The Low-redshift Lyman-continuum Survey: [S II]-deficiency and the leakage of ionizing radiation
Bingjie Wang, Timothy M. Heckman, Ricardo Amorín, Sanchayeeta Borthakur, John Chisholm, Harry Ferguson, Sophia Flury, Mauro Giavalisco, Andrea Grazian, Matthew … 展开作者
Bingjie Wang, Timothy M. Heckman, Ricardo Amorín, Sanchayeeta Borthakur, John Chisholm, Harry Ferguson, Sophia Flury, Mauro Giavalisco, Andrea Grazian, Matthew Hayes, Alaina Henry, Anne Jaskot, Zhiyuan Ji, Kirill Makan, Stephan McCandliss, M. S. Oey, Göran Östlin, Alberto Saldana-Lopez, Daniel Schaerer, Trinh Thuan, Gábor Worseck, Xinfeng Xu
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-$α$ emission and high [O III]/[O II]). Our results verify an additional technique by which reionization-era galaxies could be studied.
We have recently discovered five Lyman continuum leaking galaxies at z~0.3, selected for their compactness, intense star-formation, and high [OIII]/[OII] ratio (Izotov et al. 2016ab). Here we derive their ionizing photon production efficiency, a fundamental quantity for inferring the number of photons available to reionize the Universe, for the first time for galaxies with confirmed strong Lyman continuum escape (fesc~6-13%). We find an ionizing photon production per unit UV luminosity, which is a factor 2-6 times higher than the canonical value when reported to their observed UV luminosity. A…
While low-luminosity galaxies dominate number counts at all redshifts, their contribution to cosmic Reionization is poorly understood due to a lack of knowledge of their physical properties. We isolate a sample of 35 z~4-5 continuum-faint Lyman-alpha emitters from deep VLT/MUSE spectroscopy and directly measure their Halpha emission using stacked Spitzer/IRAC Ch. 1 photometry. Based on Hubble Space Telescope imaging, we determine that the average UV continuum magnitude is fainter than -16 (~0.01 L_star), implying a median Lyman-alpha equivalent width of 249 Angstroms. By combining the Halpha m…
A population of early star-forming galaxies is the leading candidate for the re-ionization of the universe. It is still unclear what conditions and physical processes would enable a significant fraction of the ionizing photons to escape from these gas-rich galaxies. In this paper we present the results of the analysis of HST COS far-UV spectroscopy plus ancillary multi-waveband data of a sample of 22 low-redshift galaxies that are good analogs to typical star-forming galaxies at high-redshift. We measure three parameters that provide indirect evidence of the escape of ionizing radiation: (1) t…
The connection between the escape fraction of ionizing radiation ($f_{esc}$) and the properties of galaxies, such as stellar mass (M*), age, star-formation rate (SFR), and dust content, are key inputs for reionization models, but many of these relationships remain untested at high redshift. We present an analysis of a sample of 96 z~3 galaxies from the Keck Lyman Continuum Spectroscopic Survey (KLCS). These galaxies have both sensitive Keck/LRIS spectroscopic measurements of the Lyman continuum (LyC) region, and multi-band photometry that places constraints on stellar population parameters. We…
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 conti…
The source responsible for the reionization of the Universe is believed to be the population of star-forming galaxies at $z\sim6$ to 12. The biggest uncertainty concerns the fraction of Lyman-continuum photons that actually escape from the galaxies. In recent years, several relatively small samples of "leaky" galaxies have been uncovered, and clues have begun to emerge as to both the indirect signposts of leakiness and of the conditions/processes that enable the escape of ionizing radiation. In this paper we present the results of a pilot program aimed to test a new technique for finding leaky…
Context: Studies of the production and escape of Lyman Continuum from galaxies often rely on array of indirect observational tracers in preselection of candidate leakers. Aims: Here, we investigate how much ionizing radiation might be missed due to these selection criteria by completely removing them and performing a search selected purely from rest-frame LyC emission; and how that affects our estimates of the ionizing background. Methods: We invert the conventional method and perform a bottom-up search for Lyman-continuum leaking galaxies at redshifts $2 < z < 3.5$. Using archival data from H…
