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the ngc  1023 group is one of the most extensively systems studied ( 300 related references are listed in the nasa / ipac extragalactic database ) . the first large scale study of it has been performed by @xcite . later on the presence of several outstanding dwarf galaxies drew the attention of @xcite who studied in detail four of them in the optical bands . the most peculiar feature of ngc  1023 is its proximity to the ( likely tidally interacting ) fainter galaxy ngc  1023a . the latter galaxy appears as a small companion located at the east end of ngc 1023 . it appears as a low - luminosity condensation partially embedded in the disc of the larger galaxy . this configuration gives the appearance of an asymmetric spiral arm and led @xcite to include the two galaxies in his atlas of peculiar galaxies as arp 135 . only later @xcite proposed ngc  1023a to be an interacting individual galaxy stressing also its coincidence with a cloud of a neutral hydrogen ( hi ) identified by @xcite . additional , more recent hi observations confirming the sancisi s results ( presence of tails and bridges ) has been performed by morganti et al . the separation ( and different stellar population ) of the two objects is assured today also by the uv images of the galex satellite ( morrissey et al . in particular ngc  1023a appears to be an irregular galaxy close to the sb0 ngc  1023 . moreover the surface brightness , fuv and nuv galex profiles of ngc  1023 are included in the ultraviolet atlas of nearby galaxies of gil de paz et al . ( 2007 ) . current programs ( e.g. trentham & tully 2009 ) are intended to explore the ngc  1023 environment in order to detect the faintest candidate dwarfs ( and establish their possible membership to the group ) ; the outcome of such research could indeed help to clarify , for instance , the so called fundamental missing satellite problem . moreover ngc  1023 looks to be the first known s0 for which a proper kinematical approach based on pne ( cortesi et al . 2011 ) has shown that it likely originates from the secular evolution of a parent spiral galaxy instead of a more disruptive fast merger .    in this paper we analyze uv data in the central region of the group and we use them to characterize its core with sph simulations . these are used also to investigate the dark matter distribution in the group environment as not confined in the haloes of the single galaxies . the work is structured as follows : section  2 gives a detailed description of the dominant galaxy , ngc  1023 , while section  3 provides an exhaustive description of the uv photometric analysis of both ngc  1023 and ngc  1023a . the adopted sph modeling is described in section  4 . the conclusions are finally listed in section  5 . ngc  1023 is an almost edge - on lenticular galaxy , classified as sb0 by most catalogues ( e.g. rc3 ; de vaucouleurs et al . 1991 ) , at an estimated distance of 11.4  mpc ( tonry et al . 2001 ) . it belongs to a sparse group , rich in spirals , and is the brightest member of the lgg 70 group ( garcia 1993 ) . in the optical images ( the carnegie atlas 1994 ) the galaxy appears to be composed of a faint stellar disk , a inner and brighter lens and a bar at @xmath045@xmath1 from the apparent disk major axis . a central nuclear disk is also present , with relatively young stars ( cf . debattista et al . 2002 ) . from optical line index measurements ( boroson et al . 2011 , their table 1 ) derive a velocity dispersion of 210km / s and a stellar age of 4.7 gyr . the redshift difference with the companion ngc  1023a is @xmath2v=127@xmath330 km s@xmath4 ( capaccioli et al . 1986 ) and the projected distance @xmath07.6  kpc ( barbon & capaccioli 1975 ; adopted h@xmath5 = 75 km s@xmath4 mpc@xmath4 ) . in bluer and deeper images the smaller galaxy exhibits some luminous knots that are consistent with a late - type dwarf galaxy . we used fuv and nuv background - subtracted , de - archived intensity images from the galex pipeline . in order to estimate the uncertainties of uv ( ab ) magnitudes , we propagated poisson statistical errors on source and background counts . in addition to the statistical error , we added an uncertainty to account for systematic inaccuracies in the zero - point of the absolute calibration of 0.05 and 0.03 mag for fuv and nuv respectively ( morrissey et al . 