615 research outputs found
Si and Mn Abundances in Damped Lya Systems with Low Dust Content
We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped
Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of
QSOs recorded with the Keck I telescope. In all three cases the abundances of
Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We
propose that when the proportion of refractory elements locked up in dust
grains is less than about 50 percent, it is plausible to assume an
approximately uniform level of depletion for all grain constituents and, by
applying a small dust correction, recover the intrisic abundances of Si and Mn.
We use this approach on a small sample of damped systems for which it is
appropriate, with the aim of comparing the metallicity dependence of the ratios
[Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main
conclusion is that the relative abundances of both elements in distant galaxies
are broadly in line with expectations based on Galactic data. Si displays a
mild enhancement at low metallicities, as expected for an alpha-capture
element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The
underabundance of Mn at low metallicities is possibly even more pronounced than
that in metal-poor stars, and no absorption system has yet been found where
[Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha
systems, evident even from this limited set of measurements, provide further
support for the conclusion from imaging studies that a varied population of
galaxies gives rise to this class of QSO absorbers.Comment: 29 pages, LaTex, 7 Postscript Figures. Accepted for Publication in
the Astrophysical Journa
A Multi-Species Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds
We have performed a multi-species hydrodynamical simulation of the formation
and evolution of Lyman alpha clouds in a flat CDM dominated universe with an
external flux of ionizing radiation. We solve the fully coupled non-equilibrium
rate equations for the following species: H, H^+, H^-, H_2, H_2^+, He, He^+,
He^{++}, and e^-. The statistical properties of the distribution and evolution
of both hydrogen and helium absorption lines are extracted and compared to
observed data. We find excellent agreement for the following neutral hydrogen
data: the distribution of column densities is fit well by a power law with
exponent beta=1.55 with a possible deficiency of lines above column density
10^{15} cm^{-2}; the integrated distribution matches observed data over a broad
range of column densities 10^{13} to 10^{17} cm^{-2}; a Gaussian statistical
fit to the Doppler parameter distribution yields a median of 35.6 km s^{-1};
the evolution of the number of clouds with column densities larger than 10^{14}
cm^{-2} follows a power law with exponent gamma=2.22. Analogous calculations
are presented for HeII absorption lines and we find the ratio of Doppler
parameters b_{HeII}/b_{HI} = 0.87. Our data also suggests that Ly
clouds may belong to two morphologically different groups: small column density
clouds which tend to reside in sheets or filamental structures and are very
elongated and/or flattened, and the large column density clouds which are
typically found at the intersections of these caustic structures and are
quasi-spherical.Comment: 14 pages, 4 postscript figure
A Study of Interstellar Gas and Stars in the Gravitationally Lensed Galaxy `The Cosmic Eye' from Rest-Frame Ultraviolet Spectroscopy
We report the results of a study of the rest-frame UV spectrum of the Cosmic
Eye, a luminous Lyman break galaxy at z=3.07331 gravitationally lensed by a
factor of 25. The spectrum, recorded with the ESI spectrograph on the Keck II
telescope, is rich in absorption features from the gas and massive stars in
this galaxy. The interstellar absorption lines are resolved into two components
of approximately equal strength and each spanning several hundred km/s in
velocity. One component has a net blueshift of -70 km/s relative to the stars
and H II regions and presumably arises in a galaxy-scale outflow similar to
those seen in most star-forming galaxies at z = 2-3. The other is more unusual
in showing a mean redshift of +350 km/s relative to the systemic redshift;
possible interpretations include a merging clump, or material ejected by a
previous star formation episode and now falling back onto the galaxy, or more
simply a chance alignment with a foreground galaxy. In the metal absorption
lines, both components only partially cover the OB stars against which they are
being viewed. We tentatively associate the redshifted component with the strong
damped Lyman alpha line, indicative of a column density N(H I) = (3.0 +/- 0.8)
x 10(21) atoms/cm2, and propose that it provides the dust `foreground screen'
responsible for the low ratio of far-infrared to UV luminosities of the Cosmic
Eye. Compared to other well-studied examples of strongly lensed galaxies, we
find that the young stellar population of the Cosmic Eye is essentially
indistinguishable from those of the Cosmic Horseshoe and MS 1512-cB58, while
the interstellar spectra of all three galaxies are markedly different,
attesting to the real complexity of the interplay between starbursts and
ambient interstellar matter in young galaxies (abridged).Comment: 14 pages, 6 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society after minor revision
A NJL-based study of the QCD critical line
We employ a 3 flavor NJL model to stress some general remarks about the QCD
critical line. The dependence of the critical curve on
and is discussed. The quark masses are varied to
confirm that, in agreement with universality arguments, the order of transition
depends on the number of active flavors . The slope of the critical curve
vs. chemical potential is studied as a function of . We compare our
results with those recently obtained in lattice simulations to establish a
comparison among different models.Comment: 17 pages, 5 figure
The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field
We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the
GOODS-N field. The galaxies were selected primarily because they appear to have
elongated morphologies, and slits were aligned with the major axes (as
determined from the rest-frame UV emission) of 11 of the 13. If the galaxies
are elongated because they are highly inclined, alignment of the slit and major
axis should maximize the observed velocity and reveal velocity shear, if
present. In spite of this alignment, we see spatially resolved velocity shear
in only two galaxies. We show that the seeing makes a large difference in the
observed velocity spread of a tilted emission line, and use this information to
place limits on the velocity spread of the ionized gas of the galaxies in the
sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the
velocity shear (half of the velocity range of a tilted emission line) that
would be observed under our best seeing conditions of ~0.5". When combined with
previous work, our data also indicate that aligning the slit along the major
axis does not increase the probability of observing a tilted emission line. We
then focus on the one-dimensional velocity dispersion \sigma, which is much
less affected by the seeing, and see that the elongated subsample exhibits a
significantly lower velocity dispersion than galaxies selected at random from
our total H-alpha sample, not higher as one might have expected. We also see
some evidence that the elongated galaxies are less reddened than those randomly
selected using only UV colors. Both of these results are counter to what would
be expected if the elongated galaxies were highly inclined disks. It is at
least as likely that the galaxies' elongated morphologies are due to merging
subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3
We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond
Efficient time series detection of the strong stochasticity threshold in Fermi-Pasta-Ulam oscillator lattices
In this work we study the possibility of detecting the so-called strong
stochasticity threshold, i.e. the transition between weak and strong chaos as
the energy density of the system is increased, in anharmonic oscillator chains
by means of the 0-1 test for chaos. We compare the result of the aforementioned
methodology with the scaling behavior of the largest Lyapunov exponent computed
by means of tangent space dynamics, that has so far been the most reliable
method available to detect the strong stochasticity threshold. We find that
indeed the 0-1 test can perform the detection in the range of energy density
values studied. Furthermore, we determined that conventional nonlinear time
series analysis methods fail to properly compute the largest Lyapounov exponent
even for very large data sets, whereas the computational effort of the 0-1 test
remains the same in the whole range of values of the energy density considered
with moderate size time series. Therefore, our results show that, for a
qualitative probing of phase space, the 0-1 test can be an effective tool if
its limitations are properly taken into account.Comment: 5 pages, 2 figures; accepted for publication in Physical Review
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