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Margaret M. Hanson, PhD Associate Professor of Physics The Andromeda Galaxy Our Galaxy’s Globular Clusters • Reside in Halo • Perhaps 1000s originally formed • 200 exist today • Contain 100,000 solar like stars • Variety of orbits • Ages greater than 10 billion years M10 -- Globular Cluster How do Massive Clusters form? We used to think massive cluster formation to be a thing of the past (the Globular Clusters). First, we began to see evidence of very massive clusters (proto-globular clusters) forming in other galaxies Young Globular Cluster NGC 1850 in the LMC 30 Doradus in the LMC How do Massive Clusters form? Massive clusters can form in relatively recent times. What triggers them to form? Massive clusters are seen to form when enormous dynamical stress or collisions occur within galaxies. Galaxy Collisions The Milky Way is colliding, too! Where would Young Cluster live? Stars form when gas and dust collapse under its own self gravity. Gas and dust reside ONLY in the disk of our Galaxy. Young clusters must reside in the disk of our Galaxy. Studies in our galaxy are not able to see very far in the disk and sample a very small portion of our galaxy. Luminosity of Disk Star Clusters Massive stars are rare. Massive clusters are rare, too. h & chi Per Orion Massive Clusters in our Galaxy Recent events have forced us to re-evaluate the notion of young massive clusters within the Milky Way. 1) HST imaging reveals dense, young, galactic clusters previously unresolved. NGC 3603 Massive Clusters in our Galaxy Recent events have forced us to re-evaluate the notion of young massive clusters within the Milky Way. 2) Infrared imaging discovers new, very massive clusters near the galactic center Massive Clusters in our Galaxy Recent events have forced us to re-evaluate the notion of young massive clusters within the Milky Way. 3) Deeper imaging of known galactic clusters has revealed a more substantial number of stars. Westerland 1 and its massive stars Even nearby we are uncertain! The Cyg OB2 Cluster Distance = 1.7 kpc 60 massive stars identified. Knoedlseder (2000) found the Infrared Cluster to be ‘rounder’ and containing more stars than the Optical Cluster. Contours of stellar counts Why the difference? Dust! CO intensity map The interstellar matter (gas and dust) forces us to observe at longer wavelengths. Are there super massive star clusters in our Galaxy? Westerlund 1: D = 3-5 kpc, Av= 11, >100 O stars. Consistent with being “one of the most massive young clusters in the Local Group”. Cygnus OB2: ~100 O stars, D = 1.7 kpc, Av = 5-10 Infrared Surveys: have found > 50 new galactic star clusters in the inner galaxy. We will need to move ALL observations and analyses to the infrared. Spectra of Hot Stars at 2 microns 45,000 K O3-O4 C IV N III He II 42,000 K O5-O6 40,000 K O7-O8 36,000 K O9-B0 29,000 K B1-B2 He I 2.058 He I 2.11 Br Gamma Observing time granted 8.2m VLT with ISAAC. 2 nights March 2000 2 nights June 2000. 2 nights June 2001 8.2m Subaru with IRCS. 2 nights November 2001 2 nights June 2002. Future Research Goals • Develop quantitative methods for the analysis of massive stars and massive clusters relying on infrared light only. • Continue to search for clusters deep within the Milky Way galaxy using infrared imaging. • Ultimate goal: uncover the massive clusters and the hidden structure of the Milky Way.