Hα-derived Star-formation Rates for the Z = 0.84 Galaxy Cluster Clj0023+0423b

We present Hα-derived star-formation rates (SFRs) for the galaxy cluster CL J0023+0423B at z = 0.845. Our 3σ flux limits corresponds to a star-formation rate of 0.24 h −2 100 M ⊙ yr −1 , and our minimum reliable Hα + [N II] equivalent width is > 10Å, demonstrating that near-infrared narrow-band imaging can sample the star-forming galaxy population in distant clusters. Comparison with spectroscopy shows that the number of false detections is low (9 ± 6%) and that our Hα equivalent widths are correlated with spectroscopically determined [O II] equivalent widths. A magnitude-limited spectroscopic survey conducted over the same area missed 70% of the star-forming galaxies and 65% of the integrated star formation. Using Hubble Space Telescope Wide Field Planetary Camera 2 Archive images, we fit Sersic profiles to all galaxies with significant narrow-band equivalent widths and find that equivalent width decreases as the steepness of galaxy profile increases. We find no significant population of early type galaxies with ongoing star formation. The integrated SFR per cluster mass of CL J0023+0423B is a factor of ten higher than that of the three z ∼ 0.2 clusters in the literature with available Hα observations. A larger sample of z ∼ 0.8 clusters spanning a range of cluster masses is needed to determine whether this variation is due to a difference in cluster mass or redshift. The observed variation of galaxy properties as a function of environment (Hubble & Humason 1931) is evidently an important clue to how galaxies form and evolve. The two classic observations that drive this field of study are that nearby galaxy clusters contain a higher fraction of E/S0 galaxies than observed in the field (e.g. Dressler 1980; Whitmore et al. 1993) and that the fraction of blue galaxies in a cluster increases as a function of redshift (Butcher & Oemler 1984). The subsequent interpretation of these results has been varied, often conflicting , and not yet distilled. The difficulty, in part, lies in the lack of direct observations that trace star formation in large samples of cluster and field galaxies. Broad-band colors are most often used to trace star-formation rates (for example, Butcher & Oemler 1984), but these offer a crude measure and are affected by dust and metallicity. Comparisons of emission-line measurements of star formation are complicated by the use of different lines at different redshifts, by the difficulty in getting large samples at high …