High-redshift Galaxies Research Articles

α-enhanced astrochemistry: the carbon cycle in extreme galactic conditions

ABSTRACT Astrochemistry has been widely developed as a power tool to probe the physical properties of the interstellar medium (ISM) in various conditions of the Milky Way (MW) Galaxy, and in near and distant galaxies. Most current studies conventionally apply linear scaling to all elemental abundances based on the gas-phase metallicity. However, these elements, including carbon and oxygen, are enriched differentially by stellar nucleosynthesis and the overall galactic chemical evolution, evident from α-enhancement in multiple galactic observations such as starbursts, high-redshift star-forming galaxies, and low-metallicity dwarfs. We perform astrochemical modelling to simulate the impact of an α-enhanced ISM gas cloud on the abundances of the three phases of carbon (C+, C, CO) dubbed as ‘the carbon cycle’. The ISM environmental parameters considered include two cosmic-ray ionization rates (ζCR = 10−17 and $10^{-15}\, {\rm s}^{-1}$), two isotropic FUV radiation field strengths (χ/χ0 = 1 and 102), and (sub-)linear dust-to-gas relations against metallicity, mimicking the ISM conditions of different galaxy types. In galaxies with [C/O] < 0, CO, C, and C+, all decrease in both abundances and emission, though with differential biases. The low-J CO emission is found to be the most stable tracer for the molecular gas, while C and C+ trace H2 gas only under limited conditions, in line with recent discoveries of [C i]-dark galaxies. We call for caution when using [C ii] $158\, \mu$m and [C i](1–0) as alternative H2-gas tracers for both diffuse and dense gas with non-zero [C/O] ratios.

Dark-ages reionization and galaxy formation simulation – XXI. Constraining the evolution of the ionizing escape fraction

ABSTRACT The fraction of ionizing photons that escape their host galaxies to ionize hydrogen in the intergalactic medium (IGM) is a critical parameter in analyses of the reionization era. In this paper, we use the meraxes semi-analytic galaxy formation model to infer the mean ionizing photon escape fraction and its dependence on galaxy properties through joint modelling of the observed high redshift galaxy population and existing constraints on the reionization history. Using a Bayesian framework, and under the assumption that escape fraction is primarily related to halo mass, we find that the joint constraints of the ultraviolet luminosity function, cosmic microwave background optical depth, and the Ly α forest require an escape fraction of $(18\pm 5)$ per cent for galaxies within haloes of M ≲ 109 M⊙ and $(5\pm 2)$ per cent for more massive haloes. In terms of galaxy properties, this transition in escape fraction occurs at stellar masses of M⋆ ∼ 107 M⊙, nearly independent of redshift. As a function of redshift, reionization is dominated by the smaller M⋆ ≲ 107 M⊙ galaxies with high escape fractions at z ≳ 6 and by the larger M⋆ ≳ 107 M⊙ galaxies with lower escape fractions at z ≲ 6. Galaxies with star formation rates of 10−2.5 M⊙yr−1 to 10−1.5 M⊙yr−1 provide the dominant source of ionizing photons throughout reionization. Our results are consistent with recent direct measurements of a $\sim 5~{{\ \rm per\ cent}}$ escape fraction from massive galaxies at the end of reionization and support the picture of low mass galaxies being the dominant sources of ionizing photons during reionization.

