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since the location of our solar system is within the galactic disk , studying the galactic structure of our milky way is always a challenging problem . therefore , we can not derive such comprehensive and intuitive pictures of our disk as extragalactic studies are able to do for other spiral galaxies ( e.g. , @xcite ) . nevertheless , based on a diverse set of studies over all wavelengths , in the last few decades we have derived a reasonably comprehensive picture of our galactic spiral structure ( for recent work , see , e.g. , @xcite ) . the galactic plane has been observed in the optical / near-/mid - infrared bands ( e.g. , @xcite ) as well as at longer wavelengths , e.g. , in co or cm continuum emission ( e.g. , @xcite ) . however , until the arrival of the two ( sub)mm galactic plane surveys atlasgal ( the apex telescope large area survey of the galaxy at 870@xmath0 m ) and bgps ( bolocam galactic plane survey ) @xcite , no survey at ( sub)mm wavelengths existed that trace the cold dust emission stemming from dense and young star - forming regions at adequate spatial resolution ( the cobe ( cosmic background explorer ) and wmap ( wilkinson microwave anisotropy probe ) data have too coarse resolution to isolate individual star - forming regions ) . here , we employ the 870@xmath0 m submm continuum survey atlasgal to study the general distribution of the dense dust and gas within our galactic plane . the 870@xmath0 m data are taken from the apex telescope large area survey of the galaxy ( atlasgal , @xcite ) . the @xmath4 rms of the data is @xmath350mjybeam@xmath5 and the fwhm @xmath6 . using the clumpfind source identification algorithm by @xcite with a @xmath7 threshold of 300mjybeam@xmath5 , we identified 16336 clumps within the galactic plane for longitudes @xmath1 and latitudes @xmath8 . in the context of this paper , we are not aiming for exact fluxes , column densities or masses , but we just want to evaluate source number counts within the galactic plane . therefore , the specific clump identification algorithm or the used thresholds are not of great importance . to test this , we also derived corresponding source catalogs using @xmath9 or @xmath10 thresholds . while the absolute number of sources obviously varies significantly with changed thresholds , the structural results presented below are not significantly affected by that . as an additional test , instead of deriving clumps , we just extracted the total submm fluxes above the @xmath7 threshold in the given latitude and longitude bins . again the structural distributions in longitude and latitude are very similar . since other galactic plane surveys usually also work on source counts ( e.g. , glimpse ( galactic legacy infrared midplane extraordinaire ) or msx ( midcourse space experiment ) ; @xcite ) , for the remainder of the paper we adopt the @xmath7 source catalog . the clump masses range between 100 and a few 1000m@xmath11 , and these clumps form clusters with certain star formation efficiencies . therefore , the atlasgal data largely trace gas / dust clumps capable of forming intermediate- to high - mass stars at distances between several 100 out to more than 10000pc ( @xcite ; tackenberg et al .  subm . ) . tackenberg et al . ( subm . ) analyzed the atlasgal data in the longitude range between 10 and 20deg in depth via correlating them with the glimpse and mipsgal ( mips galactic plane survey ) near- to mid - infrared surveys of the galactic plane @xcite , and at long wavelength with the nh@xmath12 spectral line data from wienen et al . ( subm . ) . out of 210 starless clump candidates , tackenberg et al  could extract nh@xmath12 spectral information   and by that kinematic distances   for 150 sources . to resolve the kinematic distance ambiguity , these targets were compared to the glimpse and mipsgal images . clumps associated with glimpse / mipsgal shadows were assigned the near distance , and the other clumps were assigned as far . this way , 115 clumps are likely on the near side of the galaxy and 35 on the far side . tackenberg et al . find that the mean distances of starless clumps in the longitude range between 10 and 20deg on the near and far side of the galaxy are 3.1 and 13.8kpc , respectively . one should keep in mind that the rotation curve of the inner galaxy is far from circular ( e.g. , @xcite ) , making the absolute determination of kinematic distances a difficult task .    for comparison with somewhat more evolved evolutionary stages , namely young stellar objects ( ysos ) , we resort to the spitzer red source catalog ( with a color criterion of @xmath13-[8.0]\geq$]1 , for more details see @xcite ) . while the intrinsically red sources are contaminated by approximately 30% agb stars , @xcite extracted statistically these agb stars and provided a yso catalog with reduced agb contamination . this yso catalog contains 11649 sources again over the galactic longitude range @xmath1 . because deriving distances and masses for individual ysos is difficult , @xcite conducted a population synthesis analysis of the sample . they find that their detected sources consist mainly of intermediate- to high - mass stars ( between 3 and 25m@xmath11 ) at distances of several kpc . again dividing their sources in near and far sources with respect to the galactic center and galactic bar , they as well find mean distances in the longitude range between 10 and 20deg of 4.9 and 13.1kpc , respectively . figure [ distances ] presents a histogram of the kinematic distances in the galactic longitude range between 10 and 20deg derived for the atlasgal sample ( tackenberg et al . subm . ) and population synthesis distances for the glimpse sources @xcite , respectively . clearly , both distributions show a near and far distance peak ( see also fig .  