Galaxies at Cosmic Noon ( z ∼ 2–3) are characterized by rapid star formation that will lead to significant metal enrichment in the interstellar medium (ISM). While much observational evidence suggests that these galaxies are chemically distinct from those in the local Universe, directly measuring the ISM chemistry in large samples of high- z galaxies is only now possible with the observational capabilities of JWST. In this first key paper of the CECILIA program, we present the direct-method physical conditions and multielement abundances in 20 galaxies at Cosmic Noon. Using a combination of archival Keck/MOSFIRE and new ∼30 hr NIRSpec spectroscopy, we measure multiple electron gas densities and the temperature structure from the O ^+ and S ^2+ ions. We find that n _e [O ii ] and n _e [S ii ] are comparable but elevated with respect to n _e in local star-forming galaxies, and the simultaneous T _e [O ii ] and T _e [S iii ] generally agree with photoionization model T _e scaling relations. The O abundances in the CECILIA galaxies range from 12 + log(O/H) = 7.76 to 8.81 (12%–131% solar O/H), representing some of the highest direct-method metallicities and lowest T _e ( T _e [O ii ] ≈ 6500 K) measured with JWST to date. The CECILIA galaxies exhibit significantly subsolar S/O and Ar/O, a signature of predominant enrichment from core-collapse supernovae. The N/O–O/H trends in the CECILIA galaxies generally agree with the abundance trends in local nebulae, but the large scatter in N/O could be sensitive to the star formation history. The CECILIA observations demonstrate that exceptionally deep JWST spectroscopy can unveil the multielement ISM abundance patterns in typical high- z galaxies.

A galaxy's metallicity and its relation to stellar mass encode the history of gas accretion, star formation, and outflows within cosmic ecosystems. We present new constraints on the low-mass end of the mass-metallicity relation (MZR) at z similar to 2-3 from ultra-deep JWST/NIRSpec spectroscopy of seven continuum-faint galaxies in the Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae (CECILIA) Faint sample. Our sample includes Ly alpha-selected and other low-luminosity star-forming galaxies with low stellar masses log(M*/M circle dot)approximate to 7.3 -9.4 and moderately faint rest-UV magnitudes (-20.7 less than or similar to MUV less than or similar to -17.3). Gas-phase oxygen abundances, calculated using empirical calibrations of [O iii]/H beta together with [N ii]/H alpha constraints, span similar to 0.04-0.5 Z circle dot. We measure a steep MZR slope of gamma = 0.52 +/- 0.13, suggesting a rapid increase in metal retention efficiency with mass, consistent with energy-driven outflows. Comparison with lower- and higher-redshift studies indicates an evolution in normalization from z similar to 0 to z similar to 2, reflecting less metal enrichment in early galaxies. We find no significant evolution in the MZR between z similar to 2 and the Epoch of Reionization, suggesting that our galaxies may serve as useful analogs of reionization-era systems. Expanded samples and direct Te-based abundance measurements will be crucial to fully trace the buildup of metals in low-mass galaxies during the peak epoch of cosmic star formation and to test the reliability of strong-line calibrations in these galaxies.

We present the large-scale spatial Ly alpha profiles of galaxies from the Keck Baryonic Structure Survey (KBSS) at 2 < z < 3. This work also describes the Ly alpha imaging for the KBSS-Ly alpha survey for the first time. Our sample includes 548 Ly alpha-selected galaxies and 120 continuum-selected galaxies with Ly alpha narrowband imaging, and we measure the spatial morphology of Ly alpha and continuum emission for stacked subsamples of these two populations. These samples allow us to study the variation in Ly alpha emission profiles over a broad range of UV continuum luminosities and Ly alpha equivalent widths (EWLy alpha), including systems with net Ly alpha absorption in slit spectroscopy. We characterize the spatial profiles using two techniques: directly fitting an exponential function to the stacked profile, and a multicomponent forward-modeling technique using the empirical large-scale point-spread function. We find that both methods yield similar results and that the forward-modeling technique self-consistently fits profiles exhibiting central Ly alpha emission or Ly alpha absorption, with the spatial scale of central Ly alpha approximately matching that of the continuum emission. We also find extended Ly alpha emission such that all of our subsamples-including central Ly alpha absorbers-are net Ly alpha emitters on scales comparable to the circumgalactic medium (R greater than or similar to 50 kpc, theta greater than or similar to 6 ''). We find that the scale length of the Ly alpha halo is not strongly dependent on the properties of the central galaxy, including its net continuum luminosity or EWLy alpha; although, we find a possible weak tendency of continuum-faint, high-EWLy alpha galaxies to exhibit larger Ly alpha halos in contrast with previous work.

