Improved distance determination to M 51 from supernovae 2011dh and 2005cs

¹ Department of Optics and Quantum Electronics, University of Szeged, Dóm tér 9, 6720 Szeged, Hungary
e-mail: vinko@physx.u-szeged.hu
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
³ Department of Astronomy, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
⁴ Konkoly Observatory of the Hungarian Academy of Sciences, POB 67, 1525 Budapest, Hungary
⁵ Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA
⁶ Department of Astronomy, Loránd Eötvös University, POB 32, 1518 Budapest, Hungary

Received: 30 October 2011
Accepted: 15 February 2012

Abstract

Aims. The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M 51, provides an opportunity to derive an improved distance to their host galaxy by combining the observations of both SNe.

Methods. We apply the Expanding Photosphere Method to get the distance to M 51 by fitting the data of these two SNe simultaneously. In order to correct for the effect of flux dilution, we use correction factors (ζ) appropriate for standard type II-P SNe atmospheres for 2005cs, but find ζ ~ 1 for the type IIb SN 2011dh, which may be due to the reduced H-content of its ejecta.

Results. The EPM analysis resulted in D_M 51 = 8.4 ± 0.7 Mpc. Based on this improved distance, we also re-analyze the HST observations of the proposed progenitor of SN 2011dh. We confirm that the object detected on the pre-explosion HST-images is unlikely to be a compact stellar cluster. In addition, its derived radius (~277 R_⊙) is too large for being the real (exploded) progenitor of SN 2011dh.

Conclusions. The supernova-based distance, D = 8.4 Mpc, is in good agreement with other recent distance estimates to M 51.