The origin of star formation gradients in rich galaxy clusters

We examine the origin of clustercentric gradients in the star formation rat es and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formatio n rates decline on a timescale of a few gigayears, the typical gas consumpt ion timescale of disk galaxies in the held. Such behavior might be expected if tides and ram pressure strip off the gaseous envelopes that normally fu el star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster f ormation in a Lambda CDM universe, we reproduce the systematic differences observed in the color distribution of cluster and held galaxies, as well as the strong suppression of star formation in cluster galaxies and its depen dence on clustercentric radius. The simulations also indicate that a signif icant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explai ns naturally why star formation in the outskirts of clusters (and as far ou t as 2 virial radii) is systematically suppressed relative to the held. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the g alaxy is accreted into the main body of the cluster. We conclude that the s tar formation rates of cluster galaxies depend primarily on the time elapse d since their accretion onto massive virialized systems and that the cessat ion of star formation may have taken place gradually over a few gigayears.