125 research outputs found
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
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