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Massive Star Clusters in Ongoing Galaxy Interactions: Clues to Cluster Formation
We present HST WFPC2 observations, supplemented by ground-based Hαdata, of the star-cluster populations in two pairs of interactinggalaxies selected for being in very different kinds of encounters seenat different stages. Dynamical information and n-body simulationsprovide the details of encounter geometry, mass ratio, and timing. InNGC 5752/4 we are seeing a weak encounter, well past closest approach,after about 2.5×108 yr. The large spiral NGC 5754 has anormal population of disk clusters, while the fainter companion NGC 5752exhibits a rich population of luminous clusters with a flatterluminosity function. The strong, ongoing encounter in NGC 6621/2, seenabout 1.0×108 yr past closest approach between roughlyequal-mass galaxies, has produced an extensive population of luminousclusters, particularly young and luminous in a small region between thetwo nuclei. This region is dynamically interesting, with such a strongperturbation in the velocity field that the rotation curve reversessign. From these results, in comparison with other strongly interactingsystems discussed in the literature, cluster formation requires athreshold level of perturbation, with stage of the interaction a lessimportant factor. The location of the most active star formation in NGC6621/2 draws attention to a possible role for the Toomre stabilitythreshold in shaping star formation in interacting galaxies. The richcluster populations in NGC 5752 and NGC 6621 show that direct contactbetween gas-rich galaxy disks is not a requirement to form luminousclusters and that they can be triggered by processes happening within asingle galaxy disk (albeit triggered by external perturbations).Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.

Is the diffuse gamma background radiation generated by Galactic cosmic rays?
We explore the possibility that the diffuse gamma-ray backgroundradiation (GBR) at high Galactic latitudes could be dominated by inverseCompton scattering of cosmic ray (CR) electrons on the cosmic microwavebackground radiation and on starlight from our own galaxy. Assuming thatthe mechanisms accelerating Galactic CR hadrons and electrons are thesame, we derive simple and successful relations between the spectralindices of the GBR above a few MeV and the CR electrons and CR nucleiabove a few GeV. We reproduce the observed intensity and angulardependence of the GBR, in directions away from the Galactic disc andcentre, without recourse to hypothetical extragalactic sources.

Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.

Molecular Gas in Strongly Interacting Galaxies. I. CO (1-0) Observations
We present observations of the CO (1-0) line in 80 interacting galaxiesas part of a program to study the role of interactions and mergers intriggering starbursts. The sample, which only includes obviouslyinteracting pairs of galaxies, is the largest such sample observed inCO. The observations were carried out at the NRAO 12 m and IRAM 30 mtelescopes. CO emission was detected in 56 galaxies (of which 32 are newdetections), corresponding to a detection rate of 70%. Because mostgalaxies are slightly larger than the telescope beam, correction factorswere applied to include CO emission outside the beam. The correctionfactors were derived by fitting a Gaussian function or an exponential CObrightness distribution to galaxies with multiple pointings and byassuming an exponential model for galaxies with single pointing. Wecompared the global CO fluxes of 10 galaxies observed by us at bothtelescopes. We also compared the measured fluxes for another 10 galaxiesobserved by us with those by other authors using the NRAO 12 m and FCRAO14 m telescopes. These comparisons provide an estimate of the accuracyof our derived global fluxes, which is ~40%. Mapping observations of twoclose pairs of galaxies, UGC 594 (NGC 317) and UGC 11175 (NGC 6621), arealso presented. In subsequent papers we will report the statisticalanalyses of the molecular properties in our sample galaxies and makecomparisons between isolated spirals and interacting galaxies.

Galaxy structures in the Hercules region
216 redshifts have been obtained in a region of 981 sq deg south of theHercules supercluster. 172 of these redshifts are of galaxies withmpg less than or equal to 15.1, 110 of which had no previousvelocity measurement. 44 new redshifts are of galaxies fainter thanmpg = 15.1. With these new data we have been able to define asample in a vast region (approximately 1700 sq deg) around Herculeslimited to mpg less than or equal to 15.1 with a velocitycompleteness of 81.5%. 189 galaxies have been morphologically classifiedso that all galaxies in the sample with known velocity now also haveknown morphology. The magnitude limited sample, including 556 galaxies,is then used to identify and describe galaxy structures in the region.We find that the overdense volume is small, that its overall appearanceis that of a coral branch floating in a sea of nothing and that earlyand late type galaxies defined different structures.

