[Paper Review] The Optical Gravitational Lensing Experiment. Ages of about 600 Star Clusters from the LMC

G. Pietrzyński, A. Udalski|arXiv (Cornell University)|Oct 18, 2000

Astronomy and Astrophysical Research3 citations

TL;DR

This study presents age determinations for approximately 600 star clusters in the Large Magellanic Cloud (LMC) using isochrone fitting on deep photometric data from the OGLE-II survey. It reveals a bursty cluster formation history with three distinct peaks at ~7 Myr, 125 Myr, and 800 Myr, indicating episodic star formation, while the SMC shows a more uniform age distribution, suggesting external triggers like Magellanic Cloud interactions may drive these bursts.

ABSTRACT

We present results of age determination based on the standard procedure of isochrone fitting for about 600 star clusters younger than about 1.2 Gyr from the central parts of the LMC. Comparison of age distributions of star clusters from the LMC and the SMC shows that cluster formation histories are different in these galaxies. The age distribution of the LMC clusters reveals bursty nature of cluster formation in this galaxy, with contrast to the relatively uniform distribution of cluster ages in the SMC. We detected three extended peaks in the distribution of ages of LMC clusters, located at about 7 Myr, 125 Myr and 0.8 Gyr. All detected peaks have complex structure. While the structure of the youngest and the oldest peaks may be spurious due to accuracy of age determination, in the middle peak two evident sub-peaks at 100 Myr and 160 Myr are clearly seen. Similar peaks are seen in the distribution of ages of clusters from the SMC, which indicates that increased cluster formation ratio during these periods might be caused by the last encounter between the Magellanic Clouds. The sample of clusters from the LMC is overabundant in very young (<10 Myr) and old (>400 Myr) clusters, and underabundant in clusters having ages in the range from 80 to 15 Myr with respect to the studied sample of clusters from the SMC.

Motivation & Objective

• To determine precise ages for a large sample of star clusters in the Large Magellanic Cloud (LMC) using photometric data.
• To compare the cluster formation histories of the LMC and the Small Magellanic Cloud (SMC) to identify differences in star formation activity.
• To investigate whether episodic star formation in the LMC is linked to dynamical interactions between the Magellanic Clouds.
• To assess the reliability and accuracy of age determination for young and old clusters using isochrone fitting in crowded fields.

Proposed method

• Utilized deep BVI photometry from the OGLE-II survey with a 1.3 m Warsaw telescope at Las Campanas Observatory.
• Applied statistical subtraction of field stars to reduce contamination in cluster color-magnitude diagrams (CMDs).
• Performed isochrone fitting using evolutionary tracks with Z = 0.008 and a distance modulus of 18.24 mag.
• Used multiple isochrones to estimate age uncertainties as half the age difference between marginally fitting models.
• Applied dereddening corrections based on interstellar extinction maps from Udalski et al. (1999).
• Focused on clusters younger than 1.2 Gyr, where the main sequence turnoff is detectable within the photometric limit (V ≈ 21.5).

Experimental results

Research questions

• RQ1 What is the detailed age distribution of star clusters in the Large Magellanic Cloud?
• RQ2 How does the cluster formation history in the LMC compare to that of the Small Magellanic Cloud?
• RQ3 Are the observed peaks in the age distribution of LMC clusters real or spurious due to photometric uncertainty?
• RQ4 Could the episodic star formation in the LMC be triggered by the interaction between the Magellanic Clouds?

Key findings

• The age distribution of LMC clusters shows three prominent peaks at approximately 7 Myr, 125 Myr, and 800 Myr, indicating bursty star formation.
• The middle peak at 125 Myr contains two distinct sub-peaks at 100 Myr and 160 Myr, which are statistically significant and likely real.
• The youngest and oldest peaks may be affected by photometric uncertainty, but the middle peak is robust and well-constrained.
• The LMC exhibits an overabundance of very young (<10 Myr) and old (>400 Myr) clusters, and an underabundance of clusters between 80 and 150 Myr, relative to the SMC.
• Similar age peaks in the SMC suggest that the 100 Myr and 160 Myr bursts may be linked to the last Magellanic Cloud encounter.
• The cluster formation history in the LMC is significantly more bursty than in the SMC, indicating different evolutionary pathways.