21 research outputs found
Optical off-nuclear spectra of quasar hosts and radio galaxies
We present optical (~3200A to ~9000A) off-nuclear spectra of 26 powerful
active galaxies in the redshift range 0.1 < z < 0.3, obtained with the Mayall
and William Herschel 4-meter class telescopes. The sample consists of
radio-quiet quasars, radio-loud quasars (all with -23 > M_V > -26) and radio
galaxies of Fanaroff & Riley Type II (with extended radio luminosities and
spectral indices comparable to those of the radio-loud quasars). The spectra
were all taken approximately 5 arcseconds off-nucleus, with offsets carefully
selected so as to maximise the amount of galaxy light falling into the slit,
whilst simultaneously minimising the amount of scattered nuclear light. The
majority of the resulting spectra appear to be dominated by the integrated
stellar continuum of the underlying galaxies rather than by light from the
non-stellar processes occurring in the active nuclei, and in many cases a 4000A
break feature can be identified. The individual spectra are described in
detail, and the importance of the various spectral components is discussed.
Stellar population synthesis modelling of the spectra will follow in a
subsequent paper (Nolan et al. 2000).Comment: 23 pages, LaTeX, uses MNRAS style file, incorporates 71 postscript
figures, to be published in MNRAS. Contact author: [email protected]
The ages of quasar host galaxies
We present the results of fitting deep off-nuclear optical spectroscopy of
radio-quiet quasars, radio-loud quasars and radio galaxies at z ~ 0.2 with
evolutionary synthesis models of galaxy evolution. Our aim was to determine the
age of the dynamically dominant stellar populations in the hos t galaxies of
these three classes of powerful AGN. Some of our spectra display residual
nuclear contamination at the shortest wavelengths, but the detailed quality of
the fits longward of the 4000A break provide unequivocal proof, if further
proof were needed, that quasars lie in massive galaxies with (at least at z ~
0.2) evolved stellar populations. By fitting a two-component model we have
separated the very blue (starburst and/or AGN contamination) from the redder
underlying spectral energy distribution, and find that the hosts of all three
classes of AGN are dominated by old stars of age 8 - 14 Gyr. If the blue
component is attributed to young stars, we find that, at most, 1% of the
baryonic mass of these galaxies is involved in star-formation activity at the
epoch of observation. These results strongly support the conclusion reached by
McLure et al. (1999) that the host galaxies of luminous quasars are massive
ellipticals which formed prior to the peak epoch of quasar activity at z ~ 2.5.Comment: 24 pages, LaTeX, uses MNRAS style file, incorporates 19 postscript
figures, final version, to be published in MNRA
The Radio Quiescence of Active Galaxies with High Accretion Rates
We present 6 cm Very Large Array observations of the Greene & Ho (2004)
sample of 19 low-mass active galaxies with high accretion rates. This is one of
the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies
with such high sensitivity and resolution. Although we detect only one source,
the entire sample is very radio-quiet down to strong limits. GH10 was found to
have a radio power of 8.5 x 10^21 W/Hz, and a ratio R = f(6 cm)/f(4400 A) of
2.8. The 3 sigma upper limits for the remaining nondetections correspond to
radio powers from 3 x 10^20 to 8 x 10^21 W/Hz and 0.47 < R <9.9. Stacking all
nondetections yields an even stronger upper limit of R < 0.27. An assessment of
existing observations in the literature confirms our finding that NLS1s are
consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is
significantly lower than the 10%-20% observed in the general quasar population.
By analogy with stellar-mass black holes, we argue that AGNs undergo a state
transition at L_bol/L_Edd~0.01. Below this value a radiatively inefficient
accretion flow effectively drives an outflow, which disappears when the flow
turns into an optically thick, geometrically thin disk, or a radiation
pressure-dominated slim disk at still higher L_bol/L_Edd.Comment: To appear in ApJ; 8 pages, 3 figures; uses emulateapj5.st
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
Custom Integrated Circuits
Contains reports on ten research projects.Analog Devices, Inc.IBM CorporationNational Science Foundation/Defense Advanced Research Projects Agency Grant MIP 88-14612Analog Devices Career Development Assistant ProfessorshipU.S. Navy - Office of Naval Research Contract N0014-87-K-0825AT&TDigital Equipment CorporationNational Science Foundation Grant MIP 88-5876
Experimental Aspects of Synthesis
We discuss the problem of experimentally evaluating linear-time temporal
logic (LTL) synthesis tools for reactive systems. We first survey previous such
work for the currently publicly available synthesis tools, and then draw
conclusions by deriving useful schemes for future such evaluations.
In particular, we explain why previous tools have incompatible scopes and
semantics and provide a framework that reduces the impact of this problem for
future experimental comparisons of such tools. Furthermore, we discuss which
difficulties the complex workflows that begin to appear in modern synthesis
tools induce on experimental evaluations and give answers to the question how
convincing such evaluations can still be performed in such a setting.Comment: In Proceedings iWIGP 2011, arXiv:1102.374
Formal Methods for Functional Verification
Formal hardware verification ranges from proving that two combinational circuits compute the same functions to the much more ambitious task of proving that a sequential circuit obeys some abstract property expressed in temporal logic. In tracing the history of work in this area, we find a few efforts in the 1970s and 1980s, with a big increase in verification capabilities the late 1980s up through today. The advent of efficient Boolean inference methods, starting with Binary Decision Diagrams (BDDs) and more recently with efficient Boolean satisfiability (SAT) checkers has provided the enabling technology for these advances.
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