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Register a new userThe Lyman Continuum Escape Survey -- II: Ionizing Radiation as a Function of the [OIII]/[OII] Line Ratio
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We discuss the rest-frame optical emission line spectra of a large (~50) sample of z=3.1 Lyman alpha emitters (LAEs) whose physical properties suggest such sources are promising analogs of galaxies in the reionization era. Reliable Lyman continuum escape fractions have now been determined for a large sample of such LAEs from the Lyman Continuum Escape Survey (LACES) undertaken via deep HST imaging in the SSA22 survey area reported in Fletcher et al. (2019). Using new measures of [OII] emission secured from Keck MOSFIRE spectra we re-examine, for a larger sample, earlier claims that Lyman continuum leakages may correlate with the nebular emission line ratio [OIII]/[OII] as expected for density-bound HII regions. We find that a large [OIII]/[OII] line ratio is indeed a necessary condition for Lyman continuum leakage, strengthening earlier claims made using smaller samples at various redshifts. However, not all LAEs with large [OIII]/[OII] line ratios are leakers and leaking radiation appears not to be associated with differences in other spectral diagnostics. This suggests the detection of leaking radiation is modulated by an additional property, possibly the viewing angle for porous HII regions. We discuss our new results in the context of the striking bimodality of LAE leakers and non-leakers found in the LACES program and the implications for the sources of cosmic reionization.
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Escaping Lyman continuum photons from galaxies likely reionized the intergalactic medium at redshifts $zgtrsim6$. However, the Lyman continuum is not directly observable at these redshifts and secondary indicators of Lyman continuum escape must be used to estimate the budget of ionizing photons. Observationally, at redshifts $zsim2-3$ where the Lyman continuum is observationally accessible, surveys have established that many objects that show appreciable Lyman continuum escape fractions $f_{esc}$ also show enhanced [OIII]/[OII] (O$_{32}$) emission line ratios. Here, we use radiative transfer analyses of cosmological zoom-in simulations of galaxy formation to study the physical connection between $f_{esc}$ and O$_{32}$. Like the observations, we find that the largest $f_{esc}$ values occur at elevated O$_{32}sim3-10$ and that the combination of high $f_{esc}$ and low O$_{32}$ is extremely rare. While high $f_{esc}$ and O$_{32}$ often are observable concurrently, the timescales of the physical origin for the processes are very different. Large O$_{32}$ values fluctuate on short ($sim$1 Myr) timescales during the Wolf-Rayet-powered phase after the formation of star clusters, while channels of low absorption are established over tens of megayears by collections of supernovae. We find that while there is no direct causal relation between $f_{esc}$ and O$_{32}$, high $f_{esc}$ most often occurs after continuous input from star formation-related feedback events that have corresponding excursions to large O$_{32}$ emission. These calculations are in agreement with interpretations of observations that large $f_{esc}$ tends to occur when O$_{32}$ is large, but large O$_{32}$ does not necessarily imply efficient Lyman continuum escape.
We present the first results of our pilot study of 8 photometrically selected Lyman continuum (LyC) emitting galaxy candidates from the COSMOS field and focus on their optical emission line ratios. Observations were performed in the H and K bands using the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) instrument at the Keck Observatory, targeting the [OII], H$beta$, and [OIII] emission lines. We find that photometrically selected LyC emitting galaxy candidates have high ionization parameters, based on their high [OIII]/[OII] ratios (O32), with an average ratio for our sample of 2.5$pm$0.2. Preliminary results of our companion Low Resolution Imaging Spectrometer (LRIS) observations, targeting LyC and Ly$alpha$, show that those galaxies with the largest O32 are typically found to also be Ly$alpha$ emitters. High O32 galaxies are also found to have tentative non-zero LyC escape fractions ($f_{esc}(LyC)$) based on $u$ band photometric detections. These results are consistent with samples of highly ionized galaxies, including confirmed LyC emitting galaxies from the literature. We also perform a detailed comparison between the observed emission line ratios and simulated line ratios from density bounded H$_{textrm{II}}$ regions modeled using the photoionization code MAPPINGS V. Estimates of $f_{esc}(LyC)$ for our sample fall in the range from 0.0-0.23 and suggest possible tension with published correlations between O32 and $f_{esc}(LyC)$, adding weight to dichotomy of arguments in the literature. We highlight the possible effects of clumpy geometry and mergers that may account for such tension.
The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S II] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S II]-deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We show that LyC leakers tend to be more [S II]-deficient and that the fraction of their detections increases as [S II]-deficiency becomes more prominent. Correlational studies suggest that [S II]-deficiency complements other LyC diagnostics (such as strong Lyman-$alpha$ emission and high [O III]/[O II]). Our results verify an additional technique by which reionization-era galaxies could be studied.
Identifying the mechanisms driving the escape of Lyman Continuum (LyC) photons is crucial to find Lyman Continuum Emitter (LCE) candidates. To understand the physical properties involved in the leakage of LyC photons, we investigate the connection between the HI covering fraction, HI velocity width, the Lyman alpha (LyA) properties and escape of LyC photons in a sample of 22 star-forming galaxies including 13 LCEs. We fit the stellar continua, dust attenuation, and absorption lines between 920 and 1300 A to extract the HI covering fractions and dust attenuation. Additionally, we measure the HI velocity widths of the optically thick Lyman series and derive the LyA equivalent widths (EW), escape fractions (fesc), peak velocities and fluxes at the minimum of the LyA profiles. Overall, we highlight strong correlations between the presence of low HI covering fractions and (1) low LyA peak velocities; (2) more flux at the profile minimum; and (3) larger EW(LyA), fesc(LyA), and fesc(LyC). Hence, low column density channels are crucial ISM ingredients for the leakage of LyC and LyA photons. Additionally, galaxies with narrower HI absorption velocity widths have higher LyA equivalent widths, larger LyA escape fractions, and lower LyA peak velocity separations. This suggests that these galaxies have low HI column density. Finally, we find that dust regulates the amount of LyA and LyC radiation that actually escapes the ISM. Overall, the ISM porosity is one origin of strong LyA emission and enables the escape of ionizing photons in low-z leakers. However, this is not enough to explain the largest fesc(LyC) observed, which indicates that the most extreme LCEs are likely density-bounded along all lines of sight to the observer. Overall, the neutral gas porosity constrains a lower limit to the escape fraction of LyC and LyA photons, providing a key estimator of the leakage of ionizing photons.
We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of five star-forming galaxies at redshifts z in the range 0.2993-0.4317 and with high emission-line flux ratios O32=[OIII]5007/[OII]3727 ~ 8-27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions fesc(LyC) in a range of 2-72 per cent. A narrow Ly-alpha emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks Vsep varying from ~153 km/s to ~345 km/s. We find a general increase of the LyC escape fraction with increasing O32 and decreasing stellar mass M*, but with a large scatter of fesc(LyC). A tight anti-correlation is found between fesc(LyC) and Vsep making Vsep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.
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