2007 ) . fuv and nuv magnitudes have been computed as @xmath6 where cr is the dead - time corrected , flat fielded count rate , and the zero points are zp=18.82 and zp=20.08 in fuv and nuv respectively ( morrissey et al . 2007 ) . we corrected uv magnitudes by adopting a foreground reddening from schlegel et al . ( 1998 ) for both ngc  1023 and ngc  1023a , namely e(b - v)=0.061 . with reference to the extinction curve of savage & mathis ( 1979 ) , this translates into the extinction values a@xmath7=0.56 and a@xmath8=0.49 respectively , adopting @xmath9=2267     and 1516    for the two above filters .    for our study , first we derive the photometic parameters of ngc 1023 and ngc 1023a and as a second step we cross - correlate the @xcite sample of objects with the galex photometric catalogue . we found 9 matches ( together with ngc 1023 and ngc 1023a ) over 28 objects . all the galaxies with know redshift are detected in uv bands while only two ( namely ngc1023 - 15 and ngc1023 - 17 ) of the dwarfs listed in @xcite have uv emission . in addition to these galaxies we discovered two new objects , identified as c1 and c2 . our galex identification of the galaxies , as listed in @xcite and @xcite , are shown in fig . [ galex_field ] . both fuv and nuv surface photometry have been performed using iraf stsdas ellipse routine . ellipse computes a fourier expansion for each successive isophote ( jedrzejewski 1987 ) , resulting in the surface photometric profiles . all corrected magnitudes m@xmath10s of ngc  1023 , ngc  1023a , and davies & kinman s and @xcite objects are given in table [ tab : fos_indb ] . in figures [ galex_field1],[galex_field2 ] we show the nuv , fuv and poss2 r images of all the objects we have detected ( in nuv or fuv ) . we computed the ultraviolet integrated photometry ( fuv@xmath11nuv ) of each uv - bright source and the color profiles of the two brightest galaxies , namely ngc 1023 and ngc 1023a ( fig [ col1 ] ) . integrated photometric properties are given in table [ tab : fos_indb ] while photometric parameters for the fits are listed in table [ re ] . errors in total magnitudes were obtained from the correlation coefficient of the model fit vs. the observed values . we fitted the luminosity profiles of ngc  1023 assuming two - components : a bulge following an r@xmath12 law , and an exponential disc . the inner regions of the profiles , affected by the galex psf ( @xmath0 5@xmath13 ) , were excluded in performing interpolation . the lens and bar are not visible in uv images and the ellipse interpolations find constant ellipticity and p.a . values . in the case of ngc 1023 they are @xmath140.23 ( nuv ) and 0.12 ( fuv ) ; p.a@xmath080@xmath1 ( nuv ) and 83@xmath1 ( fuv ) . the values of the bulge effective radii in fuv and nuv reported in table [ tab : fos_indb ] are quite similar and are in agreement with those found by optical photometry in r band , reported in the work of @xcite : r@xmath15=32 `` . the stellar disk , when observed in red light , appears larger , with an r@xmath16=62 '' compared to values lower than 50 " found in the uv . the fuv - nuv color profile ( top panel of fig . [ col1 ] ) indicate that the nucleus is bluer that the external regions . this is confirmed also by the nuv - r color profile plotted in fig . [ col ] together with the fuv - r . the r data were derived from @xcite . both uv profile components can be fitted in ngc  1023a with an exponential disk . the routine ellipse gives almost constant ellipticity and position angles . they are : @xmath170.45 ( nuv ) and 0.38 ( fuv ) , and p.a@xmath016@xmath1 ( both in nuv and fuv ) . the effective radii and surface brightness are listed in table [ re ] together with the total magnitudes of both galaxies . integrated photometric properties are given in table [ tab : fos_indb ] while photometric parameters for the fits are listed in table [ re ] . contrary to what observed in ngc 1023 the color profile in ngc 1023a is constant around a