846 research outputs found
Reverberation Mapping and the Physics of Active Galactic Nuclei
Reverberation-mapping campaigns have revolutionized our understanding of AGN.
They have allowed the direct determination of the broad-line region size,
enabled mapping of the gas distribution around the central black hole, and are
starting to resolve the continuum source structure. This review describes the
recent and successful campaigns of the International AGN Watch consortium,
outlines the theoretical background of reverberation mapping and the
calculation of transfer functions, and addresses the fundamental difficulties
of such experiments. It shows that such large-scale experiments have resulted
in a ``new BLR'' which is considerably different from the one we knew just ten
years ago. We discuss in some detail the more important new results, including
the luminosity-size-mass relationship for AGN, and suggest ways to proceed in
the near future.Comment: Review article to appear in Astronomical Time Series, Proceedings of
the Wise Observatory 25th Ann. Symposium. 24 pages including 7 figure
Red and Blue Shifted Broad Lines in Luminous Quasars
We have observed a sample of 22 luminous quasars, in the range 2.0<z<2.5, at
1.6 microns with the near-infrared (NIR) spectrograph FSPEC on the Multiple
Mirror Telescope. Our sample contains 13 radio-loud and 9 radio-quiet objects.
We have measured the systemic redshifts z_(sys) directly from the strong [O
III]5007 line emitted from the narrow-line-region. From the same spectra, we
have found that the non-resonance broad H lines have a systematic mean
redward shift of 520+/-80 km/s with respect to systemic. Such a shift was not
found in our identical analysis of the low-redshift sample of Boroson & Green.
The amplitude of this redshift is comparable to half the expected gravitational
redshift and transverse Doppler effects, and is consistent with a correlation
between redshift differences and quasar luminosity. From data in the
literature, we confirm that the high-ionization rest-frame ultraviolet broad
lines are blueshifted ~550-1050 km/s from systemic, and that these velocity
shifts systematically increase with ionization potential. Our results allow us
to quantify the known bias in estimating the ionizing flux from the
inter-galactic-medium J_(IGM) via the Proximity Effect. Using redshift
measurements commonly determined from strong broad line species, like Ly\alpha
or CIV1549, results in an over-estimation of J_(IGM) by factors of ~1.9-2.3.
Similarly, corresponding lower limits on the density of baryon Omega_b will be
over-estimated by factors of ~1.4-1.5. However, the low-ionization MgII2798
broad line is within ~50 km/s of systemic, and thus would be the line of choice
for determining the true redshift of 1.0<z<2.2 quasars without NIR
spectroscopy, and z>3.1 objects using NIR spectroscopy.Comment: 12 pages, Latex, 2 figures, 2 tables, Accepted for publication in ApJ
Letter
Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I.Observational Results
We present optical spectra and emission-line ratios of 12 Narrow-Line Seyfert
1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A
common feature in the EUV continuum of active galactic nuclei is the big blue
bump (BBB), generally associated with thermal accretion disk emission. While
Galactic absorption prevents direct access to the EUV range, it can be mapped
by measuring the strength of a variety of forbidden optical emission lines that
respond to different EUV continuum regions. We find that narrow emission-line
ratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and
[SII]6716,6731 indicate no significant difference between NLS1s and Broad-Line
Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy
distributions of their ionizing EUV - soft X-ray continua are similar. The
relative strength of important forbidden high ionization lines like [NeV]3426
compared to HeII4686 and the relative strength of [FeX]6374 appear to show the
same range as in BLS1 galaxies. However, a trend of weaker
F([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared to
BLS1s. To recover the broad emission-line profiles we used Gaussian components.
