IRS observations of a new population of massive high-z galaxies discovered by SWIRE and MAMBO
Abstract
We propose to obtain low resolution mid-IR spectra of a sample of 16 SWIRE-selected massive star-forming ULIRGs at z~2, deeply observed at 1.2 mm with IRAM-MAMBO. This sample is complete, with a selection of all sources in 0.5 square degree whose IRAC photometry peaks at 5.8 microns, S(24)>500 microJy and r>23. More than 80% of the sources have S(1.2mm)>~ 1.5 mJy, and are thus submillimeter galaxies (SMGs) with far-IR luminosities greater than a few 10^12 Lo. The sample is thus representative of a rich, Spitzer-selected special subclass of SMGs (~2000 in all SWIRE fields). Compared to sub-mm selected SMGs, these sources are brighter at 24 microns (larger PAH/FIR ratio), and have larger stellar masses. They represent thus an important stage of the assembly of elliptical galaxies already massive at z~2, with a strong, close to 'maximal', starburst probably triggered by one of the last major gaseous mergers. The sample benefits from exceptionally rich complementary data, with one of the deepest VLA observations and deep 70-160 micron data. We expect that, in most of these objects, the emission is dominated by the starburst in mid-IR (PAHs) and far-IR (thermal dust), but the parallel growth of black holes is also known to be at work in such objects and the lack of AGN signatures in our sample is somehow surprising. The proposed IRS spectra will provide essential information about this important, but poorly studied, sub-class of high z starbursts by: 1) disentangling the mid-IR PAH and AGN contribution; 2) analyzing the detailed properties of their PAH spectrum, in relation with mm, radio and far-IR properties, and comparing with classical SMGs; 3) checking any evidence of silicate absorption; 4) inferring physical and spatial properties of the starburst from this spectral information together with complete multi-wavelength data.
- Publication:
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Spitzer Proposal
- Pub Date:
- March 2008
- Bibcode:
- 2008sptz.prop50119L