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Paper text:
galaxy interactions are well known to induce bursts of star formation within the central kiloparsecs of galaxies and within their tidally - distorted features ( e.g.  @xcite ; @xcite ; @xcite ) .
it has been previously suggested that star formation is manifested in different kinds of stellar  packaging , " like isolated stars , bound groups , loose associations , globular clusters , and dwarf galaxies @xcite .
it is now evident , however , that clustered star formation is the sole product of star forming activity ( @xcite ; @xcite ) , shaped by a number of disruption mechanisms to produce field stars and associations .
tidal tails have lower extinctions and less complicated star formation histories  and resulting stellar populations  than galaxy interiors .
they therefore present a unique opportunity to study a relatively observationally simple , yet physically interesting environment , and assess how it shapes cluster sizes , color and magnitude distributions , and statistical properties .
we may also compare what is observed in these environments to prior studies of galaxy interiors .
we report our recent work in elucidating this issue with general diagnostics of cluster candidates detected in _ hst _ wfpc2 images .
this represents a pilot study of using a large sample of galaxies with limited bandpass coverage to uncover the physics of star cluster formation and survival ; a test of how to do more with less .
the sample of tidal tails is presented in figure 1 as sloan digital sky survey images with wfpc2 fields of view overlaid .
these systems were selected to represent a range of optical brightnesses , merger ages , hi column densities and masses , interaction mass ratios , and presence of tidal dwarf candidates ( mullan et al .  in preparation ) .
a full description of observations and data reduction procedures for these systems will be presented in mullan et al .
( in preparation ) . in summary , star cluster candidates were detected using the daofind program in the iraf package daophot .
the criteria for selection and follow - up extinction- and charge transfer efficiency - corrected photometry ( @xcite ; @xcite ) with the phot task of apphot were chosen to emulate @xcite , including the criterion @xmath3 @xmath4 2.0 .
this allows populations of old , metal poor globular clusters ( @xmath3 @xmath5 1 ) akin to those observed in the galaxy @xcite , as well as metal - rich clusters with @xmath3 @xmath4 1.5 ( e.g.  @xcite ) .
this also allows solar metallicity globular clusters with @xmath3 @xmath5 1.21.3 ( lee , lee , & gibson 2002 ) .
the tail regions were defined in @xmath0- band , gaussian - smoothed images as contours set to one count above the average background , corresponding to 24.3  25.7 mag arcsec@xmath6 in @xmath0 .
central galaxies , where present in the images , were excluded based on where their radial brightness profiles indicated anti - truncated disk debris @xcite .
our detections are 50% complete to @xmath7 25.7 , and k - s tests indicate that in - tail sources are not likely drawn from the same population as the out - of - tail sources ( p @xmath4 0.1 in most cases ) .
figure 2 shows an image of a representative system , ngc 2992 , with in - tail cluster candidates indicated with circles and squares for out - of - tail objects meeting the same selection criteria . a color - magnitude diagram for the in - tail objects
is also presented , with photometric model tracks of 10@xmath8 , 10@xmath9 , and 10@xmath10 m@xmath11 simple stellar populations with instantaneous - burst star formation rates @xcite overlaid .
although two photometric bands are insufficient to draw robust conclusions on individual masses and mass functions , rough estimates and limits can be made . in all systems ,
v@xmath12 and @xmath13 indicate that the most massive clusters are @xmath5 10@xmath8 m@xmath11 .
we do not find clusters in our sample with masses @xmath14 10@xmath15 m@xmath11 .    by counting the number of in - tail cluster candidates per unit area of the tail , and subtracting the number of sources found with the same selection criteria outside the tail per unit area of non - tail wfpc2 regions
, we find a cluster candidate surface density for each system .
figure 3 compares our results to those of @xcite , given an additional m@xmath2 @xmath4 -8.5 ( johnson - cousins system ) cutoff .
on the surface , our results are compatible with @xcite who find similar values , though we contend that accounting for rapid cluster fading with age by extending the limit to fainter magnitudes and improving upon background subtraction reveals clusters in every star - forming tail ( mullan et al . , in preparation ) .
also provided is a typical statistical diagnostic for the cluster populations , a plot of the brightest m@xmath16vs .
the logarithm of the number of clusters detected for each system .
the slope of this correlation , -2.50 @xmath17 -0.05 , is similar to the -2.3 @xmath17 0.2 value found for other extragalactic environments @xcite .
@xcite suggests that this statistical size - of - sample effect can be produced from a schechter cimf with a exponential slope of -2 and cutoff ( m@xmath18 ) mass of @xmath19 2.1 @xmath20 10@xmath9 m@xmath11 , given cluster disruption mechanisms from gas expulsion , relaxation , stellar evolution , and the external environment ( e.g.  @xcite ; @xcite ) . in many cases such as ngc 6872e and am 1054 - 325w , the profusion of sources between 10@xmath9  10@xmath8 m@xmath11 may indicate higher star formation efficiencies from high gmc compression by a turbulent ism @xcite or through a similar magnetohydrodynamic effect @xcite in these  ideal " star - forming tails .
this may also affect the luminosity function , whose luminosity - dependent logarithmic slope of -2.5 as sampled in this v@xmath12 range ( see the contribution by gieles ; @xcite ) may change with the altered cimf .
but without any clusters of mass 10@xmath15 m@xmath11 , the star formation efficiencies in these tails may not be as high as in the antennae , m82 , ngc 1316 , ngc 7252 , or arp 220 ( @xcite ; @xcite ; @xcite ; @xcite ) .
preliminary evidence suggests that star cluster populations are governed by similar luminosity and initial mass functions as their cousins in field galaxies .
our set of 17 tails show a maximum cluster mass of @xmath510@xmath8 m@xmath11 , while the brightest m@xmath21 vs.  log n plot is consistent with both mass - dependent and mass - independent prescriptions of cluster disruption (  infant mortality " ) , with subsequent dissolution from 2-body relaxation and stellar evolution for clusters drawn from a schechter cimf .
such a cimf , if it had a higher cutoff mass , would imply higher star formation efficiencies , aided by turbulent pressure in the tidal tails .
future work will address these areas through further analysis of the color , magnitude , and size distributions of these clusters , as well as the kinematics and ism composition of their surrounding environments .
we have used f606w ( @xmath0)- and f814w ( @xmath1)- band images from the _ hubble space telescope _ ( _ hst _ ) to identify compact stellar clusters within the tidal tails of twelve different interacting galaxies . 
 the seventeen tails within our sample span a physical parameter space of hi / stellar masses , tail pressure and density through their diversity of tail lengths , optical brightnesses , mass ratios , hi column densities , stage on the toomre sequence , and tail kinematics . our preliminary findings in this study indicate that star cluster demographics of the tidal tail environment are compatible with the current understanding of star cluster formation in quiescent systems , possibly only needing changes in certain parameters or normalization of the schechter cluster initial mass function ( cimf ) to replicate what we observe in color - magnitude diagrams and a brightest m@xmath2  log n plot .    2 2