Galaxy formation and evolution

Abstract:

We use Spitzer spectroscopic and imaging observations to study the co-evolution of black hole (BH) growth and star formation (SF) activity in a complete volume-limited sample of local Luminous Infrared Galaxies (LIRGs). Using a mid-infrared (mid-IR) spectral decomposition method we identify active galactic nuclei (AGN) with Seyfert-like luminosities and relatively small bolometric contributions (median ∼ 5%) in half of the LIRG nuclei. Combining mid-IR and AGN optical detections we obtain a 62% AGN detection rate in local LIRGs. We combine gas velocity dispersions of spectrally resolved [Ne iii]15.56 µm and [O iii]λ5007Å lines and literature stellar velocity dispersions , and we obtain typical BH masses of 3 ×107 M☉ for local LIRGs. We find that the ratios of nuclear and integrated SF rates (SFR) to BH accretion rates are higher in local LIRGs than in optically selected Seyferts. We interpret our results in a scenario where local LIRGs with an AGN represent an early phase of the (possibly episodic) growth of BHs in massive spiral galaxies with high SFR, not necessarily associated with a major merger event.

Abstract:

We report Herschel SPIRE (250, 350, and 500 μm) detections of 32 quasars with redshifts 0.5 ≤z < 3.6 from the Herschel Multi-tiered Extragalactic Survey (HerMES). These sources are from a MIPS 24 μm flux-limited sample of 326 quasars in the Lockman Hole Field. The extensive multi-wavelength data available in the field permit construction of the rest-frame spectral energy distributions (SEDs) from ultraviolet to the mid-infrared for all sources, and to the far-infrared (FIR) for the 32 objects. Most quasars with Herschel FIR detections show dust temperatures in the range of 25-60K, with a mean of 34K. The FIR luminosities range from 1011.3 to 1013.5 L, qualifying most of their hosts as ultra- or hyper-luminous infrared galaxies. These FIR-detected quasars may represent a dust-rich population, but with lower redshifts and fainter luminosities than quasars observed at 1mm. However, their FIR properties cannot be predicted from shorter wavelengths (0.3-20 μm, rest frame), and the bolometric luminosities derived using the 5100Å index may be underestimated for these FIR-detected quasars. Regardless of redshift, we observed a decline in the relative strength of FIR luminosities for quasars with higher near-infrared luminosities. © 2012. The American Astronomical Society. All rights reserved.