SDSS-V LVM: Resolving physical conditions in the Trifid Nebula

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 70-264, 04510 CDMX, Mexico
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 106, 22800 Ensenada, BC, Mexico
⁴ Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad s/n, 62210 Cuernavaca, Morelos, Mexico
⁵ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁶ Center for Astrophysics and Space Science, NYU Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁷ Universität Heidelberg, Interdiszipliäres Zentrum für Wissenschaftliches Rechnen, 69120 Heidelberg, Germany
⁸ Department of Astronomy/Departamento de Astronomía, Universidad de La Serena, La Serena, Chile
⁹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁰ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, 38200 Spain
¹¹ Universidad Católica del Norte, Núcleo UCN en Arqueología Galáctica – Inst. de Astronomía, Av. Angamos 0610, Antofagasta, Chile
¹² McDonald Observatory, The University of Texas at Austin, 1 University Station, Austin, TX 78712-0259, USA
¹³ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
¹⁴ Apache Point Observatory and New Mexico State University, PO Box 59, Sunspot, NM, 88349-0059, USA
¹⁵ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
¹⁶ Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
¹⁷ Center for Cosmology & Astro-Particle Physics, The Ohio State University, Columbus, Ohio 43210, USA
¹⁸ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
¹⁹ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
²⁰ Universidad de Chile, Av. Libertador Bernardo O’Higgins 1058, Santiago, Chile
²¹ Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA

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Received: 13 November 2025
Accepted: 2 December 2025

Abstract

Aims. The chemical abundance of the interstellar medium sets the initial conditions for star formation and provides a probe of chemical galaxy evolution models. However, unresolved inhomogeneities in the electron temperature can lead to a systematic underestimation of the abundances. We aim to directly test this effect.

Methods. We used the SDSS-V Local Volume Mapper to spatially map the physical conditions of the Trifid Nebula (M 20), a Galactic H II region ionized by a single mid-type O-star, at a 0.24 pc resolution. We exploited various emission lines (e.g., Hydrogen recombination lines and collisionally excited lines, including also faint auroral lines) and computed the spatially resolved maps of [O II] and [S II] electron densities, the [N II], [O II], [S II], and [S III] electron temperatures, and the ionic oxygen abundances.

Results. We found internal variations of electron density that result from the ionization front, along with a negative radial gradient. However, we did not find any strong gradients or structures in the electron temperature and the total oxygen abundance, making the Trifid Nebula a relatively homogeneous H II region at the observed spatial scale. We compared these spatially resolved properties with equivalent integrated measurements of the Trifid Nebula and found no significant variations between integrated and spatially resolved conditions.

Conclusions. This isolated H II region, ionized by a single O-star, represents a test case of an ideal Strömgren sphere. The physical conditions in the Trifid Nebula behave as expected, with no significant differences between integrated and resolved measurements.