Effects of Interactions on the Nuclear Near Infrared Properties of Spiral Galaxies
Using JHKL photometric measures available in the literature, we havecompared the near-infrared colour indices and central luminosities ofsamples of relatively isolated spiral galaxies with LINER and H IIregion-like nuclei (hereafter called L and H) with corresponding samplesof interacting galaxies, in order to explore the effects of interactionson their near-infrared properties. We have found that the L galaxies andthe isolated H galaxies have, in general, normal near-infrared colourswhich are fully explainable in terms of emission from late-type evolvedstars. On the other hand, the sample of interacting H galaxies displays,on average, somewhat peculiar near-infrared colours (specifically,normal J - H colours, but redward H - K and, especially, K - Lexcesses), which very probably indicate the presence of thermal emissionfrom hot dust, presumably related to intense star formation activityinduced by the interactions. Furthermore, for the same galaxy samplethere is some evidence that the emission at λ ~ 2.2 microns issomewhat more centrally concentrated than that at shorter wavelengths.Interestingly, the L galaxies appear to have greater centralnear-infrared luminosities than the H galaxies, for the samemorphological-type interval.

Effects of Interactions on the Radio Properties of Non-Seyfert Galaxies
On the basis of radio surveys published in the literature we havecompared the radio properties of samples of relatively isolated spiralgalaxies with LINER- and H II- region-like nuclei (hereafter called Land H galaxies) with corresponding samples of non-Seyfert interactinggalaxies, in order to explore the effects of interactions on their radioproperties. Basically, we have found enhanced total and central radioemission (per light unit) in interacting H galaxies (compared with theirrelatively isolated counterparts) and enhanced central radio emission(per light unit) in interacting L galaxies. Analogous enhancements inthe strength of the total and nuclear Hα emission lines areobserved in interacting galaxies. Furthermore, within a sample ofinteracting galaxies, there appears to be evidence of enhanced total andcentral radio emission (per light unit) in strongly interacting galaxieswhich are likely to have H II-region-like nuclei, compared withmoderately interacting objects of the same nuclear type. Interacting Hgalaxies contain more extended central radio sources than isolatedgalaxies, whereas no difference in this sense is observed in the case ofL galaxies. L galaxies which contain, on average, weaker total andcentral radio sources than the H galaxies have, on average, smallercentral radio sources (of greater radio surface brightness) than the Hgalaxies and follow a less-steep logarithmic radio power-radio sizerelation. As regards the Seyfert galaxies, which are known to becharacterized by powerful central radio emission, we have found thatthey contain, on average, central radio sources of intermediate size,which obey a power-size relation of intermediate steepness (with respectto the L and H galaxies). Thus our statistical study reveals basicstructural differences between the radio properties of the L, H andSeyfert galaxies, and between the effects of interactions on the radioproperties of the three classes of galaxies.

A deep redshift survey of IRAS galaxies towards the Bootes void
Redshifts were measured for a complete sample of galaxies detected bythe IRAS within 11.5 deg of the center of the void in Bootes discoveredby Kirshner et al (1981). There are 12 IRAS galaxies within the void asdefined by the above authors, seven of which were discovered in thissurvey. One of these has a companion at the same redshift. The resultingdensity of IRAS galaxies in the void is measured to be between 1/6 and1/3 of the average density; the uncertainty is dominated by Poissonstatistics. Good agreement is found between the selection function andnumber density derived from the present sample and those derived fromthe all-sky sample of Strauss (1989). The optical spectra of the newlyfound galaxies in the void are typical of IRAS galaxies in the field.

Crashing galaxies, cosmic fireworks
The study of binary systems is reviewed. The history of the study ofinteracting galaxies, the behavior of gas in binary systems, studies toidentify the processes that occur when galaxies interact, and therelationship of Seyfert galaxies and quasars to binary systems arediscussed. The development of an atlas of peculiar galaxies (Arp, 1966)and methods for modeling galaxy interactions are examined.

