AAS 201st Meeting, January, 2003
Session 23. Cosmology: The Fundamental Parameter
Oral, Monday, January 6, 2003, 10:00-11:30am, 608-609

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[23.03] The Cepheid Distance to NGC 1637: A Direct Comparison with the EPM Distance to SN 1999em

D. C. Leonard, S. M. Kanbur, C. C. Ngeow (FCAD, UMass Amherst), N. R. Tanvir (U. Hertfordshire)

Opportunities to directly compare distances estimated by two primary extragalactic distance indicators are rare. While Cepheid variables remain the most widely used and well studied extragalactic primary distance indicators, the Expanding Photosphere Method (EPM) of supernova (SN) distance determination is a powerful primary extragalactic distance indicator as well, providing distance estimates completely independent of the Cepheid scale. However, there have never been any measurements of Cepheids in a galaxy that has hosted a well-observed, spectroscopically and photometrically normal, type II-Plateau (II-P) supernova, the classic variety of core-collapse event to which EPM-derived distances are most robust.

The appearance of SN 1999em, a bright, extremely well-observed SN II-P in the nearby SBc galaxy NGC 1637 (v = 717 km s-1), offers an outstanding opportunity to test the consistency between these two techniques. Two independently-derived EPM distances exist: d = 7.5 ±0.5 Mpc (Hamuy et al. 2001, ApJ, 558, 615), and d = 8.2 ± 0.6 Mpc (Leonard et al. 2002, PASP, 114, 35). These EPM distances to SN 1999em agree with distances previously derived to NGC 1637 through analysis of the galaxy's brightest red supergiants (d = 7.8 Mpc; Sohn & Davidge 1998, AJ, 115, 130).

Using Hubble Space Telescope Cycle 10 observations, we present the discovery of several dozen Cepheid variables in NGC 1637. We derive the distance to NGC 1637 through the analysis of its Cepheid variable stars, and find that a significant discrepancy exists between the Cepheid-based distance and that derived through the EPM. Implications of this result on the extragalactic distance scale are discussed.

Support for this work was provided by NASA through grant number GO-9155 from the Space Telescope Science Institute.

The author(s) of this abstract have provided an email address for comments about the abstract: leonard@nova.astro.umass.edu

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