THE HIGH-VELOCITY SYSTEM: INFALL of A GIANT LOW-SURFACE-BRIGHTNESS GALAXY TOWARD the CENTER of the PERSEUS CLUSTER

Abstract

The high-velocity system (HVS) lies just north-west of the center and is moving at a speed of 3000 km s-1 toward NGC 1275, the central giant elliptical galaxy in the Perseus cluster. We report imaging spectroscopy of the HVS in H-α and [N ii] that resolves both the nature of this galaxy and its physical relationship with NGC 1275. The HVS exhibits a distorted disk having a projected rotational velocity that rises steadily to ∼200 km s-1 at a radius of ∼12 kpc, the same maximal extent detectable in neutral gas and dust. We discover highly blueshifted emission at relative velocities of up to ∼800 km s-1 distributed throughout and confined almost entirely within the projected area of the disk, tracing gas stripped by ram pressure. The distribution of the stripped gas implies that the HVS is moving essentially along our sightline closely toward the center of NGC 1275. We show that the speed of the HVS is consistent with it having fallen from rest at the virial radius of the Perseus cluster and reached ∼100 kpc from the cluster center. Despite having an overall metallicity (inferred from [N ii]/H-α) significantly lower than that of star-forming disk galaxies, the HVS exhibits a current star formation rate of ∼3.6 Mo yr-1 and numerous young star clusters projected against giant H ii regions. The evidence assembled implicates a progenitor giant low-surface-brightness galaxy that, because of galaxy harassment and/or the cluster tidal field, has developed two prominent spiral arms along which star formation is strongly elevated.