High-luminosity IRAS galaxies. II - Optical spectroscopy, modelling of starburst regions and comparison with structure
Moderate-resolution spectrophotometry was used to obtain variousemission-line ratios and emission-line luminosities for a completesample of predominantly high-luminosity IRAS galaxies. Most of theobjects exhibit H II region-like spectra, while about 12 percent areSeyferts or LINERs. The results show the IRAS galaxies to be of lowerionization than an optically selected sample of H II region-likegalaxies, possibly due to either high metallicities or to their highdust content. Although the estimated number of O stars present isconsistent with the observed emission-line flux, the IR to emission-lineluminosity ratio of all the IRAS galaxies is very high. The presentobservations can be reconciled using a model with two types of regions,type I clouds (with extinctions of about 20) representing very recentstar formation, and type II clouds (with extinctions of about 1)representing older starburst and/or general disk star formation.

High-luminosity IRAS galaxies. I - The proportion of IRAS galaxies in interacting systems
An analysis of CCD images of a sample of 60 high-luminosity IRASgalaxies from the redshift survey of Lawrence et al. (1986) and of acontrol sample of 87 optically selected galaxies from the Durhamredshift survey (Peterson et al., 1986) is presented. It is found that18 + or - 5 percent of the optically selected galaxies are interactingor merging systems, and that 11 + or - 8 percent of the low-luminosityIRAS galaxies are interacting or merging. At high luminosities, theproportion is shown to be 46 + or - 12 percent. The results suggest thatalthough galaxy interaction is a common causal factor in luminous IRactivity, it is far from being the ubiquitous factor suggested in recentreports.

Studies of IRAS sources at high galactic latitudes. IV - New redshifts and the spectroscopic properties of IRAS galaxies
New redshifts, H-alpha line fluxes, and optical continuum fluxes forIRAS galaxies are presented. Most of the galaxies show emission linesstronger than those found in optically selected spiral galaxies andcharacteristic of normal H II regions, suggesting a burst of starformation as the basic energy source. There is considerable reddeningtoward the emission-line regions and toward the unobserved UV sources,most of the energy emerging in the infrared. A minority of the casesshow high-excitation emission lines, and these are also distinguished bytheir infrared colors, typical luminosities, and emission-linestrengths. Type 2 Seyferts outnumber type 1s by two to one.

IRAS observations of an optically selected sample of interacting galaxies
IRAS observations of a large, morphologically selected sample ofstrongly interacting disk-type galaxies have demonstrated thatgalaxy-galaxy collisions can lead to enhanced infrared emission, but notin all cases. Infrared luminosities of the interacting galaxies span alarge range, but are about a factor of 2 higher, on average, than thoseof isolated disk galaxies. The data suggest the existence of a cutoff inblue luminosity, below which no galaxies show markedly enhanced infraredemission. Only the most strongly interacting systems in the sample showextreme values of infrared excess, suggesting that deep,interpenetrating collisions are necessary to drive infrared emission toextreme levels. Comparisons with optical indicators of star formationshow that infrared excess and color temperatures correlate with thelevel of star-formation activity in the interacting galaxies. Allinteracting galaxies in our sample that exhibit an infrared excess andhave higher than normal color temperatures also have optical indicatorsof high levels of star formation. It is not necessary to invokeprocesses other than star formation to account for the enhanced infraredluminosity in this sample of interacting galaxies.

Global properties of interacting disk-type galaxies
Optical, far-IR, and radio observations of global properties arepresented for a sample of strongly interacting disk-type galaxies.Global star formation rates (SFRs) for the galaxies span a large rangeand are, on average, a factor of 2.5 higher than similarly determinedglobal SFRs for isolated spiral galaxies. New star formation occurspreferentially in or near the nuclear regions. H I 21 cm emission-lineprofiles indicate the presence of anomalous velocity material andchaotic patterns of gas motion in many interacting systems. Few systemsshow evidence for the presence of a well-organized rotating H I disksuch as are seen in isolated spiral galaxies. Neutral hydrogen gasmass-to-blue luminosity ratios are not atypical when compared withisolated spirals. The evidence indicates that local rather than globalproperties of these galaxies govern the star-formation process. Theobservations generally support the notion that enhanced SFRs are causedby increased cloud collision rates and dissipative flows of gas to thenucleus.

