Enlightening the Universe behind the Milky Way bulge

II. Morphological classification and galaxy properties

¹ Departamento de Geofísica y Astronomía, CONICET, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), J5402DCS, Rivadavia, San Juan, Argentina
² Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
³ Instituto de Astronomía Teórica y Experimental (IATE-CONICET), Laprida 854, X5000BGR, Córdoba, Argentina
⁴ Observatorio Astronómico de Córdoba, Universidad Nacional de Córdoba (OAC-UNC), Laprida 854, X5000BGR, Córdoba, Argentina
⁵ Departamento de Astronomía, Universidad de La Serena. Av. Raul Bitrán 1305, La Serena, Chile
⁶ Association of Universities for Research in Astronomy (AURA), Av. Juan Cisternas 1500, La Serena, Chile
⁷ Grupo de Astrofísica Extragaláctica-IANIGLA, CONICET, Universidad Nacional de Cuyo (UNCuyo), Gobierno de Mendoza, Parque Gral San Martín, CC 330, CP 5500 Mendoza, Argentina
⁸ Instituto de Astrofísica, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
⁹ Vatican Observatory, V00120 Vatican City State, Italy
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 0HA, UK

^★ Corresponding author: dgaldeano@unsj-cuim.edu.ar

Received: 25 February 2025
Accepted: 29 April 2025

Abstract

Context. The detection of extragalactic sources beyond the Milky Way is limited by the location of the Solar system. In the zone of avoidance (ZOA), optical observations are hindered by stellar crowding and Galactic absorption. To fully understand the background galaxy population in this region, observations at longer wavelengths are needed.

Aims. The objective of this study is to determine the characteristics of galaxies that are situated behind the Milky Way bulge through the use of near-infrared (NIR) data obtained from the VISTA Variables in Vía Láctea (VVV) survey.

Methods. The galaxy candidates were visually confirmed, and a morphological classification of all sources was performed based on their appearance, by differentiating between elliptical, spiral, undefined, and doubtful galaxies. Additionally, the surrounding area was classified based on the number of sources that are detected in the same target field. Isolated galaxies, galaxies with close companions, and galaxies in denser environments were catalogued.

Results. Following a visual inspection, 14 480 galaxy candidates were classified morphologically: 3702 as elliptical, 1704 as spiral, 6425 as undefined, and 2649 as doubtful. The environmental classification showed that 73.9% were isolated, 12.4% had close companions, and 13.7% were in denser environments. Our catalogue shows slower proper motions than star catalogues. The majority of galaxies that are classified as elliptical and spiral manifest SExtractor parameters that are indicative of this category of sources. Additionally, elliptical galaxies in denser environments appear to be redder than spiral galaxies in isolated environments.

Conclusions. A total of 14 480 galaxy candidates were subjected to a morphological classification in the region of the Milky Way bulge. This result verifies the efficacy of the VVV/VVVX survey in identifying and characterising a diverse range of galaxies and extragalactic structures that are obscured by the Milky Way. The survey thereby contributes to a more profound understanding of the Universe in the ZOA.