GULP II: Hierarchical Distribution and Evolution of Young Stellar Structures in NGC 4449

TL;DR

This study analyzes the hierarchical distribution and evolution of young stars in NGC 4449, revealing regional differences in structure expansion and a gradual decline in clustering strength over 100 million years.

Contribution

It introduces a novel analysis of young stellar structures' hierarchical evolution in NGC 4449 using HDBSCAN clustering and TPCF, highlighting regional differences and internal dynamics effects.

Findings

Central region shows expansion of stellar structures within 60 Myr

Young stars exhibit strong hierarchical clustering that weakens over time

External perturbations may influence structural evolution in certain regions

Abstract

We investigate the hierarchical distribution and evolution of young stellar structures in the dwarf starburst galaxy NGC 4449 using data from the GULP survey. By analyzing the spatial distribution of field stars younger than 100 Myr, we identify large-scale stellar complexes and substructures using HDBSCAN -- a density-based clustering algorithm -- and trace their evolution over time. While comparing these stellar structures in different regions of the galaxy, we find that the central bar-like region shows a clear expansion of the structures within the first $\sim$ 60 Myrs, while the arm-like structure in the NE shows no discernible trend, possibly due to external perturbations from tidal interactions with a neighboring galaxy. An age-dependent two point correlation function (TPCF) analysis shows that young stars exhibit a strong hierarchical distribution, with clustering strength decreasing over time. The power-law slope of the TPCF, which starts at $\alpha \sim 0.65$ for stars younger than 5 Myr, shows a slight decline to $\alpha \sim 0.4$ for stars older than 50 Myr, though it does not reach a completely flat (random) distribution. This trend indicates a subtle weakening of structural hierarchy among young ($<$100 Myr) stars, which is primarily driven by internal stellar motions. Future work will extend this analysis to the remaining 26 galaxies in the GULP survey to better constrain the role of the galactic environment in shaping the hierarchical evolution of young stellar populations.