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

Using the near-infrared integral field spectrograph SPIFFI on the VLT, we have studied the spatially resolved dynamics in the z=3.2 strongly lensed galaxy 1E 0657-56 arc+core by observing the rest-frame optical emission lines [O III] λ5007 and Hβ. The lensing configuration suggests that the high surface brightness core is the M~20 magnified central ~1 kpc of the galaxy, whereas the fainter arc is the more strongly magnified peripheral region of the same galaxy at about a half-light radius, which otherwise appears to be a typical z~3 Lyman break galaxy. The overall shape of the position-velocity diagram resembles the rotation curves of the inner few kpc of nearby ~L^* spiral galaxies. For M=20, our data have a spatial resolution of ~200 pc in the source plane. The projected velocities v_rot rise rapidly to ~75 km s^-1 within radii ~0.5 kpc from the center and asymptotically reach a velocity of ~190 km s^-1 within the arc, at a projected radius of a few kpc radius. The rotation curve implies a dynamical mass of logM_dyn/M_solar~9.3 within the central kpc and suggests 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>~3. Approximating the circular velocity of the halo by the measured asymptotic velocity of the rotation curve, we estimate a dark matter halo mass of logM_halo/M_solar~11.7+/-0.3, in good agreement with large-scale clustering studies of Lyman break galaxies. The baryonic collapse fraction is low compared to z~2 actively star-forming BX and low-redshift galaxies, perhaps implying comparatively less gas infall to small radii or efficient feedback. Even more speculatively, the high central mass density might indicate highly dissipative gas collapse in very early stages of galaxy evolution, in approximate agreement with what is expected for ``inside-out'' galaxy formation 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).