375 research outputs found
How does flow in a pipe become turbulent?
The transition to turbulence in pipe flow does not follow the scenario
familiar from Rayleigh-Benard or Taylor-Couette flow since the laminar profile
is stable against infinitesimal perturbations for all Reynolds numbers.
Moreover, even when the flow speed is high enough and the perturbation
sufficiently strong such that turbulent flow is established, it can return to
the laminar state without any indication of the imminent decay. In this
parameter range, the lifetimes of perturbations show a sensitive dependence on
initial conditions and an exponential distribution. The turbulence seems to be
supported by three-dimensional travelling waves which appear transiently in the
flow field. The boundary between laminar and turbulent dynamics is formed by
the stable manifold of an invariant chaotic state. We will also discuss the
relation between observations in short, periodically continued domains, and the
dynamics in fully extended puffs.Comment: for the proceedings of statphys 2
Hidden in plain sight: a massive, dusty starburst in a galaxy protocluster at z=5.7 in the COSMOS field
We report the serendipitous discovery of a dusty, starbursting galaxy at
(hereafter called CRLE) in close physical association with the
"normal" main-sequence galaxy HZ10 at . CRLE was identified by
detection of [CII], [NII] and CO(2-1) line emission, making it the highest
redshift, most luminous starburst in the COSMOS field. This massive, dusty
galaxy appears to be forming stars at a rate of at least 1500
yr in a compact region only kpc in diameter. The dynamical and
dust emission properties of CRLE suggest an ongoing merger driving the
starburst, in a potentially intermediate stage relative to other known dusty
galaxies at the same epoch. The ratio of [CII] to [NII] may suggest that an
important () contribution to the [CII] emission comes from a diffuse
ionized gas component, which could be more extended than the dense,
starbursting gas. CRLE appears to be located in a significant galaxy
overdensity at the same redshift, potentially associated with a large-scale
cosmic structure recently identified in a Lyman Alpha Emitter survey. This
overdensity suggests that CRLE and HZ10 reside in a protocluster environment,
offering the tantalizing opportunity to study the effect of a massive starburst
on protocluster star formation. Our findings support the interpretation that a
significant fraction of the earliest galaxy formation may occur from the inside
out, within the central regions of the most massive halos, while rapidly
evolving into the massive galaxy clusters observed in the local Universe.Comment: 16 pages, 9 figures, 4 tables, final version to appear on ApJ
(accepted May 19, 2018
Low star formation efficiency in typical galaxies at z=5-6
Using the VLA and ALMA, we have obtained CO(2-1), [C II], [N II] line
emission and multiple dust continuum measurements in a sample of "normal"
galaxies at . We report the highest redshift detection of low- CO
emission from a Lyman Break Galaxy, at . The CO line luminosity
implies a massive molecular gas reservoir of (K km s pc,
suggesting low star formation efficiency, with a gas depletion timescale of
order 1 Gyr. This efficiency is much lower than traditionally observed in
starbursts, indicating that star forming conditions in Main
Sequence galaxies at may be comparable to those of normal galaxies
probed up to to-date, but with rising gas fractions across the entire
redshift range. We also obtain a deep CO upper limit for a Main Sequence galaxy
at with times lower SFR, perhaps implying a high
conversion factor, as typically found in low metallicity
galaxies. For a sample including both CO targets, we also find faint [N II]
205m emission relative to [C II] in all but the most IR-luminous
"normal" galaxies at , implying more intense or harder radiation fields
in the ionized gas relative to lower redshift. These radiation properties
suggest that low metallicity may be common in typical 10
galaxies at . While a fraction of Main Sequence star formation in the
first billion years may take place in conditions not dissimilar to lower
redshift, lower metallicity may affect the remainder of the population.Comment: 17 pages, 9 figures, 3 tables, accepted by the Astrophysical Journa
The DEIMOS 10k spectroscopic survey catalog of the COSMOS field
We present a catalog of 10718 objects in the COSMOS field observed through
multi-slit spectroscopy with the Deep Imaging Multi-Object Spectrograph
(DEIMOS) on the Keck II telescope in the wavelength range ~5500-9800A. The
catalog contains 6617 objects with high-quality spectra (two or more spectral
features), and 1798 objects with a single spectroscopic feature confirmed by
the photometric redshift. For 2024 typically faint objects we could not obtain
reliable redshifts. The objects have been selected from a variety of input
catalogs based on multi-wavelength observations in the field, and thus have a
diverse selection function, which enables the study of the diversity in the
galaxy population. The magnitude distribution of our objects is peaked at
I_AB~23 and K_AB~21, with a secondary peak at K_AB~24. We sample a broad
redshift distribution in the range 0<z<6, with one peak at z~1, and another one
around z~4. We have identified 13 redshift spikes at z>0.65 with chance
probabilities <4xE-4$, some of which are clearly related to protocluster
structures of sizes >10 Mpc. An object-to-object comparison with a multitude of
other spectroscopic samples in the same field shows that our DEIMOS sample is
among the best in terms of fraction of spectroscopic failures and relative
redshift accuracy. We have determined the fraction of spectroscopic blends to
about 0.8% in our sample. This is likely a lower limit and at any rate well
below the most pessimistic expectations. Interestingly, we find evidence for
strong lensing of Ly-alpha background emitters within the slits of 12 of our
target galaxies, increasing their apparent density by about a factor of 4.Comment: 28 pages, 11 figures and 5 tables. The full catalogue table is
available on http://cosmos.astro.caltech.edu. Accepted for publication in the
Astrophysical Journa
Transient growth in Taylor-Couette flow
Transient growth due to non-normality is investigated for the Taylor-Couette
problem with counter-rotating cylinders as a function of aspect ratio eta and
Reynolds number Re. For all Re < 500, transient growth is enhanced by
curvature, i.e. is greater for eta < 1 than for eta = 1, the plane Couette
limit. For fixed Re < 130 it is found that the greatest transient growth is
achieved for eta between the Taylor-Couette linear stability boundary, if it
exists, and one, while for Re > 130 the greatest transient growth is achieved
for eta on the linear stability boundary. Transient growth is shown to be
approximately 20% higher near the linear stability boundary at Re = 310, eta =
0.986 than at Re = 310, eta = 1, near the threshold observed for transition in
plane Couette flow. The energy in the optimal inputs is primarily meridional;
that in the optimal outputs is primarily azimuthal. Pseudospectra are
calculated for two contrasting cases. For large curvature, eta = 0.5, the
pseudospectra adhere more closely to the spectrum than in a narrow gap case,
eta = 0.99
Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z~5
We measure a relation between the depth of four prominent rest-UV absorption
complexes and metallicity for local galaxies and verify it up to z~3. We then
apply this relation to a sample of 224 galaxies at 3.5 = 4.8) in
COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses
from SPLASH are available. The average galaxy population at z~5 and log(M/Msun)
> 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower
metallicities than at z~2, but comparable to z~3.5. We find galaxies with
weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and
therefore may represent an evolved sub-population of z~5 galaxies. We find a
correlation between metallicity and dust in good agreement with local galaxies
and an inverse trend between metallicity and star-formation rate (SFR)
consistent with observations at z~2. The relation between stellar mass and
metallicity (MZ relation) is similar to z~3.5, however, there are indications
of it being slightly shallower, in particular for the young, Ly-alpha emitting
galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is
expected in the case of a fast (exponential) build-up of stellar mass with an
e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust
production and metal enrichment as a function of mass could be more stochastic
in the first billion years of galaxy formation compared to later times.Comment: 20 pages, 13 figures, 4 tables; Submitted to Ap
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