2,030 research outputs found
Halpha and 4000 Angstrom Break Measurements for ~3500 K-selected Galaxies at 0.5<z<2.0
We measure spectral features of ~3500 K-selected galaxies at 0.5<z<2.0 from
high quality medium-band photometry using a new technique. First, we divide the
galaxy sample in 32 subsamples based on the similarities between the full
spectral energy distributions (SEDs) of the galaxies. For each of these 32
galaxy types we construct a composite SED by de-redshifting and scaling the
observed photometry. This approach increases the signal-to-noise ratio and
sampling of galaxy SEDs and allows for model-independent stellar population
studies. The composite SEDs are of spectroscopic quality, and facilitate -- for
the first time -- Halpha measurement for a large magnitude-limited sample of
distant galaxies. The linewidths indicate a photometric redshift uncertainty of
dz<0.02x(1+z). The composite SEDs also show the Balmer and 4000 Angstrom
breaks, MgII absorption at ~2800 Angstrom, the dust absorption feature at 2175
Angstrom, and blended [OIII]+Hbeta emission. We compare the total equivalent
width of Halpha, [NII], and [SII] (W_Halpha+) with the strength of the 4000
Angstrom break (D(4000)) and the best-fit specific star formation rate, and
find that all these properties are strongly correlated. This is a reassuring
result, as currently most distant stellar population studies are based on just
continuum emission. Furthermore, the relation between W_Halpha+ and Dn(4000)
provides interesting clues to the SFHs of galaxies, as these features are
sensitive to different stellar ages. We find that the correlation between
W_Halpha+ and D(4000) at 0.5<z<2.0 is similar to z~0, and that the suppression
of star formation in galaxies at z<2 is generally not abrupt, but a gradual
process.Comment: Accepted for publication in ApJ; high-resolution version can be
downloaded at https://www.cfa.harvard.edu/~mkriek/papers
The stellar velocity dispersion of a compact massive galaxy at z=1.80 using X-Shooter: confirmation of the evolution in the mass-size and mass-dispersion relations
Recent photometric studies have shown that early-type galaxies at fixed
stellar mass were smaller and denser at earlier times. In this paper we assess
that finding by deriving the dynamical mass of such a compact quiescent galaxy
at z=1.8. We have obtained a high-quality spectrum with full UV-NIR wavelength
coverage of galaxy NMBS-C7447 using X-Shooter on the VLT. We determined a
velocity dispersion of 294 +- 51 km/s. Given this velocity dispersion and the
effective radius of 1.64 +- 0.15 kpc (as determined from HST-WFC3 F160W
observations) we derive a dynamical mass of 1.7 +- 0.5 x 10^11 Msun. Comparison
of the full spectrum with stellar population synthesis models indicates that
NMBS-C774 has a relatively young stellar population (0.40 Gyr) with little or
no star formation and a stellar mass of ~1.5 x 10^11 Msun. The dynamical and
photometric stellar mass are in good agreement. Thus, our study supports the
conclusion that the mass densities of quiescent galaxies were indeed higher at
earlier times, and this earlier result is not caused by systematic measurement
errors. By combining available spectroscopic measurements at different
redshifts, we find that the velocity dispersion at fixed dynamical mass was a
factor of ~1.8 higher at z=1.8 compared to z=0. Finally, we show that the
apparent discrepancies between the few available velocity dispersion
measurements at z>1.5 are consistent with the intrinsic scatter of the
mass-size relation.Comment: 7 pages, 4 figures, Accepted for publication in the Astrophysical
Journa
The diminishing importance of major galaxy mergers at higher redshifts
Using mass-selected galaxy samples from deep multiwavelength data we
investigate the incidence of close galaxy pairs between z=0.4-2. Many such
close pairs will eventually merge, and the pair fraction is therefore related
to the merger rate. Over this redshift range the mean pair fraction is
essentially constant (evolving as f_pair (1+z)^{-0.4 +/- 0.6}) with about
6+/-1% of massive galaxies having a 1:4 or greater companion within 30h^-1 kpc.
