEERS 4233-42232.[5]
Discovery
Platypus-type galaxies were first discovered during a study of archival data from the James Webb Space Telescope by a group of American astrophysicists led by Professor Haojin Yan from the University of Missouri. His students graduate student Banzheng Sun and undergraduate Riley Schive also actively participated in the work.[2][6][3][4][5]
The search was conducted in the spring of 2025 among a dataset of approximately 2,000 compact cosmic radiation sources from the early universe. The researchers were inspired by earlier JWST discoveries, particularly the little red dots phenomenon, which revealed that a multitude of anomalous objects are hidden at high redshifts.[2][6][3][4][5]
To isolate this atypical population, the astronomers utilized a collection of deep infrared sky surveys, including data from the international Cosmic Evolution Early Release Science Survey (CEERS). By applying a multi-stage statistical filtering method, the authors excluded instrumental artifacts and nearby Milky Way foreground stars from the catalogs.[2][6][3][4][5]
As a result of this selection process, a unique sample of nine cosmic objects was formed. The official presentation of their classification, a description of the spectral anomalies, and the introduction of the term "platypus galaxies" took place on January 6, 2026, at a press conference during the 247th meeting of the American Astronomical Society (AAS) in Phoenix, Arizona.[2][6][3][4][5]
Observed properties
Platypus galaxies represent a population of ultra-compact galaxies discovered at redshifts z = 3.624 - 5.378 with absolute magnitudes MB ≈ -18.3 to -20.7.[1] These parameters indicate that the look-back time of the detected radiation ranges from 11.9 to 12.574 billion years.[7][1]
Morphology
Photometric observations conducted with the JWST Near-Infrared Camera reveal that all detected objects appear as featureless, strictly point-like sources. Their spatial brightness distribution profile is identical to the telescope's instrumental point spread function.[1][2] The objects show no signs of diffuse halos or extended disk/spiral structures characteristic of classical galaxies, which, during the initial photometric analysis, made them indistinguishable from substellar objects or low-luminosity stars belonging to the Milky Way halo.[1][2]
Spectroscopic parameters
Their spectra exhibit intense and clean emission lines, including resolved Balmer series transitions (Hα) with widths of only 150–360 km s−1 and forbidden lines of ionized oxygen ([O III]λλ 4959,5007).[1] A notable feature of these spectra is the small full width at half maximum (FWHM) of the spectral lines. The line profiles remain narrow, indicating a low velocity dispersion of the ionized gas within the systems, ranging from several tens to hundreds of kilometers per second.[1] The spectra completely lack the broad-line components characteristic of Broad Line Regions (BLR) in Active galactic nucleus and quasars, where gas radial velocities under the gravitational influence of a supermassive black hole reach thousands or tens of thousands of kilometers per second.[1]
Emission-line diagnostics reveal that at least one of the objects hosts an active galactic nucleus.[1] However, the spectral energy distributions of most of the eight sources are well reproduced by normal galaxy templates, suggesting that they could instead be very young, star-forming systems in their early formation stages, with a median age of 120 Myr, a median star formation rate of 1.7 M⊙yr−1, and a median stellar mass of only 108.4 M⊙.[1]
The nature of these objects remains a subject of ongoing research, as their classification as either star-forming galaxies or active galactic nucleus (AGN) depends on future deep, moderate-resolution spectroscopic observations.[1] If identified as star-forming galaxies, their gas-phase metallicity is observed to fall within a narrow range of 12+log(O/H) = 8.1–8.3, suggesting a new category of objects where early-stage formation likely proceeds via secular processes originating from a highly compact core.[1][6] Conversely, if classified as AGN, they represent a novel type of low-luminosity Type 2 AGN characterized by the near-total absence of a detectable host galaxy.[1][6]
Comparison with "Green Pea" galaxies
Spectral analysis of platypus galaxies reveals a profound physical similarity to green pea galaxies at z ≲ 0.4, evidenced by identical narrow hydrogen and oxygen emission lines, which point to similar gas ionization mechanisms and comparable metallicities.[2][1] However, unlike green pea galaxies, which are extended diffuse disks with radii of several kiloparsecs, platypus galaxies are 10–30 times more compact, causing them to appear as strictly point-like sources in infrared imaging.[1] Furthermore, while the green pea class is monolithic and consists exclusively of star-forming regions, the platypus population is heterogeneous, comprising not only ultra-young nascent galaxies but also obscured low-luminosity active galactic nuclei (AGN).[1]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Yan, Haojing; Sun, Bangzheng; Shive (University of Missouri-Columbia), Riley; Dunlop, J. S.; Cullen, F.; Dickinson, M.; Arrabal Haro, P.; Taylor, A. J.; Bondestam, C.; Liu, F.-Y.; Arellano-Córdova, K. Z.; Barrufet, L.; Begley, R.; Carnall, A. C.; Golawska, H.; Leung, H.-H.; Scholte, D.; Stanton, T. M. (24 May 2026). "Point-like and Narrow-lined: A Potentially New Population of Objects Discovered by JWST". arXiv:2509.12177v2 [astro-ph.GA].
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 NASA. Baum, Stephanie; Zinin, Andrew, eds. "Webb reveals a sample of galaxies with unusual features, nicknamed 'Platypus'". Phys.org. Retrieved 2026-01-06.
- ↑ 3.0 3.1 3.2 3.3 3.4 Nola Taylor Tillman. "Strange cosmic objects spotted by the James Webb Space Telescope may be baby 'platypus' galaxies — or something entirely new". Space.com. Retrieved 2026-01-10.
- ↑ 4.0 4.1 4.2 4.3 4.4 Gough, Evan (2026-01-06). "As Puzzling As A Platypus: The JWST Finds Some Hard To Categorize Objects". Universe Today. Retrieved 2026-01-06.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 "Scientists Identify 'Astronomy's Platypus' with NASA's Webb Telescope". NASA Science. 2026-01-06. Retrieved 2026-01-06.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Eric Stann, University of Missouri. Clark, Gaby; Egan, Robert, eds. "'Platypus' objects in the early universe look like stars but behave like galaxies". Phys.org. Retrieved 2026-01-06.
- ↑ Wright, Edward L. (2022). "Ned Wright's Javascript Cosmology Calculator". University of California, Los Angeles. Retrieved 24 November 2022. (H0=67.4 and OmegaM=0.315 (see Table/Planck2018 at "Lambda-CDM model#Parameters" )
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