8,810 research outputs found
HST/WFC3 Observations of an Off-Nuclear Superbubble in Arp 220
We present a high spatial resolution optical and infrared study of the
circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging
using HST/WFC3 has resolved the previously observed peak in H+[NII]
emission into a bubble-shaped feature. This feature measures 1.6" in diameter,
or 600 pc, and is only 1" northwest of the western nucleus. The bubble is
aligned with the western nucleus and the large-scale outflow axis seen in
X-rays. We explore several possibilities for the bubble origin, including a jet
or outflow from a hidden active galactic nucleus (AGN), outflows from high
levels of star formation within the few hundred pc nuclear gas disk, or an
ultraluminous X-ray source. An obscured AGN or high levels of star formation
within the inner 100 pc of the nuclei are favored based on the alignment
of the bubble and energetics arguments.Comment: Accepted for publication in ApJ. 12 pages, 10 figure
Clustered Supernovae Drive Powerful Galactic Winds After Super-Bubble Breakout
We use three-dimensional hydrodynamic simulations of vertically stratified
patches of galactic discs to study how the spatio-temporal clustering of
supernovae (SNe) enhances the power of galactic winds. SNe that are randomly
distributed throughout a galactic disc drive inefficient galactic winds because
most supernova remnants lose their energy radiatively before breaking out of
the disc. Accounting for the fact that most star formation is clustered
alleviates this problem. Super-bubbles driven by the combined effects of
clustered SNe propagate rapidly enough to break out of galactic discs well
before the clusters' SNe stop going off. The radiative losses post-breakout are
reduced dramatically and a large fraction () of the energy
released by SNe vents into the halo powering a strong galactic wind. These
energetic winds are capable of providing strong preventative feedback and eject
substantial mass from the galaxy with outflow rates on the order of the star
formation rate. The momentum flux in the wind is only of order that injected by
the SNe, because the hot gas vents before doing significant work on the
surroundings. We show that our conclusions hold for a range of galaxy
properties, both in the local Universe (e.g., M82) and at high redshift (e.g.,
star forming galaxies). We further show that if the efficiency of
forming star clusters increases with increasing gas surface density, as
suggested by theoretical arguments, the condition for star cluster-driven
super-bubbles to break out of galactic discs corresponds to a threshold star
formation rate surface density for the onset of galactic winds
M yr kpc, of order that observed.Comment: 19 pages, 12 figures, and 3 page appendix with 6 figures. Movies
available at http://w.astro.berkeley.edu/~dfielding/#SNeDrivenWinds
A Radio Study of the Seyfert galaxy Markarian 6: Implications for Seyfert life-cycles
We have carried out an extensive radio study with the Very Large Array on the
Seyfert 1.5 galaxy Mrk 6 and imaged a spectacular radio structure in the
source. The radio emission occurs on three different spatial scales, from ~7.5
kpc bubbles to ~1.5 kpc bubbles lying nearly orthogonal to them and a ~1 kpc
radio jet lying orthogonal to the kpc-scale bubble. To explain the complex
morphology, we first consider a scenario in which the radio structures are the
result of superwinds ejected by a nuclear starburst. However, recent Spitzer
observations of Mrk 6 provide an upper limit to the star formation rate (SFR)
of ~5.5 M_sun/yr, an estimate much lower than the SFR of ~33 M_sun/yr derived
assuming that the bubbles are a result of starburst winds energized by
supernovae explosions. Thus, a starburst alone cannot meet the energy
requirements for the creation of the bubbles in Mrk 6. We show that a single
plasmon model is energetically infeasible, and we argue that a jet-driven
bubble model while energetically feasible does not produce the complex radio
morphologies. Finally, we consider a model in which the complex radio structure
is a result of an episodically-powered precessing jet that changes its
orientation. This model is the most attractive as it can naturally explain the
complex radio morphology, and is consistent with the energetics, the spectral
index and the polarization structure. Radio emission in this scenario is a
short-lived phenomenon in the lifetime of a Seyfert galaxy which results due to
an accretion event.Comment: Accepted for publication in Ap
Tightly Correlated X-ray/H Emitting Filaments in the Superbubble and Large-Scale Superwind of NGC 3079
Using Chandra and HST we show that X-ray and H filaments that form
the 1.3-kpc diameter superbubble of NGC 3079 have strikingly similar patterns
at 0."8 resolution. This tight match seems to arise from cool disk gas that has
been driven by the wind, with X-rays being emitted from upstream, stand-off
bowshocks or by conductive cooling at the cloud/wind interfaces. We find that
the soft X-ray plasma has thermal and kinetic energies of
and erg
respectively, where is the filling factor of the X-ray gas and may be
small; these are comparable to the energies of the optical line-emitting gas.
