Journal ArticleOrigin of the ring ellipticity in the black hole images of M87 ^*Dahale, Rohan; Cho, Ilje; Moriyama, Kotaro; Wiik, Kaj; Tiede, Paul; Gómez, José L; Chan, Chi-kwan; Gold, Roman; Bernshteyn, Vadim Y; Foschi, Marianna; Jeter, Britton; Pu, Hung-Yi; Georgiev, Boris; Joshi, Abhishek V; Cruz-Osorio, Alejandro; Natarajan, Iniyan; Broderick, Avery E; Salas, León_D S; Chatterjee, Koushik; Akiyama, Kazunori; Albentosa-Ruíz, Ezequiel; Alberdi, Antxon; Alef, Walter; Algaba, Juan Carlos; Anantua, Richard; Asada, Keiichi; Azulay, Rebecca; Bach, Uwe; Baczko, Anne-Kathrin; Ball, David; Baloković, Mislav; Bandyopadhyay, Bidisha; Barrett, John; Bauböck, Michi; Benson, Bradford A; Bintley, Dan; Blackburn, Lindy; Blundell, Raymond; Bouman, Katherine L; Bower, Geoffrey C; Bremer, Michael; Brissenden, Roger; Britzen, Silke; Broguiere, Dominique; Bronzwaer, Thomas; Bustamante, Sandra; Ferreira_Carlos, Douglas; Carlstrom, John E; Chael, Andrew; Chang, Dominic O; Chatterjee, Shami; Chen, Ming-Tang; Chen, Yongjun; Cheng, Xiaopeng; Christian, Pierre; Conroy, Nicholas S; Conway, John E; Crawford, Thomas M; Crew, Geoffrey B; Cui, Yuzhu; Curd, Brandon; Davelaar, Jordy; De_Laurentis, Mariafelicia; Deane, Roger; Dempsey, Jessica; Desvignes, Gregory; Dexter, Jason; Dhruv, Vedant; Dihingia, Indu K; Doeleman, Sheperd S; Dzib, Sergio A; Eatough, Ralph P; Emami, Razieh; Falcke, Heino; Farah, Joseph; Fish, Vincent L; Fomalont, Edward; Ford, H Alyson; Fraga-Encinas, Raquel; Freeman, William T; Friberg, Per; Fromm, Christian M; Fuentes, Antonio; Galison, Peter; Gammie, Charles F; García, Roberto; Gentaz, Olivier; Geertsema, Gertie; Goddi, Ciriaco; Gómez-Ruiz, Arturo I; Gu, Minfeng; Gurwell, Mark; Hada, Kazuhiro; Haggard, Daryl; Hesper, Ronald; Heumann, Dirk; Ho, Luis C; Ho, Paul; Honma, Mareki; Huang, Chih-Wei L; Huang, Lei; Hughes, David H; Ikeda, Shiro; Impellizzeri, C_M Violette; Inoue, Makoto; Issaoun, Sara; James, David J; Jannuzi, Buell T; Janssen, Michael; Jiang, Wu; Jiménez-Rosales, Alejandra; Johnson, Michael D; Jorstad, Svetlana; Jones, Adam C; Jung, Taehyun; Karuppusamy, Ramesh; Kawashima, Tomohisa; Keating, Garrett K; Kettenis, Mark; Kim, Dong-Jin; Kim, Jae-Young; Kim, Jongsoo; Kim, Junhan; Kino, Motoki; Koay, Jun Yi; Kocherlakota, Prashant; Kofuji, Yutaro; Koch, Patrick M; Koyama, Shoko; Kramer, Carsten; Kramer, Joana A; Kramer, Michael; Krichbaum, Thomas P; Kuo, Cheng-Yu; La_Bella, Noemi; Lee, Sang-Sung; Levis, Aviad; Li, Zhiyuan; Lico, Rocco; Lindahl, Greg; Lindqvist, Michael; Lisakov, Mikhail; Liu, Jun; Liu, Kuo; Liuzzo, Elisabetta; Lo, Wen-Ping; Lobanov, Andrei P; Loinard, Laurent; Lonsdale, Colin J; Lowitz, Amy E; Lu, Ru-Sen; MacDonald, Nicholas R; Mao, Jirong; Marchili, Nicola; Markoff, Sera; Marrone, Daniel P; Marscher, Alan P; Martí-Vidal, Iván; Matsushita, Satoki; Matthews, Lynn D; Medeiros, Lia; Menten, Karl M; Mizuno, Izumi; Mizuno, Yosuke; Montgomery, Joshua; Moran, James