3,252 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
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
Ionization Mechanisms in Jet-Dominated Seyferts: A Detailed Case Study
For the past 10 years there has been an active debate over whether fast
shocks play an important role in ionizing emission line regions in Seyfert
galaxies. To investigate this claim, we have studied the Seyfert 2 galaxy Mkn
78, using HST UV/optical images and spectroscopy. Since Mkn 78 provides the
archetypal jet-driven bipolar velocity field, if shocks are important anywhere
they should be important in this object. Having mapped the emission line fluxes
and velocity field, we first compare the ionization conditions to standard
photoionization and shock models. We find coherent variations of ionization
consistent with photoionization model sequences which combine optically thick
and thin gas, but are inconsistent with either autoionizing shock models or
photoionization models of just optically thick gas. Furthermore, we find
absolutely no link between the ionization of the gas and its kinematic state,
while we do find a simple decline of ionization degree with radius. We feel
this object provides the strongest case to date against the importance of shock
related ionization in Seyferts.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 222
"The Interplay among Black Holes, Stars and ISM in Galactic Nuclei", T.
Storchi Bergmann, L.C. Ho & H.R. Schmitt, ed
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
The Yang Lee Edge Singularity on Feynman Diagrams
We investigate the Yang-Lee edge singularity on non-planar random graphs,
which we consider as the Feynman Diagrams of various d=0 field theories, in
order to determine the value of the edge exponent.
We consider the hard dimer model on phi3 and phi4 random graphs to test the
universality of the exponent with respect to coordination number, and the Ising
model in an external field to test its temperature independence. The results
here for generic (``thin'') random graphs provide an interesting counterpoint
to the discussion by Staudacher of these models on planar random graphs.Comment: LaTeX, 6 pages + 3 figure
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
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