document: the measurement of the rotation curves ( rcs ) of disk galaxies is a powerful tool to investigate the nature of dark matter ( dm ) , including its content relative to the baryonic components and their distributions . in particular , dwarf galaxies are good candidates to reach this aim as their kinematics are generally dominated by the dark component , down to small galactocentric radii @xcite . 
   this leads to a reliable measurement of the dynamical contribution of the dm to the rc and hence of its density profile . 
   therefore , a dwarf galaxy like the orion dwarf provides us with an important test as to whether dm density profiles arising in @xmath3 cold dark matter ( @xmath3cdm ) numerical simulations @xcite are compatible with those detected in actual dm halos around galaxies . 
   let us comment that nfw profile arises from pure n - body dm simulations . 
   it is well known that , as effect of the baryonic infall in the cosmological dm halos and of the subsequest process of stellar disk formation , shallower profiles of the dm halo may arise ( see @xcite ) . 
   recent studies of the rcs of dwarf galaxies have tested the nfw scenario . 
   it is now clear that kinematic data are better fitted by a dm halo with a constant density core ( e.g. @xcite ) , than by one that is centrally peaked . 
   one specific example is ddo 47 , whose velocity field is clearly best fitted if the dm halo is cored ; moreover , its ( small ) detected non - circular motions can not account for the discrepancy between data and the nfw predictions @xcite . 
   + the present investigation examines the dm content of the orion dwarf galaxy . 
   this nearby system harbors an extended  disk , and thus provides us with an important test of the above paradigm . 
   as we show below , the orion dwarf is one of the few known galaxies whose kinematics _ unambiguously _ point towards a cored profile . 
   this system is thus critically important for investigating the nature of the dm particle and of the evolution of dm halos . 
   + mond accounts for the evidence that rcs of spiral galaxies are inconsistent with the corresponding distribution of the luminous matter @xcite . rather than postulating the existence of a dark halo made by massive collisionless elementary particles 
   , this scenario advocates that the gravitational force at low accelerations leaves the standard newtonian regime to enter a very different one . 
   historically mond has generally been successful in reproducing the rcs of spiral galaxies with only the ( observed ) luminous matter ( e.g. @xcite ) . 
   however , cases of tension between data and the mond formalism do exist @xcite . + it is important to stress that in order to derive the dm density profile or to test the mond formalism , we must know the distribution of the ordinary baryonic components , as well as have reliable measurements of the gas kinematics . for the orion dwarf , 21-cm  surface brightness and kinematics 
   have recently been published @xcite : their analysis provides a high quality , high resolution rc , that , in addition , can be easily corrected for asymmetric drift and tested for non - circular motions . 
   this galaxy is a very useful laboratory in that a simple inspection of the rc ensures us that it shows a large mass discrepancy at all radii . 
   moreover , the baryonic components are efficiently modeled ( i.e. , no stellar bulge is evident and the stellar disk shows a well - behaved exponential profile , see @xcite ) . 
   the distance to the galaxy , which is critical for an unambiguous test of mond @xcite , is estimated to be 5.4@xmath41.0 mpc @xcite . 
   it is important to stress that the distance of the orion dwarf remains a significant source of uncertainty . 
   @xcite estimate the distance using the brightest stars method . 
   the intrinsic uncertainty in this technique may allow a distance ambiguity much larger than the formal errors estimated by @xcite , because in their work this method yields a scatter as large as @xmath5 in distance . finally , the system s inclination ( 47@xmath1 ) is kinematically measured ( see section ( [ 3.1 ] ) ) and is high enough to not affect the estimate of the circular velocity . the properties described above 
   make the orion dwarf galaxy an attractive candidate to determine the underlying gravitational potential of the galaxy . 
   this paper is organized as follows . in sec . 
   2 we present the stellar surface photometry . in sec . 
   3 , the  surface density and kinematics data are presented and discussed ; we also provide the analysis of possible non - circular motions of the neutral gas . in sec . 
   4 we model the rc in the stellar disk using a cored / cusped halo framework . in sec . 