value @xmath00.8 . we measured the uv magnitudes of the two uv emission regions , labeled c1 and c2 ( see fig . [ galex_field ] , [ galex_field1 ] , [ galex_field2 ] ) inside an aperture of r=15 arcsec ; the values are listed in table [ tab : fos_indb ] . in fig . [ colori ] we plot the colors of the main objects in the group . we note that ngc  1023 and ngc  1023a have similar colors , whereas the `` dwarfs '' ngc  1023b , ngc  1023d , ugc 2157 and especially ngc  1023c are quite bluer in the uv ( both in fuv - nuv and nuv - b ) . ugc 2165 on the contrary is very red , in particular the fuv flux measured can be considered as an upper limit and may indicate a very old stellar population .    [ cols="<,^,^,^,^,^,^,^,^,^,^ " , ]      the present - day star formation rate ( sfr ) of each uv source can be derived  following @xcite  using its uv continuum luminosity and the relation + sfr@xmath8 ( m@xmath18 yr@xmath4 ) = 1.4 @xmath19 10@xmath20l@xmath8(ergs s@xmath4 hz@xmath4 ) . + such a relation holds for a sapleter s imf with lower and upper mass limits 0.1m@xmath18 and 100m@xmath18 respectively . we used our total fuv magnitudes to derive the sfrs which are given in table [ tab : fos_indb ] . using the previous equation we found that the sfr is not null in ngc 1023 although it provides very low values ( 2.68@xmath21m@xmath18 yr@xmath4 ) . the spitzer data for the inner regions of this galaxy @xcite indicate no signature of sf . the sfr in the other dwarf galaxies and in the two blobs ( c1 and c2 ) is even lower , with the lowest value , 0.01@xmath21m@xmath18 yr@xmath4 , for ugc 2165 . however their colors ( see fig . [ colori ] ) are bluer than those of the two brighter galaxies of the group . with the aim to match the overall seds and the global properties of ngc1023 and ngc1203a in a consistent way with the dynamical properties of the group , we select , from a large set of sph simulations , one case which gives us insights into the evolution of such a system and the group itself . our sph simulations of galaxy formation and evolution are starting from the same initial conditions as described in @xcite ( mc03 in the following ) and @xcite i.e. , collapsing triaxial systems initially composed of dark matter ( dm ) and gas in different proportions and different total masses . all the simulations include self  gravity of gas , stars and dm , radiative cooling , hydrodynamical pressure , shock heating , artificial viscosity , star formation ( sf ) and feedback from evolved stars and type ii sne , and chemical enrichment as in mc03 ( and references therein ) . they provide the synthetic sed at each evolutionary step . the sed accounted for chemical evolution , stellar emission , internal extinction and re - emission by dust in a self - consistent way , as described in @xcite and @xcite ; this extends over four order of magnitude in wavelength , i.e. , from 0.1 to 1000 @xmath22 m . so , each simulation self - consistently provides dynamic , morphological , and chemo - photometric evolution . the initial mass function ( imf ) is of salpeter type with upper mass limit 100@xmath23 and lower mass limit 0.01@xmath23 ( salpeter , 1955 ) ( see @xcite and mc03 for a discussion ) . all the model parameters here adopted had been tuned in previous papers where the integrated properties of simulated galaxies , i.e. , colors , absolute magnitudes , mass to luminosity ratios provided by different choices of model parameters after 15 gyr , had been successfully compared with those of local galaxies ( see also @xcite ; @xcite ) . in particular , the imf choice here provides a slightly higher sfr compared with the other possibilities examined , allows for the lowest feedback strength , and the expected rotational support when disk galaxies are formed . moreover , as pointed out by @xcite , its slope is almost the same as the universal mass function which links imf of galaxies and stars to those of brown dwarfs , planets and small bodies ( meteoroids , asteroids ) @xcite . with respect to mc03 , the initial particle resolution here is enhanced to more than 6@xmath1910@xmath24 ( see below ) instead of 2@xmath1910@xmath24 , and the time separation between