Galactic outflows play a significant role in regulating galaxy evolution. Scaling relations between stellar mass (M_∗), star formation (SFR), and outflow properties have been extensively studied at low redshifts (z<1) but less so beyond z ∼ 2. We construct a joint sample of 387 galaxies from z ∼ 0-9, including 98 new Cosmic Noon galaxies from the Keck Baryonic Structure Survey and the Keck Lyman Continuum Spectroscopic Survey. Using high signal-to-noise emission lines (SNR > 50 for Hα or [OIII] λ5007) from Keck/MOSFIRE spectra, we detect warm-ionized outflows by decomposing lines into narrow and broad components. With the joint sample, we explore the redshift evolution of outflow scaling relations. On average, outflows at Cosmic Noon have higher maximum velocities than those at low-z by up to a factor of 3 for a fixed M_∗, SFR, or SFR surface density. They also have higher mass outflow rates for a fixed M_∗. Despite faster and stronger outflows, there is no evolution in the mass loading factor for a fixed M_∗. We find evidence for galactic fountains, as the majority of outflowing gas is recycled at a radius of ∼0.03 R_vir. We also constrain the overall outflow occurrence rate in our galaxy sample to be at least 30% when taking galaxy orientation and outflow geometry into account. By analyzing the largest sample of warm-ionized outflows at Cosmic Noon to date and compiling large galaxy samples across all redshifts, we present a comprehensive analysis of galactic outflows throughout cosmic time.

We present the large-scale spatial Lya profiles of galaxies from the Keck Baryonic Structure Survey (KBSS) at 2 50 kpc, theta > 6”). We find that the scale length of the Lya halo is not strongly dependent on the properties of the central galaxy, including its net continuum luminosity or EW_Lya, although we find a possible weak tendency of continuum-faint, high-EW_Lya galaxies to exhibit larger Lya halos in contrast with previous work.

The metal enrichment of a galaxy is determined by the cycle of baryons in outflows, inflows, and star formation. The relative contribution and timescale of each process sets the relationship between stellar mass, metallicity, and the star formation rate (SFR). In the local Universe, galaxies evolve in an equilibrium state where the timescales on which SFR and metallicity vary are comparable, and they define a surface in mass–metallicity–SFR space known as the fundamental metallicity relation (FMR). However, high-redshift observations suggest that this state of equilibrium may not persist throughout cosmic time. Using galaxies from the Keck Baryonic Structure Survey (KBSS) observed with MOSFIRE, we explore the relationship between stellar mass, gas-phase oxygen abundance, and SFR at z ∼ 2.3. Across multiple strong-line calibrations and SFR calculation methods, KBSS galaxies are inconsistent with the locally defined FMR. We use both parametric and nonparametric methods of exploring a mass–metallicity–SFR relation. When using a parametric approach, we find no significant reduction mass–metallicity relation scatter when folding in SFR as a third parameter, although a nonparametric approach reveals that there could be a weak, redshift-dependent anticorrelation between residual gas-phase oxygen abundance and SFR. Injection-recovery tests show that a significant reduction in scatter requires a stronger anticorrelation between SFR and residual metallicity. Our results suggest that the local FMR may not persist to z ∼ 2.3, implying that z ∼ 2.3 galaxies at this redshift may not be in the equilibrium state described by the FMR and are more similar to higher-redshift galaxies.

Intrinsically faint galaxies at z similar to 2-3 offer critical insights into early galaxy formation, tracing low-metallicity, low-mass systems during cosmic noon and serving as analogs to reionization-era galaxies. We present ultradeep JWST/NIRSpec spectroscopy of nine low-luminosity galaxies (-17 less than or similar to M-UV less than or similar to -20, M-star less than or similar to 10(9) M-circle dot) at z similar to 2.5 from the CECILIA program, with similar to 29.5 hr in G235M/F170LP and 1 hr in G395M/F290LP. Our sample includes four Ly alpha emitters, three rest-UV color-selected galaxies, and two serendipitous detections, providing the most sensitive rest-optical spectra of individual faint galaxies at this epoch to date. Balmer-line measurements reveal low star formation rates (SFRs; 0.63 < SFR/(M-circle dot yr(-1)) < 5.43) and a broad range of dust reddening (0 < E(B - V) < 1), with SFRs systematically below those of continuum-selected galaxies. Electron densities are low (n(e) less than or similar to 200 cm(-3)), and emission-line diagnostics indicate low [N II]/H alpha and high [O III]/H beta, suggesting metallicities of . We also present the first O1-BPT constraints in such faint high-redshift galaxies. Notably, two galaxies show low [O III]/H beta despite high Ly alpha EWs and very low [N II]/H alpha, consistent with the predicted turnover in this ratio at very low metallicities, highlighting the need for complementary diagnostics (e.g., N2, O32) to identify metal-poor systems. Direct T-e-based abundances and expanded samples are needed to further trace metallicity and ionization trends in low-mass galaxies.

