Russell J. Lavery

High-resolution imaging of distant clusters. I : Close pairs, interactions, and the Butcher-Oemler' effect at z=0.4

We present new ground-based high-resolution images of 23 blue galaxies whose membership has been confirmed spectroscopically in the two distant rich clusters Abell 370 and Cl 0024+1654. The images, taken with the High Resolution Camera on the CFHT, have a stellar FWHM=0.45 arcsec. Ten of these blue galaxies are found to have close companions, with separations on the scale of 1 to 2 arcsec. These multiple systems represent 45% of the confirmed blue galaxy population in the two clusters. Several of the remaining blue galaxies also have peculiar morphologies and features, such as tidal tails, suggestive of interactions/mergers

A new member of the Local Group : the Tucana dwarf galaxy

Observations of the recently discovered dwarf galaxy in Tucana, which is a likely member of the Local Group, are presented. The system is highly flattened and classified as dE5. An upper limit on the distance modulus of 24.75 is determined from a V, V-I color-magnitude diagram. This galaxy has a B-R color of 1.1, and with the upper limit to the distance modulus, an absolute R magnitude of −10. This apparently isolated dwarf system, being on the opposite side of the Milky Way from most of the Local Group, may provide important constraints on Local Group dynamics

Multiaperture spectroscopy of galaxies in Abell 370

Results of spectroscopic observations of 30 galaxies in the distant rich cluster Abell 370 at z = 0.376 are presented which confirm the large fraction of blue galaxies in this cluster measured photometrically by Butcher and Oemler (1978, 1984). None of the spectra can be attributed to active galactic nuclei. All blue galaxy spectra may be described as examples of poststarburst objects with strong higher order Balmer lines and very weak or absent emission lines. A comparison of these data with synthetic spectral calculations indicates that a wide range of burst parameters is needed to describe the blue galaxies. From 1 percent to 10 percent of the galaxy mass forming stars between 30 and 200 million years prior to the epoch of observation spans the allowed range. 37 references.

Observations of a high surface brightness arclet associated with the cluster of galaxies GHO 2154+0508

We present the results of photometric and spectroscopic observations of a high surface brightness arclet associated with the cluster of galaxies GHO 2154+0508. High-resolution imaging (FWHM ≃0″.45) of this arclet, presumably the result of gravitational lensing of a background galaxy, show it to be resolved, having FWHM ≃1″.1, with lengths of 3″.3 and 4″.5 in the V band and R band, respectively. We find the arclet to have V=22.07±0.08, (V−R) KC =0.7, and a mean central surface brightness in the V band of 23.7 mag arcsec −2

Observations of the blue arcs in Abell 963

Initial spectroscopic observations of the two blue arcs in Abell 963 show no evidence for strong emission lines, though continuum radiation has been detected. Since these arcs are likely the result of gravitational lensing, this observation puts several constraints on the nature of the lensed object. The lack of any broad emission lines limits the possibility of the lensed object being a QSO unless it is at a redshift of greater than 5. If the lensed object is a galaxy with [O II] λ3727 emission, as might be expected from the arc color, the galaxy must be at a redshift of greater than 0.9. The lack of Lyman α emission constrains the redshift to be less than 2.5.

Interactions in “Butcher-Oemler” Clusters

In their pioneering photometric study on the color evolution of galaxies in rich clusters, Butcher and Oemler (1978) found that two Coma-like clusters of galaxies at redshifts near 0.4 contained a much larger population of “blue” galaxies than expected. This suggested rapid color evolution of galaxies in 1/3 of a Hubble time. In order to put this on a more quantitative level, Butcher and Oemler (1984a) undertook a photometric study of 33 clusters of galaxies in the redshift range of 0.0 to 0.5. This study confirmed their original result, showing that the cores of a number of concentrated clusters at redshifts greater than 0.2 had a blue galaxy fraction of order 20%, whereas such clusters nearby had a blue galaxy fraction of only 3%. Still, this result was not readily accepted until spectroscopic observations were made of the clusters containing this large blue galaxy population.