Stellar Populations and Kinematics of Red Galaxies at z > 2: Implications for the Formation of Massive Galaxies

We recently identified a substantial population of galaxies at z > 2 with comparatively red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion Js - Ks > 2.3. In this paper we present near-infrared spectroscopy with Keck NIRSPEC of six DRGs with previously measured redshifts 2.4 < z < 3.2, two of which were known to host an active nucleus. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of Hα in the nonactive galaxies are 20-30 Å, smaller than measured for Lyman break galaxies (LBGs) and nearby luminous infrared galaxies and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate or are very dusty. Fitting both the photometry and the Hα lines, we find continuum extinction AV = 1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 M☉ yr-1, and stellar masses (1-5) × 1011 M☉ for models with constant star formation rates. Models with a declining star formation lead to significantly lower extinction, star formation rates, and ages but similar stellar masses. From [N II]/Hα ratios we infer that the metallicities are high, 1-1.5 times solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130 to 240 km s-1, and by combining data for LBGs and DRGs, we find significant correlations between line width and rest-frame U - V color and between line width and stellar mass. The latter correlation has a similar slope and offset as the "baryonic Tully-Fisher relation" for nearby galaxies. From the line widths and effective radii we infer dynamical masses and mass-to-light (M/L) ratios. The median dynamical mass is ~2 × 1011 M☉, supporting the high stellar masses inferred from the photometry. We find that the median M/LV ≈ 0.8(M/L)☉, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal-rich, and more massive and have higher ages than z ≈ 3 LBGs of the same rest-frame V-band luminosity. Although their volume density is still uncertain, their high M/L ratios imply that they contribute significantly to the stellar mass density at z ≈ 2.5. As their stellar masses are comparable to those of early-type galaxies, they may have already assembled most of their final mass.