ABSTRACT
We utilize deep near-infrared survey data from the UltraVISTA fourth data release (DR4) and the VIDEO survey, in combination with overlapping optical and Spitzer data, to search for bright ...star-forming galaxies at z ≳ 7.5. Using a full photometric redshift fitting analysis applied to the ∼6 $\, {\rm deg}^2$ of imaging searched, we find 27 Lyman break galaxies (LBGs), including 20 new sources, with best-fitting photometric redshifts in the range 7.4 < z < 9.1. From this sample, we derive the rest-frame UV luminosity function at z = 8 and z = 9 out to extremely bright UV magnitudes (MUV ≃ −23) for the first time. We find an excess in the number density of bright galaxies in comparison to the typically assumed Schechter functional form derived from fainter samples. Combined with previous studies at lower redshift, our results show that there is little evolution in the number density of very bright (MUV ∼ −23) LBGs between z ≃ 5 and z ≃ 9. The tentative detection of an LBG with best-fitting photometric redshift of z = 10.9 ± 1.0 in our data is consistent with the derived evolution. We show that a double power-law fit with a brightening characteristic magnitude (ΔM*/Δz ≃ −0.5) and a steadily steepening bright-end slope (Δβ/Δz ≃ −0.5) provides a good description of the z > 5 data over a wide range in absolute UV magnitude (−23 < MUV < −17). We postulate that the observed evolution can be explained by a lack of mass quenching at very high redshifts in combination with increasing dust obscuration within the first ${\sim}1 \, {\rm Gyr}$ of galaxy evolution.
We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z 7 and 8, and a first estimate at z 9. An accurate determination of the form and evolution of the ...galaxy LF during this era is of key importance for improving our knowledge of the earliest phases of galaxy evolution and the process of cosmic reionization. Our analysis exploits to the full the new, deepest Wide Field Camera 3/infrared imaging from our Hubble Space Telescope (HST) Ultra-Deep Field 2012 (UDF12) campaign, with dynamic range provided by including a new and consistent analysis of all appropriate, shallower/wider area HST survey data. Our new measurement of the evolving LF at z 7 to 8 is based on a final catalogue of 600 galaxies, and involves a step-wise maximum-likelihood determination based on the photometric redshift probability distribution for each object; this approach makes full use of the 11-band imaging now available in the Hubble Ultra-Deep Field (HUDF), including the new UDF12 F140W data, and the latest Spitzer IRAC imaging. The final result is a determination of the z 7 LF extending down to UV absolute magnitudes M
1500 = −16.75 (AB mag) and the z 8 LF down to M
1500 = −17.00. Fitting a Schechter function, we find M1500
* = −19.90+0.23
−0.28, log φ* = −2.96+0.18
−0.23 and a faint-end slope α = −1.90+0.14
−0.15 at z 7, and M1500* = −20.12+0.37
−0.48, log φ* = −3.35+0.28
−0.47 and α = −2.02+0.22
+0.23 at z 8. These results strengthen previous suggestions that the evolution at z > 7 appears more akin to 'density evolution' than the apparent 'luminosity evolution' seen at z 5 − 7. We also provide the first meaningful information on the LF at z 9, explore alternative extrapolations to higher redshifts, and consider the implications for the early evolution of UV luminosity density. Finally, we provide catalogues (including derived z
phot, M
1500 and photometry) for the most robust z ∼ 6.5-11.9 galaxies used in this analysis. We briefly discuss our results in the context of earlier work and the results derived from an independent analysis of the UDF12 data based on colour-colour selection.
Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs.
), during the so-called epoch of ...reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs
, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies
and companions of rare quasars
. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z > 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z = 3-6 (refs.
). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z = 6.6813 ± 0.0005 and z = 7.3521 ± 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared C II lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z ≈ 7. This population of heavily dust-obscured galaxies appears to contribute 10-25% to the z > 6 cosmic star formation rate density.