This approach indicates that the broad Hbeta profile can be well described with
a broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component
(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typical
range of normal BLS1s. The emission-line flux that is associated with the broad
component in these NLS1s amounts to at least 60% of the total flux. Thus it
dominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the
Astrophys.Journa
ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240
We present Infrared Space Observatory mid- and far-infrared spectroscopy of
the merging galaxy NGC 6240, an object presenting many aspects of importance
for the role of star formation and AGN activity in [ultra]luminous infrared
galaxies. The mid-infrared spectrum shows starburst indicators in the form of
low excitation fine-structure line emission and aromatic `PAH' features. A
strong high excitation [OIV] line is observed which most likely originates in
the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely
powerful emission in the pure rotational lines of molecular hydrogen. We argue
that this emission is mainly due to shocks in its turbulent central gas
component and its starburst superwind. The total shock cooling in infrared
emission lines accounts for ~0.6% of the bolometric luminosity, mainly through
rotational H_2 emission and the [OI] 63micron line. We analyse several ways of
estimating the luminosities of the starburst and the AGN in NGC 6240 and
suggest that the contributions to its bolometric luminosity are most likely in
the range 50-75% starburst and 25-50% AGN.Comment: 5 figures. Accepted for publication in A&
beta-amyloid modulation of synaptic transmission and plasticity
The sequencing of β amyloid protein (Aβ) in 1984 led to the formulation of the “amyloid hypothesis” of Alzheimer's disease (AD) (Glenner and Wong, 1984). The hypothesis proposed that accumulation of Aβ is responsible for AD-related pathology, including Aβ deposits, neurofibrillary tangles, and eventual neuronal cell death (Tanzi and Bertram, 2005). Within a few years, four groups cloned the amyloid precursor protein (APP) gene from which Aβ is processed (Goldgaber et al., 1987; Kang et al., 1987; Robakis et al., 1987; Tanzi et al., 1987). Linkage analysis mapped the gene to chromosome 21, and mutations in APP were found that led to the inappropriate processing of APP into the Aβ1–42 peptide (Goate et al., 1991; Mullan et al., 1992) (for review, see Tanzi and Bertram, 2005). However, these mutations are responsible for only a small fraction of the early-onset familial AD, and the search began for other genes that might also influence the processing of Aβ. Several novel mutations were identified in the presenilins (Levy-Lahad et al., 1995; Rogaev et al., 1995; Sherrington et al., 1995), and apolipoprotein E4 was identified as a major risk factor for the most frequent form of AD (Strittmatter et al., 1993; Mahley et al., 2006)
The Nature of Associated Absorption and the UV-X-ray Connection in 3C 288.1
We discuss new Hubble Space Telescope spectroscopy of the radio-loud quasar,
3C 288.1. The data cover ~590 A to ~1610 A in the quasar rest frame. They
reveal a wealth of associated absorption lines (AALs) with no accompanying
Lyman-limit absorption. The metallic AALs range in ionization from C III and N
III to Ne VIII and Mg X. We use these data and photoionization models to derive
the following properties of the AAL gas: 1) There are multiple ionization zones
within the AAL region, spanning a factor of at least ~50 in ionization
parameter. 2) The overall ionization is consistent with the ``warm'' X-ray
continuum absorbers measured in Seyfert 1 nuclei and other QSOs. However, 3)
the column densities implied by the AALs in 3C 288.1 are too low to produce
significant bound-free absorption at any UV-X-ray wavelengths. Substantial
X-ray absorption would require yet another zone, having a much higher
ionization or a much lower velocity dispersion than the main AAL region. 4) The
total hydrogen column density in the AAL gas is log N_H (cm-2)= 20.2. 5) The
metallicity is roughly half solar. 6) The AALs have deconvolved widths of ~900
km/s and their centroids are consistent with no shift from the quasar systemic
velocity (conservatively within +/-1000 km/s). 7) There are no direct
indicators of the absorber's location in our data, but the high ionization and
high metallicity both suggest a close physical relationship to the quasar/host
galaxy environment. Finally, the UV continuum shape gives no indication of a
``blue bump'' at higher energies. There is a distinct break of unknown origin
at ~1030 A, and the decline toward higher energies (with spectral index alpha =
-1.73, for f_nu ~ nu^alpha) is even steeper than a single power-law
interpolation from 1030 A to soft X-rays.Comment: 27 pages with figures and tables, in press with Ap
Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
We present improved black hole masses for 35 active galactic nuclei (AGNs)
