17,940 research outputs found
On the [CII]-SFR relation in high redshift galaxies
After two ALMA observing cycles, only a handful of [CII]
emission line searches in z>6 galaxies have reported a positive detection,
questioning the applicability of the local [CII]-SFR relation to high-z
systems. To investigate this issue we use the Vallini et al. 2013 (V13) model,
based on high-resolution, radiative transfer cosmological simulations to
predict the [CII] emission from the interstellar medium of a z~7 (halo mass
) galaxy. We improve the V13 model by including
(a) a physically-motivated metallicity (Z) distribution of the gas, (b) the
contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic
Microwave Background on the [CII] line luminosity. We study the relative
contribution of diffuse neutral gas to the total [CII] emission () for different SFR and Z values. We find that the [CII]
emission arises predominantly from PDRs: regardless of the galaxy properties,
% since, at these early epochs, the CMB temperature
approaches the spin temperature of the [CII] transition in the cold neutral
medium ( K). Our model predicts a high-z
[CII]-SFR relation consistent with observations of local dwarf galaxies
(). The [CII] deficit suggested by actual data
( in BDF3299 at z~7.1) if confirmed by deeper
ALMA observations, can be ascribed to negative stellar feedback disrupting
molecular clouds around star formation sites. The deviation from the local
[CII]-SFR would then imply a modified Kennicutt-Schmidt relation in z>6
galaxies. Alternatively/in addition, the deficit might be explained by low gas
metallicities ().Comment: 9 pages, 6 figures, replaced with the version accepted for
pubblication in Ap
Contact-eutectic-lens fabrication technique
Method enables use of crystal or semiconductor materials with selective spectral-response characteristics (ultraviolet, visible, or infrared wavelengths) in fabrication of contact lenses, reading glasses, and photographic processing equipment
Initial mass function of intermediate mass black hole seeds
We study the Initial Mass Function (IMF) and host halo properties of
Intermediate Mass Black Holes (IMBH, 10^{4-6} Msun) formed inside metal-free,
UV illuminated atomic cooling haloes (virial temperature T_vir > 10^4 K) either
via the direct collapse of the gas or via an intermediate Super Massive Star
(SMS) stage. We achieve this goal in three steps: (a) we derive the gas
accretion rate for a proto-SMS to undergo General Relativity instability and
produce a direct collapse black hole (DCBH) or to enter the ZAMS and later
collapse into a IMBH; (b) we use merger-tree simulations to select atomic
cooling halos in which either a DCBH or SMS can form and grow, accounting for
metal enrichment and major mergers that halt the growth of the proto-SMS by gas
fragmentation. We derive the properties of the host halos and the mass
distribution of black holes at this stage, and dub it the "Birth Mass
Function"; (c) we follow the further growth of the DCBH due to accretion of
leftover gas in the parent halo and compute the final IMBH mass.We consider two
extreme cases in which minihalos (T_vir < 10^4 K) can (fertile) or cannot
(sterile) form stars and pollute their gas leading to a different IMBH IMF. In
the (fiducial) fertile case the IMF is bimodal extending over a broad range of
masses, M= (0.5-20)x10^5 Msun, and the DCBH accretion phase lasts from 10 to
100 Myr. If minihalos are sterile, the IMF spans the narrower mass range M=
(1-2.8)x10^6 Msun, and the DCBH accretion phase is more extended (70-120 Myr).
