Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies

Abstract

We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm observations of a sample of early-type galaxies in nearby clusters which are known to be either in a star-forming phase or to have had star formation which recently terminated. From the long-slit spectra, obtained with the Blanco 4-m telescope, we find that emission lines in the star-forming cluster galaxies are significantly more centrally concentrated than in a sample of field galaxies. The broadband imaging reveals that two currently star-forming early-type galaxies in the Pegasus I cluster have blue nuclei, again indicating that recent star formation has been concentrated. In contrast, the two galaxies for which star formation has already ended show no central color gradient. The Pegasus I galaxy with the most evident signs of ongoing star formation (NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen observations of that galaxy with the Arecibo radiotelescope reveal the presence of ∼4 x 10 8 M ⊙ of HI. Arecibo observations of other current or recent star-forming early-type galaxies in Pegasus I indicate smaller amounts of gas in one of them, and only upper limits in others.1 Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation -2 -The observations presented above indicate that NGC7648 in the Pegasus I cluster owes its present star formation episode to some form of tidal interaction. The same may be true for the other galaxies with centralized star formation, but we cannot rule out the possibility that their outer disks have been removed via ram pressure stripping, followed by rapid quenching of star formation in the central region.

A Comparison of the Galaxy Populations in the Coma and Distant Clusters: The Evolution of k+a Galaxies and the Role of the Intracluster Medium

“…Spectra with enhanced Balmer lines in absorption have been previously reported in early-type galaxies in Coma and other nearby clusters in a number of papers by Caldwell, Rose, and collaborators (e.g., Rose et al 2001 and references therein). Out of the 12 galaxies in the sample of Balmer-enhanced galaxies without emission studied by C93, six are in common with our Coma spectroscopic survey.…”

At the Vigintennial of the Butcher–Oemler Effect

“…This process should be a more effective mechanism than ram pressure stripping in causing activity (Byrd & Valtonen 1990;Rose et al 2001). Such a mechanism would, however, produce BO galaxies near the cluster core (Fujita 1998) which is observationally inconsistent.…”

Structural Evidence for Environment‐driven Transformation of the Blue Galaxies in Local Abell Clusters: A85, A496, and A754

“…Additionally, Bartholomew et al (2001) found that distant cluster galaxies with strong Balmer absorption have blue color gradients and are presumed to be a poststarburst population (Caldwell & Rose 1998;Poggianti et al 1999). Furthermore, Rose et al (2001) found that currently starforming galaxies in local clusters often show blue cores and have more centrally concentrated emission lines than field galaxies.…”