Galaxy kinematics and mass estimates at z ∼ 1 from ionized gas and stars

ABSTRACT We compare ionized gas and stellar kinematics of 16 star-forming galaxies (log (M⋆/M⊙) = 9.7–11.2, SFR =6 − 86 M⊙ yr−1) at z ∼ 1 using near-infrared integral field spectroscopy (IFS) of Hα emission from the KMOS3D (the K-band Multi-Object Spectrograph 3D) survey and optical slit spectroscopy of stellar absorption and gas emission from the LEGA-C (Large Early Galaxy Astrophysics Census) survey. Hα is dynamically colder than stars, with higher disc rotation velocities (by ∼45 per cent) and lower disc velocity dispersions (by a factor ∼2). This is similar to trends observed in the local Universe. We find higher rotational support for Hα relative to [O ii], potentially explaining systematic offsets in kinematic scaling relations found in the literature. Regarding dynamical mass measurements, for six galaxies with cumulative mass profiles from Jeans Anisotropic Multi-Gaussian Expansion (JAM) models the Hα dynamical mass models agree remarkably well out to ∼10 kpc for all but one galaxy (average $\Delta M_{\rm dyn}(R_{e,\rm F814W})\lt 0.1$ dex). Simpler dynamical mass estimates based on integrated stellar velocity dispersion are less accurate (standard deviation 0.24 dex). Differences in dynamical mass estimates are larger, for example, for galaxies with stronger misalignments of the Hα kinematic major axis and the photometric position angle, highlighting the added value of IFS observations for dynamics studies. The good agreement between the JAM and the dynamical models based on Hα kinematics at z ∼ 1 corroborates the validity of dynamical mass measurements from Hα IFS observations, which can be more easily obtained for higher redshift galaxies.

Detecting a disc bending wave in a barred-spiral galaxy at redshift 4.4

ABSTRACT The recent discovery of barred spiral galaxies in the early Universe (z > 2) poses questions of how these structures form and how they influence galaxy evolution in the early Universe. In this study, we investigate the morphology and kinematics of the far-infrared (FIR) continuum and [C ii] emission in BRI1335-0417 at z ≈ 4.4 from ALMA observations. The variations in position angle and ellipticity of the isophotes show the characteristic signature of a barred galaxy. The bar, $3.3^{+0.2}_{-0.2}$ kpc long in radius and bridging the previously identified two-armed spiral, is evident in both [C ii] and FIR images, driving the galaxy’s rapid evolution by channelling gas towards the nucleus. Fourier analysis of the [C ii] velocity field reveals an unambiguous kinematic m = 2 mode with a line-of-sight velocity amplitude of up to ∼30–40 km s−1; a plausible explanation is the disc’s vertical bending mode triggered by external perturbation, which presumably induced the high star formation rate and the bar/spiral structure. The bar identified in [C ii] and FIR images of the gas-rich disc galaxy (≳ 70 per cent of the total mass within radius R ≈ 2.2 disc scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies – a gravitationally unstable gas-rich disc creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disc. This may explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.

Spectral Templates Optimal for Selecting Galaxies at z > 8 with the JWST

The selection of high-redshift galaxies often involves spectral energy distribution (SED) fitting to photometric data, an expectation for contamination levels, and measurement of sample completeness—all vetted through comparison to spectroscopic redshift measurements of a sub-sample. The first JWST data are now being taken over several extragalactic fields to different depths and across various areas, which will be ideal for the discovery and classification of galaxies out to distances previously uncharted. As spectroscopic redshift measurements for sources in this epoch will not be initially available to compare with the first photometric measurements of z > 8 galaxies, robust photometric redshifts are of the utmost importance. Galaxies at z > 8 are expected to have bluer rest-frame ultraviolet (UV) colors than typically used model SED templates, which could lead to catastrophic photometric redshift failures. We use a combination of BPASS and Cloudy models to create a supporting set of templates that match the predicted rest-UV colors of z > 8 simulated galaxies. We test these new templates by fitting simulated galaxies in a mock catalog, Yung et al., which mimic expected field depths and areas of the JWST Cosmic Evolution Early Release Science Survey (m 5σ ∼ 28.6 over ∼100 arcmin2). We use EAZY to highlight the improvements in redshift recovery with the inclusion of our new template set and suggest criteria for selecting galaxies at 8 < z < 10 with the JWST, providing an important test case for observers venturing into this new era of astronomy.

