Synthetic IRIS spectra of the solar transition region: Effect of high-energy tails

The solar transition region satisfies the conditions for presence of non-Maxwellian electron energy distributions with high-energy tails at energies corresponding to the ionization potentials of many ions emitting in the EUV and UV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IV spectra in the far ultra-violet (FUV) channel of the Interface Region Imaging Spectrograph (IRIS). Ionization, recombination, and excitation rates are obtained by integration of the cross-sections or their approximations over the model electron distributions considering particle propagation from the hotter corona. The ionization rates are significantly affected by the presence of high-energy tails. This leads to the peaks of the relative abundance of individual ions to be broadened with pronounced low-temperature shoulders. As a result, the contribution functions of individual lines observable by IRIS also exhibit low-temperature shoulders, or their peaks are shifted to temperatures an order of magnitude lower than for the Maxwellian distribution. The integrated emergent spectra can show enhancements of Si IV compared toO IV by more than a factor of two. The high-energy particles can have significant impact on the emergent spectra and their presence needs to be considered even in situations without strong local acceleration

The Discovery of an Active Galactic Nucleus in the Late-type Galaxy NGC 3621: Spitzer Spectroscopic Observations

We report the discovery of an Active Galactic Nucleus (AGN) in the nearby SAd galaxy NGC 3621 using Spitzer high spectral resolution observations. These observations reveal the presence of [NeV] 14 um and 24 um emission which is centrally concentrated and peaks at the position of the near-infrared nucleus. Using the [NeV] line luminosity, we estimate that the nuclear bolometric luminosity of the AGN is ~ 5 X 10^41 ergs s^-1, which corresponds based on the Eddington limit to a lower mass limit of the black hole of ~ 4 X 10^3 Msun. Using an order of magnitude estimate for the bulge mass based on the Hubble type of the galaxy, we find that this lower mass limit does not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Mutli-wavelength follow-up observations of NGC 3621 are required to obtain more precise estimates of the bulge mass, black hole mass, accretion rate, and nuclear bolometric luminosity. The discovery reported here adds to the growing evidence that a black hole can form and grow in a galaxy with no or minimal bulge.Comment: 5 pages, 7 figures, Accepted for publication in ApJ Letter

Mid-Infrared Properties of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. I. Emission-Line Diagnostics

We compare mid-infrared emission-line properties, from high-resolution Spitzer spectra of a hard X-ray (14 -- 195 keV) selected sample of nearby (z < 0.05) AGN detected by the Burst Alert Telescope (BAT) aboard Swift. The luminosity distribution for the mid-infrared emission-lines, [O IV] 25.89 micron, [Ne II] 12.81 micron, [Ne III] 15.56 micron and [Ne V] 14.32/24.32 micron, and hard X-ray continuum show no differences between Seyfert 1 and Seyfert 2 populations, however six newly discovered BAT AGNs are under-luminous in [O IV], most likely the result of dust extinction in the host galaxy. The overall tightness of the mid-infrared correlations and BAT fluxes and luminosities suggests that the emission lines primarily arise in gas ionized by the AGN. We also compare the mid-infrared emission-lines in the BAT AGNs with those from published studies of ULIRGs, PG QSOs, star-forming galaxies and LINERs. We find that the BAT AGN sample fall into a distinctive region when comparing the [Ne III]/[Ne II] and the [O IV]/[Ne III] ratios. These line ratios are lower in sources that have been previously classified in the mid-infrared/optical as AGN than those found for the BAT AGN, suggesting that, in our X-ray selected sample, the AGN represents the main contribution to the observed line emission. These ratios represent a new emission line diagnostic for distinguishing between AGN and star forming galaxies.Comment: 54 pages, 9 Figures. Accepted for publication in The Astrophysical Journal

Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies

We report the discovery using Spitzers high resolution spectrograph of 7 Active Galactic Nuclei (AGN) in a sample of 32 late-type galaxies that show no definitive signatures of AGN in their optical spectra. Our observations suggest that the AGN detection rate in late-type galaxies is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with an input AGN and an extreme EUV-bright starburst ionizing radiation field that the observed mid-infrared line ratios cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by optical spectroscopy. The AGN bolometric luminosities in our sample range from ~3 X 10^41 - ~2 X 10^43 ergs s^-1, which, based on the Eddington limit, corresponds to a lower mass limit for the black hole that ranges from ~3 X 10^3Mdot to as high as ~1.5 X 10^5Mdot. These lower mass limits however do not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.Comment: 8 figures, 17 pages, astro-ph\0801.2766 (Abel & Satyapal 2008; ApJ accepted) and this posting designed to form a two-part investigatio

An X-ray view of 82 LINERs with Chandra and XMM-Newton data

We present the results of an homogeneous X-ray analysis for 82 nearby LINERs selected from the catalogue of Carrillo et al. (1999). All sources have available Chandra (68 sources) and/or XMM-Newton (55 sources) observations. This is the largest sample of LINERs with X-ray spectral data (60 out of the 82 objects) and significantly improves our previous analysis based on Chandra data for 51 LINERs (Gonzalez-Martin et al. 2006). It increases both the sample size and adds XMM-Newton data. New models permit the inclusion of double absorbers in the spectral fits. Nuclear X-ray morphology is inferred from the compactness of detected nuclear sources in the hard band (4.5-8.0 keV). Sixty per cent of the sample shows a compact nuclear source and are classified as AGN candidates. The spectral analysis indicates that best fits involve a composite model: absorbed primary continuum and (2) soft spectrum below 2 keV described by an absorbed scatterer and/or a thermal component. The resulting median spectra parameters and their standard deviations are: G=2.11, =0.54 keV, =21.32 and =21.93. We complement our X-ray results with our analysis of HST optical images and literature data on emission lines, radio compactness and stellar population. Adding all these multiwavelength data, we conclude that evidence do exist supporting the AGN nature of their nuclear engine for 80% of the sample (66 out of 82 objects).Comment: Accepted for publications in Astronomy and Astrophysics, 49 pages, 12 figures. Catalogs only at: http://www.star.le.ac.uk/~gmo4/O.Gonzalez-Martin-LINERs_xray.pd

A Chandra Snapshot Survey of IR-bright LINERs: A Possible Link Between Star Formation, AGN Fueling, and Mass Accretion

We present results from a high resolution X-ray imaging study of nearby LINERs observed by Chandra. This study complements and extends previous X-ray studies of LINERs, focusing on the under-explored population of nearby dust-enshrouded infrared-bright LINERs. The sample consists of 15 IR-bright LINERs (L_FIR/L_B > 3), with distances that range from 11 to 26 Mpc. Combining our sample with previous Chandra studies we find that ~ 51% (28/55) of the LINERs display compact hard X-ray cores. The nuclear 2-10 keV luminosities of the galaxies in this expanded sample range from ~ 2 X 10^38 ergs s^-1 to ~ 2 X 10^44 ergs s^-1. We find an intriguing trend in the Eddington ratio vs. L_FIR and L_FIR/L_B for the AGN-LINERs in the expanded sample that extends over seven orders of magnitude in L/L_Edd. This correlation may imply a link between black hole growth, as measured by the Eddington ratio, and the star formation rate (SFR), as measured by the far-IR luminosity and IR-brightness ratio. If the far-IR luminosity is an indicator of the molecular gas content in our sample of LINERs, our results may further indicate that the mass accretion rate scales with the host galaxy's fuel supply. We discuss the potential implications of our results in the framework of black hole growth and AGN fueling in low luminosity AGN. (Abridged)Comment: Accepted for publication by ApJ 14 pages, 13 figure