Deriving star cluster parameters with convolutional neural networks

II. Extinction and cluster-background classification

¹ Vilnius University Observatory, Saulėtekio av. 3, 10257 Vilnius, Lithuania
e-mail: jonas.bialopetravicius@ff.vu.lt, jonas.bialopetravicius@gmail.com
² Center for Physical Sciences and Technology, Saulėtekio av. 3, 10257 Vilnius, Lithuania

Received: 26 June 2019
Accepted: 19 November 2019

Abstract

Context. Convolutional neural networks (CNNs) have been established as the go-to method for fast object detection and classification of natural images. This opens the door for astrophysical parameter inference on the exponentially increasing amount of sky survey data. Until now, star cluster analysis was based on integral or resolved stellar photometry, which limit the amount of information that can be extracted from individual pixels of cluster images.

Aims. We aim to create a CNN capable of inferring star cluster evolutionary, structural, and environmental parameters from multiband images and to demonstrate its capabilities in discriminating genuine clusters from galactic stellar backgrounds.

Methods. A CNN based on the deep residual network (ResNet) architecture was created and trained to infer cluster ages, masses, sizes, and extinctions with respect to the degeneracies between them. Mock clusters placed on M 83 Hubble Space Telescope images utilizing three photometric passbands (F336W, F438W, and F814W) were used. The CNN is also capable of predicting the likelihood of the presence of a cluster in an image and quantifying its visibility (S/N).

Results. The CNN was tested on mock images of artificial clusters and has demonstrated reliable inference results for clusters of ages ≲100 Myr, extinctions A_V between 0 and 3 mag, masses between 3 × 10³ and 3 × 10⁵ M_⊙, and sizes between 0.04 and 0.4 arcsec at the distance of the M 83 galaxy. Real M 83 galaxy cluster parameter inference tests were performed with objects taken from previous studies and have demonstrated consistent results.