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Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| X-Ray Constraints on Accretion and Starburst Processes in Galactic Nuclei. I. Spectral Results
The results of the analysis of 0.4-10.0 keV ASCA spectral analysis of asample of low-luminosity active galactic nuclei (AGNs) (LLAGNs; M51, NGC3147, and NGC 4258), low-ionization nuclear emission-line regions(LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, and NGC 4594), andstarburst galaxies (M82, NGC 253, NGC 3628, and NGC 6946) are presented.In spite of the heterogeneous optical classifications of these galaxies,the X-ray spectra are fitted well by a ``canonical'' model consisting ofan optically thin Raymond-Smith plasma ``soft'' component with T~7x10^6K and a ``hard'' component that can be modeled by either a power lawwith a photon index Gamma~1.7 or a thermal bremsstrahlung with T~6x10^7K. The soft-component absorption is typically less than 10^21 cm^-2,while the hard component is typically absorbed by an additional columnon the order of 10^22 cm^-2. The soft-component 0.4-10 keV intrinsicluminosities tend to be on the order 10^39-40 ergs s^-1, while thehard-component luminosities tend to be on the order of 10^40-41 ergss^-1. The abundances inferred from the fits to the soft component aresignificantly subsolar. The Fe abundance can be measured independentlyof the other elemental abundances (dominated by alpha-process elements)in M51, M82, NGC 253, and NGC 4258. In these galaxies the Fe abundancerelative to alpha-process elements is also (statistically) significantlysubsolar. There is some indication (at a low statistical significance)that the abundance properties of starburst emission from starburstgalaxies differs from the starburst emission from low-luminosity AGNs.However, these results on abundances are model dependent. Significant FeK line emission is observed in M51, M82, NGC 3147, NGC 4258, and NGC4579. An analysis of the short-term variability properties was given inPtak et al. and detailed interpretation of these results will be givenin Paper II in this series.
| A comparative study of morphological classifications of APM galaxies
We investigate the consistency of visual morphological classificationsof galaxies by comparing classifications for 831 galaxies from sixindependent observers. The galaxies were classified on laser print copyimages or on computer screen using scans made with the Automated PlateMeasuring (APM) machine. Classifications are compared using the RevisedHubble numerical type index T. We find that individual observers agreewith one another with rms combined dispersions of between 1.3 and 2.3type units, typically about 1.8 units. The dispersions tend to decreaseslightly with increasing angular diameter and, in some cases, withincreasing axial ratio (b/a). The agreement between independentobservers is reasonably good but the scatter is non-negligible. In spiteof the scatter, the Revised Hubble T system can be used to train anautomated galaxy classifier, e.g. an artificial neural network, tohandle the large number of galaxy images that are being compiled in theAPM and other surveys.
| Redshift observations in the Hydra-Centaurus region
The paper reports 406 redshifts for galaxies in the northern galactichemisphere, south of delta = 0 deg. A substantial fraction of theobserved galaxies are located in the equatorial zone between deltavalues of -17.5 and 0 deg. By combining these new data with thoseavailable in the literature, it is possible to extend the original CfAredshift survey of galaxies brighter than m(B(0)) = 14.5 to b = 30 deg,south of delta = 0. New data taken at lower galactic latitudes alsocontribute to the existing surveys of the Hydra-Centaurus complex.
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