Measurements of chemical abundances in high-$z$ star-forming (SF) galaxies place important constraints on the enrichment histories of galaxies and the physical conditions in the early universe. JWST is beginning to enable direct chemical abundance measurements in galaxies at $z$$>$2 via the detection of the faint T$_e$-sensitive auroral line [O III]$\lambda$4364. However, abundances of other elements (e.g., S and Ar) in high-$z$ galaxies remain unconstrained due to a lack of T$_e$ data and wavelength coverage. Here, we present multiple direct abundances in Q2343-D40, a galaxy at $z=$2.9628$\pm$0.0001 observed with JWST/NIRSpec as part of the CECILIA program. We report the first simultaneous measurement of T$_e$[O III] and T$_e$[S III] in a high-$z$ galaxy, finding good agreement with the temperature trends in local SF systems. We measure a gas-phase metallicity of 12+log(O/H) $=8.07\pm0.06$, and the N/O abundance, log(N/O) $=-1.37\pm0.21$, is indicative of primary nucleosynthesis. The S/O and Ar/O relative abundances, log(S/O)$=-1.88\pm0.10$ and log(Ar/O)$=-2.80\pm0.12$, are both $>$0.3 dex lower than the solar ratios. However, the relative Ar$^{2+}$/S$^{2+}$ abundance is consistent with the solar ratio, suggesting that the relative S-to-Ar abundance does not evolve significantly with redshift. Recent nucleosynthesis models find that a significant amount of S and Ar are produced in Type Ia supernovae, such that the S/O and Ar/O abundances in Q2343-D40 could be the result of predominantly core-collapse supernovae enrichment. Future JWST observations of high-$z$ galaxies will uncover whether S/O and Ar/O are sensitive to the timescales of these different enrichment mechanisms.

We present the first results from Chemical Evolution Constrained Using Ionized Lines in Interstellar Aurorae (CECILIA), a Cycle 1 JWST NIRSpec/MSA program that uses ultra-deep ∼30 hr G235M/F170LP observations to target multiple electron temperature-sensitive auroral lines in the spectra of 33 galaxies at z ∼ 1–3. Using a subset of 23 galaxies, we construct two ∼600 object-hour composite spectra, both with and without the stellar continuum, and use these to investigate the characteristic rest-optical ( λ rest ≈ 5700–8500 Å) spectrum of star-forming galaxies at the peak epoch of cosmic star formation. Emission lines of eight different elements (H, He, N, O, Si, S, Ar, and Ni) are detected, with most of these features observed to be ≲3% the strength of H α . We report the characteristic strength of three auroral features ([N ii ] λ 5756, [S iii ] λ 6313, and [O ii ] λ λ 7322, 7332), as well as other semi-strong and faint emission lines, including forbidden [Ni ii ] λ λ 7380, 7414 and permitted O i λ 8449, some of which have never before been observed outside of the local Universe. Using these measurements, we find T e [N ii ] = 13,630 ± 2540 K, representing the first measurement of electron temperature using [N ii ] in the high-redshift Universe. We also see evidence for broad line emission with a FWHM of 536 − 167 + 45 km s −1 ; the broad component of H α is 6.01%–28.31% the strength of the narrow component and likely arises from star-formation-driven outflows. Finally, we briefly comment on the feasibility of obtaining large samples of faint emission lines using JWST in the future.

