arXiv subjects

David R. Ballantyne

Publications and source records attributed to David R. Ballantyne.

48 records · Page 3Linked to original sources

arXiv2013

The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 13 June 2012, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 -- 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low-background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than one-hundred-fold improvement in sensitivity over the collimated or coded-mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity, spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives, and will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6degree inclination orbit, the Observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of ten years, we anticipate proposing a guest investigator program, to begin in Fall 2014.

astro-ph.IM↗

arXiv2012

The Merger-Triggered Active Galactic Nuclei Contribution to the Ultraluminous Infrared Galaxy Population

It has long been thought that there is a connection between ultraluminous infrared galaxies (ULIRGs), quasars, and major mergers. Indeed, simulations show that major mergers are capable of triggering massive starbursts and quasars. However, observations by the Herschel Space Observatory suggest that, at least at high redshift, there may not always be a simple causal connection between ULIRGs and mergers. Here, we combine an evolving merger-triggered AGN luminosity function with a merger-triggered starburst model to calculate the maximum contribution of major mergers to the ULIRG population. We find that major mergers can account for the entire local population of ULIRGs hosting AGN and ~25% of the total local ULIRG luminosity density. By z ~ 1, major mergers can no longer account for the luminosity density of ULIRGs hosting AGN and contribute \lesssim 12% of the total ULIRG luminosity density. This drop is likely due to high redshift galaxies being more gas rich and therefore able to achieve high star formation rates through secular evolution. Additionally, we find that major mergers can account for the local population of warm ULIRGs. This suggests that selecting high redshift warm ULIRGs will allow for the identification of high redshift merger-triggered ULIRGs. As major mergers are likely to trigger very highly obscured AGN, a significant fraction of the high redshift warm ULIRG population may host Compton thick AGN.

astro-ph.CO↗

arXiv2012

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.

astro-ph.HE↗

arXiv2012

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 χsquared 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.

astro-ph.CO↗

arXiv2011

Are Active Galactic Nuclei the Solution to the Excess Cosmic Radio Background at 1.4 GHz?

Recently the ARCADE 2 experiment measured the cosmic radio background (CRB) and found the brightness temperature of the CRB at 1.4 GHz to be ~480 mK. Integrating the flux density from the observed 1.4 GHz radio source count produces a brightness temperature of ~100 mK---less than a quarter of the observed CRB at 1.4 GHz. Radio quiet AGN are a large fraction of the 1.4 GHz uJy sources and typically host significant star formation. Thus, it is possible that AGN and host star formation could be responsible for some fraction of the excess CRB at 1.4 GHz. Here, an X-ray background population synthesis model is used in conjunction with empirical radio to X-ray luminosity ratios to calculate the AGN contribution to the CRB at 1.4 GHz including the emission from host star formation. It is found that AGN and host star formation contribute <~9% of the CRB at 1.4 GHz. When all known 1.4 GHz radio source classes are considered, <~60% of the CRB at 1.4 GHz is accounted for; therefore, it is necessary that either known radio sources evolve significantly at flux densities below current survey sensitivity limits or a new population of low flux density radio sources exist.

astro-ph.CO↗

arXiv2011

The Young, the Old, and the Dusty: Stellar Populations of AGN Hosts

Studying the average properties of active galactic nuclei (AGN) host stellar populations is an important step in understanding the role of AGN in galaxy evolution and the processes which trigger and fuel AGN activity. Here we calculate model spectral energy distributions (SEDs) that include emission from the AGN, the host galaxy stellar population, and dust enshrouded star formation. Using the framework of cosmic X-ray background population synthesis modeling, the model AGN hosts are constrained using optical (B band) and near infrared (J band, 3.6 um, 5.7 um, 8.0 um, and 24 um) luminosity functions and number counts. It is found that at z < 1, type 1 and type 2 AGN hosts have similar stellar populations, in agreement with the orientation based unified model and indicative of secular evolution. At z > 1, type 2 AGN hosts are intrinsically different from type 1 AGN hosts, suggesting that the simple orientation based unified model does not hold at z > 1. Also, it is found that if Compton thick (CT) AGN evolve like less obscured type 2 AGN, then, on average, CT AGN hosts are similar to type 2 AGN hosts; however, if CT obscuration is connected to an evolutionary stage of black hole growth, then CT AGN hosts will also be in specific evolutionary stages. Multi-wavelength selection criteria of CT AGN are discussed.

astro-ph.CO↗

arXiv2011

Properties and Expected Number Counts of Active Galactic Nuclei and their Hosts in the Far Infrared

Telescopes like Herschel and the Atacama Large Millimeter/submillimeter Array (ALMA) are creating new opportunities to study sources in the far infrared (FIR), a wavelength region dominated by cold dust emission. Probing cold dust in active galaxies allows for study of the star formation history of active galactic nuclei (AGN) hosts. The FIR is also an important spectral region for observing AGN which are heavily enshrouded by dust, such as Compton thick AGN. By using information from deep X-ray surveys and cosmic X-ray background synthesis models, we compute Cloudy photoionization simulations which are used to predict the spectral energy distribution (SED) of AGN in the FIR. Expected differential number counts of AGN and their host galaxies are calculated in the Herschel bands. The expected contribution of AGN and their hosts to the cosmic infrared background (CIRB) and the infrared luminosity density are also computed. Multiple star formation scenarios are investigated using a modified blackbody star formation SED. It is found that FIR observations at ~500 um are an excellent tool in determining the star formation history of AGN hosts. Additionally, the AGN contribution to the CIRB can be used to determine whether star formation in AGN hosts evolves differently than in normal galaxies. The contribution of Compton thick AGN to the bright end differential number counts and to the bright source infrared luminosity density is a good test of AGN evolution models where quasars are triggered by major mergers.

