Narrow-line Regions Of Active Galactic Nuclei Research Articles

Beagle-agn I: simultaneous constraints on the properties of gas in star-forming and AGN narrow-line regions in galaxies

ABSTRACT We present the addition of nebular emission from the narrow-line regions (NLR) surrounding active galactic nuclei (AGNs) to beagle (BayEsian Analysis of GaLaxy sEds). Using a set of idealized spectra, we fit to a set of observables (emission-line ratios and fluxes) and test the retrieval of different physical parameters. We find that fitting to standard diagnostic-line ratios plus [O ii] λ3726, λ3729/[O iii] λ5007, H β/H α, [O i] λ6300/[O ii] λ3726, λ3729, and H α flux, degeneracies remain between dust-to-metal mass ratio ($\xi _\rm {d}^{\small NLR}$) and ionization parameter ($U_\rm {s}^{\small NLR}$) in the NLR gas, and between slope of the ionizing radiation ($\alpha _{\small PL}$, characterizing the emission from the accretion disc around the central black hole) and total accretion-disc luminosity ($L_\rm {acc}$). Since these degeneracies bias the retrieval of other parameters even at maximal signal-to-noise ratio (S/N), without additional observables, we suggest fixing $\alpha _{\small PL}$ and dust-to-metal mass ratios in both NLR and H ii regions. We explore the S/N in H β required for un-biased estimates of physical parameters, finding that S/N(H β) ∼ 10 is sufficient to identify an NLR contribution, but that higher S/N is required for un-biased parameter retrieval (∼20 for NLR-dominated systems, ∼30 for objects with approximately equal H β contributions from NLR and H ii regions). We also compare the predictions of our models for different line ratios to previously published models and data. By adding [He ii] λ4686-line measurements to a set of published line fluxes for a sample of 463 AGN NLR, we show that our models with $-4\lt \hbox{$\log U_{\small S}^{\small NLR}$}\lt -1.5$ can account for the full range of observed AGN properties in the local Universe.

NGC 4993 and other short gamma-ray burst host galaxies

We present the spectral detailed modelling of NGC 4993 – the host galaxy of GW 170817 – and other short gamma-ray burst (SGRB) host galaxies. In order to determine their physical conditions and the element abundances, we have gathered spectroscopic and photometric data from the literature. The observation data are sometimes missing, preventing us from fully constraining the model. However, for most of the SGRB hosts the [OIII]5007/Hβ and [NII]6548/Hα line ratios are reported. The analysis of NGC 4993 by a composite model (photoionization+shock) confirms that an active galactic nucleus (AGN), most probably a low-ionization nuclear emission-line region (LINER) or a low-luminosity AGN (LLAGN) is the gas photoionization source. Shock velocities and preshock densities are similar to those found in the narrow line region of AGN. O/H and N/H have solar values. For the other SGRB of the sample, we found that O/H ratios are nearly solar, while N/H covers a much larger range of values at redshifts close to 0.4. In NGC 4993, the relative contribution to the spectral energy distribution of an old stellar population, characterized by a black-body temperature of Tbb = 4000 K, with respect to bremsstrahlung is higher by a factor of >100 than in most of the local AGN and starburst (SB) galaxies. For the other SGRB that compose the sample, Tbb ranges between 2000 K for SGRB 100206A and 8000 K for SGRB 111117A.

O ii] as a proxy for star formation in AGN host galaxies: beware of extended emission line regions

The [OII] 3726+3728\AA\ emission line doublet is often used to estimate star formation rates within the host galaxies of active galactic nuclei (AGN), as it is known to be strongly excited by star formation, but is only weakly excited in the broad and narrow line regions of AGN. However, within AGN host galaxies, [OII] can also be excited in low-density gas located at appreciable distances from the nucleus, but still ionized by the AGN. These AGN extended emission line regions (EELRs) can contribute significant flux to integrated spectra, even in the presence of luminous AGN. Here, we identify EELRs by the presence of the [NeV] 3426\AA\ emission line, which, like [OII], is not strongly excited in the inner regions of AGN, but is a prominent emission line in the lower density EELRs. Critically, unlike [OII], [NeV] is not excited by star formation. Therefore, when strong [NeV] is present in an AGN spectrum, the flux from the EELR is not negligible, implying the [OII] flux is contaminated by emission from the EELR, and is not a good measure of star formation. After removing objects with EELRs identified by [NeV], the [OII] flux in the host galaxies of radio-loud AGN is found to be higher than that within radio-quiet AGN, which could either indicate higher star-formation rates, or the presence of moderate-velocity shocks. Being mindful of EELRs for upcoming large-area spectroscopic surveys, particularly those tied to radio continuum surveys, will be important for determining star formation rates in AGN host galaxies.

NEAR-INFRARED SPECTROSCOPY OF NEARBY SEYFERT GALAXIES: IS THERE EVIDENCE FOR SHOCK EXCITATION IN NARROW-LINE REGIONS?

ABSTRACT One of the important unsettled problems regarding active galactic nuclei (AGNs) is the major ionization mechanism of gas clouds in AGN narrow-line regions (NLRs). In order to investigate this issue, we present our J-band spectroscopic observations of a sample of 26 nearby Seyfert galaxies. In our study, we use the flux ratio of the following two forbidden emission lines, [Fe ii]1.257 μm and [P ii]1.188 μm, because it is known that this ratio is sensitive to the ionization mechanism. We obtain the [Fe ii]/[P ii] flux ratio or its lower limit for 19 objects. In addition to our data, we compile this flux ratio (or its lower limit) for 23 nearby Seyfert galaxies from the literature. Based on the collected data, we find that three Seyfert galaxies show very large lower limits of the [Fe ii]/[P ii] flux ratios (≳10): NGC 2782, NGC 5005, and Mrk 463. It is thus suggested that the contribution of the fast shock in the gas excitation is significantly large for them. However, more than half of the Seyfert galaxies in our sample show moderate [Fe ii]/[P ii] flux ratios (∼2), which is consistent with pure photoionization by power-law ionizing continuum emission. We also find that the [Fe ii]/[P ii] flux ratio shows no clear correlation with the radio loudness, suggesting that the radio jet is not the primary origin of shocks in NLRs of Seyfert galaxies.

Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

In the context of observations of the rest-frame ultraviolet and optical emission from distant galaxies, we explore the emission-line properties of photoionization models of active and inactive galaxies. Our aim is to identify new line-ratio diagnostics to discriminate between gas photoionization by active galactic nuclei (AGN) and star formation. We use a standard photoionization code to compute the emission from AGN narrow-line regions and compare this with calculations of the nebular emission from star-forming galaxies achieved using the same code. We confirm the appropriateness of widely used optical spectral diagnostics of nuclear activity versus star formation and explore new diagnostics at ultraviolet wavelengths. We find that combinations of a collisionally excited metal line or line multiplet, such as CIV 1548,1551, OIII]1661,1666, NIII]1750, [SiIII]1883+[SiIII]1892 and [CIII]1907+CIII]1909, with the HeII 1640 recombination line are individually good discriminants of the nature of the ionizing source. Diagrams involving at least 3 of these lines allow an even more stringent distinction between active and inactive galaxies, as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation. Several line ratios involving Ne-based emission lines, such as [NeIV]2424, [NeIII]3343 and [NeV]3426, are also good diagnostics of nuclear activity. Our results provide a comprehensive framework to identify the sources of photoionization and physical conditions of the ionized gas from the ultraviolet and optical nebular emission from galaxies. This will be particularly useful to interpret observations of high-redshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes.

Observational signatures of galactic winds powered by active galactic nuclei

We predict the observational signatures of galaxy scale outflows powered by active galactic nuclei (AGN). Most of the emission is produced by the forward shock driven into the ambient interstellar medium (ISM) rather than by the reverse shock. AGN powered galactic winds with energetics suggested by phenomenological feedback arguments should produce spatially extended 1-10 keV X-ray emission of 10^(41-44) erg/s, significantly in excess of the spatially extended X-ray emission associated with normal star forming galaxies. The presence of such emission is a direct test of whether AGN outflows significantly interact with the ISM of their host galaxy. We further show that even radio quiet quasars should have a radio luminosity comparable to or in excess of the far infrared-radio correlation of normal star forming galaxies. This radio emission directly constrains the total kinetic energy flux in AGN-powered galactic winds. Radio emission from AGN wind shocks can also explain the recently highlighted correlations between radio luminosity and the kinematics of AGN narrow-line regions in radio quiet quasars.

UNUSUAL BROAD-LINE Mg II EMITTERS AMONG LUMINOUS GALAXIES IN THE BARYON OSCILLATION SPECTROSCOPIC SURVEY

Many classes of active galactic nuclei (AGNs) have been observed and recorded since the discovery of Seyfert galaxies. In this paper, we examine the sample of luminous galaxies in the Baryon Oscillation Spectroscopic Survey. We find a potentially new observational class of AGNs, one with strong and broad Mg II λ2799 line emission, but very weak emission in other normal indicators of AGN activity, such as the broad-line Hα, Hβ, and the near-ultraviolet AGN continuum, leading to an extreme ratio of broad Hα/Mg II flux relative to normal quasars. Meanwhile, these objects' narrow-line flux ratios reveal AGN narrow-line regions with levels of activity consistent with the Mg II fluxes and in agreement with that of normal quasars. These AGN may represent an extreme case of the Baldwin effect, with very low continuum and high equivalent width relative to typical quasars, but their ratio of broad Mg II to broad Balmer emission remains very unusual. They may also be representative of a class of AGN where the central engine is observed indirectly with scattered light. These galaxies represent a small fraction of the total population of luminous galaxies ( 0.1%), but are more likely (about 3.5 times) to have AGN-like nuclear line emission properties than other luminous galaxies. Because Mg II is usually inaccessible for the population of nearby galaxies, there may exist a related population of broad-line Mg II emitters in the local universe which is currently classified as narrow-line emitters (Seyfert 2 galaxies) or low ionization nuclear emission-line regions.

ORIGIN AND DYNAMICAL SUPPORT OF IONIZED GAS IN GALAXY BULGES

We combine ionized gas ([N II] 6583) and stellar central velocity dispersions for a sample of 345 galaxies, with and without active galactic nuclei (AGNs), to study the dynamical state of the nuclear gas and its physical origin. The gas dispersions strongly correlate with the stellar dispersions over the velocity range of 30-350 km/s such that sigma_g/sigma_* ~ 0.6-1.4, with an average value of 0.80. These results are independent of Hubble type (for galaxies from E to Sbc), presence or absence of a bar, or local galaxy environment. For galaxies of type Sc and later and that have sigma_* < 40 km/s, the gas seems to have a minimum threshold of sigma_g ~ 30 km/s, such that sigma_g/sigma_* always exceeds 1. Within the sample of AGNs, sigma_g/sigma_* increases with nuclear luminosity or Eddington ratio, a possible manifestation of AGN feedback associated with accretion disk winds or outflows. This extra source of nongravitational line broadening should be removed when trying to use sigma_g to estimate sigma_*. We show that the mass budget of the narrow-line region can be accounted for by mass loss from evolved stars. The kinematics of the gas, dominated by random motions, largely reflect the velocity field of the hot gas in the bulge. Lastly, we offer a simple explanation for the correlation between line width and line luminosity observed in the narrow-line region of AGNs.