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because of their fairly simple geometrical and dynamical structure disk galaxies are the ideal benchmarks to test our theories of galaxy formation , gravity , and cosmology .
one notable characteristic of disk galaxies is the flatness of their rotation curves , which implies either that ( i ) disks are embedded in a massive dark matter halo , or ( ii ) our theory of gravity is incorrect .
one theory build around the latter option is modified newtonian dynamics ( mond ) , originally proposed by milgrom ( 1983a ) . in mond a characteristic acceleration @xmath0 is assumed , such that for accelerations @xmath1 the true acceleration @xmath2 , with @xmath3 the standard newtonian acceleration .
this implies flat rotation curves , with an asymptotic value @xmath4 , without the need for dark matter .    here
we examine how mond fairs in comparison to cdm when it comes to explaining global properties of disk galaxies in the context of a model for disk galaxy formation .
a more detailed treatment of the models and results presented here can be found in van den bosch ( 1998 ; 2000 ) and van den bosch & dalcanton ( 2000 ) . here
we merely summarize some of our results .
the zero - point , scatter , and slope of the tfr of disk galaxies ( @xmath5 ) all strongly depend on the photometric band in which the tfr is defined . in the near - infrared , where one is least sensitive to dust - extinction and to uncertainties regarding the ages and metallicities of stellar populations
, one finds @xmath6 ( i.e. , verheijen 1997 ) .    in the cdm scenario ,
one predicts a tfr of the form : @xmath7 ( see van den bosch 2000 and references therein ) with @xmath8 the circular velocity at the virial radius , @xmath9 the mass - to - light ratio of the disk , and @xmath10 a parameter that describes what fraction of the baryons inside the virial radius resides in the disk ( as either stars or cold gas ) .
thus , @xmath11 is related to the efficiencies of feedback and cooling .    in the case of mond , direct application of the modified acceleration results in a tfr of the form @xmath12 ( see milgrom 1983b ) .
thus , cdm and mond make distinct predictions regarding the slope @xmath13 of the tfr .
furthermore , the prediction for mond is in better agreement with the near - infrared tfr . for cdm to yield a tfr as steep
as observed the process of galaxy formation has to be such that @xmath14 .
advocates of mond have used this to argue against cdm , as they have claimed that this requires extremely difficult fine - tuning ( i.e , , mcgaugh & de blok 1998a , b ) .    in the upper panels of figure  1
we plot the @xmath15-band tfr for the data as well as for three of our models : the @xmath16cdm model , and two mond models ( @xmath17 and @xmath18 ) . as can be seen
, the @xmath16cdm tfr is as steep as the data ( @xmath19 ) , which we accomplished by adjusting the feedback efficiency .
thus , indeed some amount of tuning is required in a cdm universe .
the mond @xmath17 model , which has zero feedback , also reveals a tfr in excellent agreement with the data .
the mond @xmath18 model , however , in which feedback is included , yields a tfr that is too steep and with too much scatter at the low - mass end .
the reason for the inclusion of the feedback in this latter model becomes apparent below .
the panels in the second row of figure  1 plot the gas mass fractions ( @xmath20 ) as function of the absolute magnitude .
the data reveals a systematic decrease of @xmath21 with increasing magnitude , which is remarkably well reproduced by the models and owes in large part to the stability related threshold densities for star formation used in our models .
panels in the third row plot the central surface brightness of the disk as function of @xmath22 ( the amplitude of the flat part of the rotation curve ) .
the data indicates an absence of high surface brightness ( hsb ) galaxies at the low mass end ( indicated by the thin solid line ) .
this is nicely reproduced by the @xmath16cdm model , and owes to the particular feedback model used . in the mond @xmath17 model , however , no such deficit of low - mass hsb disks is present , in clear contradiction with the data . in model @xmath18 we included feedback , for which we tuned the parameters to reproduce the data .
note , however , that this model yields a tfr that is too steep .
finally , in the lower panels of figure  1 we plot the enclosed mass - to - light ratio @xmath23 as function of the local acceleration @xmath24 . here
@xmath25 is the ` visible ' mass of the disk ( stars and cold gas ) , and @xmath26 is the galactocentric radius .
each data point represents one resolved measurement in the rotation curve ( rc ) of a disk galaxy . as first pointed out by mcgaugh ( 1998 ) , the observed rcs of galaxies reveal a characteristic acceleration .
this is evident from the fact that the data in the lower - left panel reveals very little scatter .
the presence of such a characteristic acceleration is exactly the ` ansatz ' of mond , and , not surprisingly , the mond models @xmath17 and @xmath18 nicely reproduce the data .
however , the @xmath16cdm model is also in good agreement with the data .
therefore , the appearance of a characteristic acceleration is not a unique prediction of mond .
the @xmath16cdm model requires tuning of the feedback parameters to yield a tfr as steep as observed .
however , once this is achieved the model reproduces the gas mass fractions , the absence of low mass hsb galaxies , and reveals a characteristic acceleration as observed .
all the particular problems for cdm pointed out by mcgaugh & de blok ( 1998a ) are in fact solved by this one simple tuning .
the mond models , however , are unable to simultaneously reproduce the tfr and the absence of low - mass hsb disks .
although we do not consider this a strong proof against mond ( as our particular assumptions underlying the model for galaxy formation might not be valid under mond ) , we do want to emphasize that our results largely remove the claimed advantages of mond over cdm .
we discuss a comparison of models for the formation of disk galaxies both in a universe dominated by cold dark matter ( cdm ) and one in which the force law is given by modified newtonian dynamics ( mond ) . 
 our main aim is to address the claim made by mcgaugh & de blok that cdm suffers from severe fine - tuning problems , which are circumvented under mond . 
 as we show , cdm indeed requires some amount of tuning of the feedback efficiencies to obtain a tully - fisher relation ( tfr ) as steep as observed . 
 however , that same model is in excellent agreement with a wide variety of additional observations . 
 therefore , the modest amount of feedback needed should not be regarded a fine - tuning problem . instead , its requirement should be considered a generic prediction for cdm , which might be tested with future observations and with detailed modeling of feedback processes . 
 we also show that galaxy formation in a mond universe can not simultaneously reproduce the tfr and the lack of high surface brightness dwarf galaxies . 
 we thus conclude that cdm is a more viable theory for the formation of disk galaxies than mond .    
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