Lyman- Spectra shapes in high redshift and low redshift galaxies

Lyman-alpha (Ly) photons were first predicted by Theodore Lyman in 1906 as part of his work on the hydrogen atom spectrum. The Ly transition occurs when an electron in a hydrogen atom falls from the second energy level to the ground state, releasing a photon with a wavelength of 121.6 nanometres. It was not until the 1960s that Ly photons were observed in astronomical sources, including galaxies and quasars. Since then, the Ly line has been recognized as a powerful probe of the intergalactic medium and the properties of galaxies, providing insights into the formation and evolution of structures in the universe. With the development of advanced telescopes and spectrographs, astronomers have been able to study Ly photons in greater detail and at higher redshifts. This has led to new discoveries and a deeper understanding of the universe, including the epoch of reionization and the formation of the first galaxies. Today, Ly studies continue to be an active area of research, with the potential to uncover even more insights into the nature and history of the cosmos. Ly spectral shapes are a crucial tool for unravelling the mysteries of the universe. These spectra provide valuable information about the distribution of gas and the physical properties of galaxies at different epochs. By analysing Ly spectra from galaxies with low and high redshifts, astronomers can gain insights into the formation and evolution of galaxies and the process of cosmic reionization.

An empirical reionization history model inferred from the low-redshift Lyman continuum survey and the star-forming galaxies at z &amp;gt; 8

ABSTRACT We present a new analysis of the rest-frame ultraviolet (UV) and optical spectra of a sample of three z &amp;gt; 8 galaxies discovered behind the gravitational lensing cluster RX J2129.4+0009. We combine these observations with z &amp;gt; 7.5 galaxies from the literature, for which similar measurements are available. As already pointed out in other studies, the high [O iii]λ5007/[O ii]λ3727 ratios (O32) and steep UV continuum slopes (β) are consistent with the values observed for low-redshift Lyman continuum emitters, suggesting that such galaxies contribute to the ionizing budget of the intergalactic medium. We construct a logistic regression model to estimate the probability of a galaxy being a Lyman continuum emitter based on the measured MUV, β, and O32. Using this probability and the UV luminosity function, we construct an empirical model that estimates the contribution of high-redshift galaxies to reionization. The preferred scenario in our analysis shows that at z ∼ 8, the average escape fraction of the galaxy population [i.e. including both LyC emitters (LCEs) and non-emitters] varies with MUV, with intermediate UV luminosity (−19 &amp;lt; MUV &amp;lt; −16) galaxies having larger escape fraction. Galaxies with faint UV luminosity (−16 &amp;lt; MUV &amp;lt; −13.5) contribute most of the ionizing photons. The relative contribution of faint versus bright galaxies depends on redshift, with the intermediate UV galaxies becoming more important over time. UV bright galaxies, although more likely to be LCEs at a given log(O32) and β, contribute the least of the total ionizing photon budget.

Circumgalactic Lyα emission around submillimeter-bright galaxies with different quasar contributions

We present VLT/MUSE observations targeting the extended Lyman-α (Lyα) emission of five high-redshift (z ∼ 3-4) submillimeter galaxies (SMGs) with increasing quasi-stellar object (QSO) radiation: two SMGs; two SMGs that host a QSO; and one SMG that hosts a QSO with an SMG companion (QSO+SMG). These sources are predicted to be located in dark matter halos of comparable masses (average mass of MDM ∼ 1012.2 M⊙). We quantified the luminosity and extent of the Lyα emission, together with its kinematics, and examined four Lyα powering mechanisms: photoionization from QSOs or star formation, shocks by galactic and/or QSO outflows, gravitational cooling radiation, and Lyα photon resonant scattering. We find a variety of Lyα luminosities and extents, with the QSO+SMG system displaying the most extended and bright nebula, followed by the SMGs hosting a QSO, and finally the undetected circumgalactic medium of SMGs. This diversity implies that gravitational cooling is unlikely to be the main powering mechanism. We show that photoionization from the QSO and QSO outflows can contribute to power the emission for average densities nH &gt; 0.5 cm−3. Moreover, the observed Lyα luminosities scale with the QSO’s budget of Lyα photons modulo the dust content in each galaxy, highlighting a possible contribution from resonant scattering of QSO radiation in powering the nebulae. We find larger Lyα linewidths (FWHM ≳ 1200 km s−1) than usually reported around radio-quiet systems, pointing to large-scale outflows. A statistical survey targeting similar high-redshift massive systems with known host properties is needed to confirm our findings.

