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the key role of stellar mass to determine ( or predict ) the physical properties of present day galaxies has been established by a number of works since the last decade ( e.g. * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
indication has been provided that stellar mass density might be even more fundamental @xcite .
all these works , however , deal with _
global _ estimates of the stellar mass - to - light ratio , which is assumed to be uniform throughout a galaxy , contrary to what we should expect based on well known spatial variations of stellar population and dust properties .
hence , resolving the distribution of stellar mass is crucial to properly measure total stellar mass and mass density and to start investigating the structure and dynamics of galaxies in an unbiased way . having access to the mass distribution also allows to address questions like : is there any relation between _ local _ stellar mass density and the _ local _ sed and physical properties _ within _ a galaxy , similarly to the relations observed _ globally _ ?
if so , what does it tell us about the internal mechanisms of galaxy evolution ?    with these goals and questions in mind , we have developed a new method to build stellar mass maps of galaxies ( * ? ? ?
* zcr09 hereafter ) , which we review in this contribution ( sec . 2 ) .
we also present preliminary results on the colour - stellar mass density relation within galaxies for a small sample of different morphological types ( sec .
as described in zcr09 , our goal is to provide a computationally fast and observationally cheap method to obtain stellar mass maps .
we express the surface stellar mass density at any given position @xmath2 of a galaxy as : @xmath3 where @xmath4 is the surface brightness in the chosen ` luminance ' band of effective wavelength @xmath5 and @xmath6 is the corresponding effective m / l ratio . in turn
, we want to express @xmath7 as a function of colour indexes . despite the diversity of stellar population parameters and dust properties ,
if the luminance band and the colour indexes are appropriately chosen , the scatter of @xmath7 as a function of colour(s ) can be reasonably small ( e.g. * ? ? ?
we choose the @xmath8 band ( largely equivalent to @xmath9 or @xmath10 ) as luminance band and we use @xmath0 and @xmath1 colour indexes to predict m / l@xmath11 .
this choice is supported by the fact that m / l variations in nir are minimal with respect to shorter wavelengths , while these two colours are the best combination in terms of sensitivity and wavelength leverage . based upon the 2007 version of the @xcite code , which implements updated prescriptions for the tp - agb stellar evolutionary phase according to @xcite
, we build a monte carlo library of 50,000 stellar population synthesis ( sps ) models with a variety of star formation histories , both continuous and bursty , metallicities and dust attenuations  la @xcite . in this way
we mean to cover as uniformly as possible the parameter space occupied by individual regions in galaxies on scales of @xmath12  pc , from old , metal - rich bulges / spheroids , to young spiral arms with different degrees of dust absorption . then we bin the models in cells of @xmath13  mag@xmath14 in @xmath0 and @xmath1 and for each cell we consider the median m / l@xmath11 .
this look - up table is then used to assign the median - likelihood m / l to each pixel in a galaxy , depending on its colours .
our approach is therefore bayesian rather than frequentist and , as such , depends to some extent on the chosen prior distribution of models : a different choice of sfhs in particular can affect the m / l of the bluest stellar populations ( see zcr09 for a detailed discussion ) .
we confirm ( e.g. * ? ? ?
* ) that the treatment of the tp - agb phase in the sps models has a great systematic impact on the resulting colours and m / l in nir , but this is mainly limited to young / blue stellar populations .
apart from systematic effects , we find that the confidence half - range for the m / l is @xmath15% over most of the colour space , with the notable exception of the stellar populations with ages @xmath16  gyr , where the stellar evolution is very rapid and results in larger ranges , up to a factor 2 .
the use of two colours is crucial to obtain such accurate estimates : by neglecting the dependence on @xmath1 one would easily over / under - estimate the m / l by a factor 2 to 3.-band and the @xmath0 colour as m / l predictor gives results that are in very good agreement with those based on @xmath17 in most cases , except in stellar populations dominated by a very young burst or with very large extinction . ]    to test our stellar mass mapping method we select a sample of 9 nearby galaxies , which are also part of the sings survey @xcite and for which optical sdss imaging and medium - deep nir imaging ( either from gold mine , @xcite , or ukidss , @xcite ) are available .