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 i…
Observations of reionization-era analogs at $z\sim3$ are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 $z\sim3$ galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum (LyC) region. We present a method of removing foreground contamination from our sample using high-resolution, multiband Hubble Space Telescope imaging. We re-measure the global pr…
The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to indirectly measure the escape fraction of high-redshift galaxies, it is unlikely that we will know what reionized the universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting ga…
We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs) with deep imaging in the U band obtained with the Large Binocular Camera, mounted on the prime focus of the Large Binocular Telescope. We selected 11 LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields. We stacked the images of our sources in the R and U band, which correspond to an effective rest-frame wavelength of 1500Åand 900Årespectively, obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We der…
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 $z\simeq 3.1$. As many show intense [O III] emission characteristic of $z>6.5$ star-forming galaxies, they may represent valuable low redshift analogs of galaxies in the reionization era. Using HST Wide Field Camera 3 / UVIS $F336W$ to image Lyman continuum emission, we investigate the escape fraction of ionizing photons in this sample. For 61 sources, of which 77% are spectr…
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…
Flux estimates for ionizing radiation escaping from star-forming galaxies with characteristic UV luminosities ($L^{*}_{1500(1+z)}$) derived from GALEX and the VIMOS-VLT Deep Survey, are presented as a function of redshift and assumed escape fraction. These estimates offer guidance to the design of instrumentation and observing strategies, be they spectroscopic or photometric, attempting to detect LyC escaping star-forming galaxies for redshifts $ z < 3$. Examples are given that relate the absolution escape fraction ($f^{e}_{LyC}$) of ionizing photons, integrated over the entire extreme UV (EUV…
The observed scatter in intergalactic Lyman-$α$ opacity at $z \lesssim 6$ requires large-scale fluctuations in the neutral fraction of the intergalactic medium (IGM) after the expected end of reionization. Post-reionization models that explain this scatter invoke fluctuations in either the ionizing ultraviolet background (UVB) or IGM temperature. These models make very different predictions, however, for the relationship between Lyman-$α$ opacity and local density. Here we test these models using Lyman-$α$ emitting galaxies (LAEs) to trace the density field surrounding the longest and most opa…
We have obtained deep Hubble Space Telescope far-UV images of 15 starburst galaxies at z~1.3 in the GOODS fields to search for escaping Lyman continuum photons. These are the deepest far-UV images m_{AB}=28.7, 3σ, 1" diameter) over this large an area (4.83 arcmin^2) and provide the best escape fraction constraints for any galaxy at any redshift. We do not detect any individual galaxies, with 3σlimits to the Lyman Continuum (~700 Å) flux 50--149 times fainter (in f_nu) than the rest-frame UV (1500 Å) continuum fluxes. Correcting for the mean IGM attenuation (factor ~2), as well as an intrinsic …
Understanding the escape of ionizing (Lyman continuum) photons from galaxies is vital for determining how galaxies contributed to reionization in the early universe. While directly detecting Lyman continuum from high redshift galaxies is impossible due to the intergalactic medium, low redshift galaxies in principle offer this possibility, but requirie observations from space. The first local galaxy for which Lyman continuum escape was found is Haro11 , a luminous blue compact galaxy at z=0.02, where observations with the FUSE satellite revealed an escape fraction of 3.3 %. However the FUSE ape…
The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the HI covering fractions and column densities using ultraviolet HI and metal absorption lines of 18 star-forming galaxies which have Lyman series observations. Nine of these galaxies are confirmed LyC emitters. We fit the stellar continuum, dust attenuation, metal, and HI properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen an…
