2008ApJ...681L...1S -
Astrophys. J., 681, L1-L4 (2008/July-1)
Mid-infrared spectra of high-redshift (z > 2) radio galaxies.
SEYMOUR N., OGLE P., DE BREUCK C., FAZIO G.G., GALAMETZ A., HAAS M., LACY M., SAJINA A., STERN D., WILLNER S.P. and VERNET J.
Abstract (from CDS):
We present the first mid-infrared Spitzer/Infrared Spectrograph (IRS) observations of powerful radio galaxies at z>2. These radio galaxies, 4C +23.56 (z=2.48) and 6C J1908+7220 (z=3.53), both show strong mid-infrared continua, but with 6C J1908+7220 also showing strong PAH emission at rest-frame 6.2 and 7.7 µm. In 4C +23.56 we see no obvious PAH features above the continuum. The PAH emission in 6C J1908+7220 is the among the most distant observed to date and implies that there is a large instantaneous star formation rate (SFR). This is consistent with the strong detection of 6C J1908+7220 at far-IR and sub-mm wavelengths, indicative of large amounts of cold dust, ∼109 M☉. Powerful radio galaxies at lower redshifts tend to have weak or undetectable PAH features and typically have lower far-IR luminosities. In addition, 4C +23.56 shows moderate silicate absorption as seen in less luminous radio galaxies, indicating τ9.7µm=0.3±0.05. This feature is shifted out of the observed wavelength range for 6C J1908+7220. The correlation of strong PAH features with large amounts of cold dust, despite the presence of a powerful AGN, is in agreement with other recent results and implies that star formation at high redshift is, in some cases at least, associated with powerful, obscured AGN.
Abstract Copyright:
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Journal keyword(s):
Galaxies: Active - Galaxies: Evolution - galaxies: individual (4C +23.56) - galaxies: individual (4C +72.26)
Simbad objects:
7
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