the snapshots halved , 151myr . a new large set of galaxy encounters involving systems with 1:1 mass ratios and different mass ratios have been performed with the same initial conditions as described in mc03 ( and references therein ) . by seeking to exploit a vast range of orbital parameters , we carried out different simulations for each couple of interacting systems varying the orbital initial conditions in order to have , for the ideal keplerian orbit of two mass points of mass equal to 10@xmath25 , or 10@xmath26 , the first peri - center separation , p , equal to the initial length of the major axis of the dm triaxial halo , i.e. 88 kpc for 10@xmath27 , or equal to 1/10 , 1/5 , and 1/3 of the same axis for 10@xmath26 encounters . for each of these separations , we changed the eccentricity in order to have hyperbolic orbits of different energy . the spins of the systems are equal ( mc03 ) , generally parallel each other , and perpendicular to the orbital plane , so we studied direct encounters . however , some cases with misaligned spins have been also analyzed in order to deepen the effects of the system initial rotation on the results . moreover , for a given set of encounters with the same orbital parameters we also examined the role of increasing initial gas fractions . all these new simulations will be fully discussed elsewhere ( mazzei , in prep ) . a simulation able to match the global properties of ngc  1023 is chosen and discussed below . all the observables , i.e. absolute magnitudes , colors , which correspond to the sed over almost four order of magnitude in wavelength , and mass - to - light luminosity ratios are self - consistently derived from our simulation at the snapshot selected , and successfully compared with the available data . the whole sed of ngc  1023 and its global properties are well matched by a simulation which provides a minor merger . the total mass of the primary system is 1@xmath1910@xmath28 m@xmath18 with gas fraction 0.01 , that - one of the companion is 5 times less with the same gas fraction . so , the starting point is given by two triaxial collapsing systems with total mass 1.2@xmath1910@xmath28 m@xmath18 , and 60 times less of gas , i.e. 2@xmath1910@xmath29 m@xmath18 . the mass particle resolution is 5@xmath1910@xmath30 m@xmath18 for gas and 4.95@xmath1910@xmath31 m@xmath18 for dm particles . this requires 63838 initial particles . the triaxiality ratio , @xmath32 @xcite , is 0.84 . + the first pericenter separation , 433  kpc , corresponds to 1/3 of the major axis of the primary system ; the orbit eccentricity is 1.3 . and the anomaly corresponds to 200 degrees . the spins of the systems are equal ( @xmath33=0.06 , mc03 ) , parallel , so we are dealing with a direct encounter , and both aligned with the shorter of their principal axes . + stars are born in the inner regions of their halos after 3gyr from the beginning . galaxies grow changing their shapes step by step , as their trajectories are approaching and their halos mixing . the fit corresponds to a galaxy age of 11.9 gyr . the strong interaction phase between the systems started 7.7gyr ago and the final merger 1.8gyr ago . figure [ accgas ] shows the gas accretion history inside a sphere centered on the luminous ( v band ) centre of the galaxy system with a radius of 50kpc . the mass of gas with temperature @xmath3410000k , which represents the upper limit of the cold gas mass inside the selected radius ( its cooling time scale is very shorter than the snapshot time range , 151myr ) , is 9.50@xmath1910@xmath35 m@xmath36 at the age of our fit ( the vertical line in fig . [ accgas ] ) . such a value well agrees with that derived from eq . ( 1 ) of @xcite using d=5.93mpc ( ned from 3k cmb ) and fi , the flux integrated over line , 80jykm / s @xcite , i.e. , 7.3@xmath1910@xmath35 m@xmath36 ; this value is at least a factor 3.7 more using the distance in ( * ? ? ? * their table 2 , 11.4 mpc ) . figure [ mdist ] shows the mass distribution at the selected snapshot inside 50kpc , corresponding to 15r@xmath37 of ngc  1023 . the b band absolute magnitude of the model , @xmath38 , agrees with the value in table 1 of @xcite and with