One of the major challenges in observational cosmology is related to the redshift evolution of the average hydrogen ionization in the Universe. In order to probe the ionization level of the IGM and the ionization capabilities of bright QSOs at z=4, we have selected a sample of 2508 QSOs from the SDSS survey (DR14) at 3.6<z<4.6 and -29.0<M1450<-26.0. Starting from UV/optical rest-frame spectra of the whole QSO sample, we estimate the escape fraction and free path individually for each of them. We calculate the Lyman Continuum (LyC) escape fraction as the flux ratio blue-ward (~900 A rest-frame)…
Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high-redshifts. While observations have established that reionization occurred, observational constraints on the emissivity of ionizing photons at high-redshift remains elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and archival observations, a combined sample of 89 star-forming galaxies at z~0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6$σ$ significant) inverse correlation between the continuum slope …
The Lyman-alpha and Continuum Origins Survey II: the connection between the escape of ionizing radiation and Lyman-alpha halos in star-forming galaxies
Stockholm University（斯德哥尔摩大学）；University of Minnesota（明尼苏达大学）；Johns Hopkins University（约翰斯·霍普金斯大学）；Space Telescope Science Institute（太空望远镜科学研究所）；CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, Univ Lyon, Ens de Lyon（法国国家科学研究中心，里昂天体物理研究中心UMR5574，里昂大学，里昂高等师范学校）；Observatoire de Genève, Université de Genève（日内瓦天文台，日内瓦大学）
Subaru/SuprimeCam narrowband photometry of the SSA22 field reveals the presence of four Lyman continuum (LyC) candidates among a sample of 14 AGN. Two show offsets and likely have stellar LyC in nature or are foreground contaminants. The remaining two LyC candidates are Type I AGN. We argue that the average LyC escape fraction of high redshift low luminosity AGN is not likely to be unity, as often assumed in the literature. From direct measurement we obtain the average LyC-to-UV flux density ratio and ionizing emissivity for a number of AGN classes and find it at least a factor of two lower th…
The properties that govern the production and escape of hydrogen ionizing photons (Lyman continuum, LyC; with energies >13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relationship between the hardness of ionizing radiation and the LyC leakage in a large sample of low-$z$ star-forming galaxies from the recent Hubble Space Telescope Low-$z$ Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep XShooter observations, we investigate the hardne…
Compact starburst galaxies are thought to include many or most of the galaxies from which substantial Lyman continuum emission can escape into the intergalactic medium. Li and Malkan (2018) used SDSS photometry to find a population of such starburst galaxies at z~0.5. They were discovered by their extremely strong [OIII]4959+5007 emission lines, which produce a clearly detectable excess brightness in the i bandpass, compared with surrounding filters. We therefore used the HST/COS spectrograph to observe two of the newly discovered i-band excess galaxies around their Lyman limits. One has stron…
Star forming galaxies are believed to be a major source of Lyman Continuum (LyC) radiation responsible for reionizing the early Universe. Direct observations of escaping ionizing radiation have however been few and with low escape fractions. In the local Universe, only ~10 emitters have been observed, with typical escape fractions of a few percent. The mechanisms regulating this escape need to be strongly evolving with redshift in order to account for the Epoch of Reionization. Gas content and star formation feedback are among the main suspects, known to both regulate neutral gas coverage and …
Recent work suggests that strong emission line, star-forming galaxies may be significant Lyman Continuum leakers. We combine archival HST broadband ultraviolet and optical imaging (F275W and F606W, respectively) with emission line catalogs derived from WFC3 IR G141 grism spectroscopy to search for escaping Lyman Continuum (LyC) emission from homogeneously selected $z\simeq$2.5 SFGs. We detect no escaping Lyman Continuum from SFGs selected on [OII] nebular emission (N=208) and, within a narrow redshift range, on [OIII]/[OII]. We measure 1$σ$ upper limits to the LyC escape fraction relative to t…
