137 research outputs found
The Chemistry of SFâ‚…Br and SFâ‚…CF=CFâ‚‚ - Addition Reactions
Addition reactions of pentafluorosulfur bromide (SF5Br) with fluoroolefins were studied. Three new adducts were prepared, SF5CHFCHC1Br, SF5CH2CH(CF3)Br, SF5CHFCFC1Br. SF5Br and CFC1=CHC1 yielded a small amount of product but attempts to add SF5Br to CF2=FCOF were unsuccessful.
The mechanism for these addition reactions seems to involve a free radical addition pathway. Steric factors seem to be important in determining whether SF5Br will add to fluoroolefins.
The new compounds, SF5CF(CF3)COF and SF5CF(CF3 )CONH2 have been produced from SF5CF=CF2.
Analytical data, infrared, Raman, nmr and mass spectra are presented supporting the proposed structures for these new compounds
Two New Species for Drosera from Western Australia
Two new species of Drosera, D. marchantii and D. fimbriata, are described and illustrated from collections made in Western Australia and both are assigned to the subgenus Ergaleium
Galaxy-Scale Outflows Driven by Active Galactic Nuclei
We present hydrodynamical simulations of major mergers of galaxies and study
the effects of winds produced by active galactic nuclei (AGN) on interstellar
gas in the AGN's host galaxy. We consider winds with initial velocities ~
10,000 km/s and an initial momentum (energy) flux of ~ tau_w L/c (~ 0.01 tau_w
L), with tau_w ~ 1-10. The AGN wind sweeps up and shock heats the surrounding
interstellar gas, leading to a galaxy-scale outflow with velocities ~ 1000
km/s, peak mass outflow rates comparable to the star formation rate, and a
total ejected gas mass ~ 3 x 10^9 M_sun. Large momentum fluxes, tau_w > 3, are
required for the AGN-driven galactic outflow to suppress star formation and
accretion in the black hole's host galaxy. Less powerful AGN winds (tau_w < 3)
still produce a modest galaxy-scale outflow, but the outflow has little global
effect on the ambient interstellar gas. We argue that this mechanism of AGN
feedback can plausibly produce the high velocity outflows observed in
post-starburst galaxies and the massive molecular and atomic outflows observed
in local ultra-luminous infrared galaxies. Moreover, the outflows from local
ultra-luminous infrared galaxies are inferred to have tau_w ~ 10, comparable to
what we find is required for AGN winds to regulate the growth of black holes
and set the M_BH-sigma relation. We conclude by discussing theoretical
mechanisms that can lead to AGN wind mass-loading and momentum/energy fluxes
large enough to have a significant impact on galaxy formation.Comment: 12 pages, 7 figures; final version accepted by MNRAS; references and
minor clarifications added, conclusions unchange
The Growth of Massive Black Holes in Galaxy Merger Simulations with Feedback by Radiation Pressure
We study the growth of massive black holes (BH) in galaxies using smoothed
particle hydrodynamic simulations of major galaxy mergers with new
implementations of BH accretion and feedback. The effect of BH accretion on gas
in its host galaxy is modeled by depositing momentum at a rate ~ tau L/c into
the ambient gas, where L is the luminosity produced by accretion onto the BH
and tau is the wavelength-averaged optical depth of the galactic nucleus to the
AGN's radiation (a free parameter of our model). The accretion rate onto the BH
is relatively independent of our subgrid accretion model and is instead
determined by the BH's dynamical impact on its host galaxy: BH accretion is
thus self-regulated rather than `supply limited.' We show that the final BH
mass and total stellar mass formed during a merger are more robust predictions
of the simulations than the time dependence of the star formation rate or BH
accretion rate. In particular, the latter depend on the assumed interstellar
medium physics, which determines when and where the gas fragments to form star
clusters; this in turn affects the fuel available for further star formation
and BH growth. Simulations over a factor of ~ 30 in galaxy mass are consistent
with the observed M_BH-sigma relation for a mean optical depth of tau ~ 25.
This requires that most BH growth occur when the galactic nucleus is optically
thick to far-infrared radiation, consistent with the hypothesized connection
between ultra-luminous infrared galaxies and quasars. We find tentative
evidence for a shallower M_BH-sigma relation in the lowest mass galaxies, sigma
< 100 km/s. Our results demonstrate that feedback-regulated BH growth and
consistency with the observed M_BH-sigma relation do not require that BH
feedback terminate star formation in massive galaxies or unbind large
quantities of cold gas.Comment: 21 pages, 17 figures, submitted MNRA
The evolution of Black Hole scaling relations in galaxy mergers
We study the evolution of black holes (BHs) on the M_BH-sigma and
M_BH-M_bulge planes as a function of time in disk galaxies undergoing mergers.
We begin the simulations with the progenitor black hole masses being initially
below (Delta log M_BH=-2), on (Delta log M_BH=0) and above (Delta log M_BH=0.5)
the observed local relations. The final relations are rapidly established after
the final coalescense of the galaxies and their BHs. Progenitors with low
initial gas fractions (f_gas=0.2) starting below the relations evolve onto the
relations (Delta log M_BH=-0.18), progenitors on the relations stay there
(Delta log M_BH=0) and finally progenitors above the relations evolve towards
the relations, but still remaining above them (Delta log M_BH=0.35). Mergers in
which the progenitors have high initial gas fractions (f_gas=0.8) evolve above
the relations in all cases (Delta log M_BH=0.5). We find that the initial gas
fraction is the prime source of scatter in the observed relations, dominating
over the scatter arising from the evolutionary stage of the merger remnants.
The fact that BHs starting above the relations do not evolve onto the
relations, indicates that our simulations rule out the scenario in which
overmassive BHs evolve onto the relations through gas-rich mergers. By
implication our simulations thus disfavor the picture in which supermassive BHs
develop significantly before their parent bulges.Comment: 6 pages, 4 figures, accepted to ApJL (minor revisions to match
accepted version
The physics of galactic winds driven by active galactic nuclei
Active galactic nuclei (AGN) drive fast winds in the interstellar medium of
their host galaxies. It is commonly assumed that the high ambient densities and
intense radiation fields in galactic nuclei imply short cooling times, thus
making the outflows momentum-conserving. We show that cooling of high-velocity,
shocked winds in AGN is in fact inefficient in a wide range of circumstances,
including conditions relevant to ultra-luminous infrared galaxies (ULIRGs),
resulting in energy-conserving outflows. We further show that fast
energy-conserving outflows can tolerate a large amount of mixing with cooler
gas before radiative losses become important. For winds with initial velocity
v_in>~10,000 km s^-1, as observed in ultra-violet and X-ray absorption, the
shocked wind develops a two-temperature structure. While most of the thermal
pressure support is provided by the protons, the cooling processes operate
directly only on the electrons. This significantly slows down inverse Compton
cooling, while free free cooling is negligible. Slower winds with v_in~1,000 km
s^-1, such as may be driven by radiation pressure on dust, can also experience
energy-conserving phases but under more restrictive conditions. During the
energy-conserving phase, the momentum flux of an outflow is boosted by a factor
~v_in/2v_s by work done by the hot post-shock gas, where v_s is the velocity of
the swept-up material. Energy-conserving outflows driven by fast AGN winds
(v_in~0.1c) may therefore explain the momentum fluxes Pdot>>L_AGN/c of
galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked
wind bubbles expanding normal to galactic disks may also explain the
large-scale bipolar structures observed in some systems, including around the
Galactic Center, and can produce significant radio, X-ray, and gamma-ray
emission. [Abridged]Comment: 20 pages, 8 figures. MNRAS, in pres
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