Issue |
A&A
Volume 699, July 2025
|
|
---|---|---|
Article Number | A273 | |
Number of page(s) | 18 | |
Section | Galactic structure, stellar clusters and populations | |
DOI | https://doi.org/10.1051/0004-6361/202452614 | |
Published online | 16 July 2025 |
The completeness of the open cluster census towards the Galactic anticentre
1
Landessternwarte, Zentrum für Astronomie der Universität Heidelberg,
Königstuhl 12,
69117
Heidelberg,
Germany
2
Max-Planck-Institut für Astronomie,
Königstuhl 17,
69117
Heidelberg,
Germany
3
Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB),
Martí i Franquès, 1,
08028
Barcelona,
Spain
4
Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB),
Martí i Franquès, 1,
08028
Barcelona,
Spain
5
Institut d’Estudis Espacials de Catalunya (IEEC),
Edifici RDIT, Campus UPC,
08860
Castelldefels (Barcelona),
Spain
6
INAF, Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5,
50125
Firenze,
Italy
7
Dipartimento di Fisica e Astronomia, Università di Padova,
Vicolo dell’Osservatorio 3,
35122
Padova,
Italy
8
Leiden Observatory, Leiden University,
Einsteinweg 55,
2333
CC Leiden,
The Netherlands
9
Institute of Astronomy, University of Cambridge,
Madingley Road,
Cambridge
CB3 0HA,
UK
10
School of Physics and Astronomy, Monash University,
Clayton,
VIC 3800,
Australia
11
Centre of Excellence for Astrophysics in Three Dimensions (ASTRO-3D),
Melbourne,
Victoria,
Australia
12
Center for Computational Astrophysics, Flatiron Institute,
162 Fifth Ave,
New York,
NY
10010,
USA
13
INAF, Osservatorio Astrofisico di Torino,
Strada Osservatorio 20,
Pino Torinese
10025,
Torino,
Italy
★ Corresponding authors: emhunt@mpia.de
Received:
15
October
2024
Accepted:
12
June
2025
Context. Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution.
Aims. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions.
Methods. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia’s selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN.
Results. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are 2.97 ± 0.11 times more common at a Galactocentric radius of 13 kpc than in the solar neighbourhood - despite positive detection biases in their favour, such as hotter orbits or a higher scale height.
Conclusions. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.
Key words: methods: data analysis / Galaxy: disk / Galaxy: evolution / open clusters and associations: general
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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