AuthorMcIntosh, Daniel Howard
KeywordsPhysics, Astronomy and Astrophysics.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractWe present a data base of U,V photometric and structural properties for 642 spectroscopically confirmed members, V-band selected to Mᵥ = -18 mag, from three local Abell clusters: A85 (z = 0.055), A496 (z = 0.033) and A754 (z = 0.055). From our (U - V) half-light aperture colors and total model V-band magnitudes we construct precise color-magnitude diagrams for member galaxies out to ≳1 h⁻¹ Mpc. We measure well-defined color-magnitude relations (CMR) with low intrinsic scatter (σCMR ∼ 0.06-0.09 mag) in the cluster cores ( < 0.5h⁻¹ Mpc). We define three galaxy populations based on the relative color difference Δ(U - V) between the galaxies color and the best-fit CMR: (1) red sequence galaxies with Δ( U - V) ≥ -2σ(CMR); (2) intermediately blue (≡ bS0) galaxies with -2σ(CMR) > Δ(U - V) > -0.425 mag; and (3) blue (Butcher-Oemler ≡ B-O) galaxies with Δ( U - V) ≤ -0.425 mag. These color-magnitude cuts provide a rough galaxy age segregation assuming blueward deviations from the CMR represent mean stellar age differences. Red sequence members are the traditional cluster old, early-types (E/S0) and B-O galaxies have spiral-like colors; therefore, the bS0 population are assumed to be intermediate in age. We find a significant (∼10% in numbers) population of bS0 members in two local clusters (A85 and A754) . This is the first evidence for a quantitatively classified population of bS0 galaxies in clusters at < 1 Gyr look-back time. The bS0 populations exhibit the following characteristics: (1) Bulge-to-total morphologies intermediate between red sequence and B-O members. (2) Less morphological structure associated with star formation compared to normal, field spirals. (3) bS0 members are not found near the cluster cores which suggests more recent infall. (4) Lack of a significant color gradient which is different from both the cluster red sequence and field spirals. The observed bS0 properties are consistent with these galaxies being present-day examples midway through the predicted evolution of infalling, field spiral to red, cluster S0 via galaxy harassment and/or ram-pressure stripping. Therefore, the existence of these galaxies provides clear observational evidence for the present-day whereabouts of the blue galaxies once prevalent in rich clusters (the B-O effect) and for environmental based evolution of the cluster galaxy membership.
Degree ProgramGraduate College