arXiv subjects

Brent Groves

Publications and source records attributed to Brent Groves.

170 records · Page 10Linked to original sources

arXiv2006

The Infrared Emission from the Narrow Line Region

We present models for the mid- and far- infrared emission from the Narrow Line Region (NLR) of Active Galactic Nuclei (AGN). Using the MAPPINGS III code we explore the effect of typical NLR parameters on the spectral characteristics of the IR emission. These include useful IR emission line ratio diagnostic diagrams for the determination of these parameters, as well as Star formation--AGN mixing diagnostics. We also examine emission line to continuum correlations which would assist in separating the IR emission arising from the NLR from that coming from the inner torus. We find for AGN like NGC 1068 and NGC 4151 that the NLR only contributes ~10% to the total IRAS 25 mum flux, and that other components such as a dusty torus are necessary to explain the total AGN IR emission.

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arXiv2006

Emission-line Diagnostics of Low Metallicity AGN

Current emission-line based estimates of the metallicity of active galactic nuclei (AGN) at both high and low redshifts indicate that AGN have predominantly solar to supersolar metallicities. This leads to the question: do low metallicity AGN exist? In this paper we use photoionization models to examine the effects of metallicity variations on the narrow emission lines from an AGN. We explore a variety of emission-line diagnostics that are useful for identifying AGN with low metallicity gas. We find that line ratios involving [NII] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [SII] and [OI] are strongly affected by uncertainties in modelling the density structure of the narrow line clouds. To test our diagnostics, we turn to an analysis of AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the relative strength of [NII] with the mass of the AGN host galaxy. The metallicity of the ISM is known to be correlated with stellar mass in star-forming galaxies; our results indicate that a similar trend exists for AGN. We also find that the best-fit models for typical Seyfert narrow line regions have supersolar abundances. Although there is a mass-dependent range of a factor of 2-3 in the NLR metallicities of the AGN in our sample, AGN with sub-solar metallicities are very rare in the SDSS. Out of a sample of ~23000 Seyfert 2 galaxies we find only ~40 clear candidates for AGN with NLR abundances that are below solar.

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arXiv2006

The Host Galaxies and Classification of Active Galactic Nuclei

We present an analysis of the host properties of 85224 emission-line galaxies from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates star-forming galaxies, composite AGN-HII galaxies, Seyferts and LINERs and we study the host galaxy properties of these different classes of objects. LINERs are older, more massive, less dusty and more concentrated, and have higher velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. We consider the quantity L[OIII]/sigma^4, which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[OIII]/sigma^4, all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/L(EDD) and Seyferts dominant at high L/L(EDD). These results suggest that the majority of LINERs are AGN and that the Seyfert/LINER dichotomy is analogous to the high/low-state transition for X-ray binary systems. We apply theoretical photoionization models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than Seyfert galaxies, consistent with the low and high X-ray binary states.

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arXiv2004

Spatially Resolved Ultraviolet Spectra of the High-Velocity Nuclear Outflow of NGC 1068

We present ultraviolet emission-line maps of the narrow-line region (NLR) of NGC 1068. The maps span 115--318 nm, the biconical ionization cone, several posited jet/ISM interactions, and the compact knots whose optical spectra we reported previously resemble kinematically the quasar Associated Absorption Line systems. Across the NLR, we find that ultraviolet flux ratios are consistent with photoionization, not shock excitation, even for gas blueshifted abruptly to 3000 \kms relative to galaxy systemic velocity or for gas projected near the radio jet. The knots may be radiatively accelerated, photoablated fragments of molecular clouds.

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arXiv2004

Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: I. The role of ISM pressure & the Molecular Cloud Dissipation Timescale

In this paper, we combine the stellar spectral synthesis code STARBURST 99, the nebular modelling code MAPPINGS IIIq, a 1-D dynamical evolution model of \HII regions around massive clusters of young stars and a simplified model of synchrotron emissivity to produce purely theoretical self-consistent synthetic spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting some $10^8$ years. These SEDs extend from the Lyman Limit to beyond 21 cm. We find that two ISM parameters control the form of the SED; the pressure in the diffuse phase of the ISM (or, equivalently, its density), and the molecular cloud dissipation timescale. We present detailed SED fits to Arp 220 and NGC 6240, and we give the predicted colors for starburst galaxies derived from our models for the IRAS and the Spitzer Space Observatory MIPS and IRAC instruments. Our models reproduce the spread in observed colors of starburst galaxies. Finally, we present absolute calibrations to convert observed fluxes into star formation rates in the UV (GALEX), at optical wavelengths (H$α$), and in the IR (IRAS or the Spitzer Space Observatory). (Abstract Truncated)

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arXiv2004

Dusty, Radiation Pressure Dominated Photoionization: The Solution to the Narrow Line Region Problem

Seyfert narrow line regions (NLR) have emission line ratios which are remarkably uniform, displaying only ~0.5 dex variation between different galaxies. Existing models have been unable to explain these observations without the introduction of ad hoc assumptions, geometrical restrictions or new parameters. Here we introduce a new model: dusty radiation pressure dominated photoionization, which provides a natural self-regulating characteristic leading to an invariance of the spectrum over a very wide range (>100) of ionization parameter. The dusty model is able to reproduce both the range and the absolute value of the observational line ratios not only in the standard optical diagnostic diagrams but also in UV diagnostic plots, providing an explanation to the problem in NLR observations.

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arXiv2003

The Radio-FIR correlation: Is MHD Turbulence the Cause?

The radio - far infrared correlation is one of the tightest correlations found in astronomy. Many of the models explaining this correlation rely on the association of of global magnetic field strength with gas density. In this letter we put forward that the physical reason for this association lies within the processes of magnetohydrodynamic turbulence.

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arXiv2001

Spatial Resolution of High-Velocity Filaments in the Narrow-Line Region of NGC 1068: Associated Absorbers Caught in Emission?

Using the HST STIS spectrograph we have obtained a grid of [O III] and H-beta emission-line spectra at 0"05x0"19 and 60 km/s (FWHM) resolution that covers much of the NLR of NGC 1068. We find emitting knots that have blueshifted radial velocities up to 3200 km/s relative to galaxy systemic, are 70-150 pc NE of the nucleus and up to 40 pc from the radio jet, emit several percent of the NLR line flux but no significant continuum, span velocity extents of up to 1250 km/s but a small fraction of the sky seen from the nucleus, coincide with a region of enhanced IR coronal-line emission, and have ionized masses $\sim$200 Msun/ne4 (ne4=10^4 cm^{-3}). We argue that the blueshifted knots are ablata from disintegrating molecular clouds that are being photoionized by the AGN, and are being accelerated readiatively by the AGN or mechanically by the radio jet. In their kinematic properties, the knots resemble the associated absorbers seen projected on the UV continua of some AGN. Between 2"5-4"5 from the nucleus, emission is redshifted relative to systemic, a pattern that we interpret as gas in the galaxy disk being pushed away from us by the NE radio lobe.

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