Nicole Vogt » Morphological transformations from spirals to lenticulars
We find a strong correlation between the spiral and lenticular fractions within local clusters, and that the spiral fraction scales tightly with cluster x-ray gas luminosity. We have explored current star formation rates, and identified spirals that are (1) asymmetric, with truncated H-alpha emission and HI gas reservoirs on the leading edge of the disk, on a first pass through the dense intracluster medium in the cores of rich clusters; (2) strongly HI deficient and gas stripped, with star formation confined to the inner five kpc/h and three disk scale lengths; (3) reddened, extremely HI deficient and quenched, where star formation has been halted across the entire disk. We propose that these spirals are in successive stages of morphological transformation, between infalling field spirals and cluster lenticulars, and that the process which acts to remove the HI gas reservoir suppresses new star formation on a similarly fast timescale.
These data suggest that gas stripping plays a significant role in morphological transformation and rapid truncation of star formation across the galaxy disks. We show below the radial stripping radius, the maximum extent of H-alpha and [NII] emission along the major axes of galaxies, as a function of disk scale length for a set of local spiral galaxies. The figure is divided into early (left) versus late (right) type galaxies, and HI normal (top) versus HI deficient (bottom) spirals.
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Maximum and minimum extents of H-alpha are connected with an arrow for asymmetric spirals, giving a rough picture of their progression from the top to the bottom of the figure panels with time. Small open triangles represent isolated field galaxies, and small open circles galaxies within x-ray cool clusters or located more than 900 kpc/h from the core of a warm (kT > 4 keV) cluster. Core members of warm clusters are shown with a large solid circle; those few within the cores of three hot local clusters (A1656, A426, and A2199) are encircled. We have added as solid triangles in the upper left plot (all three fall upon the dashed line) three early type Virgo spirals undergoing stripping taken from Cayatte et al. (1994), where the stripping radius has been determined from two-dimensional maps of the HI extent. The solid lines define our criterion for gas stripped spirals, and the dashed line a model ram pressure stripping radius taken from Abadi et al. (1999).
The correlation between H-alpha maximum extent and truncation, and HI gas deficiency, is shown clearly. Early type asymmetric galaxies which still contain the bulk of the initial HI gas reservoirs are shifting down within the diagram towards the edge of the envelope in which HI normal spirals are found, though the maximum extent of H-alpha places them well within the normal distribution. Their HI deficient counterpoints have a maximum H-alpha extent which places them already at the edge of the envelope, and star formation is being suppressed across the remaining inner portion of the disk. The situation is more complicated for the few asymmetric late type spirals, which show extreme spatial truncation of star formation regardless of HI gas mass.