2,385 research outputs found
Conductivity and the current-current correlation measure
We review various formulations of conductivity for one-particle Hamiltonians
and relate them to the current-current correlation measure. We prove that the
current-current correlation measure for random Schr\"odinger operators has a
density at coincident energies provided the energy lies in a localization
regime. The density vanishes at such energies and an upper bound on the rate of
vanishing is computed. We also relate the current-current correlation measure
to the localization length
Extreme scattering events and Galactic dark matter
Extreme Scattering Events (ESEs) are attributed to radio-wave refraction by a
cloud of free-electrons crossing the line-of-sight. We present a new model in
which these electrons form the photo-ionized 'skin' of an underlying cool,
self-gravitating cloud in the Galactic halo. In this way we avoid the severe
over-pressure problem which afflicts other models. The UV flux in the Galactic
halo naturally generates electron densities of the right order. We demonstrate,
for the first time, a good reproduction of the prototypical ESE in the quasar
0954+658. The neutral clouds are a few AU in radius and have masses less than
about 10^{-3} solar. The observed rate of ESEs implies that a large fraction of
the mass of the Galaxy is in this form.Comment: 5 pp incl 3 figs, LaTeX, uses aas2pp4.sty. Minor revisions. ApJ
Letters in pres
-approximation of the integrated density of states for Schr\"odinger operators with finite local complexity
We study spectral properties of Schr\"odinger operators on \RR^d. The
electromagnetic potential is assumed to be determined locally by a colouring of
the lattice points in \ZZ^d, with the property that frequencies of finite
patterns are well defined. We prove that the integrated density of states
(spectral distribution function) is approximated by its finite volume
analogues, i.e.the normalised eigenvalue counting functions. The convergence
holds in the space where is any finite energy interval and is arbitrary.Comment: 15 pages; v2 has minor fixe
Search for cold and hot gas in the ram pressure stripped Virgo dwarf galaxy IC3418
We present IRAM 30m sensitive upper limits on CO emission in the ram pressure
stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions
in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds
of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of
molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could
correspond to a surviving nuclear gas reservoir. We estimate that there is less
molecular gas in the main body of IC3418, by at least a factor of 20, than
would be expected from the pre-quenching UV-based star formation rate assuming
the typical gas depletion timescale of 2 Gyr. Given the lack of star formation
in the main body, we think the H_2-deficiency is real, although some of it may
also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass
galaxies. The presence of HII regions in the tail of IC3418 suggests that there
must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found
in the three observed points in the outer tail. This yields an upper limit on
the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount
similar to the estimates from the observed star formation rate over the tail.
We also present strong upper limits on the X-ray emission of the stripped gas
in IC3418 from a new Chandra observation. The measured X-ray luminosity of the
IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy
in a more distant cluster with a prominent ram pressure stripped tail.
Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a
low gas content in the tail associated with its advanced evolutionary state
and/or due to a rather low thermal pressure of the surrounding intra-cluster
medium.Comment: 15 pages, 11 figures, A&A accepte
A 22 Degree Tidal Tail for Palomar 5
Using Data Release 4 of the Sloan Digital Sky Survey, we have applied an
optimal contrast, matched filter technique to trace the trailing tidal tail of
the globular cluster Palomar 5 to a distance of 18.5 degrees from the center of
the cluster. This more than doubles the total known length of the tail to some
22 degrees on the sky. Based on a simple model of the Galaxy, we find that the
stream's orientation on the sky is consistent at the 1.7 sigma level with
existing proper motion measurements. We find that a spherical Galactic halo is
adequate to model the stream over its currently known length, and we are able
to place new constraints on the current space motion of the cluster.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
New constraints on dust emission and UV attenuation of z=6.5-7.5 galaxies from millimeter observations
We have targeted two recently discovered Lyman break galaxies (LBGs) to
search for dust continuum and [CII] 158 micron line emission. The strongly
lensed z~6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the
spectroscopically confirmed z=7.508 LBG z8-GND-5296 in the GOODS-N field have
been observed with the Plateau de Bure interferometer (PdBI) at 1.2mm. These
observations have been combined with those of three z>6.5 Lya emitters (named
HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained
with the PdBI and ALMA. [CII] is undetected in both galaxies, providing a deep
upper limit for Abell1703-zD1, comparable to recent ALMA non-detections. Dust
continuum emission from Abell1703-zD1 and z8-GND-5296 is not detected with an
rms of 0.12 and 0.16 mJy/beam. From these non-detections we derive upper limits
on their IR luminosity and star formation rate, dust mass, and UV attenuation.
