3,480 research outputs found
X-ray Isophotes in a Rapidly Rotating Elliptical Galaxy: Evidence of Inflowing Gas
We describe two-dimensional gasdynamical computations of the X-ray emitting
gas in the rotating elliptical galaxy NGC 4649 that indicate an inflow of about
one solar mass per year at every radius. Such a large instantaneous inflow
cannot have persisted over a Hubble time. The central constant-entropy
temperature peak recently observed in the innermost 150 parsecs is explained by
compressive heating as gas flows toward the central massive black hole. Since
the cooling time of this gas is only a few million years, NGC 4649 provides the
most acutely concentrated known example of the cooling flow problem in which
the time-integrated apparent mass that has flowed into the galactic core
exceeds the total mass observed there. This paradox can be resolved by
intermittent outflows of energy or mass driven by accretion energy released
near the black hole. Inflowing gas is also required at intermediate kpc radii
to explain the ellipticity of X-ray isophotes due to spin-up by mass ejected by
stars that rotate with the galaxy and to explain local density and temperature
profiles. We provide evidence that many luminous elliptical galaxies undergo
similar inflow spin-up. A small turbulent viscosity is required in NGC 4649 to
avoid forming large X-ray luminous disks that are not observed, but the
turbulent pressure is small and does not interfere with mass determinations
that assume hydrostatic equilibrium.Comment: 21 pages, 9 figures, accepted for publication by Ap
Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows
We describe a new type of dynamical model for hot gas in galaxy groups and
clusters in which gas moves simultaneously in both radial directions.
Circulation flows are consistent with (1) the failure to observe cooling gas in
X-ray spectra, (2) multiphase gas observed near the centers of these flows and
(3) the accumulation of iron in the hot gas from Type Ia supernovae in the
central galaxy. Dense inflowing gas cools, producing a positive central
temperature gradient, as in normal cooling flows. Bubbles of hot, buoyant gas
flow outward. Circulation flows eventually cool catastrophically if the outward
flowing gas transports mass but no heat; to maintain the circulation both mass
and energy must be supplied to the inflowing gas over a large volume, extending
to the cooling radius. The rapid radial recirculation of gas produces a flat
central core in the gas iron abundance, similar to many observations. We
believe the circulation flows described here are the first gasdynamic,
long-term evolutionary models that are in good agreement with all essential
features observed in the hot gas: little or no gas cools as required by XMM
spectra, the gas temperature increases outward near the center, and the gaseous
iron abundance is about solar near the center and decreases outward.Comment: 17 pages (emulateapj5) with 6 figures; accepted by The Astrophysical
Journa
Galactic fountains and outflows in star forming dwarf galaxies: ISM expulsion and chemical enrichment
We investigated the impact of supernova feedback in gas-rich dwarf galaxies
experiencing a low-to-moderate star formation rate, typical of relatively
quiescent phases between starbursts. We calculated the long term evolution of
the ISM and the metal-rich SN ejecta using 3D hydrodynamic simulations, in
which the feedback energy is deposited by SNeII exploding in distinct OB
associations. We found that a circulation flow similar to galactic fountains is
generally established, with some ISM lifted at heights of one to few kpc above
the galactic plane. This gas forms an extra-planar layer, which falls back to
the plane in about yr, once the star formation stops. Very little or no
ISM is expelled outside the galaxy system for the considered SFRs, even though
in the most powerful model the SN energy is comparable to the gas binding
energy. The metal-rich SN ejecta is instead more vulnerable to the feedback and
we found that a significant fraction (25-80\%) is vented in the intergalactic
medium, even for low SN rate ( - yr).
About half of the metals retained by the galaxy are located far ( 500 pc)
from the galactic plane. Moreover, our models indicate that the circulation of
the metal-rich gas out from and back to the galactic disk is not able to erase
the chemical gradients imprinted by the (centrally concentrated) SN explosions.Comment: 19 pages, MNRAS accepte
Chaotic cold accretion on to black holes in rotating atmospheres
Chaotic cold accretion (CCA) profoundly differs from classic black hole
accretion models. Using 3D high-resolution simulations, we probe the impact of
rotation on the hot and cold accretion flow in a typical massive galaxy. In the
hot mode, with or without turbulence, the pressure-dominated flow forms a
geometrically thick rotational barrier, suppressing the accretion rate to ~1/3
of the Bondi rate. When radiative cooling is dominant, the gas loses pressure
support and quickly circularizes in a cold thin disk. In the more common state
of a turbulent and heated atmosphere, CCA drives the dynamics if the gas
velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor
number < 1. Extended multiphase filaments condense out of the hot phase via
thermal instability and rain toward the black hole, boosting the accretion rate
up to 100 times the Bondi rate. Initially, turbulence broadens the angular
momentum distribution of the hot gas, allowing the cold phase to condense with
prograde or retrograde motion. Subsequent chaotic collisions between the cold
filaments, clouds, and a clumpy variable torus promote the cancellation of
angular momentum, leading to high accretion rates. The simulated sub-Eddington
accretion rates cover the range inferred from AGN cavity observations. CCA
predicts inner flat X-ray temperature and density profiles, as
recently discovered in M 87 and NGC 3115. The synthetic H{\alpha} images
reproduce the main features of cold gas observations in massive ellipticals, as
the line fluxes and the filaments versus disk morphology. Such dichotomy is key
for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops
and boosts AGN heating; the cold mode is thus reduced and the rotating disk
remains the sole cold structure. Its consumption leaves the atmosphere in hot
mode with suppressed accretion and feedback, reloading the cycle.Comment: 18 pages, 21 figures, published in A&A; fully revised version with
new major results related to H{\alpha} and X-ray observation
Hot gaseous atmospheres in galaxy groups and clusters are both heated and cooled by X-ray cavities
Expanding X-ray cavities observed in hot gas atmospheres of many galaxy
groups and clusters generate shock waves and turbulence that are primary
heating mechanisms required to avoid uninhibited radiatively cooling flows
which are not observed. However, we show here that the evolution of buoyant
cavities also stimulates radiative cooling of observable masses of
low-temperature gas. During their early evolution, radiative cooling occurs in
the wakes of buoyant cavities in two locations: in thin radial filaments
parallel to the buoyant velocity and more broadly in gas compressed beneath
rising cavities. Radiation from these sustained compressions removes entropy
from the hot gas. Gas experiencing the largest entropy loss cools first,
followed by gas with progressively less entropy loss. Most cooling occurs at
late times, yrs, long after the X-ray cavities have disrupted
and are impossible to detect. During these late times, slightly denser low
entropy gas sinks slowly toward the centers of the hot atmospheres where it
cools intermittently, forming clouds near the cluster center. Single cavities
of energy ergs in the atmosphere of the NGC 5044 group create
of cooled gas, exceeding the mass of extended
molecular gas currently observed in that group. The cooled gas clouds we
compute share many attributes with molecular clouds recently observed in NGC
5044 with ALMA: self-gravitationally unbound, dust-free, quasi-randomly
distributed within a few kpc around the group center.Comment: 12 pages, 11 figure; accepted for publication by Ap
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