115 research outputs found
The [OIII]H Equivalent Width Distribution at z7: Implications for the Contribution of Galaxies to Reionization
We quantify the distribution of [OIII]+H line strengths at z7
using a sample of 20 bright (M 21) galaxies. We
select these systems over wide-area fields (2.3 deg total) using a new
colour-selection which precisely selects galaxies at z6.636.83, a
redshift range where blue Spitzer/IRAC [3.6][4.5] colours unambiguously
indicate strong [OIII]H emission. These 20 galaxies suggest a
log-normal [OIII]H EW distribution with median EW =
759 and standard deviation =
0.26 dex. We find no evidence for strong variation in this EW
distribution with UV luminosity. The typical [OIII]+H EW at z7
implied by our sample is considerably larger than that in massive star forming
galaxies at z2, consistent with a shift toward larger average sSFR (4.4
Gyr) and lower metallicities (0.16 Z). We also find evidence for
the emergence of a population with yet more extreme nebular emission
([OIII]+H EW1200 ) that is rarely seen at
lower redshifts. These objects have extremely large sSFR (30 Gyr), as
would be expected for systems undergoing a burst or upturn in star formation.
While this may be a short-lived phase, our results suggest that 20% of the
z7 population has such extreme nebular emission, implying that galaxies
likely undergo intense star formation episodes regularly at z6. We argue
that this population may be among the most effective ionizing agents in the
reionization era, both in terms of photon production efficiency and escape
fraction. We furthermore suggest that galaxies passing through this large sSFR
phase are likely to be very efficient in forming bound star clusters.Comment: 20 pages, 11 figures. Accepted in MNRAS with minor revision
On the ages of bright galaxies Myr after the Big Bang: insights into star formation activity at with JWST
With JWST, new opportunities to study the formation and evolution of galaxies
in the early Universe are now emerging. Spitzer constraints on rest-optical
properties of galaxies demonstrated the power of using stellar
masses and star formation histories (SFHs) of galaxies to indirectly infer the
star formation history of the Universe. However, only the brightest individual
objects at could be detected with Spitzer, making it difficult to
robustly constrain past activity at . Here, we leverage the
greatly improved rest-optical sensitivity of JWST at to constrain
the ages and SFHs of eleven UV-bright () galaxies
selected to lie at , then investigate implications for star
formation activity at . We infer the properties of individual
objects in our sample with two spectral energy distribution modelling codes,
then infer a distribution of ages for bright galaxies. We
find a median age of Myr, younger than that inferred at
with a similar analysis, which is consistent with an evolution towards larger
specific star formation rates at early times. The age distribution suggests
that only percent of bright galaxies would be
similarly luminous at , implying that the number density of
bright galaxies declines by approximately an order of magnitude between and . This evolution is challenging to reconcile with some
early JWST results suggesting that the abundance of bright galaxies does not
significantly decrease towards very early times, but we suggest this tension
may be eased if young stellar populations form on top of older stellar
components, or if bright galaxies at are observed during a burst of
star formation.Comment: 13 pages, 9 figure
Searching for Extremely Blue UV Continuum Slopes at in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies
The ultraviolet (UV) continuum slope ( where f) of galaxies is sensitive to a variety of properties, from the
metallicity and age of the stellar population to the attenuation from dust
through the galaxy. Considerable attention has focused on identifying
reionization-era galaxies with very blue UV slopes (). Not only do
such systems provide a signpost of low metallicity stars, but they also
identify galaxies that likely have ionizing photons leaking from their HII
regions as such blue UV slopes can only be seen if the reddening effect of
nebular continuum has been diminished. In this paper we present a search for
reionization-era galaxies with very blue UV colors in recent JWST/NIRCam
imaging of the EGS field. We characterize UV slopes for a large sample of
galaxies, finding a median value of . Three of the
lower luminosity (M) and lower stellar mass
(5-6M) systems exhibit both extremely blue UV slopes
( to ) and rest-optical photometry indicating weak nebular
line emission. Each system is very compact (r260 pc) with very high star
formation rate surface densities. We model the SEDs with a suite of BEAGLE
models with varying levels of ionizing photon escape. The SEDs cannot be
reproduced with our fiducial (f=0) or alpha enhanced
(Z) models. The combined blue UV slopes and weak nebular
emission are best-fit by models with significant ionizing photon escape from
HII regions (f=0.6-0.8) and extremely low metallicity massive
stars (Z=0.01-0.06 Z). The discovery of these galaxies highlights
the potential for JWST to identify large numbers of candidate Lyman Continuum
leaking galaxies in the reionization era and suggests low metallicity stellar
populations may be veryComment: 11 pages, 8 figures, 2 tables; Submitted to Ap
A JWST/NIRCam Study of Key Contributors to Reionization: The Star-forming and Ionizing Properties of UV-faint Galaxies
Spitzer/IRAC imaging has revealed that the brightest galaxies
often exhibit young ages and strong nebular line emission, hinting at high
ionizing efficiency among early galaxies. However, IRAC's limited sensitivity
has long hindered efforts to study the fainter, more numerous population often
thought largely responsible for reionization. Here we use CEERS JWST/NIRCam
data to characterize 116 UV-faint (median M)
galaxies. The SEDs are typically dominated by young (10-50 Myr), low-mass
() stellar populations, and we find no need for
extremely high stellar masses (). Considering previous
studies of UV-bright (M) galaxies, we find evidence
for a strong (5-10) increase in specific star formation rate toward
lower luminosities (median sSFR=103 Gyr in CEERS). The larger sSFRs
imply a more dominant contribution from OB stars in the relatively numerous
UV-faint population, perhaps suggesting that these galaxies are very efficient
ionizing agents (median erg Hz). In spite of their
much larger sSFRs, we find no significant increase in [OIII]H EWs
towards fainter M (median 780 ). If confirmed,
this may indicate that a substantial fraction of our CEERS galaxies possess
extremely low metallicities (3% ) where [OIII] emission is
suppressed. Alternatively, high ionizing photon escape fractions or bursty star
formation histories can also weaken the nebular lines in a subset of our CEERS
galaxies. While the majority of our objects are very blue (median
), we identify a significant tail of very dusty galaxies
() at 0.5 which may contribute significantly
to the star formation rate density.Comment: Accepted in MNRAS. Updated to use the most recent NIRCam zeropoints.
