Abstract
Galaxies that are invisible in deep optical–near-infrared imaging but detected at longer wavelengths have been the focus of several recent observational studies, with speculation that they ...could constitute a substantial missing population and even dominate the cosmic star formation rate density at
z
≳ 4. The depths now achievable with JWST at the longest wavelengths probed by the Hubble Space Telescope (HST), coupled with the transformative resolution at longer wavelengths, are already enabling detailed, spatially resolved characterization of sources that were invisible to HST, often known as “HST-dark” galaxies. However, until now, there has been little theoretical work to compare against. We present the first simulation-based study of this population, using highly resolved galaxies from the Feedback in Realistic Environments project, with multiwavelength images along several lines of sight forward-modeled using radiative transfer. We naturally recover a population of modeled sources that meet commonly used selection criteria (
H
AB
> 27 mag and
H
AB
− F444W > 2.3). These simulated HST-dark galaxies lie at high redshifts (
z
= 4–7), have high levels of dust attenuation (
A
V
= 2–4), and display compact recent star formation (
R
1/2,4.4
μ
m
≲ 1 kpc). Orientation is very important: for all but one of the 17 simulated galaxy snapshots with HST-dark sight lines, there exist other sight lines that do not meet the criteria. This result has important implications for comparisons between observations and models that do not resolve the detailed star-dust geometry, such as semianalytic models or coarsely resolved hydrodynamical simulations. Critically, we demonstrate that HST-dark sources are not an unexpected or exotic population, but a subset of high-redshift, highly dust-attenuated sources viewed along certain lines of sight.
ABSTRACT
We utilize JWST NIRCam medium-band imaging to search for extreme redshift (z ≥ 9.5) galaxy candidates in the Hubble ultra deep field (HUDF) and the additional pointing within the GOODS-South ...field provided by the second NIRCam module. Our search reveals six robust candidates, three of which have recently been spectroscopically confirmed. One of these three is the previously controversial z ≃ 12 galaxy candidate UDF-22980 which is now detected in five JWST NIRCam medium-band filters (F182M, F210M, F430M, F460M, and F480M), efficiently excluding alternative low-redshift solutions and allowing us to now report a secure photometric redshift of zphot = 11.6 ± 0.2. We also detect two galaxies at z ≥ 12.5 including a newly-detected candidate in the imaging provided by the second NIRCam module (south-west of the HUDF) at zphot = 12.6 ± 0.6. We determine the physical properties of the six galaxies by fitting the 14-band photometry with Bagpipes. We find stellar masses of log (M⋆/M⊙) ≃ 7.5−8.7 and star-formation rates of $\log (\rm {SFR}/M_{\odot }^{-1} \rm {yr}^{-1}) \simeq 0.3 - 5.0$. Despite the relatively small cosmological volume covered by the HUDF itself and the second NIRCam module imaging, we find that the existence of these galaxies is fully consistent with the latest measurements of both the ultraviolet luminosity function and cosmic star-formation rate density at z ≃ 11, supporting a gradual steady decline in the cosmic star-formation rate density out to at least z ≃ 15.
We present the results of a study which uses spectral energy distribution (SED) fitting to investigate the evolution of the equivalent width (EW) of the Hα emission line in star-forming galaxies over ...the redshift interval 1 < z < 5. After first demonstrating the ability of our SED-fitting technique to recover EW(Hα) using a sample of galaxies at z ≃ 1.3 with EW(Hα) measurements from 3D-HST grism spectroscopy, we proceed to apply our technique to samples of spectroscopically confirmed and photometric-redshift selected star-forming galaxies at z ≥ 1 in the CANDELS (Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey) UDS and GOODS-S fields. Confining our analysis to a constant stellar mass range (9.5 < log (M
⋆/M⊙) < 10.5), we find that the median EW(Hα) evolves only modestly with redshift, reaching a rest-frame value of EW(Hα) =301 ± 30 Å by redshift z ≃ 4.5. Furthermore, using estimates of star formation rate (SFR) based on both UV luminosity and Hα line flux, we use our galaxy samples to compare the evolution of EW(Hα) and specific star formation rate (sSFR). Our results indicate that over the redshift range 1 < z < 5, the evolution displayed by EW(Hα) and sSFR is consistent, and can be adequately parametrized as ∝ (1 + z)1.0 ± 0.2. As a consequence, over this redshift range, we find that the sSFR and rest-frame EW(Hα) of star-forming galaxies with stellar masses M
⋆
${\simeq }\, 10^{10}{\,\rm M_{{\odot }}}$
are related by EW(Hα)/Å = (63 ± 7) × sSFR/Gyr−1. Given the current uncertainties in measuring the SFRs of high-redshift galaxies, we conclude that EW(Hα) provides a useful independent tracer of sSFR for star-forming galaxies out to redshifts of z = 5.
