Miguel Pereira Santaella

Abstract:

© 2016. The American Astronomical Society. All rights reserved. We present a coherent database of spectroscopic observations of far-IR fine-structure lines from the Herschel/Photoconductor Array Camera and Spectrometer archive for a sample of 170 local active galactic nuclei (AGNs), plus a comparison sample of 20 starburst galaxies and 43 dwarf galaxies. Published Spitzer/IRS and Herschel/SPIRE line fluxes are included to extend our database to the full 10-600 μm spectral range. The observations are compared to a set of Cloudy photoionization models to estimate the above physical quantities through different diagnostic diagrams. We confirm the presence of a stratification of gas density in the emission regions of the galaxies, which increases with the ionization potential of the emission lines. The new [O iv] 25.9μm /[O iii] 88μm versus [Ne iii] 15.6μm /[Ne ii] 12.8μm diagram is proposed as the best diagnostic to separate (1) AGN activity from any kind of star formation and (2) low-metallicity dwarf galaxies from starburst galaxies. Current stellar atmosphere models fail to reproduce the observed [O iv] 25.9μm /[O iii] 88μm ratios, which are much higher when compared to the predicted values. Finally, the ([Ne iii] 15.6μm + [Ne ii] 12.8μm )/([S iv] 10.5μm +[S iii] 18.7μm ) ratio is proposed as a promising metallicity tracer to be used in obscured objects, where optical lines fail to accurately measure the metallicity. The diagnostic power of mid- to far-infrared spectroscopy shown here for local galaxies will be of crucial importance to study galaxy evolution during the dust-obscured phase at the peak of the star formation and black hole accretion activity (1 < z < 4). This study will be addressed by future deep spectroscopic surveys with present and forthcoming facilities such as the James Webb Space Telescope, the Atacama Large Millimeter/submillimeter Array, and the Space Infrared telescope for Cosmology and Astrophysics.

Abstract:

We investigate the evolutionary connection between local infrared (IR)-bright galaxies (logLIR ≥11.4 Lʘ) and quasars. We use high-angular resolution (∼0.3–0.4 arcsec∼few hundred parsecs) 8–13µm ground-based spectroscopy to disentangle the active galactic nuclei (AGN) id-IR properties from those of star formation. The comparison between the nuclear 11.3µm polycyclic aromatic hydrocarbon feature emission and that measured with Spitzer/Spitzer Infrared Spectrograph indicates that the star formation is extended over a few kpc in the IRbright galaxies. The AGN contribution to the total IR luminosity of IR-bright galaxies is lower than in quasars. Although the dust distribution is predicted to change as IR-bright galaxies evolve to IR-bright quasars and then to optical quasars, we show that the AGN mid-IR emission of all the quasars in our sample is not significantly different. In contrast, the nuclear emission of IR-bright galaxies with low AGN contributions appears more heavily embedded in dust although there is no clear trend with the interaction stage or projected nuclear separation. This suggests that the changes in the distribution of the nuclear obscuring material may be taking place rapidly and at different interaction stages washing out the evidence of an evolutionary path. When compared to normal AGN, the nuclear star formation activity of quasars appears to be dimming, whereas it is enhanced in some IR-bright nuclei, suggesting that the latter are in an earlier star formation-dominated phase.