Lyman Break Galaxies under a Microscope: The Small‐Scale Dynamics and Mass of an Arc in the Cluster 1E 0657−56

Abstract

Using the near-infrared integral field spectrograph SPIFFI on the VLT,we have studied the spatially resolved dynamics in the z=3.2 stronglylensed galaxy 1E 0657-56 arc+core by observing the rest-frame opticalemission lines [O III] {λ}5007 and H{β}. The lensingconfiguration suggests that the high surface brightness core is the M~20magnified central ~1 kpc of the galaxy, whereas the fainter arc is themore strongly magnified peripheral region of the same galaxy at about ahalf-light radius, which otherwise appears to be a typical z~3 Lymanbreak galaxy. The overall shape of the position-velocity diagramresembles the rotation curves of the inner few kpc of nearby~L^{*} spiral galaxies. For M=20, our data have a spatialresolution of ~200 pc in the source plane. The projected velocitiesv_{rot} rise rapidly to ~75 km s^{-1} within radii ~0.5kpc from the center and asymptotically reach a velocity of ~190 kms^{-1} within the arc, at a projected radius of a few kpcradius. The rotation curve implies a dynamical mass oflogM_{dyn}/M_{solar}~9.3 within the central kpc andsuggests that in this system the equivalent of the mass of a present-day~L^{*} bulge at the same radius was already in place by z{\gt}~3.Approximating the circular velocity of the halo by the measuredasymptotic velocity of the rotation curve, we estimate a dark matterhalo mass of logM_{halo}/M_{solar}~11.7+/-0.3, in goodagreement with large-scale clustering studies of Lyman break galaxies.The baryonic collapse fraction is low compared to z~2 activelystar-forming BX and low-redshift galaxies, perhaps implyingcomparatively less gas infall to small radii or efficient feedback. Evenmore speculatively, the high central mass density might indicate highlydissipative gas collapse in very early stages of galaxy evolution, inapproximate agreement with what is expected for ``inside-out'' galaxyformation models.Based on observations collected at the European Southern Observatory,Very Large Telescope Array, Cerro Paranal, program numbers 70.B-0545(A)and 70.A-0229(A).