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the exciting evidence for evolution in distant clusters of galaxies @xcite has diverted attention from nearby clusters .
this is unfortunate , since although nearby clusters show less evidence of galaxy activity they may be studied to much greater depth .
the findings may then be applied to studies of higher redshift clusters and to the general understanding of the cause for evolution in cluster populations with redshift .
the potential importance of dust in the active galaxies of distant clusters @xcite underscores the need for selecting samples independent of the effects of dust extinction .
radio continuum emission ( in this study we will use a frequency of 1.4 ghz ) is a powerful probe of activity caused by both active galactic nuclei ( agn ) and current star formation @xcite .
it is particularly attractive for this latter type of activity , as the radio continuum emission is directly related to the star formation rate ( sfr ) .
consequently , a radio flux - limited sample drawn from a given cluster will select all agn and galaxies forming stars at a rate greater than some adopted threshold .
we have constructed a sample of radio galaxies in 20 nearby abell clusters @xcite . for the nearer clusters ,
the radio data are taken from the nrao vla sky survey @xcite while for the more distant clusters we have collected our own vla data @xcite .
the luminosities associated with the flux limits of these data correspond roughly to that of the milky way , meaning our completeness limit is a sfr of 4m@xmath0 yr@xmath1 ( for stars with masses @xmath2 m@xmath0 , using the relationship of @xcite ) .
the more sensitive observations ( i.e. , the nearest nvss clusters and our vla data for the more distant clusters ) detect sfrs as low as 1m@xmath0 yr@xmath1 .
velocity measurements based on optical spectra indicate whether the radio galaxies are members of their respective clusters , or merely seen in projection .
the full sample contains over 400 cluster radio galaxies and over 150 foreground and background radio galaxies .
we have quality optical spectra for about half of the cluster radio galaxies , which enables us to identify the source of the radio emission as either star formation or an agn .    using this sample
, we ask the question : do any clusters show more evidence for activity and if so , why ?
the motivation is to understand the butcher - oemler effect , including both the trend and its scatter .
as noted , we also wish to establish a nearby benchmark for evolutionary studies of distant clusters .
analogous to the blue fraction of butcher - oemler studies , we calculated the fraction of cluster galaxies associated with radio emission ( @xmath3 ) .
for this calculation , we used all galaxies brighter than @xmath4 ( determined from the systemic velocities of the clusters ) .
the optical spectroscopy indicated which radio galaxies were cluster members and which were foreground / background objects , while the total number of galaxies for each cluster was corrected for estimated background counts . to place the clusters on even footing , we used only sources with radio luminosities greater than the nvss completeness limit for the most distant cluster which used nvss data ( i.e. , radio detections at lower luminosities were considered non - detections for the fractional testing ) . in each cluster
, we surveyed from the cluster center out to a radial distance of 3@xmath5 mpc , or @xmath6 . for the @xmath3 calculations we applied a radial limit of 2@xmath5 mpc in order to reduce variation caused by the background correction .
the @xmath3 values were then compared via a @xmath7 test .
two clusters showed an enhancement in activity ( larger @xmath3 than the other clusters of the sample , significant at @xmath8 confidence ) , abell 1185 and abell 2255 . to explore what types of galaxies cause these enhancements
, we investigated @xmath3 as a function of optical magnitude .
three bins in optical magnitude were created : @xmath9 , @xmath10 , and @xmath11 . for reference ,
@xmath12 for ellipticals @xcite
so these approximately correspond to bright ellipticals , larger spirals and s0s , and later - type spirals and irregulars , respectively .
the increased @xmath3 for abell 1185 was revealed to be caused by the intermediate optical magnitude galaxies . for abell 2255 ,
the increased @xmath3 was caused by both the optically brighter and fainter galaxies ( see figure [ fig - comprlf ] ) . in fact , the excess of the fainter galaxies was significant at over @xmath13 .
this result is robust against variations in the background count .
in fact , should _ all _ optically - faint galaxies in the direction of abell 2255 be cluster members ( hence no background correction and a minimum value for @xmath3 ) , the radio galaxy enhancement would still be significant at @xmath14 confidence .      with complete optical spectroscopy for abell 2255 , we further analyzed the emission source for the radio galaxies .
of the 23 optically - faint galaxies in this cluster , 21 had radio emission associated with star formation .
the other two were weak emission - line agn . among the star - forming galaxies
there were many examples of rigorous star formation , including three starbursts with ew([oii])@xmath15 40@xmath16 and four dusty starbursts with slightly weaker ew([oii ] ) but strong h@xmath17 absorption ( see @xcite ) .
additionally , one galaxy had a strong post - starburst spectrum .
thus , there is strong evidence for current and dramatic activity within the galaxy population of abell 2255 .
why is abell 2255 so special ?
we searched for correlations between @xmath3 and parameters such as cluster richness and overall galaxy distribution ( i.e. , is the cluster compact or fairly dispersed ? ) , but found none . in studies of more distant butcher - oemler clusters , an increased @xmath3 for abell 2125 as compared to abell 2645
was noted despite the nearly identical redshift and richness of these clusters @xcite .
the authors concluded that the discrepancy was likely the result of abell 2125 being a cluster - cluster merger rich in substructure whereas abell 2645 is more virialized .