astro-ph.CO↗

arXiv2007

Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*

A point-like source of ~TeV gamma-rays has recently been seen towards the Galactic center by HESS and other air Cerenkov telescopes. In recent work (Ballantyne et al. 2007), we demonstrated that these gamma-rays can be attributed to high-energy protons that (i) are accelerated close to the event horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and (iii) finally interact to produce gamma-rays. The same hadronic collision processes will necessarily lead to the creation of electrons and positrons. Here we calculate the synchrotron emissivity of these secondary leptons in the same magnetic field configuration through which the initiating protons have been propagated in our model. We compare this emission with the observed ~GHz radio spectrum of the inner few pc region which we have assembled from archival data and new measurements we have made with the Australia Telescope Compact Array. We find that our model predicts secondary synchrotron emission with a steep slope consistent with the observations but with an overall normalization that is too large by a factor of ~ 2. If we further constrain our theoretical gamma-ray curve to obey the implicit EGRET upper limit on emission from this region we predict radio emission that is consistent with observations, i.e., the hadronic model of gamma ray emission can, simultaneously and without fine-tuning, also explain essentially all the diffuse radio emission detected from the inner few pc of the Galaxy.

astro-ph↗

arXiv2004

Photoevaporation of Circumstellar Disks around Young Stars

We examine the ability of photoevaporative disk winds to explain the low-velocity components observed in the forbidden line spectra of low-mass T Tauri stars. Using the analytic model of Shu, Johnstone, & Hollenbach (1993) and Hollenbach et al. (1994) as a basis, we examine the characteristics of photoevaporative outflows with hydrodynamic simulations. General results from the simulations agree well with the analytic predictions, although some small differences are present. Most importantly, the flow of material from the disk surface develops at smaller radii than in the analytic approximations and the flow-velocity from the disk surface is only one-third the sound speed. A detailed presentation of observational consequences of the model is given, including predicted line widths, blue-shifts, and integrated luminosities of observable sulfur and nitrogen emission lines. We demonstrate that these predictions are in agreement with current observational data on the low-velocity forbidden line emission of ionized species from T Tauri stars. This is in contrast with magnetic wind models, which systematically under-predict these forbidden line luminosities. However, the present model cannot easily account for the luminosities of neutral oxygen lines in T Tauri stars.

astro-ph↗

arXiv2001

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.

astro-ph↗

arXiv2001

JHK Standard Stars for Large Telescopes: the UKIRT Fundamental and Extended Lists

We present high-precision JHK photometry with the 3.8m UK Infrared Telescope (UKIRT) of 83 standard stars, 28 from the widely used preliminary list known as the "UKIRT Faint Standards" (Casali & Hawarden, 1992), referred to here as the Fundamental List, and 55 additional stars referred to as the Extended List. The stars have 9.4<K<15.0 and most should be readily observable with imaging array detectors in normal operating modes on telescopes of up to 10m aperture. Many are accessible from the southern hemisphere. Arcsec-accuracy positions (J2000, Epoch ~1998) are given, together with optical photometry and spectral types from the literature, where available, or inferred from the J-K colour. Finding charts are provided for stars with proper motions exceeding 0.3"/yr. On 30 nights between late 1994 and early 1998 the stars from the Fundamental List, which were used as standards for the whole programme, were observed on an average of 10 nights each, and those from the Extended List 6 nights. The average internal standard error of the mean results for K is 0.005 mag; for J-H it is 0.003 mag for the Fundamental List stars and 0.006 mag for the Extended List; for H-K the average is 0.004 mag. The results are on the natural system of the IRCAM3 imager, which used a 256x256 InSb detector array with "standard" JHK filters, behind gold-coated fore-optics and a gold- or silver-dielectric coated dichroic. We give colour transformations onto the CIT, Arcetri and LCO/Palomar NICMOS systems, and preliminary transformations onto the system defined by the new Mauna Kea Observatory filter set.

astro-ph↗

arXiv1999

Emission-line Helium Abundances in Highly Obscured Nebulae

This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36$\micron$/[NeII] 12.8$\micron$, [NeIII]~15.6$\micron$ /[NeII] 12.8$\micron$, [ArIII] 9$\micron$/[ArII] 6.9$\micron$, and [ArIII] 21$\micron$/[ArII] 6.9$\micron$, that are sensitive to the He ICF. This happens because the ions cover a wide range of ionization, the line pairs are not sensitive to electron temperature, they have similar critical densities, and are formed within the He$^+$/H$^+$ region of the nebula. We compute a very wide range of photoionization models appropriate for galactic HII regions. The models cover a wide range of densities, ionization parameters, stellar temperatures, and use continua from four very different stellar atmospheres. The results show that each line pair has a critical intensity ratio above which the He ICF is always small. Below these values the ICF depends very strongly on details of the models for three of the ratios, and so other information would be needed to determine the helium abundance. The [Ar III] 9$\micron$/[ArII] 6.9$\micron$ ratio can indicate the ICF directly due to the near exact match in the critical densities of the two lines. Finally, continua predicted by the latest generation of stellar atmospheres are sufficiently hard that they routinely produce significantly negative ICFs.

astro-ph↗