32 research outputs found
The Lyman Continuum Escape Survey: Ionizing Radiation from [O III]-Strong Sources at a Redshift of 3.1
We present results from the LymAn Continuum Escape Survey (LACES), a Hubble
Space Telescope (HST) program designed to characterize the ionizing radiation
emerging from a sample of Lyman alpha emitting galaxies at redshift . As many show intense [O III] emission characteristic of
star-forming galaxies, they may represent valuable low redshift analogs of
galaxies in the reionization era. Using HST Wide Field Camera 3 / UVIS
to image Lyman continuum emission, we investigate the escape fraction of
ionizing photons in this sample. For 61 sources, of which 77% are
spectroscopically confirmed and 53 have measures of [O III] emission, we detect
Lyman continuum leakage in 20%, a rate significantly higher than is seen in
individual continuum-selected Lyman break galaxies. We estimate there is a 98%
probability that of our detections could be affected by foreground
contamination. Fitting multi-band spectral energy distributions (SEDs) to take
account of the varying stellar populations, dust extinctions and metallicities,
we derive individual Lyman continuum escape fractions corrected for foreground
intergalactic absorption. We find escape fractions of 15 to 60% for individual
objects, and infer an average 20% escape fraction by fitting composite SEDs for
our detected samples. Surprisingly however, even a deep stack of those sources
with no individual detections provides a stringent upper limit on the
average escape fraction of less than 0.5%. We examine various correlations with
source properties and discuss the implications in the context of the popular
picture that cosmic reionization is driven by such compact, low metallicity
star-forming galaxies.Comment: 26 pages, 16 figures, accepted for publication in Ap
Spectroscopic Constraints on UV Metal Line Emission at z~6-9: The Nature of Lyman-alpha Emitting Galaxies in the Reionization-Era
Recent studies have revealed intense UV metal emission lines in a modest
sample of z>7 Lyman-alpha emitters, indicating a hard ionizing spectrum is
present. If such high ionization features are shown to be common, it may
indicate that extreme radiation fields play a role in regulating the visibility
of Lyman-alpha in the reionization era. Here we present deep near-infrared
spectra of seven galaxies with Lyman-alpha emission at 5.4<z<8.7 (including a
newly-confirmed lensed galaxy at z=6.031) and three bright z>7 photometric
targets. In nine sources we do not detect UV metal lines. However in the
z=8.683 galaxy EGSY8p7, we detect a 4.6 sigma emission line in the narrow
spectral window expected for NV 1243. The feature is unresolved (FWHM<90 km/s)
and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals
non-detections of CIV, He II, and OIII]. The presence of NV requires a
substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum
powered by an AGN or fast radiative shocks. Regardless of its origin, the
intense radiation field of EGSY8p7 may aid the transmission of Lyman-alpha
through what is likely a partially neutral IGM. With this new detection, five
of thirteen known Lyman-alpha emitters at z>7 have now been shown to have
intense UV line emission, suggesting that extreme radiation fields are
commonplace among the Lyman-alpha population. Future observations with JWST
will eventually clarify the origin of these features and explain their role in
the visibility of Lyman-alpha in the reionization era.Comment: 15 pages, 3 tables and 7 figures. Accepted for publication in MNRA
Spectroscopic detection of CIV in a galaxy at z=7.045: Implications for the ionizing spectra of reionization-era galaxies
We present Keck/MOSFIRE observations of UV metal lines in four bright
gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The
spectrum of A1703-zd6, a highly-magnified star forming galaxy with a
Lyman-alpha redshift of z=7.045, reveals a confident detection of the nebular
CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not
detected in the three other galaxies. At z~2-3, nebular CIV emission is
observed in just 1% of UV-selected galaxies. The presence of strong CIV
emission in one of the small sample of galaxies targeted in this paper may
indicate hard ionizing spectra are more common at z~7. The total estimated
equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3)
are comparable to measurements of narrow-lined AGNs. Photoionization models
show that the nebular CIV line can also be reproduced by a young stellar
population, with very hot metal poor stars dominating the photon flux
responsible for triply ionizing carbon. Regardless of the origin of the CIV, we
show that the ionizing spectrum of A1703-zd6 is different from that of typical
galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity
and a larger flux density at 30-50 eV. If such extreme radiation fields are
typical in UV-selected systems at z>7, it would indicate that reionization-era
galaxies are more efficient ionizing agents than previously thought.
Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7
may trace a rare population with intense radiation fields capable of ionizing
their surrounding hydrogen distribution. Additional constraints on high
ionization emission lines in galaxies with and without Lyman-alpha detections
will help clarify whether hard ionizing spectra are common in the reionization
era.Comment: 11 pages, 6 figures, submitted to MNRA
The [OIII] equivalent width distribution at z : The redshift evolution of the extreme emission line galaxies
We determine the [OIII] equivalent width (EW) distribution of
rest-frame UV-selected (M) star-forming
galaxies in the GOODS North and South fields. We make use of deep HDUV
broadband photometry catalogues for selection and 3D-HST WFC3/IR grism spectra
for measurement of line properties. The [OIII] EW distribution
allows us to measure the abundance of extreme emission line galaxies (EELGs)
within this population. We model a log-normal distribution to the
[OIII] rest-frame equivalent widths of galaxies in our sample,
with location parameter and variance parameter . This EW distribution has a mean [OIII] EW of
168. The fractions of rest-UV-selected galaxies with
[OIII] EWs greater than and are measured to
be , , and
respectively. The EELG fractions do not vary strongly with UV luminosity in the
range () considered in this paper, consistent with
findings at higher redshifts. We compare our results to and
studies where candidate EELGs have been discovered through
Spitzer/IRAC colours, and we identify rapid evolution with redshift in the
fraction of star-forming galaxies observed in an extreme emission line phase (a
rise by a factor between and ). This
evolution is consistent with an increased incidence of strong bursts in the
galaxy population of the reionisation era. While this population makes a
sub-dominant contribution of the ionising emissivity at , EELGs
are likely to dominate the ionising output in the reionisation era.Comment: Submitted to MNRAS. 13 pages, 6 figure
Lyman-alpha and CIII] Emission in z=7-9 Galaxies: Accelerated Reionization Around Luminous Star Forming Systems?
We discuss new Keck/MOSFIRE spectroscopic observations of four luminous
galaxies at z~7-9 selected to have intense optical line emission by
Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in
two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha
is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1,
confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at
z=7.477, verifying a tentative detection presented in an earlier study. The
ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely
significantly neutral over 7<z<9. To investigate this result in more detail, we
have initiated a campaign to target UV metal emission in the four Lyman-alpha
emitters as a probe of both the radiation field and the velocity offset of
Lyman-alpha. Here we present the detection of intense CIII] emission in
EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at
z=7.73. Photoionization models indicate that an intense radiation field and low
metallicity are required to reproduce the intense CIII] and optical line
emission. We argue that this extreme radiation field is likely to affect the
local environment, increasing the transmission of Lyman-alpha through the
galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is
redshifted from the systemic value by 340 km/s. This velocity offset is larger
than that seen in less luminous systems, providing an additional explanation
for the transmission of Lyman-alpha emission through the IGM. Since the
transmission is further enhanced by the likelihood that such systems are also
situated in the densest regions with the largest ionized bubbles, the
visibility of Lyman-alpha at z>7 is expected to be strongly
luminosity-dependent, with the most effective transmission occurring in systems
with intense star formation.Comment: Submitted to MNRAS, 13 pages, 8 figure
Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation
Strong [OIII]4959,5007+H emission appears to be
typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic
reionization, these galaxies and the physical conditions within them are of
great interest. At z > 6.5, where Ly is greatly attenuated by the
intergalactic medium, rest-UV metal emission lines provide an alternative
measure of redshift and also constraints on the physical properties of
star-forming regions and massive stars. We present the first statistical sample
of rest-UV line measurements in z 2 galaxies selected as analogs of
those in the reionization era based on [OIII]4959,5007 EW or
rest-frame U-B color. Our sample is drawn from the 3D-HST Survey and spans the
