Ultra-cool Subdwarfs in the Subaru HSC survey

Ultra-cool subdwarfs (UCSD) are metal-poor low-mass stars and brown dwarfs with spectral types of M, L, T, and Y. Their old ages and long lives make them effective tracers to study the history of star formation and chemical enrichment in Milky Way. Only a handful of confirmed subdwarf brown dwarfs (L type and later) are known. Their faint optical photometry, required to identify them reliably in surveys, makes it very challenging to detect these important probes of early star formation. We are using the ongoing Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) survey to search for fainter ultracool subdwarfs. HSC-SSP allows us to push the depth of the optical photometry in a wide area matched with UKIDSS and ALLWISE near-infrared photometry. We have identified 12 strong candidate L subdwarfs on the basis of optical & near IR photometry and obvious proper motions. Five of the 12 candidates were observed by our Gemini spectroscopic follow-ups and were all confirmed as sdL or esdL type subdwarfs.