Bulgeless Evolution And the Rise of Discs (BEARD)

II. The role of mergers in shaping the Milky Way analogues in TNG50

¹ Donostia International Physics Centre (DIPC) Paseo Manuel de Lardizabal 4 20018 Donostia-San Sebastian, Spain
² Departamento de Física, Universidad de Córdoba, Campus Universitario de Rabanales Ctra. N-IV Km. 396 E-14071 Córdoba, Spain
³ Universidad de La Laguna, Departamento de Astrofísica Avda. Astrofísico Francisco Sánchez s/n E-38200 San Cristóbal de La Laguna, Spain
⁴ Instituto de Astrofísica de Canarias C/ Vía Láctea s/n E-38205 San Cristóbal de La Laguna, Spain
⁵ Isaac Newton Group of Telescopes E-38700 Santa Cruz de La Palma Canary Islands, Spain
⁶ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova Vicolo dell’Osservatorio 3 I-35122 Padova, Italy
⁷ INAF-Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 2 I-35122 Padova, Italy
⁸ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama Copayapu 485 Copiapó, Chile
⁹ Instituto Nacional de Astrofísica, Optica y Electrónica, Tonantzintla 72840 Puebla, Mexico
¹⁰ Centro de Estudios de Física del Cosmos de Aragón (CEFCA) Plaza San Juan 1 44001 Teruel, Spain
¹¹ Centro de Astrobiología, CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
¹² Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid 28040 Madrid, Spain
¹³ Departamento de Astronomía, Universidad de La Serena Av. Raúl Bitrán 1305 La Serena, Chile
¹⁴ Instituto de Física de Partículas y del Cosmos (IPARCOS), Facultad de Ciencias Físicas, Universidad Complutense de Madrid E-28040 Madrid, Spain
¹⁵ Planetarium La Enseñanza, Medellín Antioquia CP. 050022, Colombia
¹⁶ Canada–France–Hawaii Telescope Kamuela HI 96743, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 August 2025
Accepted: 10 December 2025

Abstract

We study the formation and evolution of bulgeless galaxies within the Milky Way-Andromeda analogue sample of the TNG50 simulation. Through kinematic decomposition with MORDOR, we identified bulgeless galaxies with a bulge-to-disc mass ratio of B/D ≤ 0.08, in line with the Bulgeless Evolution And the Rise of Discs (BEARD) survey and Milky Way constraints. We compared the identified bulgeless galaxies to those that are bulge-dominated, which have a bulge-to-disc mass ratio of B/D > 1. We find that 74% of bulgeless galaxies experience at least one major merger (stellar mass ratio 1:4) throughout their lifespan. Bulgeless galaxies form later (z₅₀ ∼ 0.7) compared to bulge-dominated counterparts (z₅₀ ∼ 1.2). Bulgeless galaxies have lower-mass haloes and higher specific stellar angular momentum, which is compatible with Milky Way observations. However, specific star formation rates and hydrogen gas fractions are slightly higher than Milky Way observations. Our analysis of the redshift evolution of stellar components reveals that bulgeless galaxies have gradual disc growth with high thin disc-to-total mass ratios (D/T > 0.5) since z ∼ 1 and minimal bulge growth (B/T < 0.1) since z ∼ 1.5. In contrast, bulge-dominated galaxies have earlier disc formation, which is disrupted, resulting in higher morphology evolution. Bulgeless galaxies are more likely to undergo gas-rich, coplanar, and corotating mergers, promoting disc survival, compared to bulge-dominated galaxies that encounter a broader spectrum of mergers. We also observed differences in galaxy structure between bulgeless and bulge-dominated galaxies without major mergers, suggesting the relevance of early gas accretion and alignment. Bulgeless galaxies have younger stellar populations and more extended star formation histories than bulge-dominated galaxies, which rapidly quench and have older stellar populations. These findings elucidate the distinct merger-driven and secular pathways that give rise to Milky Way galaxies.