Assuming the timescale over which pairs merge is not a strong function of
redshift, this implies a similarly constant merger rate (per unit time) out to
z=2. Since about three times as much cosmic time passes at z<1 as between
z=1-2, this implies that correspondingly more mergers occur in the low-redshift
universe. When minor companions (1:10 mass ratio or greater) are included, the
pair fraction increases to ~20% and still does not evolve strongly with
redshift. We also use a rest-frame color criterion to select pairs containing
only quiescent galaxies (major "dry merger" progenitors), and find them to be
similarly rare and constant with 4-7% of massive quiescent galaxies exhibiting
a nearby companion. Thus, even though other studies find major mergers to be
relatively uncommon since z=1, our results suggest that few additional mergers
occur in the 1<z<2 range and other mechanisms may be required to explain the
mass and size growth of galaxies over this epoch.Comment: ApJ Letters in press; minor changes to title and text per referee's
comments. 7 pages, 3 figure
Outcomes of corneal crosslinking for the treatment of keratoconus at a tertiary South African hospital
Objectives. To describe and analyse the effectiveness of using accelerated epithelium-off corneal crosslinking (A-CXL), in a cohort of patients with progressive keratoconus (KC), presenting to Groote Schuur Hospital, South Africa. Methods. A retrospective review of patients who underwent A-CXL, using 6.4 mW/cm2 ultraviolet-A irradiation for 15 min, for progressive KC between 1 May 2017 and 1 June 2018. All patients completed 6 months minimum follow-up (Mean 9months, range 6-15). The diagnosis of keratoconus was based on corneal tomography and its clinical signs. Keratometry values and visual acuities were measured to ascertain if there was improvement in acuity or corneal curvature at 6 months post-procedure. Results. Nineteen eyes of 17 patients were included. The group consisted of 6 (32%) males and 13 (68%) females, with a mean age of 22.17 years (SD = 5.8). Zero eyes showed mild keratoconus, 8(42%) showed moderate keratoconus and 11(58%) showed severe keratoconus. At 6 months follow-up, results revealed that UCDVA improved from logMAR 0.96 to 0.83 (p = 0.068) and BCVA improved from logMAR 0.40 to 0.34 (p = 0.073). The mean UCDVA and BCVA Snellen line gain was 0.43 and 0.94. Overall the change in UCDVA and BCVA trended towards being statistically significant. The median Kmax value decreased from 57.7D to 55.9D. The mean Kmax value decreased from 59.46D to 58.85D (p = 0.137). The mean Kmean anterior increased from 50.26D to 50.86D (p = 0.139), the mean Kmean posterior from -7.48D to -7.67D (p = 0.026). There was a statistically significant change in Kmean post, but not so for Kmean ant and Kmax. Both severity groups showed a decrease in thinnest point corneal pachymetry from mean 445 micrometer(μm) to mean 422 μm (Moderate: p = 0.009; Severe: p = 0.003). KC progression was stopped or stabilised in 13 eyes (68%); 6 eyes (32%) showed progression. No complications were found. Conclusions. Our results show the effectiveness of 15-minute A-CXL (irradiance of 6.4 mW/cm2) in maintaining both corneal stability and visual acuity in our patient population at 6 months follow-up. In future, a larger study with prolonged follow-up would be required to elucidate this finding
The effect of venting configuration on the blast response of an aircraft unit load device
The inclusion of venting areas in aircraft unit load devices (ULDs) as a potential blast mitigation technique is investigated in this work. Damage to the ULD, such as large deflections or container rupture, from an internal explosion threatens to tear the aircraft skin and cause fuselage decompression. The loading within a luggage container was expected to be reduced when the explosive products were vented into the adjacent ULDs. Although previous work has investigated the effect of venting on ULD blast loading, this has only considered a single venting side and not multiple venting configurations. To determine if a multiple-venting system would be beneficial in ULDs, experimental blast testing was performed by subjecting a 1:6 scaled ULD box to representative blast loads with different venting configurations. The blast response of the side of the ULD which would be positioned closest to the fuselage was measured. Numerical simulations were established to provide insight into the blast loading effects not measured experimentally. The loading within the ULD box, in terms of the number and magnitude of blast wave reflections, and internal pressure build-up, was reduced when introducing venting areas. Final deformations were reduced by 11% and 22% when using a single- and double-venting configuration, respectively. Further deformation reduction was expected if more venting area was made available: unconfined blasts tests (demonstrating complete absence of ULD confinement) reduced deformations by 44%. The fully-confined (no venting) blast test resulted in rupture failure when blasted with a 20 g explosive, whereas the vented tests exhibited no tearing when blasted with higher charge masses. The double-venting configuration demonstrated better blast mitigation than the single-venting configuration. However, since both reduced the deformations and rupture probability of the container, the implementation of a multiple-venting system within aircraft ULDs would improve the survivability of the ULD container during a blast event
Hubble Space Telescope WFC3 Grism Spectroscopy and Imaging of a Growing Compact Galaxy at z=1.9
We present HST/WFC3 grism spectroscopy of the brightest galaxy at z>1.5 in
the GOODS-South WFC3 Early Release Science grism pointing, covering the
wavelength range 0.9-1.7 micron. The spectrum is of remarkable quality and
shows the redshifted Balmer lines Hbeta, Hgamma, and Hdelta in absorption at
z=1.902, correcting previous erroneous redshift measurements from the
rest-frame UV. The average rest-frame equivalent width of the Balmer lines is
8+-1 Angstrom, which can be produced by a post-starburst stellar population
with a luminosity-weighted age of ~0.5 Gyr. The M/L ratio inferred from the
spectrum implies a stellar mass of ~4x10^11 Msun. We determine the morphology
of the galaxy from a deep WFC3 F160W image. Similar to other massive galaxies
at z~2 the galaxy is compact, with an effective radius of 2.1+-0.3 kpc.
Although most of the light is in a compact core, the galaxy has two red, smooth
spiral arms that appear to be tidally-induced. The spatially-resolved
spectroscopy demonstrates that the center of the galaxy is quiescent and the
surrounding disk is forming stars, as it shows Hbeta in emission. The galaxy is
interacting with a companion at a projected distance of 18 kpc, which also
shows prominent tidal features. The companion has a slightly redder spectrum
than the primary galaxy but is a factor of ~10 fainter and may have a lower
metallicity. It is tempting to interpret these observations as "smoking gun"
evidence for the growth of compact, quiescent high redshift galaxies through
minor mergers, which has been proposed by several recent observational and
theoretical studies. Interestingly both objects host luminous AGNs, as
indicated by their X-ray luminosities, which implies that these mergers can be
accompanied by significant black hole growth. This study illustrates the power
of moderate dispersion, low background near-IR spectroscopy at HST resolution,
which is now available with the WFC3 grism.Comment: 5 pages, 5 figures. Submitted to ApJ Letters
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