1,193 research outputs found
Experimental evidence of accelerated energy transfer in turbulence
We investigate the vorticity dynamics in a turbulent vortex using scattering
of acoustic waves. Two ultrasonic beams are adjusted to probe simultaneously
two spatial scales in a given volume of the flow, thus allowing a dual channel
recording of the dynamics of coherent vorticity structures. Our results show
that this allows to measure the average energy transfer time between different
spatial length scales, and that such transfer goes faster at smaller scales.Comment: 5 pages, 5 figure
Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7
Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins
Deep Survey, we study z_850-dropout sources in the Hubble Ultra Deep Field.
After carefully removing contaminating flux from foreground sources, we clearly
detect two z_850-dropouts at 3.6 micron and 4.5 micron, while two others are
marginally detected. The mid-infrared fluxes strongly support their
interpretation as galaxies at z~7, seen when the Universe was only 750 Myr old.
The IRAC observations allow us for the first time to constrain the rest-frame
optical colors, stellar masses, and ages of the highest redshift galaxies.
Fitting stellar population models to the spectral energy distributions, we find
photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4,
V-band luminosities L_V=0.6-3 x 10^10 L_sun, stellar masses 1-10 x 10^9 M_sun,
stellar ages 50-200 Myr, star formation rates up to ~25 M_sun/yr, and low
reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive
and evolved than Lyman break galaxies or Distant Red Galaxies at z=2-3, but
fairly similar to recently identified systems at z=5-6. The stellar mass
density inferred from our z=7 sample is rho* = 1.6^{+1.6}_{-0.8} x 10^6 M_sun
Mpc^-3 (to 0.3 L*(z=3)), in apparent agreement with recent cosmological
hydrodynamic simulations, but we note that incompleteness and sample variance
may introduce larger uncertainties. The ages of the two most massive galaxies
suggest they formed at z>8, during the era of cosmic reionization, but the star
formation rate density derived from their stellar masses and ages is not nearly
sufficient to reionize the universe. The simplest explanation for this
deficiency is that lower-mass galaxies beyond our detection limit reionized the
universe.Comment: 4 pages, 3 figures, emulateapj, Accepted for publication in ApJ
Letter
Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization
We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame
optical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5
z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allow
accurate removal of contaminating foreground light, enabling reliable flux
measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to
[3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection
for the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. We
construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes
and fit stellar population synthesis models to derive mean redshifts, stellar
masses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1},
(U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9
M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and
SSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies,
indicating the galaxies are smaller but not younger. The sub-L* galaxies
observed here contribute significantly to the stellar mass density and under
favorable conditions may have provided enough photons for sustained
reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts
have stellar masses that are uncertain by 1.5 dex due to the near-complete
reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the
stellar mass density to M_{UV,AB} < -18 declines from
rho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sun
Mpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signify
more dramatic evolution, which can be established with deeper IRAC
observations, long before the arrival of the James Webb Space Telescope.Comment: 6 pages, 3 figures, 2 tables, emulateapj, accepted for publication in
ApJ
The evolution of the specific star formation rate of massive galaxies to z ~ 1.8 in the E-CDFS
We study the evolution of the star formation rate (SFR) of mid-infrared (IR)
selected galaxies in the extended Chandra Deep Field South (E-CDFS). We use a
combination of U-K GaBoDS and MUSYC data, deep IRAC observations from SIMPLE,
and deep MIPS data from FIDEL. This unique multi-wavelength data set allows us
to investigate the SFR history of massive galaxies out to redshift z ~ 1.8. We
determine star formation rates using both the rest-frame ultraviolet luminosity
from young, hot stars and the total IR luminosity of obscured star formation
obtained from the MIPS 24 um flux. We find that at all redshifts the galaxies
with higher masses have substantially lower specific star formation rates than
lower mass galaxies. The average specific star formation rates increase with
redshift, and the rate of incline is similar for all galaxies (roughly
(1+z)^{n}, n = 5.0 +/- 0.4). It does not seem to be a strong function of galaxy
mass. Using a subsample of galaxies with masses M_*> 10^11 M_sun, we measured
the fraction of galaxies whose star formation is quenched. We consider a galaxy
to be in quiescent mode when its specific star formation rate does not exceed
1/(3 x t_H), where t_H is the Hubble time. The fraction of quiescent galaxies
defined as such decreases with redshift out to z ~ 1.8. We find that, at that
redshift, 19 +/-9 % of the M_* > 10^11 M_sun sources would be considered
quiescent according to our criterion.Comment: 7 pages, 6 figures, accepted for publication in Ap
The Star Formation Rate Function for Redshift z~4-7 Galaxies: Evidence for a Uniform Build-Up of Star-Forming Galaxies During the First 3 Gyr of Cosmic Time
We combine recent estimates of dust extinction at z~4-7 with UV luminosity
function (LF) determinations to derive star formation rate (SFR) functions at
z~4, 5, 6 and 7. SFR functions provide a more physical description of galaxy