   5 we test the orion kinematics against the mond formalism . 
   our conclusions are given in sec . 
   following the discussion in @xcite , the underlying stellar mass in the orion dwarf is estimated using the near - infrared ( ir ) photometry ( j and  bands ) presented by @xcite . 
   those authors find ( j@xmath6 ) @xmath0 @xmath20.80 and a total  magnitude of @xmath210.90 . 
   when comparing to models ( see below ) we assume that the color difference between k and  is negligible ; further , we assume l@xmath73.33 @xcite . accounting for extinction , 
   the total k - band luminosity of the orion dwarf is @xmath83.5@xmath910@xmath10 l@xmath11 . 
   the mass of the stellar component was estimated by @xcite to be ( 3.7@xmath41.5)@xmath910@xmath12 . 
   the stellar surface brightness profile is well fitted by an exponential thin disk , with a scale length of @xmath13= 25  @xmath4 1  ( equivalent to 1.33 @xmath4 0.05 kpc at the adopted distance ) 
   . moreover , there are no departures from an exponential profile that would be indicative of a prominent central bulge . 
   spectral line imaging was acquired with the _ very large array _ and presented in @xcite . 
   we refer the reader to that work for a full discussion of the data handling , and we summarize salient details here . 
   the final data cubes have a circular beam size of 20 , with a 3@xmath14  column density sensitivity of n@xmath15= 1.5 @xmath910@xmath16@xmath17 . 
   the first three moment maps ( i.e. the integrated  intensity , the velocity field , and the velocity dispersion ) are shown in figure ( [ figcap3 ] ) . 
   + the neutral gas disk of the orion dwarf shows rich morphological and kinematic structure at this physical resolution . 
   the outer disk contains tenuous  gas , but column densities rise above the 5@xmath910@xmath18 @xmath17 level at intermediate radii . 
   there is plentiful high - column density ( @xmath1910@xmath20 @xmath17 )  throughout the disk . 
   the more or less parallel iso - velocity contours at inner radii are indicative of linear rotation ( although almost certainly not solid body ) and the curving of the outer contours suggests that the outer rotation curve has a fairly constant velocity . 
   the outer disk contours show no evidence for a decrease in rotational velocity at large radii . in the central regions of the disk , however , some  `` holes '' or `` depressions '' manifest a pronounced kink in these contours ( consider the contours at 370@xmath420 ) . 
   the intensity weighted velocity dispersion averages to @xmath87 - 8  throughout the disk , although the innermost regions show dispersions above 10 . 
   + the total  flux integral , proportional to the  disk mass , was found to be 50.3@xmath45.1 jykm s@xmath21 , a value somewhat lower than the single - dish flux measure of 80.6@xmath47.72 jykm s@xmath21 by @xcite ; the difference may arise from the lack of short interferometric spacings that provide sensitivity to diffuse structure . 
   the total mass is found to be m@xmath15 @xmath0 ( 3.5@xmath40.5)@xmath910@xmath22 . after applying the usual 35% correction for helium and molecular material , 
   we adopt m@xmath23 @xmath0 ( 4.7@xmath40.7)@xmath910@xmath22 as the total gas mass . 
   + in figure ( [ reswri ] ) we plot the 10  /20  resolution  surface density , throughout the gas disk . 
   a simple fit ( valid out to the last measured point and for the scope of this work ) yields : _ 
   m_/pc^2 , where @xmath24 is in kpc . 
   the related fitting uncertainty on @xmath25 is about @xmath26 . 
   figure ( [ reswri ] ) shows that the  surface density rises from the center of the galaxy , reaches a maximum , and then declines exponentially . at the last measured point , 
   i.e. out to @xmath8 7 kpc , the profile has almost ( though not completely ) reached the edge of the  disk and rapidly converges to zero . 
   note that , in newtonian gravity , the outer gaseous disk contributes in a negligible way to the galaxy total gravitational potential . 
   the channel maps of the orion dwarf provide evidence of well - ordered rotation throughout the  disk ( see cannon et al . 
   the intensity - weighted - mean velocity field ( figure ( [ figcap3]b ) ) exhibits symmetric structure in the outer disk . 
   twisted iso - velocity contours at inner radii coincide with the  holes near the centre of the disk . the disk is therefore dominated by circular motion . 
   the rc of the galaxy was derived by fitting a tilted ring model to the intensity - weighted - mean velocity field using the gipsy task rotcur . 