PRUSSIC

We present deep ALMA Band 3 observations of the HCN, HCO+, and HNC(4–3) emission in SDP.81, a well-studied z = 3.042, strongly lensed galaxy. These lines trace the high-density gas, which remains almost entirely unexplored in z ≥ 1 galaxies. Additionally, these dense-gas tracers are potentially powerful diagnostics of the mechanical heating of the interstellar medium. While the HCN(4–3) and HNC(4–3) lines are not detected, the HCO+(4–3) emission is clearly detected and resolved. This is the third detection of this line in a high-redshift star-forming galaxy. We find an unusually high HCO+/HCN intensity ratio of ≥2.2. Based on the modelling of the photodissociation region, the most likely explanation for the elevated HCO+/HCN ratio is that SDP.81 has low mechanical heating, making up less than 10% of the total energy budget, along with a sub-solar metallicity of Z ≈ 0.5 Z⊙. While such conditions might not be representative of the general population of high-redshift dusty galaxies, a lower-than-solar metallicity might significantly impact gas masses inferred from CO observations. In addition, we report the detection of CO(0–1) absorption from the foreground lensing galaxy and CO(1–0) emission from a massive companion to the lensing galaxy, approximately 50 kpc to the south-east.

UNCOVER: Illuminating the Early Universe—JWST/NIRSpec Confirmation of z > 12 Galaxies

Observations of high-redshift galaxies provide a critical direct test to the theories of early galaxy formation, yet to date, only three have been spectroscopically confirmed at z > 12. Due to strong gravitational lensing over a wide area, the galaxy cluster field A2744 is ideal for searching for the earliest galaxies. Here we present JWST/NIRSpec observations of two galaxies: a robust detection at , and a plausible candidate at . The galaxies are discovered in JWST/NIRCam imaging and their distances are inferred with JWST/NIRSpec spectroscopy, all from the JWST Cycle 1 UNCOVER Treasury survey. Detailed stellar population modeling using JWST NIRCam and NIRSpec data corroborates the primeval characteristics of these galaxies: low mass (∼108 M ⊙), young, rapidly assembling, metal-poor, and star-forming. Interestingly, both galaxies are spatially resolved, having lensing-corrected rest-UV effective radii on the order of 300–400 pc, which are notably larger than other spectroscopically confirmed systems at similar redshifts. The observed dynamic range of z ≳ 10 sizes spans over 1 order of magnitude, implying a significant scatter in the size–mass relation at early times. Deep into the epoch of reionization, these discoveries elucidate the emergence of the first galaxies.

Observational signatures of forming young massive clusters: continuum emission from dense H ii regions

ABSTRACT Young massive clusters (YMCs) are the most massive star clusters forming in nearby galaxies and are thought to be a young analogue to the globular clusters. Understanding the formation process of YMCs leads to looking into very efficient star formation in high-redshift galaxies suggested by recent JWST observations. We investigate possible observational signatures of their formation stage, particularly when the mass of a cluster is increasing via accretion from a natal molecular cloud. To this end, we study the broad-band continuum emission from ionized gas and dust enshrouding YMCs, whose formation is followed by recent radiation hydrodynamics simulations. We perform post-process radiative transfer calculations using simulation snapshots and find characteristic spectral features at radio and far-infrared frequencies. We show that a striking feature is long-lasting, strong free–free emission from a ∼10-pc-scale H ii region with a large emission measure of ≳107 cm−6 pc, corresponding to the mean electron density of ≳103 cm−3. There is a turnover feature below ∼10 GHz, a signature of the optically thick free–free emission, often found in Galactic ultracompact H ii regions. These features come from the peculiar YMC formation process, where the cluster’s gravity effectively traps photoionized gas for a long duration and enables continuous star formation within the cluster. Such large and dense H ii regions show distinct distribution on the density–size diagram, apart from the standard sequence of Galactic H ii regions. This is consistent with the observational trend inferred for extragalactic H ii regions associated with YMCs.