the sample spans all range of morphologies , from ellipticals to late spirals . as fluxes and colours in individual pixels
have to be accurate at few percent level in order for our method to work , it is necessary to pre - process the images to ensure that sufficiently high s / n can be reached also in the regions with surface brightness as low as 24 - 25 mag@xmath18 arcsec@xmath19 .
this is done using a newly developed adaptive smoothing code adaptsmooth ( zibetti in prep . , zcr09 ) , which preserves the maximum spatial resolution compatible with the requested s / n .
figure [ fig1 ] shows the maps of effective @xmath20 for the 9 galaxies of the sample .
earlier type galaxies have more uniform and higher
m / l , on average .
the young stellar populations that characterize spiral arms result in lower m / l ratios , which in turn decrease the spiral arm contrast .
the resulting stellar mass maps reveal that the structure of spiral galaxies is significantly smoother than it appears at any optical or nir wavelength .
the colour gradients , especially visible in spiral galaxies , result in m / l gradients , which in turn affect the structural parameters and make them differ from the ones derived from light distribution .
in particular , the effective radius of spirals is smaller when the stellar mass distribution is used .    in fig .
[ fig1 ] one can see lanes of much higher m / l ( especially ngc4569 and ngc4536 ) , which correspond to dust lanes .
these regions can hide a significant amount of stellar mass , although the emerging light hardly affects the global luminosity and colours of the galaxy .
as we show in zcr09 , this causes stellar masses obtained from global photometry to be biased low with respect to what one gets from integrating resolved stellar mass maps : the latter may miss up to 40% of the total stellar mass of a galaxy if dust obscured regions are very extended ( as in ngc4569 and ngc4536 ) .
based on the analysis of the previous section , we study the correlation between colours ( namely the optical @xmath0 ) and the surface stellar mass density _ within _ each galaxy .
the distribution of pixels as a function of these two quantities are shown in fig .
[ fig2 ] , where the grey scale denotes the density of pixels . as a general result
, we see that the colour positively correlates with surface stellar mass density : galaxies are redder in higher density regions .
although this holds for all galaxies , the slope and the dispersion of the correlation varies a lot along the morphological sequence . for a typical elliptical galaxy
the relation is very flat and very tight ( consistent with a scatter due to photometric errors only ) .
as we move to later types the colour - mass relation steepens , with low - density regions becoming increasingly bluer , while the highest density regions have roughly constant red colours .
we can interpret this as a sign that star formation prefers low density and that the colour - morphology relation is mainly set by the relative weight of the younger stellar populations in the lower density regions with respect to the red high - density ` cores ' .
in addition , we see that the scatter around the mean relation increases going to later types , which have a more disomogeneous distribution of physical properties at given surface mass density .
the presence of dust also increases the scatter , especially at high surface mass density .
these preliminary results based on a small sample of only 9 galaxies will be put on a much more solid ground , both from the statistical and physical point of view , by the forthcoming analysis of a larger sample of galaxies for which multiwavelength observations , from uv to radio , are available .
we report on the method developed by zibetti , charlot & rix ( 2009 ) to construct resolved stellar mass maps of galaxies from optical and nir imaging . 
 accurate pixel - by - pixel colour information ( specifically @xmath0 and @xmath1 ) is converted into stellar mass - to - light ratios with typical accuracy of 30% , based on median likelihoods derived from a monte carlo library of 50,000 stellar population synthesis models that include dust and updated tp - agb phase prescriptions . 
 hence , surface mass densities are computed . in a pilot study 
 , we analyze 9 galaxies spanning a broad range of morphologies . among the main results 
 , we find that : i ) galaxies appear much smoother in stellar mass maps than at any optical or nir wavelength ; ii ) total stellar mass estimates based on unresolved photometry are biased low with respect to the integral of resolved stellar mass maps , by up to 40% , due to dust obscured regions being under - represented in global colours ; iii ) within a galaxy , _ on local scales _ colours correlate with surface stellar mass density ; iv ) the slope and tightness of this correlation reflect / depend on the morphology of the galaxy .