The Extragalactic Distance Scale
Determining the distances to galaxies is of fundamental importance to understanding galaxies, and the Universe in general. There are numerous techniques used to derive the distances to the galaxies -- for an excellent review of many of these please see Jacoby et al 1992. This field is filled with controversy, simply because it is not an easy task to derive galactic distances! The all-important Hubble constant Ho (which is a measure of the expansion of the universe, and thus can be an indicator of the age of the universe) can be derived based on galactic distances and their measured recessional velocities. The difficulty in getting accurate distances thus leads to large uncertainties in the Hubble constant.
I have been involved in research projects using 2 methods of deriving galactic distances -- the TRGB (tip-of-the-red-giant-branch) method, and the PNLF (planetary nebula luminosity function) method. Both are excellent distance indicators, and often lead to distances that are internally precise to better than 10%.
Tip of the Red Giant Branch
The tip-of-the-red-giant-branch (or TRGB) method is a simple technique, and uses the luminosity of thebrightest red-giant branch stars in old stellar populations as a `standard candle'. For old (> 2-3 Gyr), metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the absolute magnitude of these stars in the I band (only) is roughly constant (MI = -4.1+\- 0.1, based on distances to nearby globular clusters). This technique is as internally precise as the Cepheid variables that are often used for distance determinations, and thus provides an excellent check on the distance scale using these stars (as well as other techniques). The TRGB method (which was really used first by W. Baade in 1944 to get distances to Local Group dwarf galaxies) has been successfully used recently to derive distances to galaxies in the Local Group (Lee et al 1993a,b; ApJ 417, 553 + 408, 409) and as far as 11 Mpc from us (Sakai et al. 1997; ApJ 478, 49 ).
Pushing this method even further out to the all-important Virgo cluster would be an excellent independent check on the Cepheid distance scale. We (W. Harris, M. Pierce, J. Secker and myself) have undertaken a project to extend this technique to the Virgo cluster, for which differing distances have been derived using the battery of other distance indicators available. The Virgo cluster is an important piece in the puzzle of the distance scale, for it is the closest large cluster of galaxies (within which essentially all `distance indicators' can be used).
Noting that stellar populations must be old and metal-poor we used a type of galaxy for which these conditions are readily met -- dwarf elliptical (dE) galaxies. As an added bonus, the nucleated dE's (or dE,Ns) are known to lie in the cores of galaxy clusters (Ferguson & Sandage 1989; ApJ 346, L53). Thus by choosing a dE,N we have the best chance of deriving a distance to the Virgo cluster core using this method. We have Cycle 6 HST I images of the dE,N galaxy VCC 1104; a total of 12 images totalling almost 9 hours of data. Part of the resulting co-added image is to the left, and clearly shows the resolution of the galaxy into its constituent stars (which are the brightest red-giant branch stars).
From this image, we have found that the brightest resolved stars are at I=26.8 (limiting magnitudeis I=27.5). From this, we derive (m-M)o= 31.0 (with an uncertainty of 0.2 mag), or a distance of 15.7 Mpc (with a 10% error). This is consistent with results using numerous other techniques (eg. Cepheids, surface brightness fluctuations, planetary nebulae...). The resulting value of Ho=77 +/- 9 km/s/Mpc. Full details of this work has been published in 1998: Nature, vol. 395, pg. 45.
Planetary Nebula Luminosity Function
TO BE ADDED.....UNDER CONSTRUCTION
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