UV-continuum β slopes of individual z ∼ 2–6 clumps and their evolution

ABSTRACT We study the ultraviolet (UV) continuum β slope of a sample of 166 clumps, individual star-forming regions observed in high-redshift galaxies. They are hosted by 67 galaxies with redshift between 2 and 6.2, strongly lensed by the Hubble Frontier Fields cluster of galaxies MACS J0416.1 − 2403. The β slope is sensitive to a variety of physical properties, such as the metallicity, the age of the stellar population, the dust attenuation throughout the galaxy, the stellar initial mass function (IMF), and the star formation history (SFH). The aim of this study is to compare the β-values of individual clumps with those measured on the entire galaxy, to investigate possible physical differences between these regions and their hosts. We found a median value of β ∼ −2.4, lower than that of integrated galaxies. This result confirms that clumps are sites of intense star formation, populated by young, massive stars, whose spectrum strongly emits in the UV. This is also consistent with the assumption that the dust extinction at the location of the clumps is lower than the average extinction of the galaxy, or that clumps have a different IMF or SFH. We made use of the correlations, discovered for high-redshift galaxies, of the β-value with those of redshift and UV magnitude, MUV, finding that clumps follow the same relations, extended to much fainter magnitudes (MUV &amp;lt; −13). We also find evidence of eight clumps with extremely blue (β ≲ −2.7) slopes, which could be the signpost of low-metallicity stars and constrain the emissivity of ionizing photons at high redshift.

The power spectrum of extended [C ii] haloes around high redshift galaxies

ABSTRACT Atacama Large Millimeter/sub-millimeter Array (ALMA) observations have detected extended (≃10 kpc) [C$\scriptstyle \rm II$] haloes around high-redshift (z ≳ 5) star-forming galaxies. If such extended structures are common, they may have an impact on the line intensity mapping (LIM) signal. We compute the LIM power spectrum including both the central galaxy and the [C $\scriptstyle \rm II$] halo, and study the detectability of such signal in an ALMA LIM survey. We model the central galaxy and the [C $\scriptstyle \rm II$] halo brightness with a $\rm S\acute{e}rsic$ + exponential profile. The model has two free parameters: the effective radius ratio $f_{R_{\rm e}}$, and the central surface brightness ratio, fΣ, between the two components. [C $\scriptstyle \rm II$] haloes can significantly boost the LIM power spectrum signal. For example, for relatively compact [C $\scriptstyle \rm II$] haloes ($f_\Sigma =0.4$, $f_{R_{\rm e}}=2.0$), the signal is boosted by ≃20 times; for more extended and diffuse haloes ($f_\Sigma =0.1, f_{R_{\rm e}}=6.0$), the signal is boosted by ≃100 times. For the ALMA ASPECS survey (resolution θbeam = 1.13 arcsec, survey area $\Omega _{\rm survey}=2.9\, \rm arcmin^{2}$) the [C $\scriptstyle \rm II$] power spectrum is detectable only if the deL14d [C $\scriptstyle \rm II$]–SFR relation holds. However, with an optimized survey (θbeam = 0.232 arcsec, $\Omega _{\rm survey}=2.0\, \rm deg^{2}$), the power spectrum is detectable for all the [C $\scriptstyle \rm II$]–SFR relations considered in this paper. Such a survey can constrain $f_\Sigma$ ($f_{R_{\rm e}}$) with a relative uncertainty of $\sim 15~{{\ \rm per\ cent}}$ ($\sim 10~{{\ \rm per\ cent}}$). A successful LIM experiment will provide unique constraints on the nature, origin, and frequency of extended [C $\scriptstyle \rm II$] haloes, and the [C $\scriptstyle \rm II$]–SFR relation at early times.

