B2 0902+34: A collapsing protogiant elliptical galaxy at z = 3.4

Abstract

We have used the visible integral-field replicable unit spectrograph prototype, a new integral field spectrograph, to study the spatially and spectrally resolved Lyman-α emission line structure in the radio galaxy B2 0902+34 at z = 3.4. We observe a halo of Lyman-α emission with a velocity dispersion of 250 km s-1 extending to a radius of 50 kpc. A second feature is revealed in a spatially resolved region where the line profile shows blueshifted structure. This may be viewed as either HI absorption at ≈-450 km s-1 or secondary emission at -900 km s-1 from the primary peak. B2 0902+34 is also the only high-redshift radio galaxy with a detection of 21 cm absorption. Our new data, in combination with the 21 cm absorption, suggest two important and unexplained discrepancies. First, nowhere in the line profiles of the Lyman-α halo is the 21 cm absorber population evident. Second, the 21 cm absorption redshift is higher than the Lyman-α emission redshift. In an effort to explain these two traits, we have undertaken the first three-dimensional Monte Carlo simulations of resonant scattering in radio galaxies. We have created a simple model with two photoionized cones embedded in a halo of neutral hydrogen. Lyman-α photons propagate from these cones through the optically thick HI halo until reaching the virial radius. Though simple, the model produces the features in the Lyman-α data and predicts the 21 cm properties. To reach agreement between this model and the data, global infall of the HI is strictly necessary. The amount of gas necessary to match the model and data is surprisingly high, ≥1012M ̇, an order of magnitude larger than the stellar mass. The collapsing structure and large gas mass lead us to interpret B2 0902+34 as a protogiant elliptical galaxy. This interpretation is a falsifiable alternative to the presence of extended HI shells ejected through feedback events such as starburst superwinds. An understanding of these gas features and a classification of this system's evolutionary state give unique observational evidence of the formation events in massive galaxies. © 2009. The American Astronomical Society. All rights reserved.