The Nature of Early-Type Galaxies at the Redshift Circa 0.5 Epoch

Details The Nature of Early-Type Galaxies at the Redshift Circa 0.5 Epoch The Nature of Early-Type Galaxies at the Redshift Circa 0.5 Epoch

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt.........4w&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, SANTA CRUZ, 1996.Source: Dissertation Abstracts International, Volume: 57-05, Section

Mathematics

Logic

4

Cosmology

Scientific paper

Over the last twenty years, studies of distant galaxy clusters have provided tantalizing evidence for changes in both the color and spectral type of cluster galaxies since redshifts z~0.5: more distant clusters apparently contain greater fractions of blue galaxies (the "Butcher-Oemler effect") or galaxies with spectra showing recent or ongoing star formation. Forming a complete picture of cluster galaxies is important to understanding the processes governing their evolution. To address the connection between spectral type and morphology in distant cluster galaxies, we studied Hubble Space Telescope images of the clusters C10939+47 (Abell 851) at z=0.41 and C10016+16 at z=0.55. We measured "metric" sizes and brightnesses plus a quantitative profile shape indicator for galaxies in both clusters and used realistic galaxy simulations to help interpret the results. The spectral class and membership status of galaxies were determined from previously-published low-resolution spectroscopy, recent narrow multiband photometry, and new high-quality Keck Telescope spectra, yielding the largest existing set of spectrally-classified galaxies in these distant clusters. As expected, objects with emission-line spectra generally show disk morphology, while spectra dominated by late-type stars correspond to bulge-profile objects. In contrast with previous studies, we find the class of Balmer-strong ("E+A") galaxies in each cluster to be morphologically diverse, containing some disklike or interacting objects and others which appear bulge-dominated. The observed existence of dynamically-young "E+A" galaxies appears incompatible with a merger hypothesis for their origin. These data also permit a test of whether the universe truly expands. We performed the Tolman surface-brightness test by comparing the locally-observed correlation between galaxy size and surface brightness to the relations observed in both distant clusters, yielding the change in surface brightness with redshift. The observed differences of Delta=1.08 +/-0.10 for Cl0939+47 and Delta =1.77+/-0.12 for Cl0016+16 are consistent with the predicted (1+z) ^4 change in surface flux due to the expansion, plus moderate (Delta m _sp{ ~}< 0.3) amounts of luminosity evolution. Unless the slope index of the stellar initial-mass function is very high (x _sp{~}> 10), these findings are inconsistent with a non-expanding universe.

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