398 research outputs found
Young stars and non-stellar emission in the aligned radio galaxy 3C 256
We present ground-based images of the z=1.824 radio galaxy 3C 256 in the
standard BVRIJHK filters and an interference filter centered at 8800A, a Hubble
Space Telescope image in a filter dominated by Ly-alpha emission (F336W), and
spectra covering rest-frame wavelengths from Ly-alpha to [O III] 5007. Together
with published polarimetry observations, we use these to decompose the overall
spectral energy distribution into nebular continuum emission, scattered quasar
light, and stellar emission. The nebular continuum and scattered light together
comprise half (one third) of the V-band (K-band) light within a 4-arcsec
aperture, and are responsible for the strong alignment between the
optical/near-infrared light and the radio emission. The stellar emission is
dominated by a population estimated to be 100-200 Myr old (assuming a Salpeter
IMF), and formed in a short burst with a peak star formation rate of 1-4x10^3
Msun/yr. The total stellar mass is estimated to be no more than 2x10^{11} Msun,
which is far less than other luminous radio galaxies at similar redshifts, and
suggests that 3C 256 will undergo further star formation or mergers.Comment: 35 pages including 10 figures; to appear in Nov 10 Ap
The black hole mass distribution in early-type galaxies: cusps in HST photometry interpreted through adiabatic black hole growth
The surface brightness profiles of early-type galaxies have central cusps.
Two characteristic profile types are observed with HST: `core' profiles have a
break at a resolved radius and logarithmic cusp slope gamma < 0.3 inside that
radius; `power-law' profiles have no clear break and gamma > 0.3. With few
exceptions, galaxies with M_V
-20.5 have power-law profiles. Both profile types occur in galaxies with -22 <
M_V < -20.5. We show that these results are consistent with the hypothesis
that: (i) all early-type galaxies have black holes (BHs) that grew
adiabatically in homogeneous isothermal cores; and (ii) these `progenitor'
cores followed scaling relations similar to those of the fundamental plane.
The models studied here are the ones first proposed by Young. Models with BH
masses and progenitor cores that obey established scaling relations predict (at
Virgo) that galaxies with M_V < -21.2 have core profiles and galaxies with M_V
> -21.2 have power-law profiles. This reproduces both the sense and the
absolute magnitude of the observed transition. Intrinsic scatter in BH and
galaxy properties can explain why both types of galaxies are observed around
the transition magnitude. The observed bimodality in cusp slopes may be due to
a bimodality in M_bh/L, with rapidly rotating disky galaxies having larger
M_bh/L than slowly rotating boxy galaxies.
Application to individual galaxies with HST photometry yields a roughly
linear correlation between BH mass and V-band galaxy luminosity, log M_bh =
-1.83 + log L (solar units). This agrees with the average relation for nearby
galaxies with kinematically determined BH masses, and also with predictions
from quasar statistics (shortened abstract).Comment: 41 pages, LaTeX, with 11 PostScript figures. Submitted to the
Astronomical Journal. Postscript version also available from
http://sol.stsci.edu/~marel/abstracts/abs_R23.htm
QSO's from Galaxy Collisions with Naked Black Holes
In the now well established conventional view (see Rees [1] and references
therein), quasi-stellar objects (QSOs) and related active galactic nuclei (AGN)
phenomena are explained as the result of accretion of plasma onto giant black
holes which are postulated to form via gravitational collapse of the high
density regions in the centers of massive host galaxies. This model is
supported by a wide variety of indirect evidence and seems quite likely to
apply at least to some observed AGN phenomena. However, one surprising set of
new Hubble Space Telescope (HST) observations [2-4] directly challenges the
conventional model, and the well known evolution of the QSO population raises
some additional, though not widely recognized, difficulties. We propose here an
alternative possibility: the Universe contains a substantial independent
population of super-massive black holes, and QSO's are a phenomenon that occurs
due to their collisions with galaxies or gas clouds in the intergalactic medium
(IGM). This hypothesis would naturally explain why the QSO population declines
very rapidly towards low redshift, as well as the new HST data.Comment: plain TeX file, no figures, submitted to Natur
Deformation and Grain Growth of Low-Temperature-Sintered High-Purity Alumina
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66091/1/j.1151-2916.1990.tb06489.x.pd
Thick tori around AGN: the case for extended tori and consequences for their X-ray and IR emission
Two families of models of dusty tori in AGNs (moderately thick and extended
versus very thick and compact) are tested against available observations. The
confrontation suggests that the former class better explains the IR broad-band
spectra of both broad and narrow line AGNs, the anisotropy of the emission
deduced by comparing IR properties of Seyfert 1 and 2 nuclei, the results of IR
spectroscopy and those of high spatial resolution observations. There is
however clear evidence for a broad distribution of optical depths. We also
examine the relationship between IR and X-ray emission. The data support a view
in which the matter responsible for the X-ray absorption is mostly dust free,
lying inside the dust sublimation radius. The consequences of these results for
the hard X-ray background as well as IR counts and background are discussed.Comment: 33 pages, 9 Postscript figures, to appear in ApJ, September 199
Constraints on QSO models from a relation between the QSO luminosity function and the local black hole mass function
QSOs are believed to be powered by accretion onto massive black holes (BHs).
In this paper, assuming that each central BH in nearby galaxies has experienced
the QSO phase and ignoring BH mergers, we establish a relation between the QSO
luminosity function (LF) and the local BH mass function (MF). The QSOLF is
jointly controlled by the luminosity evolution of individual QSOs and the
triggering history of the accretion onto seed BHs. By comparing the time
integral of the QSOLF with that inferred from local BHs, we separate the effect
of the luminosity evolution of individual QSOs from the effect of the
triggering history. Assuming that the nuclear luminosity evolution includes two
phases (first increasing at the Eddington luminosity with growth of BHs and
then declining), we find that observations are generally consistent with the
expected relation between the QSOLF and the local BHMF and obtain the
constraints on QSO models and BH growth. We point out that it is hard to
accurately estimate the value of the QSO lifetime from the QSOLF and/or the
local BHMF, if it is longer than a certain value (e.g., four times of the
Salpeter timescale in this study). We discuss the importance of accurate
measurements of the intrinsic scatter in the BH mass and velocity dispersion
relation of local galaxies and the scatter in the bolometric correction of
QSOs. We also discuss some possible applications of the work in this paper,
such as to the study of the demography of QSOs and the demography of normal
galaxies at intermediate redshift (abridged).Comment: 36 pages, including 9 figures; minor revisions to match the published
versio
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Age-Related Degradation of Nuclear Power Plant Structures and Components
This paper summarizes and highlights the results of the initial phase of a research project on the assessment of aged and degraded structures and components important to the safe operation of nuclear power plants (NPPs). A review of age-related degradation of structures and passive components at NPPs was performed. Instances of age-related degradation have been collected and reviewed. Data were collected from plant generated documents such as Licensing Event Reports, NRC generic communications, NUREGs and industry reports. Applicable cases of degradation occurrences were reviewed and then entered into a computerized database. The results obtained from the review of degradation occurrences are summarized and discussed. Various trending analyses were performed to identify which structures and components are most affected, whether degradation occurrences are worsening, and what was the most common aging mechanisms. The paper also discusses potential aging issues and degradation-susceptible structures and passive components which would have the greatest impact on plant risk
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