We present results of a deep spectroscopic survey designed to quantify the statistics of the escape of ionizing photons from star-forming galaxies at z~3. We measure the ratio of ionizing to non-ionizing UV flux density <f900/f1500>_obs, where f900 is the mean flux density evaluated over the range [880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux density by analyzing high-S/N composite spectra formed from sub-samples with common observed properties and numbers sufficient to reduce the statistical uncertainty in the modeled IGM+CGM correction to obtain precise valu…
In this work, we investigate upper limits on the global escape fraction of ionizing photons ($f_{\rm esc/global}^{\rm abs}$) from a sample of galaxies probed for Lyman-continuum (LyC) emission characterized as non-LyC and LyC leakers. We present a sample of 9 clean non-contaminated (by low redshift interlopers, CCD problems and internal reflections of the instrument) galaxies which do not show significant ($>$ $3σ$) LyC flux between 880Å $<λ_{rest}<$ 910Å. The 9 galaxy stacked spectrum reveals no significant LyC flux with an upper limit of $f_{\rm esc}^{\rm abs} \leq 0.06$. In the next step of…
We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity, star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(CIII])$=11.7 \pm 2.9 Å$ rest-frame), although their equivalent widths should be reduced due to the loss of ionizing phot…
Fluctuations in Lyman-$α$ (Ly$α$) forest transmission towards high-$z$ quasars are partially sourced from spatial fluctuations in the ultraviolet background (UVB), the level of which are set by the mean free path of ionizing photons ($λ_{\text{mfp}}$). The auto-correlation function of Ly$α$ forest flux characterizes the strength and scale of transmission fluctuations and, as we show, is thus sensitive to $λ_{\text{mfp}}$. Recent measurements at $z \sim 6$ suggest a rapid evolution of $λ_{\text{mfp}}$ at $z>5.0$ which would leave a signature in the evolution of the auto-correlation function. Fo…
BlastBerries: How Supernovae Affect Lyman Continuum Escape Fractions and Ionizing Photon Production in Local Analogs of High-Redshift Galaxies
University of Hawaiʻi at Hilo（夏威夷希洛大学）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；Flatiron Institute（平坦研究所）；Gemini Observatory（双子座天文台）；NSF NOIRLab（美国国家科学基金会NOIR实验室）；University of California, Davis（加州大学戴维斯分校）；MIT Kavli Institute for Astrophysics and Space Research（麻省理工学院卡弗里天体物理与空间研究所）
We spectroscopically confirmed two narrow-band selected redshift 7.0 Ly$α$ galaxies and studied their restframe UV spectra. The Ly$α$ and other UV nebular lines are very useful to confirm the galactic redshifts and diagnose the different mechanisms driving the ionizing emission. We observed two narrowband-selected $z$=7.0 Ly$α$ candidates in the LAGER Chandra Deep Field South (CDFS) field with IMACS at Magellan telescope and confirmed they are Ly$α$ emitters at $z$=6.924 and 6.931. In one galaxy, we also obtained deep NIR spectroscopy, which yields non-detections of the high-ionization UV nebu…
We use the wealth of deep archival optical spectroscopy on the GOODS-South field from Keck, the VLT, and other facilities to select candidate high-redshift Lyman continuum (LyC) leakers in the Hubble Deep UV Legacy Survey (HDUV) dataset. We select sources at $2.35 < z < 3.05$, where the HST/WFC3 F275W filter probes only the redshifted LyC. We find five moderately F275W-bright sources (four detected at $\gtrsim3σ$ significance) in this redshift range. However, two of these show evidence in their optical spectra for contamination by foreground galaxies along the line-of-sight. We then perform an…
The James Webb Space Telescope (JWST) is expected to enable transformational progress in studying galaxy populations in the very early Universe, during the Epoch of Reionization (EoR). A critical parameter for understanding the sources that reionized the Universe is the Lyman-continuum production efficiency, $ξ_{\rm ion}$, defined as the rate of production of ionizing photons divided by the intrinsic UV luminosity. In this work, we combine self-consistent star formation and chemical enrichment histories predicted by semi-analytic models of galaxy formation with stellar population synthesis (SP…