9 in @xcite ) . for the atlasgal sources , the far peak has less sources because , at the given spatial resolution of @xmath14 , close clumps that would be spatially resolved at the near distance merge and appear as one clump at the far side of the galaxy ( tackenberg et al . subm . ) . to be more precise , tackenberg et al . ( subm . ) simulated the distance smoothing effect for the atlasgal data , and they found that an artificial sample of 328 clumps at 3kpc distance would appear as only 20 clumps when put to 15kpc distance ( see also the corresponding discussion in @xcite ) . this effect is far less severe for the much better resolved glimpse sources . additionally , glimpse sources easier saturate at the near side of the galaxy . combining these effects with the larger observed volume at the far side of the galaxy , more sources are found at the far side for that sample . although the near peaks of the atlasgal and glimpse samples are shifted with respect to each other a little bit , taking into account the inherent uncertainties of kinematic distances for the atlasgal sample and population synthesis for the yso sample ( @xmath31kpc each ) , the two source types cover comparable distance ranges . another difference between the two samples is that atlasgal barely detects any sources between 5 and 10kpc whereas the glimpse distribution shows sources there . the latter is due to the model used for the population synthesis with an axisymmetric ring around the center of the galaxy ( see fig .  2 in @xcite ) . the non - detection of atlasgal sources in that distance regime is likely attributed to the large extend of the galactic bar ( see also fig .  [ sketch ] ) which is not taken into account in galactic rotation models to derive the kinematic distances ( e.g. , @xcite ) . while the atlasgal clumps are in absolute terms more massive than the glimpse ysos , considering typical star formation rates and an initial mass function for each cluster - forming region , the atlasgal clumps form intermediate- to high - mass stars in the mass range of the glimpse ysos .    in combination , the two samples are well suited for comparison of the different evolutionary populations . in addition to this , since the galactic longitude range between 10 and 20 represents a fraction of the galactic plane that covers near and far spiral arms , we consider it to first order as representative to extrapolate the distance similarities for both samples also for the rest of the surveys . additional similarities and differences will be discussed in section [ lat ] . figure [ histo_lon ] presents the longitude distribution of the submm continuum and yso sources within our galactic plane ( see @xcite for a first version of such a plot based on a far smaller initial dataset ) . while the yso distribution based on the spitzer data is relatively flat , the submm continuum emission shows a series of distinctive peaks . the most prominent one is toward the galactic center where the source count increases approximately by a factor of 4 . in addition to this , there are a few more clear submm source count peaks at positive and negative longitudes . they can mainly be attributed to tangential points of spiral arms ( for a discussion of older cobe 240@xmath0 m data , see @xcite ) as well as to a few prominent star formation complexes in the nearby sagittarius arm . the most important of these are marked in figure [ histo_lon ] . for comparison , a sketch of our galaxy as it would be viewed face - on is shown in fig . [ sketch ] where several lines of sight are marked corresponding to increased number counts in fig . [ histo_lon ] . for longitudes @xmath15deg , a similar distribution was found in the bgps survey @xcite . the implications suggested by this result will be discussed further in section [ general ] .            another way to represent the 870@xmath0 m source distribution is in a 2-dimensional binning in galactic longitude and latitude ( fig . [ lon_lat ] top panel ) . in addition to an increase in source counts toward specific galactic longitudes , we also identify a tight confinement to the galactic mid - plane with only a narrow spread north and south of the mid - plane . to derive the approximate latitude for which the submm emission peaks in each longitude bin , we fitted gaussians to their latitude distribution in each longitude bin . the gaussian fit peak positions are marked in fig . [ lon_lat ] ( top panel ) . these fits indicate that the dominant dust and gas distribution is slightly shifted to negative galactic latitudes with a mean offset over the whole plane of @xmath16deg ( the mean values are derived from gaussian fits to 10deg longitude bins , see below ) . +     