ALMAGAL

IV. Morphological comparison of molecular and thermal dust emission using the histogram of oriented gradients method

¹ INAF-Istituto di Astrofisica e Planetologia Spaziale, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² Institut de Ciències de l’Espai (ICE), CSIC, Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860, Castelldefels (Barcelona), Spain
⁴ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Rome, Italy
⁵ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁷ University of Connecticut, Department of Physics, 2152 Hillside Road, Unit 3046 Storrs, CT 06269, USA
⁸ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of ASMAB, AS/NTU No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
⁹ East Asian Observatory, 660 N. A’ohoku, Hilo, Hawaii, HI 96720, USA
¹⁰ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹² Jodrell Bank Centre for Astrophysics, Oxford Road, The University of Manchester, Manchester M13 9PL, UK
¹³ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Heidelberg, Germany
¹⁴ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Heidelberg, Germany
¹⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
¹⁶ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹⁷ Center for Data and Simulation Science, University of Cologne, Germany
¹⁸ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands
¹⁹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
²⁰ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
²¹ UK ALMA Regional Centre Node, M13 9PL, UK
²² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²³ Max-Planck-Institute for Extraterrestrial Physics (MPE), Garching bei München, Germany
²⁴ Laboratory for the study of the Universe and eXtreme phenomena (LUX), Observatoire de Paris, Meudon, France
²⁵ Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
²⁶ School of Engineering and Physical Sciences, Isaac Newton Building, University of Lincoln, Brayford Pool, Lincoln LN6 7TS, UK
²⁷ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
²⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China
³⁰ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³¹ INAF-Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
³² Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
³³ Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan
³⁴ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁵ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum – Università di Bologna, Bologna, Italy
³⁶ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
³⁷ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
³⁸ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³⁹ Zhejiang Laboratory, Hangzhou 311100, PR China
⁴⁰ Universidad Autonoma de Chile, Pedro de Valdivia 425, 9120000 Santiago de Chile, Chile

^★ Corresponding author: asanchez@ice.csic.es

Received: 22 October 2024
Accepted: 10 April 2025

Abstract

Context. The study of molecular line emission is crucial to unveil the kinematics and the physical conditions of gas in star-forming regions. We use data from the ALMAGAL survey, which provides an unprecedentedly large statistical sample of high-mass star-forming clumps that helps us to remove bias and reduce noise (e.g., due to source peculiarities, selection, or environmental effects) to determine how well individual molecular species trace continuum emission.

Aims. Our aim is to quantify whether individual molecular transitions can be used reliably to derive the physical properties of the bulk of the H₂ gas, by considering morphological correlations in their overall integrated molecular line emission with the cold dust. We selected transitions of H₂CO, CH₃OH, DCN, HC₃N, CH₃CN, CH₃OCHO, SO, and SiO and compared them with the 1.38 mm dust continuum emission at different spatial scales in the ALMAGAL sample. We included two transitions of H₂CO to understand whether the validity of the results depends on the excitation condition of the selected transition of a molecular species. The ALMAGAL project observed more than 1000 candidate high-mass star-forming clumps in ALMA band 6 at a spatial resolution down to 1000 au. We analyzed a total of 1013 targets that cover all evolutionary stages of the high-mass star formation process and different conditions of clump fragmentation.

Methods. For the first time, we used the method called histogram of oriented gradients (HOG) as implemented in the tool astroHOG on a large statistical sample to compare the morphology of integrated line emission with maps of the 1.38 mm dust continuum emission. For each clump, we defined two masks: the first mask covered the extended more diffuse continuum emission, and the second smaller mask that only contained the compact sources. We selected these two masks to study whether and how the correlation among the selected molecules changes with the spatial scale of the emission, from extended more diffuse gas in the clumps to denser gas in compact fragments (cores). Moreover, we calculated the Spearman correlation coefficient and compared it with our astroHOG results.

Results. Only H₂CO, CH₃OH, and SO of the molecular species we analyzed show emission on spatial scales that are comparable with the diffuse 1.38 mm dust continuum emission. However, according the HOG method, the median correlation of the emission of each of these species with the continuum is only ~24–29%. In comparison with the dusty dense fragments, these molecular species still have low correlation values that are below 45% on average. The weak morphological correlation suggests that these molecular lines likely trace the clump medium or outer layers around dense fragments on average (in some cases, this might be due to optical depth effects) or also trace the inner parts of outflows at this scale. On the other hand DCN, HC₃N, CH₃CN₃ and CH₃OCHO are well correlated with the dense dust fragments at above 60%. The lowest correlation is seen with SiO for the extended continuum emission and for compact sources. Moreover, unlike other outflow tracers, in a large fraction of the sources, SiO does not cover the area of the extended continuum emission well. This and the results of the astroHOG analysis reveal that SiO and SO do not trace the same gas, in contrast to what was previously thought. From the comparison of the results of the HOG method and the Spearman correlation coefficient, the HOG method gives much more reliable results than the intensity-based coefficient when the level of similarity of the emission morphology is estimated.