M; Moscibrodzka, Monika; Mulaudzi, Wanga; Müller, Cornelia; Müller, Hendrik; Mus, Alejandro; Musoke, Gibwa; Myserlis, Ioannis; Nagai, Hiroshi; Nagar, Neil M; Nair, Dhanya G; Nakamura, Masanori; Narayanan, Gopal; Nathanail, Antonios; Navarro_Fuentes, Santiago; Neilsen, Joey; Ni, Chunchong; Nowak, Michael A; Oh, Junghwan; Okino, Hiroki; Olivares_Sánchez, Héctor Raúl; Oyama, Tomoaki; Özel, Feryal; Palumbo, Daniel_C M; Paraschos, Georgios Filippos; Park, Jongho; Parsons, Harriet; Patel, Nimesh; Pen, Ue-Li; Pesce, Dominic W; Piétu, Vincent; PopStefanija, Aleksandar; Porth, Oliver; Prather, Ben; Principe, Giacomo; Psaltis, Dimitrios; Ramakrishnan, Venkatessh; Rao, Ramprasad; Rawlings, Mark G; Rezzolla, Luciano; Ricarte, Angelo; Ripperda, Bart; Röder, Jan; Roelofs, Freek; Romero-Cañizales, Cristina; Ros, Eduardo; Roshanineshat, Arash; Rottmann, Helge; Roy, Alan L; Ruiz, Ignacio; Ruszczyk, Chet; Rygl, Kazi_L J; Sánchez, Salvador; Sánchez-Argüelles, David; Sánchez-Portal, Miguel; Sasada, Mahito; Satapathy, Kaushik; Saurabh; Savolainen, Tuomas; Peter_Schloerb, F; Schonfeld, Jonathan; Schuster, Karl-Friedrich; Shao, Lijing; Shen, Zhiqiang; Silpa, Sasikumar; Small, Des; Sohn, Bong Won; SooHoo, Jason; Souccar, Kamal; Stanway, Joshua S; Sun, He; Tazaki, Fumie; Tetarenko, Alexandra J; Tilanus, Remo_P J; Titus, Michael; Toma, Kenji; Torne, Pablo; Toscano, Teresa; Traianou, Efthalia; Trent, Tyler; Trippe, Sascha; Turk, Matthew; van_Bemmel, Ilse; van_Langevelde, Huib Jan; van_Rossum, Daniel R; Vos, Jesse; Wagner, Jan; Ward-Thompson, Derek; Wardle, John; Washington, Jasmin E; Weintroub, Jonathan; Wharton, Robert; Wielgus, Maciek; Witzel, Gunther; Wondrak, Michael F; Wong, George N; Wu, Qingwen; Yadlapalli, Nitika; Yamaguchi, Paul; Yfantis, Aristomenis; Yoon, Doosoo; Young, André; Younsi, Ziri; Yu, Wei; Yuan, Feng; Yuan, Ye-Fei; Zeng, Ai-Ling; Anton_Zensus, J; Zhang, Shuo; Zhao, Guang-Yao; Zhao, Shan-Shan<p>We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87^*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT)'s 2018 array, including improved (<italic>u</italic>,<italic>v</italic>)-coverage from the Greenland Telescope, we measured the ring's ellipticity using five independent imaging methods, obtaining a consistent average value of<italic>τ</italic> = 0.08_−0.02^+0.03with a position angle of<italic>ξ</italic> = 50.1_−7.6^+6.2 degrees. To interpret this measurement, we compared it to general relativistic magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including the thermal or nonthermal electron distribution function, spins, and ion-to-electron temperature ratios in both low- and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in nonthermal models and models with higher jet emission. These results indicate that the ellipticity measured from the M87^*emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects.</p>A&A2025-07-0110626757Astronomy & Astrophysics699A2790004-6361https://doi.org/10.1051/0004-6361/2025552352332483; 1935980; 2034306; 2034318National Science Foundation