A Tale of Two Populations: The Contribution of Merger and Secular Processes to the Evolution of Active Galactic Nuclei

Due to the co-evolution of supermassive black holes and their host galaxies, understanding the mechanisms that trigger active galactic nuclei (AGN) are imperative to understanding galaxy evolution and the formation of massive galaxies. It is observationally difficult to determine the trigger of a given AGN due to the difference between the AGN lifetime and triggering timescales. Here, we utilize AGN population synthesis modeling to determine the importance of different AGN triggering mechanisms. An AGN population model is computed by combining an observationally motivated AGN triggering rate and a theoretical AGN light curve. The free parameters of the AGN light curve are constrained by minimizing a \chi squared test with respect to the observed AGN hard X-ray luminosity function. The observed black hole space density, AGN number counts, and X-ray background spectrum are also considered as observational constraints. It is found that major mergers are not able to account for the entire AGN population. Therefore, non-merger processes, such as secular mechanisms, must also trigger AGN. Indeed, non-merger processes are the dominant AGN triggering mechanism at z \lesssim 1--1.5. Furthermore, the shape and evolution of the black hole mass function of AGN triggered by major mergers is intrinsically different from the shape and evolution of the black hole mass function of AGN triggered by secular processes.Comment: Accepted Ap

CO Line Emission from Compact Nuclear Starburst Disks Around Active Galactic Nuclei

There is substantial evidence for a connection between star formation in the nuclear region of a galaxy and growth of the central supermassive black hole. Furthermore, starburst activity in the region around an active galactic nucleus (AGN) may provide the obscuration required by the unified model of AGN. Molecular line emission is one of the best observational avenues to detect and characterize dense, star-forming gas in galactic nuclei over a range of redshift. This paper presents predictions for the carbon monoxide (CO) line features from models of nuclear starburst disks around AGN. These small scale (\la 100 pc), dense and hot starbursts have CO luminosities similar to scaled-down ultra-luminous infrared galaxies and quasar host galaxies. Nuclear starburst disks that exhibit a pc-scale starburst and could potentially act as the obscuring torus show more efficient CO excitation and higher brightness temperature ratios than those without such a compact starburst. In addition, the compact starburst models predict strong absorption when J_{\mathrm{Upper}} \ga 10, a unique observational signature of these objects. These findings allow for the possibility that CO SLEDs could be used to determine if starburst disks are responsible for the obscuration in z \la 1 AGN. Directly isolating the nuclear CO line emission of such compact regions around AGN from galactic-scale emission will require high resolution imaging or selecting AGN host galaxies with weak galactic-scale star formation. Stacking individual CO SLEDs will also be useful in detecting the predicted high-$J$ features.Comment: 27 pages, 5 figures, accepted by ApJ, updated to suit referee's suggestion

Iron K-alpha Emission from X-ray Reflection: Predictions for Gamma-Ray Burst Models

Recent observations of several gamma-ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line emitting material, possibly arising from ionized iron. A significant detection of an X-ray spectral feature, such as that found in the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence on progenitor models. The large Fe K-alpha equivalent widths inferred from the X-ray observations favor models in which the line is produced when the primary X-ray emission from the source strikes Thomson-thick material and Compton scatters into our line of sight. We present such reflection spectra here, computed in a fully self-consistent manner, and discuss the range of ionization parameters that may be relevant to different models of GRBs. We argue that the presence of a strong hydrogen-like K-alpha line is unlikely, because Fe-XXVI photons would be trapped resonantly and removed from the line core by Compton scattering. In contrast, a strong narrow emission line from He-like Fe-XXV is prominent in the model spectra. We briefly discuss how these constraints may affect the line energy determination in GRB 991216.Comment: 8 pages, 3 figures, Ap.J. Letters accepte

Ecological processes dominate the 13C land disequilibrium in a Rocky Mountain subalpine forest

pre-printFossil fuel combustion has increased atmospheric CO2 by ≈ 115 μmol mol1 since 1750 and decreased its carbon isotope composition (δ13C) by 1.7-2‰(the 13C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ13C of CO2 released by terrestrial ecosystems is expected to differ from the δ13C of CO2 assimilated by land plants during photosynthesis. This isotopic difference between land-atmosphere respiration (δR) and photosynthetic assimilation (δA) fluxes gives rise to the 13C land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus, D is generally positive (δR>δA). A 7 year record of biosphere-atmosphere carbon exchange was used to evaluate the seasonality of δA and δR, and the 13C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ13C of net land-atmosphere exchange during day and night and combined with tower-based flux observations to assess δA and δR. The disequilibrium varied seasonally and when flux-weighted was opposite in sign than expected from the Suess effect (D =0.75 ± 0.21‰or 0.88 ± 0.10‰depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Δcanopy) responding to environmental factors. Possible explanations for negative D include (1) changes in Δcanopy over decades as CO2 and temperature have risen, and/or (2) post-photosynthetic fractionation processes leading to sequestration of isotopically enriched carbon in long-lived pools like wood and soil