Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...461..546c&link_type=abstract
Astrophysical Journal v.461, p.546
Astronomy and Astrophysics
Astronomy
9
Galaxies: Distances And Redshifts, Galaxies: Interactions, Galaxies: Spiral
Scientific paper
We discuss pairs of spiral galaxies with projected separations r_p_ ranging from very wide (r_p_ ~ 1.0 Mpc) to very small (r_p_ <~ 75 kpc). Single-dish H I data for the wide pairs and H I aperture synthesis as well as CCD images for the close pairs were presented in earlier papers. From pairs (regardless of isolation criteria) in redshift catalogs, we find that the kinematics of spiral-spiral pairs in low-density regions is qualitatively different from that in high-density regions. The velocity distribution function f({DELTA}V) in low-density regions has two components: (i) a narrow component (which is related to isolated pairs) with a very small rms value σ_v,n_ of the "one-dimensional random cosmic peculiar velocity" δν, and (ii) a broad, slowly decreasing component (which is related to pairs in loose groups) with a large rms value of the "one-dimensional random cosmic peculiar velocity" σnu,b_. Thus, we confirm that the distribution of the "one- dimensional cosmic peculiar velocity" {DELTA}ν is not Gaussian and its rms value σ_V_ varies with environment. The median {DELTA}V_med_ of the absolute value of the velocity difference of isolated wide pairs is ~30 km s^-1^, considerably smaller than all estimates of σ_v_. This small {DELTA}V_med_ suggests that the pairs are on low total energy, almost radial orbits, and it is also a constraint on cosmological models of structure formation. We find some circumstantial evidence for a negative correlation between the peculiar velocity δν and the differential Hubble flow velocity {DELTA}V_H_ which would be the case for bound orbits near the maximum separation (" turnaround ``) at the current epoch. If future larger samples (e.g., the CfA2 and the SSRS2 surveys) confirm this, then a total galaxy mass ~10^12^ M_sun_ is implied. All the six close pairs for which we have detailed H I and optical CCD images show some signs of interaction, even though the selection criteria were specifically independent of known morphological peculiarities. For three of the six pairs there is good evidence for the pairs also being on high eccentricity, low total energy orbits, and there is some evidence that the impact parameter for one pair is ~1 disk radius. The median velocity difference {DELTA}V_med_ of the close pairs sample is also ~30 km s^-1^. This small value of {DELTA}V_med_ is probably an indirect effect of dynamical friction.
Chengalur Jayaram N.
Salpeter Edwin E.
Terzian Yervant
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