table 1 of ( * ? ? ? * and references therein , @xmath39 ) ; r band absolute magnitude , @xmath40 , agrees with the estimate of @xcite , @xmath41 ( their table 2 ) . furthermore , all the available data , extending over three order of magnitude in wavelength , are well matched by the sed provided by our simulation , as shown in fig . [ modsed ] . the fir region is not strongly constrained by the data , since upper limits of 25 , 60 and 100@xmath22 m fluxes are only available ; dust components ( warm and cold with pah as discussed in @xcite ) with the same average properties as derived by @xcite and @xcite for a complete sample of nearby early - type galaxies are included ( r@xmath42=100r@xmath43 , @xcite ) . figure [ morfo_ii ] shows r band maps of different projections of the selected snapshot . figure [ morfo ] compares , on the same scale , the uv galex image with superimposed the hi map of ngc  1023 , with r map of xy projection as in fig . [ morfo_ii ] . star populations inside the previous maps are 5.0 gyr old in average , in agreement with @xcite ( see section  2 ) . the luminosity weighted age of the model does not depend strongly on the radius nor on the band , it is 2.5 - 3 gyr going from the inner regions up to 50kpc .    from the study of @xcite , the virial mass of the ngc  1023 group inside 315kpc ( their virial / harmonic radius ) , is 6.4@xmath33@xmath1910@xmath25 m@xmath44 , and l@xmath45=2.0@xmath1910@xmath46 l@xmath44 . we predict a total mass of 5.54@xmath1910@xmath25 m@xmath44 with a stellar mass of 1.20@xmath1910@xmath29 m@xmath44 , and l@xmath47=2.56@xmath1910@xmath46 l@xmath44 inside the same radius . the instantaneous sfr in the inner region of the simulated galaxy agrees with sauron results by shapiro et al . ( 2010 ) who measure sfr@xmath00 @xmath48  yr@xmath4 ( see section 3.1 ) , moreover the total sfr at the selected snapshot , 0.045  m@xmath44  yr@xmath4 , is in well agreement with our estimates from fuv luminosity . note that to compare these values with those in table [ tab : fos_indb ] a multiplying factor of @xmath02.4 has to be applied to the data in such a table to account for the lower mass limit of our simulation . figure [ vel ] compares our predicted average velocity distribution of stars at each galaxy radius with the same from the data in fig . 4 of cortesi et al . velocities from our simulations are higher than 7% ; such a reduction factor allow us to account for the inclination angle of ngc  1023 that is @xmath49 . note that the green line in fig . [ vel ] corresponds to the average velocity distribution of planetary nebulae ; we select their velocities along the major axis of such a galaxy ( pa=@xmath50 ) accounting for a recession velocity of 637km / s . first , we reanalyzed the inner group ngc  1023 by means of deep galex images . so we extend to uv range the detailed optical observations of both the four dwarfs studied by davies & kinman ( 1984 ) and the sample of @xcite . these observations led to the discovery of two other very faint uv - emitting objects , possibly members of the group itself ( see fig . [ galex_field2 ] and table [ tab : fos_indb ] ) . all the five dwarfs show nuv - b colors bluer than those of the two brighter members of the group . in particular ugc 2165 is very faint in fuv showing extremely red fuv - nuv colors . moreover , with the aim at matching the specific overall seds and the global properties of ngc1023 and ngc1203a in a consistent way with the dynamical properties of the group , we select , from a large set of sph simulations , one case which gives us insights into the evolution of such a system and the group itself . such sph simulations of galaxy formation and evolution are starting from a collapsing triaxial systems initially composed of dark matter ( dm ) and gas in different proportions and different total masses . these simulations allowed to investigate the dm distribution in the group environment finding that this is not confined to the halo of single galaxies . the whole sed of ngc  1023 and its global properties are well matched by the simulation which provides a minor merger , 1:5 @xmath51 ) . the merger phase started 1.8 gyr before the