The resonantly scattered Ly α line illuminates the extended halos of neutral hydrogen in the circumgalactic medium of galaxies. We present integral field Keck Cosmic Web Imager observations of double-peaked, spatially extended Ly α emission in 12 relatively low-mass ( M ⋆ ∼ 10 9 M ⊙ ) z ∼ 2 galaxies characterized by extreme nebular emission lines. Using individual spaxels and small bins as well as radially binned profiles of larger regions, we find that for most objects in the sample the Ly α blue-to-red peak ratio increases, the peak separation decreases, and the fraction of flux emerging at line center increases with radius. We use new radiative transfer simulations to model each galaxy with a clumpy, multiphase outflow with radially varying outflow velocity, and self-consistently apply the same velocity model to the low-ionization interstellar absorption lines. These models reproduce the trends of peak ratio, peak separation, and trough depth with radius, and broadly reconcile outflow velocities inferred from Ly α and absorption lines. The galaxies in our sample are well-described by a model in which neutral, outflowing clumps are embedded in a hotter, more highly ionized inter-clump medium (ICM), whose residual neutral content produces absorption at the systemic redshift. The peak ratio, peak separation, and trough flux fraction are primarily governed by the line-of-sight component of the outflow velocity, the H i column density, and the residual neutral density in the ICM respectively. The azimuthal asymmetries in the line profile further suggest nonradial gas motions at large radii and variations in the H i column density in the outer halos.

We report two low-frequency measurements of the power-law index for the amplitudes of giant radio pulses from the Crab pulsar. The two observations were taken with the Arecibo and Green Bank radio telescopes at center frequencies of 327 MHz and 350 MHz, respectively. We find best-fit values for the differential power-law index beta (where dN/dS proportional to S-beta and S is the pulse amplitude) of -2.63 +/- 0.05 and -3.6 +/- 0.5 from the Arecibo and Green Bank data sets, respectively. Both values are broadly consistent with other values previously measured for the Crab pulsar at low radio frequencies. These reported values may be useful in future giant pulse studies of the Crab pulsar.

The combination of the MOSDEF and KBSS-MOSFIRE surveys represents the largest joint investment of Keck/MOSFIRE time to date, with ~3000 galaxies at 1.4<=z<=3.8, roughly half of which are at z~2. MOSDEF is photometric- and spectroscopic-redshift selected with a rest-optical magnitude limit, while KBSS-MOSFIRE is primarily selected based on rest-UV colors and a rest-UV magnitude limit. Analyzing both surveys in a uniform manner with consistent spectral-energy-distribution (SED) models, we find that the MOSDEF z~2 targeted sample has a higher median M_* and redder rest U-V color than the KBSS-MOSFIRE z~2 targeted sample, and a smaller median SED-based SFR and sSFR (SFR(SED) and sSFR(SED)). Specifically, MOSDEF targeted a larger population of red galaxies with U-V and V-J >=1.25, while KBSS-MOSFIRE contains more young galaxies with intense star formation. Despite these differences in the z~2 targeted samples, the subsets of the surveys with multiple emission lines detected and analyzed in previously published work are much more similar. All median host-galaxy properties with the exception of stellar population age -- i.e., M_*, SFR(SED), sSFR(SED), A_V, and UVJ colors -- agree within the uncertainties. Additionally, when uniform emission-line fitting and stellar Balmer absorption correction techniques are applied, there is no significant offset between the two samples in the [OIII]$\lambda$5008/H$\beta$ vs. [NII]$\lambda$6585/H$\alpha$ diagnostic diagram, in contrast to previously-reported discrepancies. We can now combine the MOSDEF and KBSS-MOSFIRE surveys to form the largest z~2 sample with moderate-resolution rest-optical spectra and construct the fundamental scaling relations of star-forming galaxies during this important epoch.

The chemical abundance patterns of gas and stars in galaxies are powerful probes of galaxies’ star formation histories and the astrophysics of galaxy assembly but are challenging to measure with confidence in distant galaxies. In this paper, we report the first measurements of the correlation between stellar mass (M *) and multiple tracers of chemical enrichment (including O, N, and Fe) in individual z ∼ 2–3 galaxies, using a sample of 195 star-forming galaxies from the Keck Baryonic Structure Survey. The galaxies’ chemical abundances are inferred using photoionization models capable of reconciling high-redshift galaxies’ observed extreme rest-UV and rest-optical spectroscopic properties. We find that the M *–O/H relation for our sample is relatively shallow, with moderately large scatter, and is offset ∼0.35 dex higher than the corresponding M *–Fe/H relation. The two relations have very similar slopes, indicating a high level of α-enhancement—O/Fe ≈ 2.2 × (O/Fe)⊙—across two decades in M *. The M *–N/H relation has the steepest slope and largest intrinsic scatter, which likely results from the fact that many z ∼ 2 galaxies are observed near or past the transition from “primary” to “secondary” N production, and may reflect uncertainties in the astrophysical origin of N. Together, these results suggest that z ∼ 2 galaxies are old enough to have seen substantial enrichment from intermediate-mass stars, but are still young enough that Type Ia supernovae have not had time to contribute significantly to their enrichment.