   the routine carries out a least - squares fit to @xmath27 , the line of sight - velocity . to derive the best - fitting model , an iterative approach was adopted in which the various combinations of the parameters were fitted . 
   the final rc was extract by fixing all other parameters . 
   the receding and approaching sides of the galaxy were fitted separately . 
   the best fitting parameters are @xmath28 , @xmath29 , @xmath30  km / s , and ( @xmath31,@xmath32 ) = ( 05:45:01.66 , 05:03:55.2 ) for the dynamical centre . 
   we have realized that the inclination is not dependent on the radius , and the fit is shown in figure ( [ inclination ] ) . 
   its weighted value is @xmath33 . 
   notice that because the errors reported by gipsy / rotcur include only errors on the fits and systematics are not included , the @xmath34 error estimate comes from attempting the rotcur fits in various orders ( e.g. , holding each variable fixed in turn ) . 
   + the resulting rc is shown in figure ( [ asymm ] ) . notice that in this object the disk inclination is determined kinematically and therefore it is quite accurate . 
   no result of this paper changes by adopting different values of @xmath35 , inside the quoted errorbar . 
   the second - order moment map for the galaxy is shown in figure ( [ figcap3]c ) . throughout most of the disk , 
   the velocity dispersion is roughly constant at @xmath14 @xmath36 7@xmath42 km / s , with a more complex behaviour near the galaxy centre and at the outermost radii . 
   this velocity dispersion estimate allows us to derive the asymmetric drift correction to the rc yielded by the tilted ring model . the observed rotation velocity , @xmath37 , is related to the circular velocity @xmath38 via v^2_c(r)= v^2_rot(r ) - ^2(r )   . [ circ ]    from an examination of figure ( [ asymm ] ) it is clear that the @xmath37 and @xmath38 profiles differ by less than 1@xmath39 . 
   throughout this paper , we use the latter for the purposes of mass modelling . 
   we notice that in very small dwarfs this correction is not negligible ( @xmath40 ) and it introduces an uncertainty in the analysis , e.g. @xcite .    in summary 
   , the orion dwarf rc has a spatial resolution of 0.26 kpc ( i.e. 0.2 @xmath13 ) , and extends out to 5.1 r@xmath41 . 
   the uncertainties on the rc are few km / s and the error on the rc slope @xmath42 . 
   is the circular velocity given by eq . 
   ( [ circ ] ) a proper estimate of the gravitational field ? to further investigate the presence of non - circular motions within the disk that jeopardize the kinematics , we carried out a harmonic decomposition of the intensity - weighted velocity field to search for any significant non - circular components . 
   this test is necessary in that the undetected presence of non - circular motions can lead to incorrect parametrization of the total mass distribution . 
   + following @xcite , the line - of - sight velocities from the  velocity field are decomposed into harmonic components up to order @xmath43 according to @xmath44 where @xmath45 is the systemic velocity , @xmath46 and @xmath47 are the magnitudes of the harmonic components , @xmath48 the harmonic number , and @xmath49 the azimuthal angle in the plane of the galaxy . 
   the gipsy task reswri was used to carry out the decomposition by fitting a purely circular model to the velocity field , subtracting it from the data , and then determining from the residual the magnitudes of the non - circular components . the tilted ring model fitted by reswri had its kinematic centre fixed to that of the purely circular tilted ring model used to derive the rc above . 
   the position angles and inclinations were fixed to constant values of 20 and @xmath50 , respectively . 
   the parameters of the best - fitting model are shown in figure ( [ reswri4 ] ) . 
   adjacent points are separated by a beam width in order to ensure that they are largely independent of one another . 
   we argue that because the standard tilted ring model has fewer free parameters than the model incorporating the higher order fourier components , it is not as essential to space the points on the rotation curve by a full beam width . 
   then in this model only @xmath51 points are considered instead of the @xmath52 points used in fitting the rc . + at inner radii the inferred non - circular motions are not negligible , but this is almost certainly due to the fact that the  distribution over this portion of the disk is irregular , being dominated by the large central  under - densities . 
   the harmonic components of the outer disk are , instead , reliable and demonstrate the gas flow to be dominated by circular kinematics . 