Emission-line properties of IllustrisTNG galaxies: from local diagnostic diagrams to high-redshift predictions for JWST

ABSTRACT We compute synthetic, rest-frame optical and ultraviolet (UV) emission-line properties of galaxy populations at redshifts from z ≈ 0 to = 8 in a full cosmological framework. We achieve this by coupling, in post-processing, the cosmological IllustrisTNG simulations with new-generation nebular-emission models, accounting for line emission from young stars, post-asymptotic giant branch (PAGB) stars, accreting black holes (BHs) and, for the first time, fast radiative shocks. The optical emission-line properties of simulated galaxies dominated by different ionizing sources in our models are largely consistent with those expected from classical diagnostic diagrams and reflect the observed increase in [O iii]/Hβ at fixed [N ii]/Hα and the evolution of the Hα, [O iii]λ5007, and [O ii] λ3727 luminosity functions from z ≈ 0 to ∼ 2. At higher redshift, we find that the emission-line galaxy population is dominated by star-forming and active galaxies, with negligible fractions of shock- and PAGB-dominated galaxies. We highlight 10 UV-diagnostic diagrams able to robustly identify the dominant ionizing sources in high-redshift galaxies. We also compute the evolution of several optical- and UV-line-luminosity functions from z = 4 to 7, and the number of galaxies expected to be detectable per field of view in deep, medium-resolution spectroscopic observations with the NIRSpec instrument on board the JWST. We find that 2-h-long exposures are sufficient to achieve unbiased censuses of Hα and [O iii]λ5007 emitters, while at least 5 h are required for Hβ, and even 10 h will detect only progressively smaller fractions of [O ii] λ3727, O iii] λ1663, C iii] λ1908, C iv λ1550, [N ii]λ6584, Si iii] λ1888, and He ii λ1640 emitters, especially in the presence of dust.

High-redshift metallicity calibrations for JWST spectra: insights from line emission in cosmological simulations

ABSTRACT Optical emission-line ratios are traditionally used to estimate gas metallicities from observed galaxy spectra. While such estimators have been calibrated primarily at low redshift, they are commonly used to interpret observations of high-redshift galaxies, where their applicability may be questioned. We use comprehensive emission-line catalogues of galaxies from the IllustrisTNG simulation including ionization by stars, active galactic nuclei, and shocks to reassess the calibrations of both optical and ultraviolet metallicity estimators at redshifts $0\lesssim z \lesssim 8$. For present-day galaxies, the predicted optical-line calibrations are consistent with previously published ones, while we find different ultraviolet-line ratios, such as He ii λ1640/C iii] λ1908, can provide powerful metallicity diagnostics. At fixed metallicity, most emission-line ratios are predicted to strongly increase or decrease with redshift (with the notable exception of N2O2 = [N ii]λ6584/[O ii] λ3727), primarily because of a change in ionization parameter. The predicted dependence of R3 = [O iii]λ5007/H β and R23 = ([O ii] λ3727 + [O iii]λ5007)/H β, and to a slightly lesser extent R2 = [O ii] λ3727/H β and O32 = [O iii]λ5007/[O ii] λ3727, on O abundance for galaxies at z = 4–8 agrees remarkably well with Te-based measurements in 14 galaxies observed with JWST. This success motivates us to provide new calibrations of optical and ultraviolet metallicity estimators specifically designed for galaxies at z &amp;gt; 4, to guide interpretations of future, high-redshift spectroscopic surveys. We further demonstrate that applying classical z = 0 calibrations to high-redshift galaxies can bias oxygen abundance estimates downward by up to 1 dex, leading to the inference of stronger evolution of the mass–metallicity relation than is actually occurring.

