ABSTRACT The intrinsic escape fraction of ionizing Lyman continuum photons ( ) is crucial to understanding whether galaxies are capable of reionizing the neutral hydrogen in the early universe at z > ...6. Unfortunately, it is not possible to access at z > 4 with direct observations, and the handful of measurements from low-redshift galaxies consistently find < 10%, while at least ∼ 10% is necessary for galaxies to dominate reionization. Here, we present the first empirical prediction of at z > 6 by combining the (sparsely populated) relation between / and with the redshift evolution of / as predicted from local high-z analogs selected by their H equivalent width. We find at z = 6 and at z = 9 for galaxies with (errors given as 1 ). However, there is a negative correlation with stellar mass and we find up to 50% larger per 0.5 dex decrease in stellar mass. The population-averaged escape fraction increases according to , with fesc,0 = (2.3 0.05)% and = 1.17 0.02 at z > 2 for . With our empirical prediction of (thus fixing an important, previously unknown variable) and further reasonable assumptions on clumping factor and the production efficiency of Lyman continuum photons, we conclude that the average population of galaxies is just capable of reionizing the universe by z ∼ 6.
ABSTRACT
The infrared (IR) spectral energy distributions (SEDs) of main-sequence galaxies in the early Universe (z > 4) is currently unconstrained as IR continuum observations are time-consuming and ...not feasible for large samples. We present Atacama Large Millimetre Array Band 8 observations of four main-sequence galaxies at z ∼ 5.5 to study their IR SED shape in detail. Our continuum data (rest-frame 110 $\rm \mu m$, close to the peak of IR emission) allows us to constrain luminosity-weighted dust temperatures and total IR luminosities. With data at longer wavelengths, we measure for the first time the emissivity index at these redshifts to provide more robust estimates of molecular gas masses based on dust continuum. The Band 8 observations of three out of four galaxies can only be reconciled with optically thin emission redward of rest-frame $100\, {\rm \mu m}$. The derived dust peak temperatures at z ∼ 5.5 ($30\!-\!43\, {\rm K}$) are elevated compared to average local galaxies, however, $\sim 10\, {\rm K}$ below what would be predicted from an extrapolation of the trend at z < 4. This behaviour can be explained by decreasing dust abundance (or density) towards high redshifts, which would cause the IR SED at the peak to be more optically thin, making hot dust more visible to the external observer. From the $850{\hbox{-}}{\rm \mu m}$ dust continuum, we derive molecular gas masses between 1010 and $10^{11}\, {\rm M_{\odot }}$ and gas fractions (gas over total mass) of $30\!-\!80{{\ \rm per\ cent}}$ (gas depletion times of $100\!-\!220\, {\rm Myr}$). All in all, our results provide a first measured benchmark SED to interpret future millimetre observations of normal, main-sequence galaxies in the early Universe.
Abstract
The redshift range
z
= 4–6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. ...The study of galaxies in this redshift range is therefore important to understanding early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star formation history (SFH) of 221
z
∼ 4.5 main-sequence galaxies at
by comparing their ultra-violet (UV) continuum, H
α
luminosity, and H
α
equivalent-width (EW). The H
α
properties are derived from the
Spitzer
3.6
μ
m−4.5
μ
m broadband color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H
α
- and UV-derived luminosities and star formation rates (SFRs). About half of the galaxies show a significant excess in H
α
compared to expectations from a constant smooth SFH. We also find a tentative anticorrelation between H
α
EW and stellar mass, ranging from 1000 Å at
to below 100 Å at
. Consulting models suggests that most
z
∼ 4.5 galaxies had a burst of star formation within the last 50 Myr, increasing their SFRs by a factor of >5. The most massive galaxies on the other hand might decrease their SFRs and may be transitioning to a quiescent stage by
z
= 4. We identify differential dust attenuation (
f
) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H
α
EW (reaching values similar to high-
z
galaxies), we predict that
f
approaches unity at
z
> 4, consistent with the extrapolation of measurements out to
z
= 2.
We present the first stellar velocity dispersion measurement of a massive quenching galaxy at z = 4. The galaxy is first identified as a massive z ≥ 4 galaxy with suppressed star formation from ...photometric redshifts based on deep multiband data. A follow-up spectroscopic observation with MOSFIRE on Keck revealed strong multiple absorption features, which are identified as Balmer lines, giving a secure redshift of z = 4.01. This is the most distant quiescent galaxy known to date. Thanks to the high S/N of the spectrum, we are able to estimate the stellar velocity dispersion, , making a significant leap from the previous highest redshift measurement at z = 2.8. Interestingly, we find that the velocity dispersion is consistent with that of massive galaxies today, implying no significant evolution in velocity dispersion over the last 12 Gyr. Based on a stringent upper limit on its physical size from deep optical images (reff < 1.3 kpc), we find that its dynamical mass is consistent with the stellar mass inferred from photometry. Furthermore, the galaxy is located on the mass fundamental plane extrapolated from lower redshift galaxies. The observed no strong evolution in suggests that the mass in the core of massive galaxies does not evolve significantly, while most of the mass growth occurs in the outskirts of the galaxies, which also increases the size. This picture is consistent with a two-phase formation scenario in which mass and size growth is due to accretion in the outskirts of galaxies via mergers. Our results imply that the first phase may be completed as early as z ∼ 4.
