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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.
| 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.
| A catalog of stellar velocity dispersions. I - Compilation and standard galaxies
A catalog of central stellar velocity dispersion measurements ispresented, current through June 1984. The catalog includes 1096measurements of 725 galaxies. A set of 51 standard galaxies is definedwhich consists of galaxies with at least three reliable, concordantmeasurements. It is suggested that future studies observed some of thesestandard galaxies in the course of their observations so that differentstudies can be normalized to the same system. Previous studies arecompared with the derived standards to determine relative accuracies andto compute scale factors where necessary.
| A search for environmental effects on the optical properties of galaxies in groups
Environmental density-related modifications of basic optical properties(luminosities, sizes, axial ratios, and colors) of galaxies belonging toGeller and Huchra's (1983) groups have been investigated. Remarkably, itis found that the broad maxima of the distributions of luminosities anddiameters of spirals and the whole corresponding distributions oflenticulars tend to move to lower values as one goes to groups of highcompactness, whereas the luminosity-diameter relationship of spiralstends to become flatter. No color and axial ratio differences betweengalaxies of high- and low-compactness groups have been detected.
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