we can produce the same plot for the yso distribution derived from the spitzer data ( fig . [ lon_lat ] bottom panel ) . similarly , the mean value of the peaks of the yso distribution is also shifted below the galactic mid - plane , again at @xmath17deg ( the mean values are also derived from gaussian fits to 10deg longitude bins ) . already a visual inspection of the two distributions indicates that the ysos appear to cover a broader range in galactic latitude than the dust and gas clumps traced by the submm continuum emission . fitting gaussians to the latitude distributions in each longitude bin for the submm clumps as well as the ysos allows us to better quantify this effect . since the latitude distributions are not as smooth on the scales of individual degrees in galactic longitude , we average over 10 deg in longitude for smoothing purposes . figure [ gauss ] presents gaussian example fits at different galactic longitudes outlining the applicability of the gaussian assumption to these distributions . the corresponding gaussian full width half maximum ( fwhm ) for the two distributions are shown in figure [ width ] . one clearly sees that the yso distributions is broader over the whole galactic plane than the dense gas and dust distribution . below galactic longitudes of -30deg , the atlasgal distribution shows a tendency of increased fwhm . however , this effect is confined to only three bins , one with a particularly large error - bar . therefore , in the context of this paper we refrain from further interpretation . the mean values of the fwhm for the dust continuum and yso distributions are @xmath30.67@xmath18 and @xmath31.09@xmath18deg , respectively . this corresponds to characteristic scale heights @xmath19 ( distance where distribution has dropped to 1/e , @xmath200.6@xmath21fwhm ) of @xmath30.4 and @xmath30.7deg for the submm continuum and yso distributions , respectively . @xcite fitted their source distribution with a mean physical scale height of 80pc . since we have no explicit distances for individual atlasgal sources over the whole range of the galactic plane , deriving directly a physical scale height from our data alone is hardly feasible . however , because the distance and mass regimes of the atlasgal sources and the ysos are similar ( see section [ obs ] ) , we can use the linear scale height of the ysos to derive a first - order estimate of the linear scale height for the atlasgal clumps ( see section [ lat ] for additional discussion ) . with the measured fwhms of the ysos and the atlasgal sources , and using the modeled 80pc mean physical scale height of the ysos , we estimate , proportionally , an approximate physical scale height of the submm dust continuum emission of 46pc . since the distribution of near and far sources favors near sources for atlasgal and far sources for glimpse ( see fig . [ distances ] ) , the 46pc should be considered as an upper limit . +      +    another estimate of the atlasgal scale height can be derived from the sample discussed in section [ obs ] for the galactic longitude range between 10 and 20deg ( tackenberg et al . subm ) . using only the near kinematic distance sample ( the far distance sample has too low number statistics ) , we find a fwhm and scale height of 23 and 14pc , respectively . this is considerably lower than the estimate of 46pc derived above . the most important reason for this difference is based on the way near and far distance sources are separated in the study by tackenberg et al . ( subm . ) : while the kinematic distances always give near and far solutions at this and the other side of the galaxy , the near solution was favored for infrared dark clouds if absorption shadows were detected in the spitzer mid - infrared data ( e.g. , @xcite ) . however , at high galactic latitudes the infrared background is significantly reduced and by default it is more difficult to identify infrared dark clouds . therefore , from a statistical point of view , at high latitude , clumps are potentially shifted to the far distance . this implies that the derived scale height of 14pc from this sample has to be considered as a lower limit . to first order , it appears surprising that the cold dust clump distribution and the yso distribution do not resemble each other more closely . while the submm continuum emission is optically thin and should therefore trace almost all cold dust along each line of sight , the yso distribution is more strongly affected by extinction . it may well be that a considerable fraction of ysos are not identified because of too high extinction . regarding the different properties of the atlasgal clumps and ysos toward the galactic center ( fig . [ histo_lon ] ) , a similar emission increase toward the galactic center was recently also found in the dense gas emission of nh@xmath12 ( the hops survey , e.g. , @xcite , purcell et al . , longmore et al .  in prep . ) . the lack of a prominent peak toward the galactic center in the glimpse yso data may be partly an observational bias but also partly a real physical effect . observationally , the extinction toward the galactic center increases which increases the glimpse detection threshold . however , in contrast to that , also surveys of hii regions , tracing more evolved high - mass star - forming regions , as well as h@xmath22o and ch@xmath12oh maser surveys exhibit no strong emission peaks toward the galactic center ( e.g. , @xcite ) . does that imply a relatively low gas - to - star conversion in that specific part of our galaxy ? for a more detailed discussion , see longmore et al .  