AAS 205th Meeting, 9-13 January 2005
Session 73 Distances, the CMB and the Sunyaev-Zeldovich Effect
Oral, Tuesday, January 11, 2005, 10:00-11:30am, Golden Ballroom

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[73.02] HST/ACS Observations of Cepheid Variables in NGC 4258

L.M. Macri (NOAO), D. Bersier (STScI), L.J. Greenhill, M.J. Reid, K.Z. Stanek (CfA)

We present the discovery of Cepheid variables in the spiral galaxy NGC 4258, a key galaxy for the absolute calbration of the Extragalactic Distance Scale.

The data was obtained using the Advanced Camera for Surveys onboard the Hubble Space Telescope. We observed two fields using the B, V and I filters on twelve epochs over a period of 46 days. We used the DAOPHOT/ALLFRAME software package to perform PSF-fitting photometry and detected about 2x105, 4x105 and 7x105 stars in B, V and I, respectively, with completeness limits of B~27 mag, V~26.5 mag and I~26 mag. We discovered over 300 Cepheids with periods ranging from 3 to 48 days, a significant improvement over the previous sample of 15 Cepheids discovered with HST/WFPC2.

We present color-magnitude diagrams for the complete stellar sample and Period-Luminosity relations for the Cepheid variables in both fields, and discuss the distance estimated from them. This forms one of three presentations that describe distance estimates to NGC 4258 and ramifications for the extragalactic distance scale. This research has been funded in part by NASA grants HST-GO-09810.01-A and HST-HF-01153.01-A.

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