A Morphology--Cosmology Connection for X--Ray Clusters

We employ N--body/$3D$ gas dynamic simulations of the formation of galaxy clusters to determine whether cluster X--ray morphologies can be used as cosmological constraints. Confirming the analytic expectations of Richstone, Loeb, \& Turner, we demonstrate that cluster evolution is sensitive to the cosmological model in which the clusters form. We further show that evolutionary differences are echoed in the gross morphological features of the cluster X--ray emission. We examine current--epoch X--ray images of models originating from the same initial density fields evolved in three different cosmologies: (i) an unbiased, low density universe with \Omega_o \se 0.2; (ii) an unbiased universe dominated by vacuum energy with \Omega_o \se 0.2 and \lambda_o \se 0.8 and (iii) a biased Einstein--deSitter model (\Omega \se 1, $\sigma_8=0.59$). Using measures of X--ray morphology such as the axial ratio and centroid shifting, we demonstrate that clusters evolved in the two low $\Omega_o$ models are much more regular, spherically symmetric, and centrally condensed than clusters evolved in the Einstein--deSitter model. This morphology--cosmology connection, along with the availability of a large body of cluster X--ray observations, makes cluster X--ray morphology both a powerful and a practical cosmological discriminant.Comment: (uuencoded, compressed postscript, 9 pages including figures), CFA-370

Building a CCD Spectrograph for Educational or Amateur Astronomy

We discuss the design of an inexpensive, high-throughput CCD spectrograph for a small telescope. By using optical fibers to carry the light from the telescope focus to a table-top spectrograph, one can minimize the weight carried by the telescope and simplify the spectrograph design. We recently employed this approach in the construction of IntroSpec, an instrument built for the 16-inch Knowles Telescope on the Harvard College campus.Comment: 17 pages including 7 figures, PASP, accepted (higher resolution figures at http://cfa-www.harvard.edu/~sheila/introspec.ps.gz

Spectrophotometry of nearby field galaxies: the data

We have obtained integrated and nuclear spectra, as well as U, B, R surface photometry, for a representative sample of 196 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M_B=-14 to -22). Here we present the spectrophotometry for these galaxies. The selection of the sample and the U, B, R surface photometry is described in a companion paper (Paper I). Our goals for the project include measuring the current star formation rates and metallicities of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt (1992a) to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift. We describe the observing, data reduction and calibration techniques, and demonstrate that our spectrophotometry agrees well with that of Kennicutt. The spectra span the range 3550--7250 A at a resolution (FWHM) of ~6 A, and have an overall relative spectrophotometric accuracy of +/- 6 per cent. We present a spectrophotometric atlas of integrated and nuclear rest-frame spectra, as well as tables of equivalent widths and synthetic colors. We study the correlations of galaxy properties determined from the spectra and images. Our findings include: (1) galaxies of a given morphological class display a wide range of continuum shapes and emission line strengths if a broad range of luminosities are considered, (2) emission line strengths tend to in- crease and continua tend to get bluer as the luminosity decreases, and (3) the scatter on the general correlation between nuclear and integrated H_alpha emission line strengths is large.Comment: Accepted for publication in ApJS (scheduled for Vol.127, 2000 March); 63 pages, LateX, 9 figures and 6 tables included, a spectrophotometric atlas is provided as GIF images, fig 1 as a JPEG image, in a single tar-file; a full 600 dpi version is available at http://www.astro.rug.nl/~nfgs

Galaxy Morphologies in the Cluster CL1358+62 at z=0.33

We describe the morphological composition of a sample of 518 galaxies in the field of CL1358+62 at z=0.33, drawn from a large HST mosaic covering 53 sq. arcmin. The sample is complete to I=22, corresponding to M_V=-18.5 in the rest frame. The galaxies have been independently classified by the authors of this paper and by Alan Dressler. For galaxies with I<21, the two sets of classifiers agree on the total early-type population, but disagree on the S0/E ratio. We discuss the constraints on morphological evolution and the implication of the differing S0/E ratios. We use our large body of spectra to make the correspondence between morphological and spectral type.Comment: includes 10 fig