in prep .. @xcite presented a similar plot to our fig . [ histo_lon ] for the distribution of irdcs in the galactic plane . while the general features for the irdcs appear similar to the dust continuum distribution , the spiral arm and star - forming regions are less pronounced . @xcite performed a similar study of the hii region distribution of the galactic plane accessible to the northern hemisphere , whose results are noticeably different to what we find . the hii region distribution does not exhibit a peak toward the galactic center region , but a clear peak is found at approximately + 30deg , corresponding to our peak for the scutum arm . previous hii region surveys show a similar hii number increase at negative longitudes around -30deg @xcite . recently , @xcite report on the ch@xmath12oh maser distribution in the galactic plane between longitude @xmath23deg . among other source count peaks , in particular they report an increased detection rates at longitudes around @xmath24 and @xmath25deg , very similar to what we find . while the peaks at @xmath24 and @xmath26 deg are likely associated with the end of the long galactic bar and the beginning of the scutum - centaurus spiral arm , the @xmath25deg peak should be associated with a tangent point of the 3kpc arm ( fig . [ sketch ] ) . in summary , the submm continuum emission is an excellent tracer of the galactic dense gas structure , even in our milky way where our location within the plane complicates the picture so severely . the finding that the average peak of the gaussian latitude distribution is below the galactic plane has already been inferred by other groups , e.g. , for ( sub)mm continuum emission @xcite , in the infrared @xcite , for co emission @xcite , clusters @xcite , hii regions @xcite or infrared bubbles ( kendrew priv . , simpson et al .  in prep . ) . even the galactic center itself is located at 0.05deg below the galactic plane @xcite . while a common interpretation of that effect is based on a poor definition of the galactic plane where neither the sun nor the galactic center itself are located directly in the plane at 0deg latitude ( e.g. , @xcite ) , @xcite recently suggested that this effect may also simply be caused by individual star formation complexes and not so much reflect a global galactic property . however , they state that the offset is mainly a feature of the molecular interstellar medium , whereas different studies of the glimpse survey indicate that the stellar component shows the same effect ( e.g. , @xcite , this study ) . although we can not conclusively differentiate these scenarios , the data here are indicative of a real global offset of the galactic midplane from its conventional position where the axis between the sun and the galactic center are located at @xmath27deg . are the different galactic latitude distributions of the submm clumps and the ysos a real physical effect or could they be caused by observational biases ? as outlined in section [ obs ] , the mass distributions and the distances on the near and far side of the galaxy of the two samples are similar . one may now ask whether the number of near sources were larger for the ysos than for the submm clumps . however , there are several effects that counteract this : at the relatively coarse spatial resolution of atlasgal ( @xmath14 ) , clumps that would be separate entities on the near side of the galaxy merge into single objects on the far side , and the total number of sources on the far side is lower than that on the near side ( tackenberg et al . in contrast to that , at the higher spatial resolution of spitzer ( @xmath28 ) , most sources remain point sources independent of the distance , and therefore less suffer from the `` merging - problem '' . furthermore , the observed volume at the far side of our galaxy is larger than that on the near side , and glimpse sources on the near side more easily saturate . these combined effects even cause a yso number increase on the far side compared to the near side of our galaxy ( fig . [ distances ] , @xcite ) . therefore , the mass and distance distributions of the submm clumps and ysos are unlikely to be the cause for the difference in the latitude distribution . furthermore , @xcite statistically excluded the agb star population from their catalogue , which implies that contamination by post - main - sequence sources is not responsible for the difference in latitude distribution as well . similar to the effect seen for the longitude distribution discussed in the previous section (  [ lon ] ) , extinction may cause part of the broader latitude distribution because toward the highest extinction regions , that are traced by the submm continuum emission , infrared emission is hardly detectable and hence the most deeply embedded yso population is likely to be missed by the spitzer data or the selection criteria in @xcite . herschel longer - wavelength data may identify such embedded population