E. E. Salpeter

Dynamics of binary galaxies. I: Wide pairs

We analyze pairs of galaxies selected from the CfA magnitude limited and SSRS diameter-limited redshift catalogs. The pairs are chosen only from regions of low galaxy density and have projected separations of up to 1.0 Mpc. This is almost an order of magnitude larger than the typical projected separations of samples of binary galaxies used in earlier studies. Accurate H I velocities for these pairs were obtained at both Arecibo and Parkes Observatories, and we tabulate these new velocities along with total H I flux and other quantities of interest. The velocity measurements are very homogeneous, and we estimate the typical internal error in the measured systemic velocity to be under 5 km s −1

Close galaxy pairs in low and medium density regions: the Southern sky

We extend to the southern hemisphere a continuing program of optical and HI observations of galaxy pairs (Chengalur \etal 1993, 1994, 1995 and Nordgren \etal 1997). These pairs are drawn from published redshift catalogs and represent a complete sample. We present new data of 15 pairs observed with the Palomar 5-meter telescope, Mount Stromlo Siding Spring 40-inch telescope, Australia Telescope Compact Array and VLA D synthesis array. These galaxy pairs are all defined as close pairs (projected separations < 100 kpc). HI companions are found near five of 15 pairs.

Dynamics of Binary Galaxies. III. Details of the Close Pairs

In a previous paper [Chengalur et al. ApJ, 107, 1984(1994)] H I synthesis data and CCD images were presented for an objectively chosen sample of six close spiral-spiral galaxy pairs. In this paper we present a detailed analysis of the data. Although the presence of tidal features was not an explicit selection criterion, five of our six pairs show tidal tails and bridges. The remaining pair also shows signs of interaction: both galaxies in the pair have disturbed velocity fields and one of the two galaxies has a central concentration of molecular gas and is undergoing a starburst. The morphology of three of the six pairs implies that the galaxies are on high eccentricity, low total energy orbits, and for one of these pairs there is some evidence that the impact parameter is as small as ~1 disk radius. For the remaining three pairs the tidal features are not as pronounced, and the observations do not yield as much information on the orbit. However, even for these pairs, the observed morphology is consistent with the galaxies being on slow, small impact parameter orbits, provided the galaxies are at a relatively early phase of the orbit, i.e., prior to the first close encounter. We also find weak evidence for preferential alignment in the pairs (with the lack of preferential alignment being ruled out at the 97% level), in the sense that the major axis of a galaxy tends to point toward the companion galaxy. This sample of close pairs is complemented by a larger sample of wide pairs (Chengalur et al. 1993). The median velocity difference {DELTA}V of the sample of close pairs in low-density regions is not much different from that for the wide pairs sample, in particular, there is no evidence for positive energy impacts. The effect of dynamical friction on infall velocities is controversial and will be discussed in the following paper [Chengalur et al., in preparation (1995)] which combines the data on close and wide pairs.

Dark matter and dynamics in the Hercules Cluster (A2151)

We present new 21 cm observations of faint (15.7<m pg <16.5) spiral galaxies in the Hercules cluster (A2151). These results, when combined with the large body of velocities available in the literature for Hercules, permit us to study the dynamics throughout the cluster core, out to a projected radius of 1.8 h −1 Mpc. We calculate the global dynamical mass of Hercules using two different, but related, methods: the virial theorem and the projected mass estimator. These masses lie in the range 3-9×10 14 M ○ .. We investigate the importance of subclustering in A2151 by eye, by using the statistical test of Dressler & Shectman, and by considering the effects of the detected substructure on the dynamical mass determination

A catalog of galaxies in Hercules from the Palomar Sky Survey and the Minnesota Automated Plate Scanner

The development of galaxies catalogs from the Palomar Sky Survey using the Minnesota Automated Plate Scanner (APS) is examined. The design and operation of APS are described. The algorithm used to distinguish stars from galaxies is discussed, and the completeness and confusion of the algorithm as a function of magnitude are estimated. The quality of the photometric calibration for galaxy magnitudes and surface-brightness thresholds is evaluated. The relation between the distribution of colors and morphological types is studied. The luminosity functions for Hercules galaxies are calculated and presented. The catalog generated using the APS is 80 percent complete; photometry good to 0.23 mag rms is possible; and J-F colors are good to 0.17 mag rms.

1.4 GHz continuum sources in the Cancer cluster

Results of 1.4-GHz continuum observations are presented for 11 VLA fields, using the D-configuration, which contain the A group of the Cnc cluster (CC). Sixteen Zwicky spiral galaxies in the CC were detected, but no ellipticals, confirming the finding that spiral galaxies with close companions tend to have enhanced radio emission. Over 200 continuum sources beyond the CC are tabulated. The spectral index (relative to 610 MHz) is given for many of the sources, including some of the Zwicky galaxies. There is a suggestion for a nonuniform number surface-density distribution of the sources, not correlated with the CC. Possible predictions of such nonuniformities, from assumptions on super-superclusters, are discussed. 31 references.

DARK MATTER AND DYNAMICS IN HERCULES CLUSTER

We present new 21-cm observations of faint (15.7<mpg<16.5) spiral galaxies in the Abell cluster 2151. These results, when combined with the large body of velocities available in the literature for Hercules, permit us to study the dynamics throughout the cluster core, out to a projected radius of 1.8h−1 Mpc. We calculate the global dynamical mass of Hercules using 3 different but related methods: two versions of the virial theorem and the projected mass estimator. These masses lie in the range 3–6×1014 M⊙. We investigate the importance of subclustering in A2151 using the statistical test of Dressler and Shectman1 and the effects of the detected substructure on the dynamical mass determination. The clumpy distribution of galaxies is interpreted as a sign that the galaxies in the cluster have not reached dynamical equilibrium in the gravitational potential, which means that dynamical mass estimates are prone to significant errors. In spite of this uncertainty, we estimate that the virial theorem errors due to the presence of substructure are not larger than 30% in A2151. Finally, we use the 21-cm linewidths to estimate the minimum total mass in the cluster which is contained within the HI radii of the cluster galaxies is about 3×1014 M⊙. This number may be compared with the dynamical mass and used to separate the contribution of dark matter inside and outside the HI envelopes of galaxies, and the fraction of DM which cannot be associated with individual galaxies, about 90–95%.

Selection effects and binary galaxy velocity differences

Measurements of the velocity differences (delta vs) in pairs of galaxies from large statistical samples have often been used to estimate the average masses of binary galaxies. A basic prediction of these models is that the delta v distribution ought to decline monotonically. However, some peculiar aspects of the kinematics have been uncovered, with an anomalous preference for delta v approx. equal to 72 km s(sup-1) appearing to be present in the data. The authors examine a large sample of binary galaxies with accurate redshift measurements and confirm that the distribution of delta vs appears to be non-monotonic with peaks at 0 and approx. 72 km s (exp -1). The authors suggest that the non-zero peak results from the isolation criteria employed in defining samples of binaries and that it indicates there are two populations of binary orbits contributing to the observed delta v distribution.