The Visibility of Galactic Bars and Spiral Structure at High Redshifts

We investigate the visibility of galactic bars and spiral structure in the distant universe by artificially redshifting 101 B-band CCD images of local spiral galaxies from the Ohio State University Bright Spiral Galaxy Survey. These local galaxy images represent a much fairer statistical baseline than the galaxy atlas images presented by Frei et al. in 1995, the most commonly used calibration sample for morphological work at high redshifts. Our artificially redshifted images correspond to Hubble Space Telescope I814-band observations of the local galaxy sample seen at z = 0.7, with integration times matching those of both the very deep northern Hubble Deep Field (HDF) data and the much shallower HDF flanking field observations. The expected visibility of galactic bars is probed in two ways: (1) using traditional visual classification and (2) by charting the changing shape of the galaxy distribution in "Hubble space," a quantitative two-parameter description of galactic structure that maps closely onto Hubble's original tuning fork. Both analyses suggest that over two-thirds of strongly barred luminous local spirals (i.e., objects classified as SB in the Third Reference Catalogue) would still be classified as strongly barred at z = 0.7 in the HDF data. Under the same conditions, most weakly barred spirals (classified SAB in the Third Reference Catalogue) would be classified as regular spirals. The corresponding visibility of spiral structure is assessed visually, by comparing luminosity classifications for the artificially redshifted sample with the corresponding luminosity classifications from the Revised Shapley-Ames Catalog. We find that for exposure times similar to that of the HDF, spiral structure should be detectable in most luminous (MB ∼ M*) low-inclination spiral galaxies at z = 0.7 in which it is present. However, obvious spiral structure is only detectable in ∼30% of comparable galaxies in the HDF flanking field data using the Wide Field Planetary Camera 2. Our study of artificially redshifted local galaxy images suggests that, when viewed at similar resolution, noise level, and redshift-corrected wavelength, barred spirals are less common at z ∼ 0.7 than they are at z = 0.0, although more data are needed to definitively rule out the possibility that cosmic variance is responsible for much of this effect.