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the two most frequently applied early - type galaxy distance indicators are the fundamental plane ( fp , and the related @xmath0-@xmath1 ) and surface brightness fluctuations ( sbf ) methods . in a recent study ( blakeslee  2001,2002 )
, we used @xmath2- and @xmath3-band data from the ground - based sbf survey ( tonry  2001 ) to calculate fp photometric parameters for 170 galaxies with velocity dispersions available in the homogenized smac catalogue ( hudson  2001 ) . to our knowledge , this is the largest galaxy - by - galaxy comparison of different standard candle / rod distance methods to date .
fig.1a shows the comparison .
overall the distance agreement was good , but several low - luminosity , s0 galaxies had systematically low fp distances , probably due in part to younger ages and lower mass - to - light ratios , although aperture effects may also contribute .
the sbf distances are tied to the cepheids via measurements in spiral bulges , while the fp distances are tied to the hubble flow via distant clusters ; the hubble constant that results from this comparison is @xmath4 .
however , we also derived independent distances for these galaxies based on their velocities and the gravity field inferred from the redshift - space galaxy density ; the resulting comparison with sbf yields @xmath5 ( fig.1b ) , formally discordant at the 2@xmath6 level with the fp - sbf result , but within the range of the systematic uncertainties in the various ties .
another interesting facet of this work relates to the `` fluctuation number '' @xmath7 , which measures the galaxy luminosity in units of the weighted mean stellar luminosity .
correlates tightly with stellar velocity dispersion ; it also correlates with galaxy color and is independent of galactic extinction .
interestingly , sbf distances calibrated using the properties of , such as those shown in fig.1b , can be viewed as a hybrid of sbf and fp distances , and may be more accurate than those calibrated from galaxy color alone .
we plan to investigate these issues in more detail .
cepheids occur only in spirals and other late - type , star - forming galaxies . however , the most massive virialized structures in the nearby universe ( e.g. , the virgo , fornax , and centaurus clusters ) , are overwhelmingly dominated by early - type galaxies . although some spirals appear in projection against the virgo core ,
the various secondary indicators tied to the cepheid scale indicate that these galaxies are not at the same distance as the core ellipticals ( e.g. , tonry  2000 ; ferrarese  2000 ; kelson  2000 ; blakeslee  2002 ) .
alternatively , it may be that the secondary indicators are yielding systematically different results for the calibrating spirals and the target ellipticals .
we have an ongoing cycle  10 wfpc2 program to calibrate the early - type galaxy distance scale via cepheid distances to late - type galaxies that are physically associated with ellipticals .
the target galaxies are the ngc4647/ngc4649 pair ( fig.2a ) and ngc5128 ( cena ) , an elliptical with a central dust lane and associated star formation , apparently resulting from the incursion of a gas - rich dwarf .
the cena cepheid observations have yielded more than 60 superb cepheids , making this one of largest high - quality hst cepheid data sets . at present
, we are still finalizing the analysis , but fig.2b shows some example light curves ; differential extinction within cena is a major issue for this program .
sbf and type ia supernovae ( sneia ) studies have in the past disagreed on @xmath8 at the @xmath920% level , which is surprising for two methods that routinely achieve 510% internal accuracy .
the excellent resolution of hst provides an enormous advantage over ground - based data for sbf studies , and we have recently used wfpc2 to measure high - quality hst / wfpc2 sbf distances to seven early - type galaxies that have hosted well - observed sneia ( ajhar  2001 ) .
the results showed excellent agreement in the relative distances , but an offset of @xmath90.25 mag in zero points , which we traced to the different , and indeed dissonant , compilations of cepheid distances used in the past for the respective zero - point calibrations of the two methods .
when calibrated consistently , sbf and sneia also agree in an absolute sense ( fig.3a ) and give @xmath10 .
this is the first time the agreement has been demonstrated through a direct comparison of statistically significant samples of sbf and sneia galaxy distances .
the most pressing problem in the measurement of extragalactic distances appears to be systematic uncertainties in the zero points .
we have seen that there is significant uncertainty in the zero - point tie of the early - type galaxy distance scale to cepheids , but perhaps even greater is the uncertainty in the cepheid zero point itself , in part due to the poorly - constrained lmc distance .
stellar population models can be used to predict sbf magnitudes and colors for a large range of metallicities and ages ( e.g. , blakeslee , vazdekis , & ajhar 2001b ) .
these models reproduce the observed sbf colors and behaviors very well , but predict an sbf zero point fainter than the cepheid - calibrated one by @xmath11mag in @xmath3 ( the only band in which sbf is directly tied to the cepheids via spiral bulges ) .
however , the model and empirical zero points would come into close agreement if the cepheid scale were revised to agree with the dynamical distance to the ngc4258 water maser ( herrnstein  1999 ) , for example , by changing the assumed lmc distance modulus from 18.5 to 18.3 mag .
further refinements of the models should provide more stringent tests of the distance scale and guide future sbf programs ( fig.3b ) .
i summarize recent work comparing relative distances measured to individual galaxies with independent methods . 
 the comparisons include : ground - based surface brightness fluctuation ( sbf ) and fundamental plane distances to 170 galaxies , distances predicted from galaxy velocities and the inferred gravity field , hst sbf measurements to seven early - type hosts of type ia supernovae , and ties of the cepheid distance scale to early - type galaxies . 
 independent calibrations for some methods provide interesting constraints on the cepheid zero point .    
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