ABSTRACT
We report a robust sample of 10 massive quiescent galaxies at redshift, z > 3, selected using the first data from the JWST Cosmic Evolution Early Release Science programme. Three of these ...galaxies are at 4 < z < 5, constituting the best evidence to date for quiescent galaxies significantly before z = 4. These extreme galaxies have stellar masses in the range log10(M*/M⊙) = 10.1–11.1, and formed the bulk of their mass around z ≃ 10, with two objects having star formation histories that suggest they had already reached log10(M*/M⊙) > 10 by z ≳ 8. We report number densities for our sample, demonstrating that, based on the small area of JWST imaging so far available, previous work appears to have underestimated the number of quiescent galaxies at 3 < z < 4 by a factor of 3–5, due to a lack of ultra-deep imaging data at $\lambda \gt 2\, \mu$m. This result deepens the existing tension between observations and theoretical models, which already struggle to reproduce previous estimates of z > 3 quiescent galaxy number densities. Upcoming wider-area JWST imaging surveys will provide larger samples of such galaxies and more robust number densities, as well as providing opportunities to search for quiescent galaxies at z > 5. The galaxies we report are excellent potential targets for JWST NIRSpec spectroscopy, which will be required to understand in detail their physical properties, providing deeper insights into the processes responsible for forming massive galaxies and quenching star formation during the first billion years.
In recent years, credit unions have increasingly purchased loan-participation agreements in order to diversify their loan portfolios and manage loan growth. Responding to high charge-off rates for ...these loans and concern for systemic risk, the National Credit Union Administration imposed in 2013 new rules on federally insured credit unions that limited the purchase of loan participations from a single originator to the greater of $5 million or 100% of net worth. This study uses a difference-in-difference framework to examine the effect the policy had on returns. As a result of the policy change, credit unions with a high share of net worth in loan participations earned, on average, a return on assets 47 basis points less than their counterparts. Further, we find evidence that suggests these lower returns were driven by a decline in participation loans with recourse provisions, liquidity issues, and relatively higher interest expenses.
ABSTRACT
We present a study designed to measure the average Lyman-continuum escape fraction (〈fesc〉) of star-forming galaxies at z ≃ 3.5. We assemble a sample of 148 galaxies from the VANDELS ...spectroscopic survey at 3.35 ≤ zspec ≤ 3.95, selected to minimize line-of-sight contamination of their photometry. For this sample, we use ultra-deep, ground-based, U-band imaging and Hubble Space Telescope V-band imaging to robustly measure the distribution of $\mathcal {R_{\rm obs}}\, =(L_{\rm LyC}/L_{\rm UV})_{\rm obs}$. We then model the $\mathcal {R_{\rm obs}}$ distribution as a function of 〈fesc〉, carefully accounting for attenuation by dust, the intergalactic medium and the circumgalactic medium. A maximum likelihood fit to the $\mathcal {R_{\rm obs}}$ distribution returns a best-fitting value of $\langle f_{\rm esc}\rangle =0.07^{+0.02}_{-0.02}$, a result confirmed using an alternative Bayesian inference technique (both techniques exclude 〈fesc〉 = 0.0 at >3σ). By splitting our sample in two, we find evidence that 〈fesc〉 is positively correlated with Ly α equivalent width (Wλ(Ly α)), with high and low Wλ(Lyα) subsamples returning values of $\langle f_{\rm esc}\rangle =0.12^{+0.06}_{-0.04}$ and $\langle f_{\rm esc} \rangle =0.02^{+0.02}_{-0.01}$, respectively. In contrast, we find evidence that 〈fesc〉 is anticorrelated with intrinsic UV luminosity and UV dust attenuation; with low UV luminosity and dust attenuation subsamples both returning best fits in the range 0.10 ≤ 〈fesc〉 ≤ 0.22. We do not find a clear correlation between fesc and galaxy stellar mass, suggesting stellar mass is not a primary indicator of fesc. Although larger samples are needed to further explore these trends, our results suggest that it is entirely plausible that the low dust, low-metallicity galaxies found at z ≥ 6 will display the 〈fesc〉 ≥ 0.1 required to drive reionization.
This study uses a two-part model with firm-level fixed effects to examine the decisions of Korean firms to issue domestic and foreign denominated debt (extensive margin), and among those firms using ...external finance, their level of leverage (intensive margin) in each denomination. We find that less profitable and fast-growing firms adopt a pecking order in their leverage decisions, which is evident in their preference for domestic, relative to foreign denominated debt. Interestingly, this relation is shown to be stronger following the 2007 financial crisis and suggests demand side factors are important to leverage decisions. Our results also indicate the fixed costs of issuing debt are important to debt use along the extensive margin and vary in strength by denomination. The use of foreign debt, with its higher fixed costs, is shown to be largely determined by factors associated with these costs.