Identifying non-contaminated sample of high-redshift galaxies with escaping Lyman continuum (LyC) flux is important for understanding the sources and evolution of cosmic reionization. We present CLAUDS $u$-band photometry of the COSMOS field to probe LyC radiation from spectroscopically confirmed galaxies at $z\geq3.5$ and outside the standard Lyman-break galaxy colour selection expectations. Complementary to the CLAUDS data, we use Subaru multi-filter photometry, \textit {Hubble Space Telescope} ($HST$) multi-filter imaging, and the spectroscopic surveys D10K, VUDS and 3D-HST. We present a sa…
Motivated by the claimed detection of a large population of faint active galactic nuclei (AGN) at high redshift, recent studies have proposed models in which AGN contribute significantly to the z > 4 H I ionizing background. In some models, AGN are even the chief sources of reionization. If correct, these models would make necessary a complete revision to the standard view that galaxies dominated the high-redshift ionizing background. It has been suggested that AGN-dominated models can better account for two recent observations that appear to be in conflict with the standard view: (1) large op…
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 th…
Star forming galaxies have long been considered the dominant sources of the cosmic ultraviolet background radiation at early epochs. However, observing and characterizing the galaxy population with significant ionizing emission has proven to be challenging. In particular, the fraction of ionizing radiation that escapes the local environment to the intergalactic medium is poorly known. We investigate the relation between the escape fraction and galaxy luminosity. We combine the deep ultraviolet observations of Hubble Ultra Deep Field (UVUDF) with the deep Multi Unit Spectroscopic Explorer (MUSE…
The high redshift Lyman-alpha forest, in particular the Gunn-Peterson trough, is the most unambiguous signature of the neutral to ionized transition of the intergalactic medium (IGM) taking place during the Epoch of Reionization (EoR). Recent studies, e.g. Kulkarni et al. (2019a) and Keating et al. (2019), showed that reproducing the observed Lyman-alpha opacities after overlap required a non-monotonous evolution of cosmic emissivity: rising, peaking at z=6, and then decreasing onwards to z=4. Such an evolution is puzzling considering galaxy build-up and the cosmic star formation rate are stil…
Questions as to what drove the bulk reionization of the Universe, how that reionization proceeded, and how the hard ionizing radiation reached the intergalactic medium remain open and debated. Observations probing that epoch are severely hampered by the increasing amounts of neutral gas with increasing redshift, so a small, but growing number of experiments are targeting star forming galaxies ($z\sim3$) as proxies. However, these studies, while providing fantastic detail, are time intensive, contain relatively few targets, and can suffer from selection biases. As a complementary alternative, w…
The neutral intergalactic medium above redshift 6 is opaque to ionizing radiation, therefore one needs indirect measurements of the escape fraction of ionizing photons from galaxies of this epoch. Low-ionization state absorption lines are a common feature in the spectrum of galaxies, showing a diversity of strengths and shapes. Since these lines indicate the presence of neutral gas in front of the stars, they have been proposed to carry information on the escape of ionizing radiation from galaxies. We study which processes are responsible for the shape of the absorption lines, to better unders…
Throughout the epoch of reionization the most luminous Lyα emitters are capable of ionizing their own local bubbles. The CR7 galaxy at $z \approx 6.6$ stands out for its combination of exceptionally bright Lyα and HeII 1640 Angstrom line emission but absence of metal lines. As a result CR7 may be the first viable candidate host of a young primordial starburst or direct collapse black hole. High-resolution spectroscopy reveals a +160 km s$^{-1}$ velocity offset between the Lyα and HeII line peaks while the spatial extent of the Lyα emitting region is $\sim 16$ kpc. The observables are indicativ…