this explanation is attractive in the case of abell 2255 , as it is also a strong merger candidate ( e.g. , @xcite ) .
the main problem with this explanation is that other clusters in our sample are also probable mergers . as a case example , we discuss abell 2256 ( see @xcite for merger evidence ) .
radio observations of this cluster were performed concurrently with those of abell 2255 , and the two clusters are also comparable in richness and redshift . however , with the possible exception of the optically - brightest galaxies the @xmath3 values for abell 2256 were normal ( see figure [ fig - comprlf ] ) .
consistent with this result is the measured blue fraction for abell 2256 , @xmath18 @xcite .
the key appears to be a combination of the merger stage and large - scale structure .
we assessed the substructure in each cluster using a variety of techniques , including the dressler - schectman test @xcite and a kmm analysis to separate galaxies into respective substructures @xcite .
each cluster showed evidence for substructure , but the specific distributions of the star - forming galaxies are illustrative . in abell 2255 , the star - forming galaxies were found in a strong north - south alignment which proved statistically significant through monte carlo testing .
this alignment is perpendicular to the merger axis , which is east - west and fairly close to the plane of the sky @xcite .
we would expect such an arrangement if the cluster - cluster merger initiated bursts of star formation as galaxies experienced spikes in ram pressure as they crossed the shock front of a very recent merger @xcite .
presumably , the precursors to these starbursts were field - like spirals at the peripheries of the pre - merger systems . in abell 2256
, the star - forming galaxies are clustered slightly to the north of the cluster center , and prove to be almost exclusively associated with a high velocity group composed of around 30 members .
interestingly enough , this substructure is not one of the culprits suggesting a large - scale merger in abell 2256 ; many more galaxies are associated with the primary cluster and a large infalling subcluster , the two obvious merger partners from the x - ray @xcite . that the star - forming galaxies originate in an infalling group
underscores that the merger of a pair of rich clusters alone is likely not enough to cause an increase in the active galaxy population relative to other clusters .
some `` fuel '' is required , in particular a population of gas - rich galaxies which can undergo a starburst .
these are apparently supplied by outlying groups and field galaxies which are thrown into the mix by the merger .
note that the main population responsible for the increased @xmath3 in abell 2255 are galaxies one to two magnitudes fainter than @xmath19 , similar to the magnitudes of late - type field spirals and the blue galaxies observed in butcher - oemler studies @xcite . in summary
, we apparently see an enhancement in activity for abell 2255 as a result of both timing ( viewing the system after very recent core passage ) and large - scale structure ( having available galaxies which could undergo a starburst ) .
the relationship of this explanation to the butcher - oemler effect is also quite attractive .
currently favored cosmological models indicate that clusters are actively being assembled from smaller clusters and groups since @xmath20 , with the rate of such mergers increasing with redshift .
consequently , the trend of increased blue fraction ( and @xmath3 ) with redshift is indicative of the assembly history of clusters .
by @xmath21 , most clusters are fairly well assembled and virialized and consequently contain fewer active galaxies .
the spread in the fraction of active galaxies at any given redshift is a reflection of the diversity of cluster dynamical states .
more relaxed clusters would have fewer active galaxies , while clusters in the process of assembly would have more active galaxies .
in fact , this explanation has been noted to alleviate the discrepancy in galaxy activity observed for the morphs and cnoc clusters , as the cnoc sample are x - ray selected and consequently more frequently associated with relaxed clusters @xcite .
lastly , this spread in dynamical state indicates that galaxy activity may be observed even in nearby clusters .
of course , this explanation requires a much larger sample of clusters to fill in necessary details .
similar results are being found for very rich clusters at higher redshift , in a sample which includes the aforementioned abell 2125 @xcite .
we have also obtained deep , wide - field radio and optical images of several nearby clusters ( out to @xmath22 ) in a range of merger stages .
the author thanks the carnegie observatories for an informative and enjoyable conference , and frazer owen , bill oegerle , and john hill for their involvement in this work .
the author also acknowledges the national radio astronomy observatory for a predoctoral appointment , and the national research council for postdoctoral support through an associateship award held at nasa goddard space flight center .
radio continuum emission at 1.4 ghz was used to identify active galaxies in a sample of 20 nearby abell clusters . 
 the radio emission indicates either an active galactic nucleus or current star formation , and optical spectroscopy was used to evaluate which of these dominates for each of the radio - selected galaxies . in an analysis which parallels the blue fractions of butcher - oemler studies , we calculated radio galaxy fractions for the clusters . 
 one cluster in particular shows a dramatic increase in activity relative to the others : the cluster - cluster merger abell 2255 . 
 we compare the results for abell 2255 with those for abell 2256 to assess the role of cluster - cluster mergers on the star formation activity of member galaxies . from these clusters , as well as 
 several identified in other studies , a picture emerges in which substructure and cluster dynamical activity are of great importance in understanding evolutionary phenomenon such as the butcher - oemler effect . 
 increased fractions of active galaxies ( be they the blue galaxies of butcher - oemler studies or radio galaxies as discussed here ) naturally result as clusters are built through mergers of smaller groups in hierarchical formation scenarios . 
 the observed spread in fractions of active galaxies at any given redshift reflects the spread in cluster dynamical state .    
 [ 1996/06/01 ]    a&a