Vera C. Rubin

Virgo Cluster Galaxies with Disturbed Kinematics, and the Cluster Approach to Equilibrium

We live in an era in which galaxies are forming into clusters. In a galaxy cluster, the high central galaxy density overcomes the expansion of the universe, and the galaxies are not separating, one from the other, but are gravitationally bound to the group. Some clusters, formed early, appear to be in dynamical equilibrium, with a deep central potential, massive elliptical galaxies in the core, and a fairly smooth spherical distribution of constituent galaxies. Younger clusters, like our nearby Virgo cluster, still contain irregular clumps of galaxies, which are expected to smooth out during their consequent evolution.

It has long been known that the Virgo cluster galaxies exhibit a curious distribution of central velocities when the galaxies are separated into ellipticals and spirals. The ellipticals, a population probably well on their way toward dynamical equilibrium, exhibit a Gaussian distribution of observed velocities with many velocities near 1,100 km/second. In contrast, the spirals have velocities that range equally from about -400 km/second (i.e., approach with respect to the cluster mean velocity) to about +2,600 km/second (i.e., recession). Hence the cluster, well defined by the ellipticals, is poorly defined by the spiral velocities.

Over the past decade, I have observed internal rotational velocities for the ionized gas component in 100 spiral galaxies in the Virgo cluster. Together with high school student Andrew Waterman and Yale University astronomer Jeff Kenney, I have analyzed these data. We divide the spirals into two groups, roughly equal in number: those with normal rotation properties, and those with significantly disturbed rotation. Independent of galaxy rotation properties, parameters such as galaxy brightness and Hubble type are the same for both groups.

However, the central velocities are strikingly different. Spirals with normal rotation curves have central velocities distributed evenly from -400 to +2,600 km/second, like the previously known spirals. In contrast, spirals with disturbed rotation have a distribution of central velocities that matches, almost exactly, the velocity distribution of the ellipticals (Fig. 1). Moreover, the distribution on the sky of the galaxies with disturbed rotation shows that these galaxies are preferentially on elongated radial orbits, which carry them close to the central core of the cluster. Here, tidal interactions are strong and more common.

We believe that the disturbed rotation properties arise from galaxy-galaxy or galaxy-cluster field interactions. Galaxies with disturbed rotation have likely endured such an interaction within the last billion years. Interactions and mergers will alter the morphology of some galaxies, puffing the disks into a spheroidal or an elliptical morphology. The interactions will also play a role in driving the Virgo cluster toward dynamical equilibrium.

Fig. 1. The distribution of 43 spirals with disturbed rotation curves, as a function of systemic velocity, are superposed here on a histogram that shows the distribution of 164 ellipticals in Virgo. Note the exceptional similarity. (Elliptical velocities from the work of John Huchra of the Harvard-Smithsonian Center for Astrophysics.)