Disc galaxies are still settling

Discovery of the smallest nuclear discs and their young stellar bars

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Muenchen, Germany
e-mail: camila.desafreitas@eso.org
² Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
³ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁴ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Rua do Matão 1226, 05508-090 São Paulo, SP, Brazil
⁵ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, Calle Padre Herrera, 38200 La Laguna, Tenerife, Spain
⁷ Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, L3 5RF Liverpool, UK
⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, Av. Séneca, 28040 Madrid, Spain
¹⁰ Instituto de Física de Partículas y del Cosmos (IPARCOS), Universidad Complutense de Madrid, Av. Séneca, 28040 Madrid, Spain
¹¹ Observatorio Astronómico Nacional, C/Alfonso XII 3, Madrid 28014, Spain
¹² Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 27 May 2023
Accepted: 4 August 2023

Abstract

When galactic discs settle and become massive enough, they are able to form stellar bars. These non-axisymmetric structures induce shocks in the gas, causing it to flow to the centre where nuclear structures, such as nuclear discs and rings, are formed. Previous theoretical and observational studies have hinted at the co-evolution of bars and nuclear discs, suggesting that nuclear discs grow ‘inside-out’ and thereby proposing that smaller discs reside in younger bars. Nevertheless, it remains unclear how the bar and the nuclear structures form and evolve over time. The smallest nuclear discs discovered to date tend to be larger than ∼200 pc, even though some theoretical studies have reported that when nuclear discs form, they can be much smaller. Using MUSE archival data, we report, for the first time, two extragalactic nuclear discs with radius measurements below 100 pc. Additionally, our estimations reveal the youngest bars found to date. We estimate that the bars in these galaxies formed 4.50_−1.10^+1.60(sys)_−0.75^+1.00(stat) and 0.7^+2.60(sys)_−0.05^+0.05(stat) Gyr ago, for NGC 289 and NGC 1566, respectively. This suggests that at least some disc galaxies in the Local Universe may still be dynamically settling. By adding these results to previous findings in the literature, we are able to retrieve a stronger correlation between nuclear disc size and bar length. We also derive a tentative exponential growth scenario for nuclear discs.