The effects of interactions on spiral galaxies. II - Disk star-formation rates
H-alpha emission-line and IRAS far-IR observations of interacting spiraland irregular galaxies are here used to assess the influence ofinteractions on their global star-formation rates. Two samples ofinteracting galaxies were observed: a complete sample of close pairs,and an Arp atlas sample of peculiar systems. When compared to a controlsample of single galaxies, both samples of interacting systems exhibitsystematically higher levels of H-alpha and infrared emission onaverage, and a larger dispersion in emission properties. Emission levelsin the very active system are much more strongly correlated with theproperties of the interaction than with the internal properties of thegalaxies themselves. Strong disk emission is almost always accompaniedby unusually strong nuclear activity. Simple star-formation burst modelscan reproduce the observed H-alpha equivalent widths and broadbandcolors of most of the galaxies. The bursts are relatively short (fewtimes 10 million yr) and rarely involve more than 1-2 percent of agalaxy's total mass.

The effects of interactions on spiral galaxies. III - A radio continuum survey of galactic nuclei at 1.49 GHz
The radio continuum emission from the central region of a sample ofinteracting spiral galaxies (92 galaxies of which 60 in a completesample) and of a control sample of more isolated spiral galaxies (94)was observed with the Very Large Array at 1.49 GHz. The angularresolution of the observations is about 1.3 arcsec, and the detectionlimits are about 0.6 and 1.5 mJy for point sources and extended sourceswith a half power size of 10 arcsec, respectively. This survey, incombination with published optical spectroscopy, provides the data for adetailed comparison of the central region in interacting and moreisolated spiral galaxies.

Star-formation rates in the nuclei of violently interacting galaxies
It has now been recognized that galaxy-galaxy interactions represent animportant evolutionary process in many extragalactic systems, takinginto account large-scale star-formation bursts, Seyfert activity, andQSO phenomena. The present paper provides results of aspectrophotometric survey of the nuclear regions of a large sample ofviolently interacting galaxies. Attention is given to sample selection,observational procedures and data reduction, data analysis, aspects ofspectral classification, ionization mechanisms, emission-line strengths,correlations with integral properties, comparisons with other studies,and a comparison with theoretical models.

The effects of interactions on spiral galaxies. I - Nuclear activity and star formation
When the present results of spectrophotometry for the nuclei of 161(mostly spiral) galaxies with bright companions and emission lineimaging of 63 galaxies were compared to a similarly observed sample ofisolated-spiral nuclei, both samples of interacting galaxies exhibitedsignificant excesses of nuclear emission. The rate of nuclear starformation is significantly above average even in systems withoutnoticeable tidal distortion in the outer disks, suggesting that thenear-nuclear gas is only marginally stable in isolated galaxies. Theresults obtained suggest that nuclear phenomena are triggered by atidally induced influx of gas from the disk into the nuclear regions,rather than gas transfer between the galaxies.

Recent star formation in interacting galaxies. I - Evidence from JHKL photometry
A survey has been carried out using JHKL photometry to investigaterecent star formation in interacting galaxies. The objective was to lookfor a K-L excess produced by 'warm' dust heated by a putative burst ofstar formation. K-L excesses are found suggesting that interactionsinduce starbursts with an efficiency approaching 100 percent. Theappearance of these inferred starbursts in interacting systems ofdifferent morphological types is qualitatively consistent with dynamicalstudies of galaxy interactions. However, the common occurrence of suchstarbursts shows that interactions have implications for theastrophysics of galaxies well beyond purely morphological effects.

Atlas of Peculiar Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966ApJS...14....1A&db_key=AST

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Observation and Astrometry data

Right ascension:14h45m24.40s
Aparent dimensions:0.692′ × 0.479′

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NGC 2000.0NGC 5755

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