Dynamics of Lyman Break Galaxies and Their Host Halos

James D. Lowenthal, Smith College
David C. Koo, University of California, Santa Cruz
Luc Simard, National Research Council Canada
Eelco Van Kampen, European Southern Observatory

Peer reviewed accepted manuscript.


We present deep two-dimensional spectra of 22 candidate and confirmed Lyman break galaxies (LBGs) at redshifts 2 < z < 4 in the Hubble Deep Field (HDF) obtained at the Keck II telescope. The targets were preferentially selected with spatial extent and/or multiple knot morphologies, and we used slitmasks and individual slits tilted to optimize measurement of any spatially resolved kinematics. Our sample is more than 1 mag fainter and is at higher redshift than the kinematic LBG targets previously studied by others. The median target magnitude was I = 25.3, and total exposure times ranged from 10 to 50 ks. We measure redshifts, some new, ranging from z = 0.2072 to z = 4.056, including two interlopers at z < 1, and resulting in a sample of 14 LBGs with a median redshift z = 2.424. The morphologies and kinematics of the close pairs and multiple knot sources in our sample are generally inconsistent with galaxy formation scenarios postulating that LBGs occur only at the bottom of the potential wells of massive host halos; rather, they support "collisional starburst" models with significant major merger rates and a broad halo occupation distribution. For 13 LBGs with possible kinematic signatures, we estimate a simple dynamical mass, subject to numerous caveats and uncertainties, of the galaxies and/or their host dark matter halos. Dynamical mass estimates of individual galaxies range from 4 × 10 h M to 1.1 × 10 h M and mass estimates of halos, based on close LBG pairs, range from <10 h to 10 h M with a median value 1 × 10 M . Comparison with a recent numerical galaxy formation model implies that indeed the pairwise velocities might not reflect true dynamical masses. We compare our dynamical mass estimates directly to stellar masses estimated for the same galaxies from SEDs, and find no evidence for a strong correlation. The diversity of morphologies and dynamics implies that LBGs represent a broad range of galaxy or protogalaxy types in a variety of evolutionary or merger stages rather than a uniform class with a narrow range of mass. © 2009. The American Astronomical Society. All rights reserved. 814 ⊙ ⊙ ⊙ ⊙ 9 -1 11 -1 10 -1 14 -1 13