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Session 8 - Distance Indicators.
Display session, Monday, January 15
North Banquet Hall, Convention Center
A weakness of current extragalactic distance scale is the small amount of cross-checks between some of the techniques. In fact, there is very little overlap between the Population II scale, defined by the planetary nebula luminosity function (PNLF), the surface brightness fluctuation method, the globular cluster luminosity function, and the elliptical galaxy fundamental plane method, and the Population I scale, formed from Cepheid variables, supernovae, and the Tully-Fisher relation. Additional links between the two systems are theoretically possible using the PNLF technique in late-type systems, but such observations are difficult, due to the possible confusion of planetary nebulae with H II regions.
Here, we present the results of an [O III] \lambda 5007 planetary nebula survey of three large spiral galaxies: M51 (Hubble-type Sc, face-on), M96 (Sab, with intermediate inclination), and M101 (Sc, face-on). By restricting our survey to the inter-arm and outer regions of each galaxy, and using H\alpha\ and deep continuum images to exclude low-excitation sources and objects with bright, exciting stars, we create PNLFs that are homogeneous and uncontaminated by H II regions. We then compare these luminosity functions to the empirical function derived from observations in M31, and compute the distance to each galaxy. Our distances are in excellent agreement with those found using other methods; this strongly suggests that the PNLF cutoff is insensitive to population age.
With the addition of M101 and M96, there are now 7 galaxies that have both PNLF and Cepheid distance measurements. We use these data, and PNLF-Cepheid comparisons in the Virgo Cluster and Triangulum Group to test for systematic differences between the two methods. Our data indicate that the Population I and Population II distance scales are in close agreement, and measurements using the techniques on either distance ladder are reliable.
Program listing for Monday