selected snapshot . the whole outcome of the model is in strict agreement with @xcite results : their estimate of the the r band absolute magnitude is @xmath41 to be compared with our @xmath40 . in addition from their study of the ngc 1023 group , the virial mass inside 315kpc ( their virial / harmonic radius ) , is 6.4@xmath33@xmath1910@xmath25 @xmath48 , and l@xmath47=2.0@xmath1910@xmath46 @xmath52 , according to our prediction of a total mass of 5.54@xmath1910@xmath25 @xmath48 with a stellar mass of 1.20@xmath1910@xmath29 @xmath48 , and l@xmath47=2.56@xmath1910@xmath46 @xmath52 inside the same radius . looking at its evolution , we predict the group more loose and poor , still no - virialized when the final merger phase is not yet begun , 2 gyr before our fit , which corresponds to an age of 11.9 gyr . b and r luminosities are almost the same as at the age of the fit , but there is 20% less mass within the same reference radius , i.e. 315 kpc @xcite . furthermore , b and r luminosities reduce by  1 and  0.5 mag respectively 2 gyr after the age of our fit , the total mass being the same . we thank the referee dr . n. trentham whose comments greatly improved the paper . we acknowledge the financial contribution from university of padova ( ex 60% ) and the agreement asi - inaf i/009/10/0 . galex is a nasa small explorer , launched in april 2003 . galex is operated for nasa by california institute of technology under nasa contract nas-98034 . 99 arp , h.  1966 , apjs , 14 , 1 barbon , r. , & capaccioli , m.  1975 , a&a , 42 , 103 bettoni d. , galletta , g. , rampazzo , r. , marino , a. , mazzei , p. & buson , l. m.  2011 , a&a , 534 , a24 binggeli , b. & hascher , t.  2007 , pasp , 119 , 592 boroson , b. , dong - woo , k. , & fabbiano , g.   2011 , apj 729 , 12 capaccioli , m. , lorenz , h. , & afanasjev , v.  l.  1986 , a&a , 169 , 54 cappellari , m. , emsellem , e. , bacon , r. et al . 2007 , mnras , 379 , 418 cortesi , a. , merrifield , m.  r. , arnaboldi , m. , et al . 2011 , mnras , 414 , 642 curir , a. , mazzei , p.  1999 , , new astron . , 4 , 1 davies , r.  d. , & kinman , t.  d.  1984 , mnras , 207 , 173 debattista , v.  p. , corsini , e.  m. , & aguerri , j.  a.  l.  2002 , mnras , 332 , 65 de vaucouleurs , g. , de vaucouleurs , a. , corwin , h.  g. , jr . , et al .  1991 ( rc3 ) emsellem , e. , cappellari , m. , krajnovic , d. , et al . 2007 , mnras , 379 , 401 . garcia , a.  m.  1993 , a&as , 100 , 47 gil de paz , a. , boissier , s. , madore , b.  f. , et al . 2007 , apjs , 173 , 185 jedrzejewski , r.  i.  1987 , mnras , 226 , 747 jeong , h. , sukyoung , k. , bureau , m. et al . 2009 , mnras , 398 , 2028 kennicutt , r.  c. , jr . 1998 , ara&a , 36 , 189 kennicutt , r.  c. , jr . 1998 , ara&a , 36 , 189 knapp , g. , turner , e. , cunniffe , p.e  1985 , aj , 90 , 454 mazzei , p.  2003 sait , 74 , 498 mazzei , p. , xu , c. , & de zotti , g.  1992 , a&a , 256 , 45 mazzei , p. & de zotti , g. , xu , c. 1994 , apj , 422 , 81 mazzei , p. & de zotti , g. 1994 , apj , 426 , 97 mazzei , p. & curir , a. 2003 , apj , 591 , 784 : mc03 mazzei , p.   2004 , recent reaserch dev . in astron . & astrop . , 1 , 457 ed . pandalai , s.g . , research signpost ; 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the ngc  1023 group is one of the most studied nearby groups . 
 we want to give an insight into the evolution of its innermost region by means of ultraviolet observations and proper models . 
 we used the fuv and nuv galex archival data as well as a large set of sph simulations with chemo - photometric implementation . from the uv observations we found that several , already known , dwarf galaxies very close to ngc  1023 
 are also detected in uv and two more objects ( with no optical counterpart ) can be added to the group . using these data we construct exhaustive models to account for their formation . 
 we find that the whole sed of ngc  1023 and its global properties are well matched by a simulation which provides a minor merger with a companion system 5 times less massive . 
 the strong interaction phase started 7.7gyr ago and the final merger 1.8gyr ago . 
 [ firstpage ]    galaxies : structure  galaxies : individual : ngc 1023