There are no significant changes to the conclusions relative to v
Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z6.8: A Radio-Loud AGN in the Reionization Era?
We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and
sub-mm (24-850m) emission at the position of one of 41 UV-bright
(M) Lyman-break galaxy
candidates in the 1.5 deg COSMOS field. This source, COS-87259, exhibits a
sharp flux discontinuity (factor 3) between two narrow/intermediate bands at
9450 and 9700 Angstroms and is undetected in all nine bands blueward of 9600
Angstroms, as expected from a Lyman-alpha break at . The full
multi-wavelength (X-ray through radio) data of COS-87529 can be
self-consistently explained by a very massive (M
M) and extremely red (rest-UV slope )
galaxy with hyperluminous infrared emission (L
L) powered by both an intense burst of highly-obscured star formation
(SFR1800 M yr) and an obscured () radio-loud (L W
Hz) AGN. The radio emission is compact (1.040.12 arcsec) and
exhibits an ultra-steep spectrum between 1.32-3 GHz
() that flattens at lower frequencies
( between 0.144-1.32 GHz), consistent with known
radio galaxies. We also demonstrate that COS-87259 may reside in a
significant (11) galaxy overdensity at , as common for
systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will
ultimately be required to establish the true nature of COS-87259 as we cannot
yet completely rule out low-redshift solutions. If confirmed to lie at
, the properties of COS-87259 would be consistent with a picture
wherein AGN and highly-obscured star formation activity are fairly common among
very massive (M M) reionization-era galaxies.Comment: 14 pages, 6 figures. Accepted in MNRAS with minor revisions. This
accepted version considers very recent data from LOFAR and MeerKAT which
improve our analyses on the radio slope and luminosit
ALMA Confirmation of an Obscured Hyperluminous Radio-Loud AGN at Associated with a Dusty Starburst in the 1.5 deg COSMOS Field
We present band 6 ALMA observations of a heavily-obscured radio-loud
( W Hz) AGN candidate at
found in the 1.5 deg COSMOS field. The ALMA
data reveal detections of exceptionally strong [CII]158m
() and underlying dust continuum emission from this
object (COS-87259), where the [CII] line luminosity, line width, and 158m
continuum luminosity are comparable to that seen from sub-mm galaxies
and quasar hosts. The 158m continuum detection suggests a total infrared
luminosity of with corresponding very large obscured
star formation rate (1300 /yr) and dust mass (
). The strong break seen between the VIRCam and IRAC photometry
perhaps suggests that COS-87259 is an extremely massive reionization era galaxy
with . Moreover, the MIPS, PACS, and
SPIRE detections imply that this object harbors an AGN that is heavily obscured
() with a bolometric luminosity of
approximately . Such a very high AGN luminosity
suggests this object is powered by an 1.6 10
black hole if accreting near the Eddington limit, and is effectively a
highly-obscured version of an extremely UV-luminous ()
quasar. Notably, these quasars are an exceedingly rare
population (0.001 deg) while COS-87259 was identified over a
relatively small field. Future very wide-area surveys with, e.g., Roman and
Euclid have the potential to identify many more extremely red yet UV-bright
objects similar to COS-87259, providing richer insight into the
occurrence of intense obscured star formation and supermassive black hole
growth among this population.Comment: 12 pages, 7 figures, 1 table. Updated to accepted version (MNRAS
Unveiling the Phase Transition of the Universe During the Reionization Epoch with Lyman-alpha
The epoch of reionization (6 < z < 10) marks the period in our universe when
the first large galaxies grew to fruition, and began to affect the universe
around them. Massive stars, and potentially accreting supermassive black holes,
filled the universe with ionizing radiation, burning off the haze of neutral
gas that had filled the intergalactic medium (IGM) since recombination
(z~1000). The evolution of this process constrains key properties of these
earliest luminous sources, thus observationally constraining reionization is a
key science goal for the next decade. The measurement of Lyman-alpha emission
from photometrically-identified galaxies is a highly constraining probe of
reionization, as a neutral IGM will resonantly scatter these photons, reducing
detectability. While significant work has been done with 8-10m telescopes,
these observations require extremely large telescopes (ELTs); the flux limits
available from today's 10m class telescopes are sufficient for only the
brightest known galaxies (m < 26). Ultra-deep surveys with the Giant Magellan
Telescope (GMT) and Thirty Meter Telescope (TMT) will be capable of detecting
Lyman-alpha emission from galaxies 2-3 magnitudes fainter than today's deepest
surveys. Wide-field fiber spectroscopy on the GMT combined with narrow-field
AO-assisted slit spectroscopy on the TMT will be able to probe the expected
size of ionized bubbles throughout the epoch of reionization, following up
degree scale deep imaging surveys with the Wide Field Infrared Space Telescope.
These data will provide the first resolved Lyman-alpha-based maps of the
ionized intergalactic medium throughout the epoch of reionization, constraining
models of both the temporal and spatial evolution of this phase change.Comment: White paper submitted to the Astro2020 Decadal Review of the National
Academy of Science
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