X-rays are also seen from the base of the radio counterbubble that is obscured
optically by the galaxy disk, and from the nucleus (whose spectrum shows the Fe
K line). Hydrodynamical simulations reproduce the obbservations well
using large filling factors within both filament systems; assuming otherwise
seriously underestimates the mass loss in the superwind. The superbubble is
surrounded by a fainter conical halo of X-rays that fill the area delineated by
high angle, H-emitting filaments, supporting our previous assertion
that these filaments form the contact discontinuity/shock between galaxy gas
and shocked wind. About 40\arcsec (3 kpc) above the disk, an X-ray arc may
partially close beyond the bubble, but the north-east quadrant remains open,
consistent with the superwind having broken out into at least the galaxy halo.Comment: 8 pages, 3 figures, to appear in Sept. 10 Ap
Competition Between Fractional Quantum Hall Liquid, Bubble and Wigner Crystal Phases in the Third Landau Level
Magnetotransport measurements were performed in a ultra-high mobility
GaAs/AlGaAs quantum well of density . The
temperature dependence of the magnetoresistance was studied in detail
in the vicinity of . In particular, we discovered new minima in
at filling factor and , but only at
intermediate temperatures mK. We interpret these as
evidence for a fractional quantum Hall liquid forming in the N=2 Landau level
and competing with bubble and Wigner crystal phases favored at lower
temperatures. Our data suggest that a magnetically driven insulator-insulator
quantum phase transition occurs between the bubble and Wigner crystal phases at
T=0.Comment: Phys. Rev. Lett.93 266804 (2004
Toward connecting core-collapse supernova theory with observations: I. Shock revival in a 15 Msun blue supergiant progenitor with SN 1987A energetics
We study the evolution of the collapsing core of a 15 Msun blue supergiant
supernova progenitor from the core bounce until 1.5 seconds later. We present a
sample of hydrodynamic models parameterized to match the explosion energetics
of SN 1987A.
We find the spatial model dimensionality to be an important contributing
factor in the explosion process. Compared to two-dimensional simulations, our
three-dimensional models require lower neutrino luminosities to produce equally
energetic explosions. We estimate that the convective engine in our models is
4% more efficient in three dimensions than in two dimensions. We propose that
the greater efficiency of the convective engine found in three-dimensional
simulations might be due to the larger surface-to-volume ratio of convective
plumes, which aids in distributing energy deposited by neutrinos.
We do not find evidence of the standing accretion shock instability nor
turbulence being a key factor in powering the explosion in our models. Instead,
the analysis of the energy transport in the post-shock region reveals
characteristics of penetrative convection. The explosion energy decreases
dramatically once the resolution is inadequate to capture the morphology of
convection on large scales. This shows that the role of dimensionality is
secondary to correctly accounting for the basic physics of the explosion.
We also analyze information provided by particle tracers embedded in the
flow, and find that the unbound material has relatively long residency times in
two-dimensional models, while in three dimensions a significant fraction of the
explosion energy is carried by particles with relatively short residency times.Comment: accepted for publication in Astrophysical Journa
Magnetic substructure in the northern Fermi Bubble revealed by polarized WMAP emission
We report a correspondence between giant, polarized microwave structures
emerging north from the Galactic plane near the Galactic center and a number of
GeV gamma-ray features, including the eastern edge of the recently-discovered
northern Fermi Bubble. The polarized microwave features also correspond to
structures seen in the all-sky 408 MHz total intensity data, including the
Galactic center spur. The magnetic field structure revealed by the polarization
data at 23 GHz suggests that neither the emission coincident with the Bubble
edge nor the Galactic center spur are likely to be features of the local ISM.
On the basis of the observed morphological correspondences, similar inferred
spectra, and the similar energetics of all sources, we suggest a direct
connection between the Galactic center spur and the northern Fermi Bubble.Comment: Accepted for publication in The Astrophysical Journal Letters after
minor change
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