Thanks to strong gravitational lensing the limit for Abell1703-zD1 is probing
the sub-LIRG regime ( Lsun) and very low dust
masses ( Msun). We find that all five galaxies are
compatible with the Calzetti IRX- relation, their UV attenuation is
compatible with several indirect estimates from other methods (the UV slope,
extrapolation of the attenuation measured from the IR/UV ratio at lower
redshift, and SED fits), and the dust-to-stellar mass ratio is not incompatible
with that of galaxies from z=0 to 3. For their stellar mass the high-z galaxies
studied here have an attenuation below the one expected from the mean relation
of low redshift (z<1.5) galaxies. More and deeper (sub)-mm data are clearly
needed to directly determine the UV attenuation and dust content of the
dominant population of high-z star-forming galaxies and to establish more
firmly their dependence on stellar mass, redshift, and other properties.Comment: 10 pages, 7 figures. Minor revisions. Accepted for publication in A&
The Mass Inflow Rate in the Barred Galaxy NGC 1530
Mass inflow in barred galaxies has been invoked to account for a wide variety
of phenomena, but until now direct evidence for inflow has been lacking. We
present Fabry-Perot H-alpha observations of the barred spiral galaxy NGC 1530
from which we determine velocities of the ionized gas for the entire region
swept by the bar. We compare the velocity field to models of gas flow in barred
spirals and show that it is well reproduced by ideal gas hydrodynamic models.
Inspection of the models and observations reveals that gas entering the bar
dust lanes streams directly down the dust lanes toward the 2 kpc radius nuclear
ring. The models predict that approximately 20% of the gas flowing down the
dust lane enters the nuclear ring; the remaining gas sprays around the ring to
the other bar dust lane. The fraction of the gas entering the ring is
relatively insensitive to the shape or size of the bar. Our observations of the
velocity field and dust optical depth yield a mass inflow rate into the nuclear
ring of 1 solar mass per year.Comment: 13 pages, 3 figures, aastex 4.0, accepted for publication in Ap J
Letter
Why Buckling Stellar Bars Weaken in Disk Galaxies
Young stellar bars in disk galaxies experience a vertical buckling
instability which terminates their growth and thickens them, resulting in a
characteristic peanut/boxy shape when viewed edge on. Using N-body simulations
of galactic disks embedded in live halos, we have analyzed the bar structure
throughout this instability and found that the outer third of the bar dissolves
completely while the inner part (within the vertical inner Lindblad resonance)
becomes less oval. The bar acquires the frequently observed peanut/boxy-shaped
isophotes. We also find that the bar buckling is responsible for a mass
injection above the plane, which is subsequently trapped by specific 3-D
families of periodic orbits of particular shapes explaining the observed
isophotes, in line with previous work. Using a 3-D orbit analysis and surfaces
of sections, we infer that the outer part of the bar is dissolved by a rapidly
widening stochastic region around its corotation radius -- a process related to
the bar growth. This leads to a dramatic decrease in the bar size, decrease in
the overall bar strength and a mild increase in its pattern speed, but is not
expected to lead to a complete bar dissolution. The buckling instability
appears primarily responsible for shortening the secular diffusion timescale to
a dynamical one when building the boxy isophotes. The sufficiently long
timescale of described evolution, ~1 Gyr, can affect the observed bar fraction
in local universe and at higher redshifts, both through reduced bar strength
and the absence of dust offset lanes in the bar.Comment: 7 pages, 4 figures, ApJ Letters, in pres
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