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2.4 Independent local distance-scale methods

2.4.1 Masers

One exception to the rule that Cepheids are necessary for tying local and more global distance determinations is provided by the study of masers, the prototype of which is the maser system in the galaxy NGC 4258 [23]. This galaxy has a shell of masers which are oriented almost edge-on [9650] and apparently in Keplerian rotation. As well as allowing a measurement of the mass of the central black hole, the velocity drift (acceleration) of the maser lines from individual maser features can also be measured. This allows a measurement of absolute size of the maser shell, and hence the distance to the galaxy. This has become steadily more accurate since the original work [54663]. Macri et al. [92Jump To The Next Citation Point] also measure Cepheids in this object to determine a Cepheid distance (see Figure 3View Image) and obtain consistency with the maser distance provided that the LMC distance, to which the Cepheid scale is calibrated, is 48 ± 2 kpc.

View Image

Figure 3: Positions of Cepheid variables in HST/ACS observations of the galaxy NGC 4258, reproduced from [92Jump To The Next Citation Point] (upper panel). Typical Cepheid lightcurves are shown in the lower panel.

Further discoveries and observations of masers could in principle establish a distance ladder without heavy reliance on Cepheids. The Water Maser Cosmology Project
(External Linkhttp://www.cfa.harvard.edu/wmcp/index.html) is conducting monitoring and high-resolution imaging of samples of extragalactic masers, with the eventual aim of a maser distance scale accurate to ∼ 3%.

2.4.2 Other methods of establishing the distance scale

Several other different methods have been proposed to bypass some of the early rungs of the distance scale and provide direct measurements of distance to relatively nearby galaxies. Many of these are reviewed in the article by Olling [105Jump To The Next Citation Point].

One of the most promising methods is the use of detached eclipsing binary stars to determine distances directly [107]. In nearby binary stars, where the components can be resolved, the determination of the angular separation, period and radial velocity amplitude immediately yields a distance estimate. In more distant eclipsing binaries in other galaxies, the angular separation cannot be measured directly. However, the light-curve shapes provide information about the orbital period, the ratio of the radius of each star to the orbital separation, and the ratio of the stars’ luminosities. Radial velocity curves can then be used to derive the stellar radii directly. If we can obtain a physical handle on the stellar surface brightness (e.g. by study of the spectral lines) then this, together with knowledge of the stellar radius and of the observed flux received from each star, gives a determination of distance. The DIRECT project [16] has used this method to derive a distance of 964 ± 54 kpc to M33, which is higher than standard distances of 800 – 850 kpc [4687]. It will be interesting to see whether this discrepancy continues after further investigation.

A somewhat related method, but involving rotations of stars around the centre of a distant galaxy, is the method of rotational parallax [117106105Jump To The Next Citation Point]. Here one observes both the proper motion corresponding to circular rotation, and the radial velocity, of stars within the galaxy. Accurate measurement of the proper motion is difficult and will require observations from future space missions.

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