redshift range 1.36 z 2.49. We find that the median
Ly and CIII]1907,1909 EWs of our sample are
significantly greater than those of z 2 UV-continuum-selected
star-forming galaxies. Measurements from both individual and composite spectra
indicate a monotonic, positive correlation between CIII] and [OIII], while a
lack of trend is observed between Ly and [OIII] at [OIII] EW <
1000\unicode{x212B}. At higher [OIII] EW, extreme Ly emission starts
to emerge. Using stacked spectra, we find that Ly and CIII] are
significantly enhanced in galaxies with lower metallicity. Two objects in our
sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV
metal line emission. These objects have significant
CIV1548,1550, HeII1640, and
OIII]1661,1665 emission in addition to intense Ly or
CIII]. Detailed characterization of these lower-redshift analogs provides
unique insights into the physical conditions in z > 6.5 star-forming regions,
motivating future observations of reionization-era analogs at lower redshifts.Comment: 22 pages, 9 figures, accepted for publication in Ap
JADES NIRSpec Spectroscopy of GN-z11: Lyman- emission and possible enhanced nitrogen abundance in a luminous galaxy
We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous
candidate Lyman break galaxy in the GOODS-North field with
. We derive a redshift of (lower than previous
determinations) based on multiple emission lines in our low and medium
resolution spectra over m. We significantly detect the continuum
and measure a blue rest-UV spectral slope of . Remarkably, we see
spatially-extended Lyman- in emission (despite the highly-neutral IGM
expected at this early epoch), offset 555 km/s redward of the systemic
redshift. From our measurements of collisionally-excited lines of both low- and
high-ionization (including [O II] , [Ne III] and C
III] ) we infer a high ionization parameter (). We
detect the rarely-seen N IV] and N III] lines in
both our low and medium resolution spectra, with other high ionization lines
seen in low resolution spectrum such as He II (blended with O III]) and C IV
(with a possible P-Cygni profile). Based on the observed rest-UV line ratios,
we cannot conclusively rule out photoionization from AGN. The high C III]/He II
ratios, however, suggest a likely star-formation explanation. If the observed
emission lines are powered by star formation, then the strong N III]
observed may imply an unusually high abundance. Balmer
emission lines (H, H) are also detected, and if powered by star
formation rather than an AGN we infer a star formation rate of (depending on the IMF) and low dust attenuation. Our
NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme
properties seen 430 Myr after the Big Bang.Comment: Submitted to Astronomy & Astrophysics, 14 pages, 9 figure
JWST reveals a possible galaxy merger in triply-lensed MACS0647JD
MACS0647JD is a triply-lensed galaxy originally discovered with
the Hubble Space Telescope. Here we report new JWST imaging, which clearly
resolves MACS0647JD as having two components that are either merging
galaxies or stellar complexes within a single galaxy. Both are very small, with
stellar masses and radii . The brighter
larger component "A" is intrinsically very blue (), likely due
to very recent star formation and no dust, and is spatially extended with an
effective radius . The smaller component "B" appears redder
(), likely because it is older () with mild dust
extinction (), and a smaller radius . We
identify galaxies with similar colors in a high-redshift simulation, finding
their star formation histories to be out of phase. With an estimated stellar
mass ratio of roughly 2:1 and physical projected separation ,
we may be witnessing a galaxy merger 400 million years after the Big Bang. We
also identify a candidate companion galaxy C away, likely
destined to merge with galaxies A and B. The combined light from galaxies A+B
is magnified by factors of 8, 5, and 2 in three lensed images JD1, 2, and
3 with F356W fluxes , , (AB mag 25.1, 25.6, 26.6).
MACS0647JD is significantly brighter than other galaxies recently discovered
at similar redshifts with JWST. Without magnification, it would have AB mag
27.3 (). With a high confidence level, we obtain a photometric
redshift of based on photometry measured in 6 NIRCam filters
spanning , out to rest-frame. JWST NIRSpec
observations planned for January 2023 will deliver a spectroscopic redshift and
a more detailed study of the physical properties of MACS0647JD.Comment: 27 pages, 14 figures, submitted to Natur
Robust estimation of bacterial cell count from optical density
Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data