We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $\rm Ly\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $\rm Ly\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $\rm Ly\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling.

ABSTRACT We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\,{\small I}}$$\rm Ly\,\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\,{\small I}}$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\,{\small I}}$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$^{-1}\,$ dominate the kinematics of circumgalactic $\rm{H\,{\small I}}$ out to Dtran ≃ 50 kpc, while $\rm{H\,{\small I}}$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $\rm{H\,{\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\,\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.

Out-of-hospital cardiac arrest (OHCA) and opioid overdose (OD) are two emergencies where prompt recognition and response—typically by untrained bystanders—are critical to ensure positive outcomes. The median incidence of emergency medical services–treated OHCA across 10 urban centers in North America is 52 per 100,000,1 while the U.S. incidence of fatal OD is around 14 per 100,000.2 In both emergencies, bystanders are the first link in the chain of survival. In OHCA, bystander-initiated cardiopulmonary resuscitation (CPR) and automated external defibrillator (AED) application increases survival and functional neurologic outcomes in OHCA, and for every 1-minute delay, there is a 12% decrease in favorable neurologic outcomes.3 In OD, bystander-administered naloxone decreases mortality.4 Although public agencies have allocated significant resources providing educational courses for laypersons to conduct CPR, not all members of the public can even recognize OHCA,5-10 and it is likewise uncertain whether they could recognize or treat OD. We conducted a multimodal in-person survey to describe the general public's ability to recognize OHCA or OD, as well as investigate the knowledge of and willingness to administer appropriate treatments. We hypothesized that the majority of respondents would be able to recognize a person experiencing OHCA or OD. The Providence Health Care Research Ethics Board provided approval. We selected 17 urban British Columbia locations in greater Vancouver (population 2.4 million), Victoria (390,000), Prince George (74,000), and Kelowna (132,000). We screened participants from outside public transportation stations, shopping malls, sports venues, community centers, and university campuses. Trained research assistants conducted 3-hour recruitment shifts at times of anticipated high pedestrian traffic at these locations. Participants were required to be at least 18 years or older, proficient in English, and provide informed verbal consent. Assistants then administered a 36-item survey (Appendix S1, available as supporting information in the online version of this paper, which is available at http://onlinelibrary.wiley.com/doi/10.1111/acem.13916/full) with two embedded videos (Videos S1 and S2). We based our instrument on prior similar surveys regarding OHCA5-10 and extrapolated OD-based questions from these. We covered several domains, including respondent demographics, prior witness of an OHCA/OD, prior bystander training, knowledge of appropriate actions (including chest compressions, use of AED, and administration of naloxone), and willingness to provide these. Since laypersons are typically trained in resuscitation via multihour classroom instruction, we inquired whether participants would be willing to view short videos while waiting in line at kiosks at banks, airports, or community centers or when renewing their driver's license, as a low-barrier alternative. The survey was expected to take approximately 20 minutes and participants were compensated $5 (Canadian) for their time. To ascertain whether laypersons could recognize OHCA or OD, we used publicly available videos created by public service agencies. We showed footage demonstrating a person experiencing OHCA (Heart and Stroke Foundation of Canada [Video S1]) and a person experiencing OD (British Columbia Centre for Disease Control "Toward the Heart" [Video S2]). We screened videos among five emergency physicians to ensure appropriate and recognizable content. Respondents were asked to choose the condition from a list of medical emergencies. We piloted the survey for content, clarity, and length among five volunteer emergency physicians across three iterations. Coprimary outcomes were the correct identification of the video depiction of OHCA and OD. Other outcomes included knowledge of what treatments to apply in case of the videos, as well willingness to administer evidence-based treatments in response to hypothetical OHCA and OD scenarios and desire to undergo additional training. A priori, we felt that 196 respondents would provide a margin of error ±7% (95% CI) around a hypothetical 50% recognition rate. We used descriptive statistics including frequencies with counts and proportions and describe continuous variables as means with standard deviations if normally distributed or medians with interquartile ranges if otherwise (STATA 11). Between March 1, 2018, and December 22, 2018, we approached 980 people, of whom 582 (59.4%) endorsed a lack of time or desire to participate and 164 (16.7%) did not speak sufficient English, leaving 234 for enrollment (23.9%). Respondents were 48% female, the median (IQR) age was 38 (28–49) years, and 62% reported at least some university education (Table 1). Almost one-quarter reported having witnessed an OHCA (23%) or OD (24%), and one-third (34%) reported using opioids or knowing someone who did. For OHCA, 26 respondents (11%, 95% CI = 7% to 15%) correctly identified this from the video clip, and 54 (23.1%) would perform chest compressions (selected from a list of options). If presented with a hypothetical scenario where a patient had OHCA, 62% were willing to perform CPR, 76% were willing to perform dispatch assisted CPR, and 47% were comfortable using an AED. For OD, 89 respondents (38%, 95% CI = 32% to 43%) correctly identified this from the video, while 93 would administer naloxone and 33 would provide assisted ventilations. Over half (53%) were aware of naloxone kits; if provided with a hypothetical OD scenario, 16% would be willing to administer naloxone. For further training, almost all respondents (89%) were willing to watch a 1-minute video on how to perform CPR while waiting at a kiosk. For the 197 respondents who had not received naloxone training, 54% were receptive to receiving a full naloxone training course (Table 1). This in-person survey of British Columbia residents in a variety of urban locations found that few could correctly identify a patient having a cardiac arrest or OD from a short video clip, and few could provide the appropriate lifesaving therapy. However, when presented with hypothetical scenarios, two-thirds were willing to provide CPR, and one-sixth were willing to administer naloxone in OD. Importantly, this mismatch between recognition and willingness to assist demonstrates an urgent need for focused educational interventions to assist with recognition. Our findings are similar to those of prior work. Breckwoldt et al.5 interviewed German bystanders who attended an OHCA, and nearly half did not appear to appreciate that a cardiac arrest had taken place. Likewise, a survey of Lebanese youth demonstrated that most could not identify signs of cardiac arrest.6 However, bystanders are willing to provide medical assistance in OHCA, and questionnaires in the United States7 and Japan8 have confirmed this. Gonzalez et al.9 surveyed laypersons at two Philadelphia train stations and found that 66% could identify an AED while 58% were willing to use one in an emergency, results similar to those in a Viennese telephone survey.10 There has been little investigation of community ability to recognize and willingness to assist in OD. Given that 47,000 Americans died from OD in 2017,2 this information is critical to public health and emergency educators. Our data indicate that less than half of respondents can even recognize OD, but some would be willing to administer naloxone, and most would be willing to undergo training. There is significant potential for improvement in both these statistics, and training laypersons in recognition and management of OD is likely a worthwhile investment. Training is critical to bystander interventions, and numerous agencies have invested substantial resources in first aid and medical response public education courses over the past few decades. Unfortunately, our results demonstrate that most laypersons cannot even recognize OHCA or OD: if the emergency cannot be recognized, even the most rigorous training will be of little assistance. Therefore, we advocate that current first aid training programs provide ample and diverse opportunities for recognition of OHCA and OD in addition to teaching appropriate bystander treatments. Since the cost and time of many first aid courses may be a barrier to laypersons, it is worth assessing whether current educational opportunities are maximized. Planners may consider from our results that people seem willing to engage in low-barrier training in public spaces, such as video kiosks while people wait in line, or at sporting or community events. Such relatively low-cost methods have the potential to reach large numbers of people quickly and at multiple times. These may be a viable avenue to address public knowledge and recognition deficits in order to align the skills and desire of the public to help those with medical emergencies. We note some limitations. We surveyed the public in numerous urban locations in a single province with the majority of respondents declining. A shorter survey, or one conducted via telephone or online or better-compensated, might have achieved a higher response rate. Our participants were younger with higher levels of postsecondary education and income, and some subgroups may be more able to recognize OHCA or provide assistance;10 social desirability bias may have influenced respondents. Our recognition rate was lower than anticipated, potentially affecting power of the survey. Our choice of videos—although they were developed and are used by public agencies—may have influenced the recognition rate; other videos may have led to different results. Naloxone can also be administered intranasally and public acceptance rates may be higher using this route. This is a snapshot taken over a few months during an opioid epidemic that has been heavily featured in the media and cannot measure changes over time. Appendix S1. Questionnaire. Video S1. Heart stroke Video S2. Toward the heart. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.