better ( e.g. , @xcite ) . @xcite recently reported a galactic average scale height for h@xmath22o masers of approximately 0.4deg , earlier finding a scale height for the ch@xmath12oh class ii masers in a similar range @xcite . assuming a comparable distance distribution for the maser as well as the submm continuum sources , the mean physical scale - height of the masers should also be @xmath2946pc . for ultracompact hii regions , @xcite found a scale height of 0.6deg , intermediate between our values for the submm clumps and the ysos . later , @xcite reported a smaller mean physical scale - height for ultracompact hii regions of @xmath330pc ( about 40% of the @xcite value ) , claiming that the @xcite sample is biased by its large fraction of b - stars . similar mean physical scale heights for high - mass star - forming regions of @xmath344pc and @xmath329pc were reported by @xcite and @xcite . hence masers as typical tracers of star - forming regions ( a fraction of the h@xmath22o masers may also stem from agb stars ) , dust continuum emission as a tracer of the dense gas , and young high - mass stars exhibit similar scale height distributions in the milky way . in comparison , @xcite find an approximate scale height of 70pc for co ( rescaled to a galactocentric solar distance of 8.5kpc ) , and @xcite derive a values of @xmath3120pc for the thick co disk ( which is an average of their 3 cited values ) . the reported cold hi scale height is around 150pc @xcite . therefore , while tracers of the youngest evolutionary stages of star formation ( submm continuum emission and masers ) are all found closest to the galactic plane , more evolved evolutionary stages like ysos as well as the less dense atomic and molecular gas appear to be located on average slightly further from the plane .    using the estimated mean physical scale heights for the dust continuum and yso distributions of 46 and 80pc , respectively , we can calculate approximate velocities required to move the 30pc difference in the given yso lifetimes of 12myrs . this estimate results in required yso velocities between 15 and 30kms@xmath5 . while velocities of that order are found ( e.g. , pv cephei , @xcite ) , they are apparently not the rule . therefore , we propose that the most likely explanation for the spread in scale height for different populations appears to be a combination of extinction effects and dissolving young clusters from their natal birth sites . we present a study of the galactic distribution of submm dust continuum emission from the northern and southern hemisphere . the submm continuum emission , which traces the dense gas emission from star - forming regions , excellently traces the structure within our galaxy . spiral arms and prominent structures of star formation are easily distinguished by significantly increased source counts toward these galactic longitudes . the galactic latitude distribution is skewed slightly below the galactic plane , and the mean physical scale height is estimated to 46pc . this scale height corresponds well to other star - formation tracers like ch@xmath12oh and h@xmath22o maser emission , and it is more confined to the galactic plane than most of the other populations in our galaxy . we compare the submm continuum emission with several other tracers of galactic structure , in particular with the yso population identified by spitzer observations . the yso population has a significantly larger scale height , and we propose that this may be attributed to a combined effect of extinction and dissolving of protostellar clusters after their birth . we thank the referee for a careful review improving the paper significantly . likes to thanks sarah kendrew for discussion about galactic distributions . lb acknowledges support from conicyt through projects fondap no . 15010003 and basal pfb-06 .
the atlasgal 870@xmath0 m continuum survey conducted with the apex telescope is the first survey covering the whole inner galactic plane ( @xmath1 & @xmath2 ) in submm continuum emission tracing the cold dust of dense and young star - forming regions . here , we present the overall distribution of sources within our galactic disk . 
 the submm continuum emission is confined to a narrow range around the galactic plane , but shifted on average by @xmath30.07deg below the plane . 
 source number counts show strong enhancements toward the galactic center , the spiral arms and toward prominent star - forming regions . comparing the distribution of atlasgal dust continuum emission to that of young intermediate- to high - mass young stellar objects ( ysos ) derived from spitzer data , 
 we find similarities as well as differences . in particular 
 , the distribution of submm dust continuum emission is significantly more confined to the plane than the yso distribution ( fwhm of 0.7 and 1.1deg , corresponding to mean physical scale heights of approximately 46 and 80pc , respectively ) . while this difference may partly be caused by the large extinction from the dense submm cores , gradual dispersal of stellar distributions after their birth 
 could also contribute to this effect . 
 compared to other tracers of galactic structure , the atlasgal data are strongly confined to a narrow latitude strip around the galactic plane .