   the circular velocity so obtained well matches that found by means of the tilted ring model presented above . 
   the amplitudes of @xmath53 and @xmath54 are too small to hide a cusp inside an apparently solid body rc ( as suggested by @xcite ) . 
   these results provide further decisive support for the use of @xmath37 of the orion dwarf as a tracer of its mass distribution . 
   we model the orion dwarf as consisting of two `` luminous '' components , namely the stellar and the gaseous disks , embedded in a dark halo . 
   the stellar component is modelled as an exponential thin disk @xcite with a scale length of 1.33  kpc . 
   any bulge component is assumed to be negligible in terms of mass . 
   the dynamical contribution of the gas to the observed rc is derived from the  total intensity map . 
   a scaling factor of 1.33 is incorporated to account for the presence of helium and other elements . for the dark halo we consider two different parametrizations of the mass distribution : an nfw profile @xcite and the cored profile of the halo universal rotation curve ( urch ) @xcite . 
   it is well known that the nfw profile is one outcome in numerical simulations of cold dark matter structure formation , whereas the cored profile ( an empirical result ) , by design , fits the broad range of rc shapes of spiral galaxies . 
   the rc is modelled as the quadrature sum of the rcs of the individual mass components :    @xmath55    for the cored halo parametrization we adopt the urch profile : v^2_urch(r)&=&6.4 , where the disk mass , the core radius @xmath56 and the central halo density @xmath57 are free parameters . 
   it is evident that this model yields a total rc that fits the data extremely well ( see figure ( [ urcnfw ] ) left panel ) , with best - fitting parameters of @xmath58 kpc , m@xmath59 m@xmath60  and @xmath61g/@xmath62 . 
   more accurate statistics is not necessary ; the mass model predicts all the @xmath63 data points within their observational uncertainty . 
   notice that the derived value of the disk mass agrees with the photometric estimate discussed above . 
   the corresponding virial mass and radius of the dm halo are m@xmath64 m@xmath60 ( see eq . 10 in @xcite ) and r@xmath65 kpc @xcite , respectively . 
   we note that the orion dwarf has a mass 20 times smaller than that of the milky way , with the dm halo dominating the gravitational potential at all galactocentric radii . 
   the baryonic fraction is @xmath66 , while the gas fraction is @xmath67 . 
   the rc for the nfw dark matter profile is v^2_nfw(r)=v^2_vir , where @xmath68 , @xmath69^{-1}$ ] and @xmath70 is the concentration parameter ( see @xcite ) . 
   we fitted the rc of the orion dwarf by adjusting m@xmath71 and m@xmath72 . 
   the resulting best - fit values are m@xmath73 m@xmath60 and m@xmath74 m@xmath60 , but since @xmath75 , i.e. the fit is unsuccessful , the best - fit values of the free parameters and those of their fitting uncertainties do not have a clear physical meaning . 
   we plot the results in the right panel of figure ( [ urcnfw ] ) . 
   the nfw model , at galactocentric radii r@xmath762 kpc , overestimates the observed circular velocity ( see figure ( [ urcnfw ] ) right panel ) . 
   an alternative to newtonian gravity was proposed by @xcite to explain the phenomenon of mass discrepancy in galaxies . 
   it was suggested that the true acceleration @xmath77 of a test particle , at low accelerations , is different from the standard newtonian acceleration , @xmath78 :    a= , where @xmath79 is an interpolation function and @xmath80 cm s@xmath81 is the critical acceleration at which the transition occurs ( see @xcite ) . for this 
   we adopt the following form of the interpolation function ( see @xcite ) : ( a / a_0)=.    in this framework the circular velocity profile can be expressed as a function of @xmath82 and of the standard newtonian contribution of the baryons to the rc , v@xmath83=(v@xmath84+v@xmath85)@xmath86 , obtaining for it v^2_mond = v^2_bar(r ) ( ) , [ mond ] ( see @xcite ) 
   ( [ mond ] ) shows that in the mond framework the resulting rc is similar to the no - dm standard newtonian one , with an additional term that works to mimic and substitute for the dm component @xcite . 
   no result of this paper changes by adopting the `` standard '' mond interpolation function ( see @xcite ) . 