based on a complete and consistent reanalysis of broad emission-line
reverberation-mapping data. From objects with multiple line measurements, we
find that the highest precision measure of the virial product is obtained by
using the cross-correlation function centroid (as opposed to the
cross-correlation function peak) for the time delay and the line dispersion (as
opposed to full width half maximum) for the line width and by measuring the
line width in the variable part of the spectrum. Accurate line-width
measurement depends critically on avoiding contaminating features, in
particular the narrow components of the emission lines. We find that the
precision (or random component of the error) of reverberation-based black hole
mass measurements is typically around 30%, comparable to the precision attained
in measurement of black hole masses in quiescent galaxies by gas or stellar
dynamical methods. Based on results presented in a companion paper by Onken et
al., we provide a zero-point calibration for the reverberation-based black hole
mass scale by using the relationship between black hole mass and host-galaxy
bulge velocity dispersion. The scatter around this relationship implies that
the typical systematic uncertainties in reverberation-based black hole masses
are smaller than a factor of three. We present a preliminary version of a
mass-luminosity relationship that is much better defined than any previous
attempt. Scatter about the mass-luminosity relationship for these AGNs appears
to be real and could be correlated with either Eddington ratio or object
inclination.Comment: 61 pages, including 8 Tables and 16 Figures. Accepted for publication
in The Astrophysical Journa
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability
We present results of an intensive two-month campaign of approximately daily
spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with
HST. The fractional variability amplitude of the continuum variations between
1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1
galaxies over a similar period of time. However, large amplitude, short
time-scale flaring behavior is evident, with trough-to-peak flux changes of
about 18% in approximately 3 days. We present evidence for wavelength-dependent
continuum time delays, with the variations at 3000 A lagging behind those at
1365 A by about 1 day. These delays may be interpreted as evidence for a
stratified continuum reprocessing region, possibly an accretion-disk structure.
The Lyman-alpha 1216 emission-line exhibits flux variations of about 1%
amplitude.Comment: 27 pages, 14 figures. Accepted by Astrophysical Journa
Average UV Quasar Spectra in the Context of Eigenvector 1: A Baldwin Effect Governed by Eddington Ratio?
We present composite UV spectra for low redshift Type 1 AGN binned to exploit
the information content of the Eigenvector 1 (E1) parameter space. Composite
spectra allow a decomposition of the CIV1549 line profile - one of the
strongest high-ionization lines. The simplest CIV decomposition into narrow
(NLR), broad (BLR) and very broad (VBLR) components suggests that different
components have an analog in Hb with two major exceptions. VBLR emission is
seen only in population B (FWHM(Hb)>4000 km/s) sources. A blue
shifted/asymmetric BLR component is seen only in pop. A (FWHM(Hb)<4000 km/s)
HIL such as CIV. The blueshifted component is thought to arise in a wind or
outflow. Our analysis suggests that such a wind can only be produced in pop. A
(almost all radio-quiet) sources where the accretion rate is relatively high.
Comparison between broad UV lines in radio-loud (RL) and radio-quiet (RQ)
sources shows few significant differences. Clear evidence is found for a narrow
CIV component in most radio-loud sources. We find also some indirect
indications that the black hole (BH) spin, rather than BH mass or accretion
rate is a key trigger in determining whether an object will be RL or RQ. We
find a ten-fold decrease in EW CIV with Eddington ratio (decreasing from ~1 to
\~0.01) while NV shows no change. These trends suggest a luminosity-independent
"Baldwin effect" where the physical driver may be the Eddington ratio.Comment: 39 pages, 6 figures. To appear in Ap
A Hierarchical Approach to Protein Molecular Evolution
Biological diversity has evolved despite the essentially infinite complexity
of protein sequence space. We present a hierarchical approach to the efficient
searching of this space and quantify the evolutionary potential of our approach
with Monte Carlo simulations. These simulations demonstrate that non-homologous
juxtaposition of encoded structure is the rate-limiting step in the production
of new tertiary protein folds. Non-homologous ``swapping'' of low energy
secondary structures increased the binding constant of a simulated protein by
relative to base substitution alone. Applications of our approach
include the generation of new protein folds and modeling the molecular
evolution of disease.Comment: 15 pages. 2 figures. LaTeX styl
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