We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 Hubble Space Telescope (HST) Wide Field Camera 3/IR campaign to explore the rest-frame ...ultraviolet (UV) properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, 〈β〉, (f
λ ∝ λβ) for faint galaxies at z 7, the first meaningful measurements of 〈β〉 at z 8, and tentative estimates for a new sample of galaxies at z 9. Utilizing galaxy selection in the new F140W (J
140) imaging to minimize colour bias, and applying both colour and power-law estimators of β, we find 〈β〉 = −2.1 ± 0.2 at z 7 for galaxies with M
UV −18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z 8 we find a similar value, 〈β〉 = −1.9 ± 0.3. At z 9, we find 〈β〉 = −1.8 ± 0.6, essentially unchanged from z 6 to 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in β within our new, robust z 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either solar metallicity and zero dust, or moderately sub-solar ( 10-20 per cent) metallicity with modest dust obscuration (A
V
0.1-0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with β −2.5 may not emerge until M
UV > −16, a regime likely to remain inaccessible until the James Webb Space Telescope.
Abstract
We present new Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) imaging of 25 extremely luminous (−23.2 ≤ M
UV ≲ −21.2) Lyman-break galaxies (LBGs) at z ≃ 7. The sample was initially ...selected from 1.65 deg2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman α emitters, ‘Himiko’ and ‘CR7’. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW0(Hβ + O iii) > 600 Å). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (f
multi > 0.4) in bright z ≃ 7 galaxies, and ubiquitous at the very bright end (M
UV < −22.5). The galaxies have half-light radii in the range r
1/2 ∼ 0.5–3 kpc. The size measurements provide the first determination of the size–luminosity relation at z ≃ 7 that extends to M
UV ∼ −23. We find the relation to be steep with r
1/2 ∝ L
1/2. Excluding clumpy, multicomponent galaxies however, we find a shallower relation that implies an increased star formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ≃ 7 favours a power-law decline at the bright end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect >1000 similarly bright galaxies at z ≃ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.
We present the H
I
emission project within the MIGHTEE survey, currently being carried out with the newly commissioned MeerKAT radio telescope. This is one of the first deep, blind, medium-wide ...interferometric surveys for neutral hydrogen (H
I
) ever undertaken, extending our knowledge of H
I
emission to
z
= 0.6. The science goals of this medium-deep, medium-wide survey are extensive, including the evolution of the neutral gas content of galaxies over the past 5 billion years. Simulations predict nearly 3000 galaxies over 0 <
z
< 0.4 will be detected directly in H
I
, with statistical detections extending to
z
= 0.6. The survey allows us to explore H
I
as a function of galaxy environment, with massive groups and galaxy clusters within the survey volume. Additionally, the area is large enough to contain as many as 50 local galaxies with H
I
mass < 10
8
M
⊙
, which allows us to study the low-mass galaxy population. The 20 deg
2
main survey area is centred on fields with exceptional multi-wavelength ancillary data, with photometry ranging from optical through far-infrared wavelengths, supplemented with multiple spectroscopic campaigns. We describe here the survey design and the key science goals. We also show first results from the Early Science observations, including kinematic modelling of individual sources, along with the redshift, H
I
, and stellar mass ranges of the sample to date.
Abstract
The Reionization Era Bright Emission Line Survey (REBELS) is a cycle-7 ALMA Large Program (LP) that is identifying and performing a first characterization of many of the most luminous ...star-forming galaxies known in the
z
> 6.5 universe. REBELS is providing this probe by systematically scanning 40 of the brightest UV-selected galaxies identified over a 7 deg
2
area for bright C
ii
158
μ
m
and O
iii
88
μ
m
lines and dust-continuum emission. Selection of the 40 REBELS targets was done by combining our own and other photometric selections, each of which is subject to extensive vetting using three completely independent sets of photometry and template-fitting codes. Building on the observational strategy deployed in two pilot programs, we are increasing the number of massive interstellar medium (ISM) reservoirs known at
z
> 6.5 by ∼4–5× to >30. In this manuscript, we motivate the observational strategy deployed in the REBELS program and present initial results. Based on the first-year observations, 18 highly significant ≥ 7
σ
C
ii
158
μ
m
lines have already been discovered, the bulk of which (13/18) also show ≥3.3
σ
dust-continuum emission. These newly discovered lines more than triple the number of bright ISM-cooling lines known in the
z
> 6.5 universe, such that the number of ALMA-derived redshifts at
z
> 6.5 rival Ly
α
discoveries. An analysis of the completeness of our search results versus star formation rate (SFR) suggests an ∼79% efficiency in scanning for C
ii
158
μ
m
when the SFR
UV+IR
is >28
M
⊙
yr
−1
. These new LP results further demonstrate ALMA’s efficiency as a “redshift machine,” particularly in the Epoch of Reionization.
