5,121 research outputs found
An H alpha Survey of 8 Abell Clusters: the dependence of tidally-induced star formation on cluster density
We have undertaken a survey of H alpha emission in a substantially complete
sample of CGCG galaxies of types Sa and later within 1.5 Abell radii of the
centres of 8 low-redshift Abell clusters. Some 320 galaxies were surveyed, of
which 116 were detected in emission (39% of spirals, 75% of peculiars).
Detected emission was classified as `compact' or `diffuse'. From an analysis of
the full survey sample, we reconfirm our previous identification of compact and
diffuse emission with circumnuclear starburst and disk emission respectively.
The circumnuclear emission is associated either with the presence of a bar, or
with a disturbed galaxy morphology indicative of on-going tidal interactions.
The frequency of such tidally-induced (circumnuclear) starburst emission in
spirals increases from regions of lower to higher local galaxy surface density,
and from clusters with lower to higher central galaxy space density. We
conclude that tidal interactions are likely to be the main mechanism for the
transformation of spirals to S0s in clusters. Finally, for regions of
comparable local density, the frequency of tidally-induced starburst emission
is greater in clusters with higher central galaxy density. This implies that,
for a given local density, morphological transformation of disk galaxies
proceeds more rapidly in clusters of higher central galaxy density. This effect
is considered to be due to subcluster merging and could account for the
previously considered anomalous absence of a significant type - local surface
density relation for irregular clusters at intermediate redshift.Comment: 22 pages including 4 figures. Accepted for publication in MNRA
Between social policy and Union citizenship: the Framework Directive on equal treatment in employment
In December 2000, the Council adopted the Framework Directive forbidding discrimination on grounds of religion or belief, disability, age and sexual orientation in the field of employment. The Directive adopted Article 13 EC as its legal basis. However, there are strong arguments suggesting that this was not the correct choice of legal basis; in particular, the Social Chapter of the EC Treaty (Title XI) provided an alternative legal foundation, including different legislative processes (co-decision and the social dialogue). This article first examines the legal grounds requiring a different legal basis for the Directive and then explores the wider political imperatives that may explain the preference of the EU institutions for relying instead on Article 13 EC.</p
Minimal energy control of a nanoelectromechanical memory element
The Pontryagin minimal energy control approach has been applied to minimise the switching energy in a nanoelectromechanical memory system and to characterise global stability of the oscillatory states of the bistable memory element. A comparison of two previously experimentally determined pulse-type control signals with Pontryagin control function has been performed, and the superiority of the Pontryagin approach with regard to power consumption has been demonstrated. An analysis of global stability shows how values of minimal energy can be utilized in order to specify equally stable states
The Locus of Highly Accreting AGNs on the M_BH--sigma Plane: Selections, Limitations, and Implications
We re-examine the locus of narrow line Seyfert 1 galaxies on the M_BH--sigma
(black hole mass--bulge velocity dispersion) plane in the light of the results
from large new optically selected samples. We find that (1) soft X-ray selected
NLS1s have a lower ratio of BH mass to \sigma^{4}_{[OIII]} than broad line
Seyfert 1 galaxies; this remains a robust statistical result contrary to recent
claims otherwise; (2) optically selected NLS1s have systematically lower
Eddington luminosity ratio compared to X-ray selected NLS1s; and (3) as a
result, the locus of NLS1s on the M_BH--sigma plane is affected by selection
effects. We argue that there is no single explanation for the origin of the
M_BH--sigma relation; instead tracks of galaxies on the M_BH--sigma plane
differ with redshift, consistent with the downsizing of AGN activity. If these
results at face value are incorrect, then the data imply that AGNs with high
Eddington accretion reside preferentially in relatively late type galaxies at
the present epoch, perhaps a more interesting result and a challenge to
theoretical models.Comment: To appear in Ap
An obstacle to a decomposition theorem for near-regular matroids
Seymour's Decomposition Theorem for regular matroids states that any matroid
representable over both GF(2) and GF(3) can be obtained from matroids that are
graphic, cographic, or isomorphic to R10 by 1-, 2-, and 3-sums. It is hoped
that similar characterizations hold for other classes of matroids, notably for
the class of near-regular matroids. Suppose that all near-regular matroids can
be obtained from matroids that belong to a few basic classes through k-sums.