   the best - fitting mond mass model is shown in figure ( [ mond1 ] ) . 
   the model total rc ( cyan line ) completely fails to match the observations . 
   we fix the stellar mass m@xmath41 at m@xmath87 . 
   if we let the disk mass becomes higher , covering the mass range estimated in @xcite , the fit is not even able to reproduce the rc at inner radii . 
   note that in the mond formalism , the distance of the galaxy and the amount of gaseous mass are both crucial in deriving the model rc . to quantify the discrepancy of these observations with the mond formalism , note that only if the orion dwarf were 1.9 times more distant than the current estimate we would obtain a satisfactory fit to the rc ( see figure ( [ mond2 ] ) ) . 
   the orion dwarf galaxy is representative of a population of dwarfs with a steep inner rc that gently flattens at the edge of the gas disk . 
   the observed kinematics imply the presence of large amounts of dm also in the central regions . 
   we have used new  observations of the orion dwarf to analize its kinematics and derive the mass model . 
   the derived rc is very steep and it is dominated by dm at nearly all galactocentric radii . 
   baryons are unable to account for the observed kinematics and are only a minor mass component at all galactic radii . 
   we have used various mass modeling approaches in this work . using the nfw halo , 
   we find that this model fails to match the observed kinematics ( as occurs in other similar dwarfs ) . 
   we show that non - circular motions can not resolve this discrepancy . 
   then we modelled the galaxy by assuming the urch parametrization of the dm halo . 
   we found that this cored distribution fits very well the observed kinematics . 
   orion is a typical dwarf showing a cored profile of the dm density and the well - known inability of dm halo cuspy profiles to reproduce the observed kinematics . finally , we find that the mond model is discrepant with the data if we adopt the literature galaxy distance and gas mass . 
   the kinematic data can be reproduced in the mond formalism if we allow for significant adjustments of the distance and/or value of the gas mass . 
   let us point out that the present interferometric observations may miss some of the objects flux , although this may be limited in that the cubes do not have significant negative bowls . 
   obviously , for bigger values of the mass , the distance at which the baryon components would well fit the data will also somewhat decrease . + it is worth stressing that there is a galaxy distance ( albeit presently not - favoured ) for which mond would strike an extraordinary success in reproducing the observed kinematics of the orion dwarf 
   .    the orion dwarf has a favorable inclination , very regular gas kinematics , a small asymmetric drift correction , a well - understood baryonic matter distribution , and a large discrepancy between luminous and dynamical mass . 
   all of these characteristics make this system a decisive benchmark for the mond formalism and a promising target for further detailed studies . of particular value 
   would be a direct measurement of the distance ( for example , infrared observations with the hubble space telescope would allow a direct distance measurement via the magnitude of the tip of the red giant branch ) . 
   the authors would like to thank the referee , gianfranco gentile , for his very fruitful comments that have increased the level of presentation of the paper .
summary: dwarf galaxies are good candidates to investigate the nature of dark matter , because their kinematics are dominated by this component down to small galactocentric radii . 
   we present here the results of detailed kinematic analysis and mass modelling of the orion dwarf galaxy , for which we derive a high quality and high resolution rotation curve that contains negligible non - circular motions and we correct it for the asymmetric drift . 
   moreover , we leverage the proximity ( d @xmath0 5.4 kpc ) and convenient inclination ( 47@xmath1 ) to produce reliable mass models of this system . 
   we find that the universal rotation curve mass model ( freeman disk @xmath2 burkert halo @xmath2 gas disk ) fits the observational data accurately . 
   in contrast , the nfw halo + freeman disk @xmath2 gas disk mass model is unable to reproduce the observed rotation curve , a common outcome in dwarf galaxies . finally , we attempt to fit the data with a modified newtonian dynamics ( mond ) prescription . with the present data and with the present assumptions on distance , stellar mass , constant inclination and reliability of the gaseous mass , the mond `` amplification '' of the baryonic component appears to be too small to mimic the required `` dark component '' . 
   the orion dwarf reveals a cored dm density distribution and a possible tension between observations and the canonical mond formalism . 
   [ firstpage ]    dark matter ; galaxy : orion dwarf ; mass profiles