•This study examines firms' decisions to have domestic and foreign debt and their intensity of debt use.•The study uses a two-part model with firm-level fixed effects.•Less profitable and fast-growing firms prefer domestic to foreign debt.•Business risk is the only factor affecting the intensity of foreign debt use.•The use of foreign debt is largely determined by the fixed costs of issuing debt.
ABSTRACT
We present the first results from a study of O vi absorption around galaxies at z < 1.44 using data from a near-infrared grism spectroscopic Hubble Space Telescope Large Programme, the ...Quasar Sightline and Galaxy Evolution (QSAGE) survey. QSAGE is the first grism galaxy survey to focus on the circumgalactic medium at z ∼ 1, providing a blind survey of the galaxy population. The galaxy sample is H α flux limited (f(H α) > 2 × 10−17 erg s−1 cm−2) at 0.68 < z < 1.44, corresponding to ≳0.2–0.8 M⊙ yr−1. In this first of 12 fields, we combine the galaxy data with high-resolution STIS and COS spectroscopy of the background quasar to study O vi in the circumgalactic medium. At z ∼ 1, we find O vi absorption systems up to b ∼ 350 kpc (∼4Rvir) from the nearest detected galaxy. Further, we find ${\sim }50{{\ \rm per\ cent}}$ of ≳1 M⊙ yr−1 star-forming galaxies within 2Rvir show no associated O vi absorption to a limit of at least N(O vi) = 1013.9 cm−2. That we detect O vi at such large distances from galaxies and that a significant fraction of star-forming galaxies show no detectable O vi absorption disfavours outflows from ongoing star formation as the primary medium traced by these absorbers. Instead, by combining our own low- and high-redshift data with existing samples, we find tentative evidence for many strong (N(O vi) > 1014 cm−2) O vi absorption systems to be associated with M⋆ ∼ 109.5–10 M⊙ mass galaxies (Mhalo ∼ 1011.5–12 M⊙ dark matter haloes), and infer that they may be tracing predominantly collisionally ionized gas within the haloes of such galaxies.
Abstract
We constrain the distribution of spatially offset Lyman-alpha emission (Ly α) relative to rest-frame ultraviolet emission in ∼300 high redshift (3 < z < 5.5) Lyman-break galaxies (LBGs) ...exhibiting Ly α emission from VANDELS, a VLT/VIMOS slit-spectroscopic survey of the CANDELS Ultra Deep Survey and Chandra Deep Field South fields (≃0.2 deg2 total). Because slit spectroscopy only provides one spatial dimension, we use Bayesian inference to recover the underlying two-dimensional Ly α spatial offset distribution. We model the distribution using a two-dimensional circular Gaussian, defined by a single parameter σr,Ly α, the standard deviation expressed in polar coordinates. Over the entire redshift range of our sample (3 < z < 5.5), we find $\sigma _{r,\mathrm{Ly}\,\alpha }=1.70^{+0.09}_{-0.08}$ kpc ($68\hbox{ per cent}$ conf.), corresponding to ∼0${^{\prime\prime}_{.}}$25 at 〈z〉 = 4.5. We also find that σr,Ly α decreases significantly with redshift. Because Ly α spatial offsets can cause slit losses, the decrease in σr,Ly α with redshift can partially explain the increase in the fraction of Ly α emitters observed in the literature over this same interval, although uncertainties are still too large to reach a strong conclusion. If σr,Ly α continues to decrease into the reionization epoch, then the decrease in Ly α transmission from galaxies observed during this epoch might require an even higher neutral hydrogen fraction than what is currently inferred. Conversely, if spatial offsets increase with the increasing opacity of the intergalactic medium, slit losses may explain some of the drop in Ly α transmission observed at z > 6. Spatially resolved observations of Ly α and UV continuum at 6 < z < 8 are needed to settle the issue.
ABSTRACT
We present results from the NIRVANDELS survey on the gas-phase metallicity (Zg, tracing O/H) and stellar metallicity (Z⋆, tracing Fe/H) of 33 star-forming galaxies at redshifts 2.95 < z < ...3.80. Based on a combined analysis of deep optical and near-IR spectra, tracing the rest-frame far-ultraviolet (FUV; 1200–2000 Å) and rest-frame optical (3400–5500 Å), respectively, we present the first simultaneous determination of the stellar and gas-phase mass–metallicity relationships (MZRs) at z ≃ 3.4. In both cases, we find that metallicity increases with increasing stellar mass (M⋆) and that the power-law slope at M⋆ ≲ 1010M⊙ of both MZRs scales as $Z \propto M_{\star }^{0.3}$. Comparing the stellar and gas-phase MZRs, we present direct evidence for super-solar O/Fe ratios (i.e. α-enhancement) at z > 3, finding (O/Fe) = 2.54 ± 0.38 × (O/Fe)⊙, with no clear dependence on M⋆.
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