ABSTRACT
We include a fully coupled treatment of metal and dust enrichment into the Delphi semi-analytic model of galaxy formation to explain the dust content of 13 Lyman break galaxies (LBGs) ...detected by the Atacama Large millimetre Array (ALMA) REBELS Large Program at z ≃ 7. We find that the galaxy dust mass, Md, is regulated by the combination of Type II supernova dust production, astration, shock destruction, and ejection in outflows; grain growth (with a standard time-scale τ0 = 30 Myr) plays a negligible role. The model predicts a dust-to-stellar mass ratio of $\sim 0.07\!-\!0.1{{\ \rm per\ cent}}$ and a UV-to-total star formation rate relation such that log(ψUV) = −0.05 log(ψ)2 + 0.86 log(ψ) − 0.05 (implying that 55–80 per cent of the star formation is obscured) for REBELS galaxies with stellar mass $M_* = 10^{9}\!-\!10^{10} \rm M_\odot$. This relation reconciles the intrinsic UV luminosity of LBGs with their observed luminosity function at z = 7. However, 2 out of the 13 systems show dust-to-stellar mass ratios ($\sim 0.94\!-\!1.1{{\ \rm per\ cent}}$) that are up to 18 times larger than expected from the fiducial relation. Due to the physical coupling between dust and metal enrichment, even decreasing τ0 to very low values (0.3 Myr) only increases the dust-to-stellar mass ratio by a factor of ∼2. Given that grain growth is not a viable explanation for such high observed ratios of the dust-to-stellar mass, we propose alternative solutions.
We present the results of a search for bright (−22.7 ≤ M
UV ≤ −20.5) Lyman-break galaxies at z ≃ 6 within a total of 1.65 deg2 of imaging in the UltraVISTA/Cosmological Evolution Survey (COSMOS) and ...United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) fields. The deep near-infrared imaging available in the two independent fields, in addition to deep optical (including z
′-band) data, enables the sample of z ≃ 6 star-forming galaxies to be securely detected longward of the break (in contrast to several previous studies). We show that the expected contamination rate of our initial sample by cool Galactic brown dwarfs is ≲3 per cent and demonstrate that they can be effectively removed by fitting brown dwarf spectral templates to the photometry. At z ≃ 6, the galaxy surface density in the UltraVISTA field exceeds that in the UDS by a factor of ≃ 1.8, indicating strong cosmic variance even between degree-scale fields at z > 5. We calculate the bright end of the rest-frame Ultraviolet (UV) luminosity function (LF) at z ≃ 6. The galaxy number counts are a factor of ∼1.7 lower than predicted by the recent LF determination by Bouwens et al. In comparison to other smaller area studies, we find an evolution in the characteristic magnitude between z ≃ 5 and z ≃ 7 of ΔM* ∼ 0.4, and show that a double power law or a Schechter function can equally well describe the LF at z = 6. Furthermore, the bright end of the LF appears to steepen from z ≃ 7 to z ≃ 5, which could indicate the onset of mass quenching or the rise of dust obscuration, a conclusion supported by comparing the observed LFs to a range of theoretical model predictions.
ABSTRACT
ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature Td remains mostly unconstrained due to the few ...available FIR continuum data at redshift $z$ > 5. This introduces large uncertainties in several properties of high-$z$ galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous C $\scriptstyle \rm II$ 158-μm line and underlying dust continuum measurements, we derive Td in the continuum and C $\scriptstyle \rm II$ detected $z$ ≈ 7 galaxies in the ALMA Large Project REBELS sample. We find 39 < Td < 58 K, and dust masses in the narrow range Md = (0.9−3.6) × 107 M⊙. These results allow us to extend for the first time the reported Td($z$) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing Td($z$) trend with the decrease of gas depletion time, tdep = Mg/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields Td ∝ (1 + $z$)0.4. The model also explains the observed Td scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, Td only depends on the gas column density (metallicity), $T_{\rm d} \propto N_{\rm H}^{1/6}$ (Td ∝ Z−1/6). REBELS galaxies are on average relatively transparent, with effective gas column densities around NH ≃ (0.03−1) × 1021 cm−2. We predict that other high-$z$ galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated Td ≫ 60 K, are significantly obscured, low-metallicity systems. In fact, Td is higher in metal-poor systems due to their smaller dust content, which for fixed LIR results in warmer temperatures.
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