Also suppose that these basic classes are such that, whenever a class contains
all graphic matroids, it does not contain all cographic matroids. We show that
in that case 3-sums will not suffice.Comment: 11 pages, 1 figur
A Note on the Viability of Gaseous Ionization in Active Galaxies by Fast Shocks
Observational evidence suggest that shocks may affect the spatial and
velocity distributions of gas in the NLR/ENLR of some active galaxies. It thus
seemed plausible that shocks may also energize the NLR. The observed emission
line ratios strongly favor photoionization as the heating source, but it is not
clear whether the ionizing radiation is generated in the NLR by "photoionizing
shocks" or whether it originates at the central continuum source. Here I point
out that shocks are highly inefficient in producing line emission. Shocks in
the NLR can convert at most 10^{-6} of the rest mass to ionizing radiation,
compared with a maximum conversion efficiency of ~0.1 for the central continuum
source. The required mass flow rate through shocks in the NLR is thus a few
orders of magnitude higher than the mass accretion rate required to power the
NLR by the central continuum source. Since gravity appears to dominate the NLR
cloud dynamics, shocks must lead to an inflow, and the implied high inflow
rates can be ruled out in most active galaxies. NLR dynamics driven by a
thermal wind or by some jet configurations may produce the mass flux through
shocks required for photoionizing shocks to be viable, but the mass flux inward
from the NLR must be kept ~100-1000 times smaller. Photoionizing shocks are a
viable mechanism in very low luminosity active galaxies if they are highly
sub-Eddington (<~10^{-4}) and if they convert mass to radiation with a very low
efficiency (<~10^{-4}).Comment: 6 pages, aas2pp4.sty, accepted for publication in ApJ Letter
The Nuclear Outflow in NGC 2110
We present a HST/STIS spectroscopic and optical/radio imaging study of the
Seyfert NGC 2110 aiming to measure the dynamics and understand the nature of
the nuclear outflow in the galaxy. Previous HST studies have revealed the
presence of a linear structure in the Narrow-Line Region (NLR) aligned with the
radio jet. We show that this structure is strongly accelerated, probably by the
jet, but is unlikely to be entrained in the jet flow. The ionisation properties
of this structure are consistent with photoionisation of dusty, dense gas by
the active nucleus. We present a plausible geometrical model for the NLR,
bringing together various components of the nuclear environment of the galaxy.
We highlight the importance of the circum-nuclear disc in determining the
appearance of the emission line gas and the morphology of the jet. From the
dynamics of the emission line gas, we place constraints on the accelerating
mechanism of the outflow and discuss the relative importance of radio source
synchrotron pressure, radio jet ram pressure and nuclear radiation pressure in
accelerating the gas. While all three mechanisms can account for the energetics
of the emission line gas, gravitational arguments support radio jet ram
pressure as the most likely source of the outflow.Comment: 15 pages, 7 figures; accepted to MNRA
HST Observations of the Double-Peaked Emission Lines in the Seyfert Galaxy Markarian 78: Mass Outflows from a Single AGN
Previous ground based observations of the Seyfert 2 galaxy Mrk 78 revealed a
double set of emission lines, similar to those seen in several AGN from recent
surveys. Are the double lines due to two AGN with different radial velocities
in the same galaxy, or are they due to mass outflows from a single AGN?We
present a study of the outflowing ionized gas in the resolved narrow-line
region (NLR) of Mrk 78 using observations from Space Telescope Imaging
Spectrograph (STIS) and Faint Object Camera (FOC) aboard the Hubble Space
Telescope(HST) as part of an ongoing project to determine the kinematics and
geometries of active galactic nuclei (AGN) outflows. From the spectroscopic
information, we deter- mined the fundamental geometry of the outflow via our
kinematics modeling program by recreating radial velocities to fit those seen
in four different STIS slit positions. We determined that the double emission
lines seen in ground-based spectra are due to an asymmetric distribution of
outflowing gas in the NLR. By successfully fitting a model for a single AGN to
Mrk 78, we show that it is possible to explain double emission lines with
radial velocity offsets seen in AGN similar to Mrk 78 without requiring dual
supermassive black holes.Comment: 22 pages, 7 figures (2 color), accepted for publication in The
Astrophysical Journa
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