B2 0902+34: A Collapsing Protogiant Elliptical Galaxy At Z=3.4

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Date

2009-03

Authors

Adams, Joshua J.
Hill, Gary J.
MacQueen, Phillip J.

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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-alpha emission line structure in the radio galaxy B2 0902+34 at z = 3.4. We observe a halo of Lyman-alpha emission with a velocity dispersion of approximate to 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 H I absorption at approximate to-450 km s(-1) or secondary emission at approximate to-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-alpha halo is the 21 cm absorber population evident. Second, the 21 cm absorption redshift is higher than the Lyman-alpha 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-alpha photons propagate from these cones through the optically thick H I halo until reaching the virial radius. Though simple, the model produces the features in the Lyman-alpha data and predicts the 21 cm properties. To reach agreement between this model and the data, global infall of the H I is strictly necessary. The amount of gas necessary to match the model and data is surprisingly high, >= 10(12) M(circle dot), 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 H I shells ejected through feedback events such as starburst superwinds. An understanding of these gas features and a classification of this system's evolutionary Stategive unique observational evidence of the formation events in massive galaxies.

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Adams, Joshua J., Gary J. Hill, and Phillip J. MacQueen. "B2 0902+ 34: a collapsing protogiant elliptical galaxy at z= 3.4." The Astrophysical Journal, Vol. 694, No. 1 (Mar., 2009): 314.