Using the Very Large Array and ALMA, we have obtained CO(2-1), C ii, and N ii line emission and multiple dust continuum measurements in a sample of "normal" galaxies at z = 5-6. We report the ...highest-redshift detection of low-J CO emission from a Lyman break galaxy, at z ∼ 5.7. The CO line luminosity implies a massive molecular gas reservoir of (1.3 0.3)( CO/4.5 M (K km s−1 pc2)−1) × 1011 M , suggesting low star formation efficiency with a gas depletion timescale of order ∼1 Gyr. This efficiency is much lower than traditionally observed in z 5 starbursts, indicating that star-forming conditions in main-sequence galaxies at z ∼ 6 may be comparable to those of normal galaxies probed up to z ∼ 3 to date but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a main-sequence galaxy at z ∼ 5.3 with an approximately three times lower star formation rate, perhaps implying a high CO conversion factor, as typically found in low-metallicity galaxies. For a sample including both CO targets, we also find faint N ii 205 m emission relative to C ii in all but the most IR-luminous "normal" galaxies at z = 5-6, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical ∼1010 M galaxies at z = 5-6. While a fraction of main-sequence star formation in the first billion yr may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.
Abstract
We use the unprecedented resolution and depth of the JWST NIRCam Early Release Observations at 1–5
μ
m to study the stellar mass, age, and metallicity of compact star clusters in the ...neighborhood of the host galaxies in the SMACS J0723.3–7327 galaxy cluster field at
z
= 0.39. The measured colors of these star clusters show a similar distribution as quiescent galaxies at the same redshift, but are >3 mag fainter than the current depths of wide-field galaxy survey. The star clusters are unresolved in the NIRCam/F150W data suggesting sizes smaller than 50 pc. This is significantly smaller than star-forming clumps or dwarf galaxies in local galaxies. From fitting their photometry with simple stellar population (SSP) models, we find stellar metallicities consistent with 0.2–0.3
Z
⊙
and ages of
1.5
−
0.5
+
0.5
Gyr
. We rule out metallicities <0.2
Z
⊙
and solar/supersolar at 4
σ
significance. Assuming mass-to-light ratios obtained from the best-fit SSP, we estimate stellar masses of
2.4
−
1.5
+
3.0
×
10
6
M
⊙
. These are between average masses of local globular clusters and dwarf galaxies. Our analysis suggests middle-aged globular cluster with relatively recent formation times at
z
= 0.5–0.7, which could have been subsequently stripped away from their host galaxies due to interactions in the cluster environment or formed in cold flows onto the cluster core. However, we cannot rule out these objects being compact cores of stripped dwarf galaxies.
We report the serendipitous discovery of a dusty, starbursting galaxy at z = 5.667 (hereafter called CRLE) in close physical association with the "normal" main-sequence galaxy HZ10 at z = 5.654. CRLE ...was identified by detection of C ii, N ii, and CO(2-1) line emission, making it the highest-redshift, most luminous starburst in the COSMOS field. This massive, dusty galaxy appears to be forming stars at a rate of at least 1500 M yr−1 in a compact region only ∼3 kpc in diameter. The dynamical and dust emission properties of CRLE suggest an ongoing merger driving the starburst, which is in a potentially intermediate stage relative to other known dusty galaxies at the same epoch. The ratio of C ii to N ii may suggest that an important (∼15%) contribution to the C ii emission comes from a diffuse ionized gas component, which could be more extended than the dense, starbursting gas. CRLE appears to be located in a significant galaxy overdensity at the same redshift, potentially associated with a large-scale cosmic structure recently identified in a Lyman -emitter survey. This overdensity suggests that CRLE and HZ10 reside in a protocluster environment, offering the tantalizing opportunity to study the effect of a massive starburst on protocluster star formation. Our findings support the interpretation that a significant fraction of the earliest galaxy formation may occur from the inside out, within the central regions of the most massive halos, while rapidly evolving into the massive galaxy clusters observed in the local universe.
Abstract
We present the results of a systematic search for candidate quiescent galaxies in the distant universe in 11 JWST fields with publicly available observations collected during the first 3 ...months of operations and covering an effective sky area of ∼145 arcmin
2
. We homogeneously reduce the new JWST data and combine them with existing observations from the Hubble Space Telescope. We select a robust sample of ∼80 candidate quiescent and quenching galaxies at 3 <
z
< 5 using two methods: (1) based on their rest-frame
UVJ
colors, and (2) a novel quantitative approach based on Gaussian mixture modeling of the near-UV −
U
,
U
−
V
, and
V
−
J
rest-frame color space, which is more sensitive to recently quenched objects. We measure comoving number densities of massive (
M
⋆
≥ 10
10.6
M
⊙
) quiescent galaxies consistent with previous estimates relying on ground-based observations, after homogenizing the results in the literature with our mass and redshift intervals. However, we find significant field-to-field variations of the number densities up to a factor of 2–3, highlighting the effect of cosmic variance and suggesting the presence of overdensities of red quiescent galaxies at
z
> 3, as could be expected for highly clustered massive systems. Importantly, JWST enables the robust identification of quenching/quiescent galaxy candidates at lower masses and higher redshifts than before, challenging standard formation scenarios. All data products, including the literature compilation, are made publicly available.
We report two secure ( ) and one tentative (z 3.767) spectroscopic confirmations of massive and quiescent galaxies through K-band observations with Keck/MOSFIRE and Very Large Telescope/X-Shooter. ...The stellar continuum emission, absence of strong nebular emission lines, and lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M /M ) ∼ 11 and ongoing star formation rates placing these galaxies 1-2 dex below the main sequence at their redshifts. The adopted parameterization of the star formation history suggests that these sources experienced a strong ( M yr−1) and short (∼50 Myr) burst of star formation, peaking ∼150-500 Myr before the time of observation, all properties reminiscent of the characteristics of submillimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest submillimeter surveys detecting not only the most extreme starbursts but also more normal galaxies. We support these findings by further exploring the Illustris TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3-4 and SMGs at z ∼ 4−5, with number densities and properties in agreement with the observations at z ∼ 3 but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all of the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past, and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves.
PDF
