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in many current and upcoming extragalactic photometric surveys it will be necessary to use spectral energy distribution ( sed ) fitting techniques to characterize most objects ( e.g. , redshifts , k - corrections , stellar masses and bolometric luminosities ) , because spectroscopic observations are too expensive .
seds of galaxies are typically either empirically obtained ( e.g. , * ? ? ? * ) or theoretically created ( e.g. , * ? ? ?
the former have the advantage of giving an accurate representation of the seds but generally lack the large wavelength ranges of theoretical ones .
empirical seds also have the advantage of easily including non - stellar emission , like active nuclei and dust / pah emission , which are hard to model from first principles .
in particular , most available agn seds are empirically obtained ( e.g. , * ? ? ? * ) by averaging photometric observations of a small number of quasars .    in this proceeding
we discuss a non - negative basis of empirically obtained sed templates for galaxies and agns that accurately represents the colors of such objects .
these templates are derived from a combination of 14448 galaxies and 5347 likely agns in the ndwfs botes field with spectroscopic redshifts and photometry spanning the far - uv to the mid - ir .
all results shown in here have been presented and are discussed in detail by @xcite , @xcite and @xcite .
our data set consists on 14448 galaxies and 5347 likely agns in the noao deep wide - field survey ( ndwfs ; * ? ? ?
* ) botes field .
each object has a spectroscopic redshift from the agn and galaxy evolution survey and has at least an upper limit in 8 of the 14 following bands : bw , r , i and k from ndwfs , j and ks from flamex , z from zbotes , irac [ 3.6 ] , [ 4.5 ] , [ 5.8 ] and [ 8.0 ] from sdwfs , galex fuv and nuv , and mips 24@xmath0 m from @xcite .
the agn classification scheme is conservative , and objects classified as agns are either spectroscopically or photometrically classified according to their mid - ir , x - ray and radio properties .
we refer the reader to @xcite for details on the sample selection .
the methods we use to derive the sed templates are described in detail by @xcite and @xcite . in this section
we give a brief summary of how our methods work and refer the reader to the aforementioned papers for detailed discussions .
we assume that the spectrum of any object in our sample can be modeled as a linear combination of a small set of unknown spectral templates .
we construct these unknown templates using the data sets described in the previous section . for `` pure '' galaxies ,
we assume the majority of them have seds that can be described as a linear combination of three templates : one similar to an elliptical galaxy ( an old stellar population ) , one similar to a spiral galaxy ( a continuously star forming population ) , and a third similar to an irregular galaxy ( a starburst population ) . for objects with active nuclei ,
we assume that every agn sed can be described by the same spectral template with varying amounts of reddening and absorption by the intergalactic medium ( igm ) , combined with the galaxy templates to describe the host .
we use an iterative algorithm to derive the sed templates .
this algorithm is based on the method proposed by @xcite for deriving principal component seds .
we start with an initial guess for each of our sed templates and sequentially improve it to best fit the photometric data .
our initial guess for the `` pure '' galaxy seds corresponds to the e , sbc and i m templates of , extended into the uv and mid - ir with the models of @xcite and with the addition of pah and dust emission of the vcc1003 and m82 seds obtained by @xcite to the star forming templates . for the agn component we use a combination of power - laws that broadly resembles the agn sed template of @xcite .
we divide every sed template in 300 logarithmically spaced wavelength bins and fit each bin independently to our data set , with a restriction to force their smoothness .
the top left panel of figure [ fg : temps ] shows the resulting sed templates alongside with the initial guesses .
we have pursued several applications of our sed templates , including the estimation of photometric redshifts and k - corrections for galaxies and agns , the study of biases in mid - ir selection of agns , predictions of colors of agns and galaxies in the pass - bands of the recently launched wise mission , and the determination of luminosity function of mid - ir and x - ray selected agns from redshift 0 to 6 .
the former three applications are discussed in detail by @xcite and @xcite , while our results on the agn luminosity function were presented by @xcite . in this section
we discuss two of these applications , namely the biases in the mid - ir color selection of agns and the colors of agns and galaxies in wise .
figure [ fg : temps ] shows the mid - ir colors of all sources in sdwfs with @xmath1 , the agn selection diagrams of @xcite ( top right panel ) and of @xcite ( bottom left panel ) and the color tracks of our templates .
the boundaries of the selection region of @xcite do a remarkable job of excluding inactive galaxies at @xmath2 , but also exclude unreddened agns at @xmath3 . in a more subtle manner
, the selection region of @xcite also excludes agns that are faint in comparison to their host galaxies , as the latter will dominate the mid - ir colors .
the selection region proposed by @xcite is not strongly affected by the biases of the @xcite criteria , but their agn selection region is heavily contaminated by intermediate redshift star forming galaxies and galaxies at @xmath4 .
these problems are partially solved by the revised version of this criteria by @xcite .
it can be seen , however , that if the left - most boundary in this criterion is pushed further to the right to limit contamination by inactive galaxies , similar biases to those of the @xcite selection criteria arise .
using our sed templates , we have also studied the color distribution of sources in the wise mission .
the selection boundaries shown in the bottom right panel of figure [ fg : temps ] correspond to those proposed by @xcite , and would be able to classify in between @xmath5 and @xmath6 agns , depending on whether a detection in the two longest wavelength bands is required or not .
this criterion would keep the contamination by inactive galaxies and ulirgs to a minimum , as shown by the color tracks of our seds and of the sed of arp 220 produced by grasil @xcite .
we note , however , that the wise colors of @xmath7 agns are similar to those of intermediate redshift galaxies and ulirgs , leading to a color degeneracy that must be broken by the inclusion of other data ( e.g. , optical broad - band photometry ) .
we present a set of low resolution empirical sed templates for agns and galaxies in the wavelength range from 0.03 to 30@xmath0 m . 
 these templates form a non - negative basis of the color space of such objects and have been derived from a combination 14448 galaxies and 5347 likely agns in the ndwfs botes field . 
 we briefly describe how the templates are derived and discuss some applications of them . 
 in particular , we discuss biases in commonly used agn mid - ir color selection criteria and the expected distribution of sources in the current wise satellite mission .