Astrophys. J., Suppl. Ser., 171, 353-375 (2007/August-0)
The massive hosts of radio galaxies across cosmic time.
SEYMOUR N., STERN D., DE BREUCK C., VERNET J., RETTURA A., DICKINSON M., DEY A., EISENHARDT P., FOSBURY R., LACY M., McCARTHY P., MILEY G., ROCCA-VOLMERANGE B., ROTTGERING H., STANFORD S.A., TEPLITZ H., VAN BREUGEL W. and ZIRM A.
Abstract (from CDS):
We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Using IRAC (3.6-8.0 µm), IRS (16 µm), and MIPS (24-160 µm) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H band from stars is >60% for ∼75% of the high-redshift radio galaxies. As expected from unified models of AGNs, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5 µm) luminosity. In addition, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anticorrelates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 1011-1011.5 M☉ even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGNs, despite the stellar contribution to their mid-IR SEDs at shorter wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly accreting AGN. We find a weak correlation of stellar mass with radio luminosity.
Galaxies: Active - Galaxies: Evolution - Galaxies: High-Redshift
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