A photon burst clears the earliest dusty galaxies: modelling dust in high-redshift galaxies from ALMA to JWST

ABSTRACT The generation and evolution of dust in galaxies are important tracers for star formation, and can characterize the rest-frame ultraviolet to infrared emission from the galaxies. In particular understanding dust in high-redshift galaxies are important for observational cosmology, as they would be necessary to extract information on star formation in the early Universe. We update the public semi-analytical model Ancient Stars and Local Observables by Tracing Halos (a-sloth) to model the evolution of dust, focusing on high-redshift star-forming galaxies with stellar masses of ∼108–1010 M⊙ observed by ALMA (z ≈ 7) and JWST (z ≈ 11). We find that these galaxies should qualitatively differ in their star formation properties; while the samples in ALMA are explained by dust growth in normal star-forming galaxies, the lack of dust in the samples by JWST requires dust ejection by radiation pressure due to recent highly efficient star formation within a few 10 Myr, with order 100 times higher efficiency than normal galaxies calibrated by a-sloth. Depending on where the JWST galaxies locate on the luminosity function, their bursty star formation histories inferred from our model can have impacts for rates of star formation, supernova explosion, stellar feedback, and detectability of dusty, mature galaxies in the very early Universe.

H i content of selected mid-infrared bright, starburst blue compact dwarf galaxies

ABSTRACT We report measurements of H i content in 11 nearby, actively star-forming, blue compact dwarf galaxies (BCDs) from 21 cm observations with the Arecibo telescope. These BCDs, selected by their red (W2[4.6 $\mu$m]−W3[12 $\mu$m]&amp;gt;3.8 mag) and bright mid-infrared (MIR) emission (W4[22 $\mu$m]&amp;lt;7.6 mag), have high specific star formation rates (median sSFR ∼10−7.8 yr−1), similar to high redshift galaxies. H i emission was detected in six sources. We analyse our new detections in the context of previous H i observations of 218 dwarf irregulars (dIs) and BCDs in the literature. The MH i–M* relation resulting from our observations confirms the dominating fraction of H i gas among baryons in galaxies with lower stellar masses. This Arecibo BCD sample has significantly lower median H i depletion time-scales (τH i ∼ 0.3 Gyr) than other dIs/BCDs (∼ 6.3 Gyr) in the literature. The majority of the sources (10/11) in the Arecibo sample are very red in W1[3.4 $\mu$m]−W2[4.6 $\mu$m] colour (&amp;gt;0.8 mag) implying the presence of warm dust. We investigate the relation of τHI with stellar mass (M*) and sSFR. We find that τH i is significantly anticorrelated with M* for higher sSFR (&amp;gt;10−8.5 yr−1) and with sSFR for higher stellar mass ($\gt 10^{7.5}\, {\rm M}_{\odot }$) dwarf galaxies. The high sSFR for the BCDs in the Arecibo observed sample is mainly due to their high atomic gas star formation efficiency (SFE) or low τH i. The low τH i or high SFE in these sources is possibly due to runaway star formation in compact and dense super star clusters.

A Near-infrared-faint, Far-infrared-luminous Dusty Galaxy at z ∼ 5 in COSMOS-Web

A growing number of far-infrared (FIR) bright sources completely invisible in deep extragalactic optical surveys hint at an elusive population of z > 4 dusty, star-forming galaxies. Cycle 1 JWST surveys are now detecting their rest-frame optical light, which provides key insight into their stellar properties and statistical constraints on the population as a whole. This work presents the JWST Near Infrared Camera (NIRCam) counterpart from the COSMOS-Web survey to an FIR SCUBA-2 and Atacama Large Millimeter/submillimeter Array (ALMA) source, AzTECC71, which was previously undetected at wavelengths shorter than 850 μm. AzTECC71, among the reddest galaxies in COSMOS-Web with F277W − F444W ∼ 0.9, is undetected in NIRCam/F150W and F115W and fainter in F444W than other submillimeter galaxies identified in COSMOS-Web by 2–4 magnitudes. This is consistent with the system having both a lower stellar mass and higher redshift than the median dusty, star-forming galaxy. With deep ground- and space-based upper limits combined with detections in F277W, F444W, and the FIR including ALMA Band 6, we find a high probability (99%) that AzTECC71 is at z > 4 with . This galaxy is massive () and infrared-luminous (), comparable to other optically undetected but FIR-bright dusty, star-forming galaxies at z > 4. This population of luminous, infrared galaxies at z > 4 is largely unconstrained but comprises an important bridge between the most extreme dust-obscured galaxies and more typical high-redshift star-forming galaxies. If further FIR-selected galaxies that drop out of the F150W filter in COSMOS-Web have redshifts z > 4 like AzTECC71, then the volume density of such sources may be ∼3–10 × greater than previously estimated.

