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Session 57 -- Galaxies III

Oral presentation, Thursday, January 13, 10:15-11:45, Salon IV Room (Crystal Gateway)

[57.05] The Theta-z Relationship for HST bulges and disks out to z$\leq$0.6

We present a systematic project as part of the HST Medium-Deep Survey (``MDS'') to determine scale-lengths for about 72 galaxies down to V$\le$22.5 (I$\le$21) mag with the Wide Field/Planetary Camera (at deconvolved FWHM$\le$0.2''). We measured redshifts with the MMT and the 100 inch duPont out to z=0.6. The HST galaxies have scale-lengths in the range 0.3$ We compare the sample of HST galaxy bulges with $r^{1/4}$ profiles to the Seven Samurai sample of nearby ellipticals (Burstein et al. 1987). For $H_{o}$=75 km/s/Mpc, the latter follow the $\Theta$-z relation out to z=0.03 for a best fit $r_{e}$=5.7 kpc. In the redshift range 0.2$< z<$0.6 our HST/MDS sample shows a lack of bulges with scale-lengths larger than the $r_{e}$=5.7 kpc models for $q_{o}$=0.01---1.0. We discuss various possible selection effects, such as the selection against faint low surface brightness galaxies in both HST images and MMT spectra at the faintest magnitudes. Our result may nontheless indicate that elliptical galaxy scale-lengths have evolved with cosmic time, in that they were smaller in the past, due to e.g. an epoch dependent merger rate or dynamical evolution. We similarly analyzed our HST/MDS sample of galaxy disk scale-lengths out to z=0.6, and compared this to the RC3 catalog of nearby spirals. The latter has more complicated selection effects due to its limits in both diameter and $B_{T}$-mag. Taking as best value for the local disk scale-length $r_{e}$=3.4 kpc (for $H_{o}$=75km/s/Mpc), we find a more symmetrical distribution of disk scale-lengths around the models out to z=0.6. This implies that galaxy disks may have been stable since z$\sim$1.0.

This work was supported by NASA/HST grants GO-2684-0*-87A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.

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