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Mid-Infrared Spectra of High Redshift (z>2) Radio Galaxies

Seymour, N; Ogle, P; Breuck, CD; Fazio, GG; Galametz, A; Haas, M; ... Vernet, J; + view all (2008) Mid-Infrared Spectra of High Redshift (z>2) Radio Galaxies. 10.1086/590081.

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Abstract

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 7.7um. In 4C+23.56 we see no obvious PAH features above the continuum. The PAH emission in 6C J1908+7220 is the amongst 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, ~10^9Msun. 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 tau_{9.7um}=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.

Type:Article
Title:Mid-Infrared Spectra of High Redshift (z>2) Radio Galaxies
DOI:10.1086/590081
Publisher version:http://dx.doi.org/10.1086/590081
Additional information:6 pages, 3 figures, includes emulateapj.cls, in press ApJL
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Space and Climate Physics

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