Cross Correlation of Pencil-beam Galaxy Surveys and Line-intensity Maps: An Application of the James Webb Space Telescope

Line-intensity mapping (IM) experiments seek to perform statistical measurements of large-scale structure with spectral lines such as 21 cm, CO, and Lyα. A challenge in these observations is to ensure that astrophysical foregrounds, such as galactic synchrotron emission in 21 cm measurements, are properly removed. One method that has the potential to reduce foreground contamination is to cross correlate with a galaxy survey that overlaps with the IM volume. However, telescopes sensitive to high-redshift galaxies typically have small field of views compared to IM surveys. Thus, a galaxy survey for cross correlation would necessarily consist of pencil beams that sparsely fill the IM volume. In this paper, we develop the formalism to forecast the sensitivity of cross correlations between IM experiments and pencil-beam galaxy surveys. We find that a random distribution of pencil beams leads to very similar overall sensitivity as a lattice spaced across the IM survey and derive a simple formula for random configurations that agrees with the Fisher matrix formalism. We explore examples of combining high-redshift James Webb Space Telescope (JWST) observations with both an SPHEREx-like Lyα IM survey and a 21 cm experiment based on the Hydrogen Epoch of Reionization Array (HERA). We find that the JWST-SPHEREx case is promising, leading to a total signal-to-noise ratio of ∼5 after 100 total hours of JWST (at z = 7). We find that HERA is not well-suited for this approach owing to its drift-scan strategy, but that a similar experiment that can integrate down on one field could be.

Seen and unseen: bursty star formation and its implications for observations of high-redshift galaxies with JWST

ABSTRACT Both observations and simulations have shown strong evidence for highly time-variable star formation in low-mass and/or high-redshift galaxies, which has important observational implications because high-redshift galaxy samples are rest-ultraviolet (rest-UV) selected and therefore particularly sensitive to the recent star formation. Using a suite of cosmological ‘zoom-in’ simulations at z &amp;gt; 5 from the Feedback in Realistic Environments project, we examine the implications of bursty star formation histories for observations of high-redshift galaxies with JWST. We characterize how the galaxy observability depends on the star formation history. We also investigate selection effects due to bursty star formation on the physical properties measured, such as the gas fraction, specific star formation rate, and metallicity. We find the observability to be highly time-dependent for galaxies near the survey’s limiting flux due to the star formation rate variability: as the star formation rate fluctuates, the same galaxy oscillates in and out of the observable sample. The observable fraction $f_\mathrm{obs} = 50~{{\ \rm per\ cent}}$ at z ∼ 7 and M⋆ ∼ 108.5–$10^{9}\, {\rm M}_{\odot }$ for a JWST/NIRCam survey reaching a limiting magnitude of $m^\mathrm{lim}_\mathrm{AB} \sim 29{\!-\!}30$, representative of surveys such as JADES and CEERS. JWST-detectable galaxies near the survey limit tend to have properties characteristic of galaxies in the bursty phase: on average, they show approximately 2.5 times higher cold, dense gas fractions and 20 times higher specific star formation rates at a given stellar mass than galaxies below the rest-UV detection threshold. Our study represents a first step in quantifying selection effects and the associated biases due to bursty star formation in studying high-redshift galaxy properties.