We built a grid of photoionization models and compiled already available observational emission line intensities ($\rm 1000 \: < \: λ(Å) \: < \: 2000$) of confirmed star formation regions and Active Galactic Nucleus (AGNs) in order to classify five Ly$α$ emitter (LAE) objects at high redshift $(5.7 \: < \: z \: < \:7.2)$. We selected objects for which at least one metal emission-line was measured. The resulting sample is composed by the objects RXCJ2248.7-4431-ID3, HSCJ233408+004403, COSY, A1703-zd6, and CR7 (clump C). The photoionization models were built assuming a Power Law (associated with…
The fraction of ionizing photons escaping from galaxies, $f_{esc}$, is at the same time a crucial parameter in modelling reionization and a very poorly known quantity, especially at high redshift. Recent observations are starting to constrain the values of $f_{esc}$ in low-z star-forming galaxies, but the validity of this comparison remains to be verified. Applying at high-z the empirical relation between $f_{esc}$ and the UV slope trends derived from the Low-z Lyman Continuum Survey, we use the DELPHI semi-analytical galaxy formation model to estimate the global ionizing emissivity of high-z …
We present Keck/MOSFIRE spectra of the diagnostic nebular emission lines [OIII]5007,4959, [OII]3727, and H-beta for a sample of 15 redshift z=3.1-3.7 Ly-alpha emitters (LAEs) and Lyman break galaxies (LBGs). In conjunction with spectra from other surveys, we confirm earlier indications that LAEs have a much higher [OIII]/[OII] line ratio than is seen in similar redshift LBGs. By comparing their distributions on a [OIII]/[OII] versus R23 diagram, we demonstrate that this difference cannot arise solely because of their lower metallicities but most likely is due to a harder ionizing spectrum. Usi…
Star-forming galaxies are the sources likely to have reionized the universe. As we cannot observe them directly due to the opacity of the intergalactic medium at $z\gtrsim5$, we study $z\sim3\text{--}5$ galaxies as proxies to place observational constraints on cosmic reionization. Using new deep \textit{Hubble Space Telescope} rest-frame UV F336W and F435W imaging (30-orbit, $\sim40$~arcmin$^2$, $\sim29\text{--}30$~mag depth at 5$σ$), we attempt to identify a sample of Lyman continuum galaxies (LCGs). These are individual sources that emit ionizing flux below the Lyman break ($<912~\textÅ$). T…
We present $U_{336}V_{606}J_{125}H_{160}$ follow-up $HST$ observations of 16 $z\sim3$ candidate LyC emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 150…
We study the production rate of ionizing photons of a sample of 588 H$α$ emitters (HAEs) and 160 Lyman-$α$ emitters (LAEs) at $z=2.2$ in the COSMOS field in order to assess the implied emissivity from galaxies, based on their UV luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/$NUV$ data, we find f$_{\rm esc} < 2.8\, (6.4)$% through median (mean) stacking. By combining the H$α$ luminosity density with IGM emissivity measurements from absorption studies, we find a globally averaged $\langle$f$_{\rm esc}\rangle$ of $5.9^{+14.5}_{-4.2}$ % at $z=2.2$ if we assume HAEs are t…
We present the first measurements of the Lyman-continuum photon production efficiency $ξ_{\textrm{ion,0}}$ at $z\sim4$-5 for galaxies fainter than 0.2 $L^*$ ($-$19 mag). $ξ_{\textrm{ion,0}}$ quantifies the production rate of ionizing photons with respect to the UV luminosity density assuming a fiducial escape fraction of zero. Extending previous measurements of $ξ_{\textrm{ion,0}}$ to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe $ξ_{\textrm{ion,0}}$ to such faint magnitudes by taking advantage of 200-hour de…
The James Webb Space Telescope (JWST) NIRSpec instrument will allow rest-frame ultraviolet/optical spectroscopy of galaxies in the epoch of reionization (EoR). Some galaxies may exhibit significant leakage of hydrogen-ionizing photons into the intergalactic medium, resulting in faint nebular emission lines. We present a machine learning framework for identifying cases of very high hydrogen-ionizing photon escape from galaxies based on the data quality expected from potential NIRSpec observations of EoR galaxies in lensed fields. We train our algorithm on mock samples of JWST/NIRSpec data for g…
Recent observations have shown that the scatter in opacities among coeval segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ~ 5.5 requires a short spatially averaged mean free path of < 15 comoving Mpc/h, a factor of > 3 shorter than direct measurements at z ~ 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the glo…