build-up at high redshift and allow for direct comparisons to SFRs at lower
redshifts determined by a variety of techniques. Our SFR functions are derived
from well-established z~4-7 UV LFs, UV-continuum slope trends with redshift and
luminosity, and IRX-beta relations. They are well-described by Schechter
relations. We extend the comparison baseline for SFR functions to z~2 by
considering recent determinations of the H{\alpha} and mid-IR luminosity
functions. The low-end slopes of the SFR functions are flatter than for the UV
LFs, \Delta\alpha\sim+0.13, and show no clear evolution with cosmic time
(z~0-7). In addition, we find that the characteristic value SFR* from the
Schechter fit to SFR function exhibits consistent, and substantial, linear
growth as a function of redshift from ~5 M_sun/yr at z~8, 650 Myr after the Big
Bang, to ~100 M_sun/yr at z~2, ~2.5 Gyr later. Recent results at z~10, close to
the onset of galaxy formation, are consistent with this trend. The uniformity
of this evolution is even greater than seen in the UV LF over the redshift
range z~2-8, providing validation for our dust corrections. These results
provide strong evidence that galaxies build up uniformly over the first 3 Gyr
of cosmic time.Comment: Added an appendix, 1 figure and 3 tables: 9 pages, 5 figures, 4
tables, ApJ, in pres
The GREATS H+[OIII] Luminosity Function and Galaxy Properties at : Walking the Way of JWST
The James Webb Space Telescope will allow to spectroscopically study an
unprecedented number of galaxies deep into the reionization era, notably by
detecting [OIII] and H nebular emission lines. To efficiently prepare
such observations, we photometrically select a large sample of galaxies at
and study their rest-frame optical emission lines. Combining data from
the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and
from HST, we perform spectral energy distribution (SED) fitting, using
synthetic SEDs from a large grid of photoionization models. The deep
Spitzer/IRAC data combined with our models exploring a large parameter space
enables to constrain the [OIII]+H fluxes and equivalent widths for our
sample, as well as the average physical properties of galaxies, such
as the ionizing photon production efficiency with
. We
find a relatively tight correlation between the [OIII]+H and UV
luminosity, which we use to derive for the first time the [OIII]+H
luminosity function (LF) at . The [OIII]+H LF is higher
at all luminosities compared to lower redshift, as opposed to the UV LF, due to
an increase of the [OIII]+H luminosity at a given UV luminosity from
to . Finally, using the [OIII]+H LF, we make
predictions for JWST/NIRSpec number counts of galaxies. We find that
the current wide-area extragalactic legacy fields are too shallow to use JWST
at maximal efficiency for spectroscopy even at 1hr depth and JWST
pre-imaging to mag will be required.Comment: 13 pages, 9 figures, accepted for publication in MNRA
The Evolution of the Fractions of Quiescent and Star-forming Galaxies as a Function of Stellar Mass Since z=3: Increasing Importance of Massive, Dusty Star-forming Galaxies in the Early Universe
Using the UltraVISTA DR1 and 3D-HST catalogs, we construct a
stellar-mass-complete sample, unique for its combination of surveyed volume and
depth, to study the evolution of the fractions of quiescent galaxies,
moderately unobscured star-forming galaxies, and dusty star-forming galaxies as
a function of stellar mass over the redshift interval . We
show that the role of dusty star-forming galaxies within the overall galaxy
population becomes more important with increasing stellar mass, and grows
rapidly with increasing redshift. Specifically, dusty star-forming galaxies
dominate the galaxy population with at . The ratio of dusty and non-dusty star-forming galaxies as
a function of stellar mass changes little with redshift. Dusty star-forming
galaxies dominate the star-forming population at , being a factor of 3-5 more common,
while unobscured star-forming galaxies dominate at . At , red
galaxies dominate the galaxy population at all redshift , either because
they are quiescent (at late times) or dusty star-forming (in the early
universe).Comment: 7 pages, 4 figures, 1 table. Accepted by Astrophysical Journal
Letters after minor revisio
X-ray properties of K-selected galaxies at 0.5<z<2.0: Investigating trends with stellar mass, redshift and spectral type
We examine how the total X-ray luminosity correlates with stellar mass,
stellar population, and redshift for a K-band limited sample of ~3500 galaxies
at 0.5<z<2.0 from the NEWFIRM Medium Band Survey in the COSMOS field. The
galaxy sample is divided into 32 different galaxy types, based on similarities
between the spectral energy distributions. For each galaxy type, we further
divide the sample into bins of redshift and stellar mass, and perform an X-ray
stacking analysis using the Chandra COSMOS (C-COSMOS) data. We find that full
band X-ray luminosity is primarily increasing with stellar mass, and at similar
mass and spectral type is higher at larger redshifts. When comparing at the
same stellar mass, we find that the X-ray luminosity is slightly higher for
younger galaxies (i.e., weaker 4000\AA breaks), but the scatter in this
relation is large. We compare the observed X-ray luminosities to those expected
from low and high mass X-ray binaries (XRBs). For blue galaxies, XRBs can
almost fully account for the observed emission, while for older galaxies with
larger 4000\AA breaks, active galactic nuclei (AGN) or hot gas dominate the
measured X-ray flux. After correcting for XRBs, the X-ray luminosity is still
slightly higher in younger galaxies, although this correlation is not
significant. AGN appear to be a larger component of galaxy X-ray luminosity at
earlier times, as the hardness ratio increases with redshift. Together with the
slight increase in X-ray luminosity this may indicate more obscured AGNs or
higher accretion rates at earlier times.Comment: 9 pages, 9 figures, ApJ accepte
- âŠ