We conclude that a good seeding prescription is to populate halos (a) of mass
7.5 < log (M_h/Msun) < 8, (b) in the redshift range 8 < z < 17, (c) with IMBH
in the mass range 4.75 < log (M_BH/Msun) < 6.25.Comment: MNRAS, in press. Comments welcom
Efficiency of Fish Propulsion
It is shown that the system efficiency of a self-propelled flexible body is
ill-defined unless one considers the concept of quasi-propulsive efficiency,
defined as the ratio of the power needed to tow a body in rigid-straight
condition over the power it needs for self-propulsion, both measured for the
same speed. Through examples we show that the quasi-propulsive efficiency is
the only rational non-dimensional metric of the propulsive fitness of fish and
fish-like mechanisms. Using two-dimensional viscous simulations and the concept
of quasi-propulsive efficiency, we discuss the efficiency two-dimensional
undulating foils. We show that low efficiencies, due to adverse body-propulsor
hydrodynamic interactions, cannot be accounted for by the increase in friction
drag
Mapping metals at high redshift with far-infrared lines
Cosmic metal enrichment is one of the key physical processes regulating
galaxy formation and the evolution of the intergalactic medium (IGM). However,
determining the metal content of the most distant galaxies has proven so far
almost impossible; also, absorption line experiments at become
increasingly difficult because of instrumental limitations and the paucity of
background quasars. With the advent of ALMA, far-infrared emission lines
provide a novel tool to study early metal enrichment. Among these, the [CII]
line at 157.74 m is the most luminous line emitted by the interstellar
medium of galaxies. It can also resonant scatter CMB photons inducing
characteristic intensity fluctuations () near the peak of the
CMB spectrum, thus allowing to probe the low-density IGM. We compute both [CII]
galaxy emission and metal-induced CMB fluctuations at by using
Adaptive Mesh Refinement cosmological hydrodynamical simulations and produce
mock observations to be directly compared with ALMA BAND6 data ( GHz). The [CII] line flux is correlated with as
. Such
relation is in very good agreement with recent ALMA observations (e.g. Maiolino
et al. 2015; Capak et al. 2015) of galaxies. We predict that a
() galaxy can be detected at in
(2000) hours, respectively. CMB resonant scattering can produce Jy/beam emission/absorptions features that are very challenging to be
detected with current facilities. The best strategy to detect these signals
consists in the stacking of deep ALMA observations pointing fields with known
galaxies. This would allow to simultaneously detect both
[CII] emission from galactic reionization sources and CMB fluctuations produced
by metals.Comment: 13 pages, 6 figure
Multifractality and scale invariance in human heartbeat dynamics
Human heart rate is known to display complex fluctuations. Evidence of
multifractality in heart rate fluctuations in healthy state has been reported
[Ivanov et al., Nature {\bf 399}, 461 (1999)]. This multifractal character
could be manifested as a dependence on scale or beat number of the probability
density functions (PDFs) of the heart rate increments. On the other hand, scale
invariance has been recently reported in a detrended analysis of healthy heart
rate increments [Kiyono et al., Phys. Rev. Lett. {\bf 93}, 178103 (2004)]. In
this paper, we resolve this paradox by clarifying that the scale invariance
reported is actually exhibited by the PDFs of the sum of detrended healthy
heartbeat intervals taken over different number of beats, and demonstrating
that the PDFs of detrended healthy heart rate increments are scale dependent.
Our work also establishes that this scale invariance is a general feature of
human heartbeat dynamics, which is shared by heart rate fluctuations in both
healthy and pathological states
Zero-gravity growth of NaF-NaCl eutectics in the NASA Skylab program
Continuous and discontinuous NaF fibers, embedded in a NaCl matrix, were produced in space and on earth. The production of continuous fibers in a eutectic mixture is attributed to the absence of convection current in the liquid during solidification in space. Image transmission and optical transmittance measurements of transverse sections of the space-grown and earth-grown ingots were made with a light microscope and a spectrometer. It is shown that better optical properties were obtained from samples grown in space. This was attributed to a better alignment of NaF fibers along the ingot axis. A new concept is advanced to explain the phenomenon of transmittance versus far infrared wavelength of the directionally solidified NaCl-NaF eutectic in terms of the two-dimensional Bragg Scattering and the polarization effect of Rayleigh scattering. This concept can be applied to other eutectic systems as long as the index of refraction of the matrix over a range of wavelengths is known. Experimental data are in agreement with the theoretical prediction
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