Large Escape Fraction Research Articles

An empirical reionization history model inferred from the low-redshift Lyman continuum survey and the star-forming galaxies at z > 8

ABSTRACT We present a new analysis of the rest-frame ultraviolet (UV) and optical spectra of a sample of three z > 8 galaxies discovered behind the gravitational lensing cluster RX J2129.4+0009. We combine these observations with z > 7.5 galaxies from the literature, for which similar measurements are available. As already pointed out in other studies, the high [O iii]λ5007/[O ii]λ3727 ratios (O32) and steep UV continuum slopes (β) are consistent with the values observed for low-redshift Lyman continuum emitters, suggesting that such galaxies contribute to the ionizing budget of the intergalactic medium. We construct a logistic regression model to estimate the probability of a galaxy being a Lyman continuum emitter based on the measured MUV, β, and O32. Using this probability and the UV luminosity function, we construct an empirical model that estimates the contribution of high-redshift galaxies to reionization. The preferred scenario in our analysis shows that at z ∼ 8, the average escape fraction of the galaxy population [i.e. including both LyC emitters (LCEs) and non-emitters] varies with MUV, with intermediate UV luminosity (−19 < MUV < −16) galaxies having larger escape fraction. Galaxies with faint UV luminosity (−16 < MUV < −13.5) contribute most of the ionizing photons. The relative contribution of faint versus bright galaxies depends on redshift, with the intermediate UV galaxies becoming more important over time. UV bright galaxies, although more likely to be LCEs at a given log(O32) and β, contribute the least of the total ionizing photon budget.

Insights into the reionization epoch from cosmic-noon-C IV emitters in the VANDELS survey

Recently, intense emission from nebular C III] and C IV emission lines have been observed in galaxies in the epoch of reionization (z > 6) and have been proposed as the prime way of measuring their redshift and studying their stellar populations. These galaxies might represent the best examples of cosmic reionizers, as suggested by recent low-z observations of Lyman continuum emitting galaxies, but it is hard to directly study the production and escape of ionizing photons at such high redshifts. The ESO spectroscopic public survey VANDELS offers the unique opportunity to find rare examples of such galaxies at cosmic noon (z ∼ 3), thanks to the ultra deep observations available. We have selected a sample of 39 galaxies showing C IV emission, whose origin (after a careful comparison to photoionization models) can be ascribed to star formation and not to active galactic nuclei. By using a multiwavelength approach, we determined their physical properties including metallicity and the ionization parameter and compared them to the properties of the parent population to understand what the ingredients are that could characterize the analogs of the cosmic reionizers. We find that C IV emitters are galaxies with high photon production efficiency and there are strong indications that they might also have a large escape fraction: given the visibility of C IV in the epoch of reionization, this could become the best tool to pinpoint the cosmic reioinzers.

The role of galaxies and AGNs in reionizing the IGM – II. Metal-tracing the faint sources of reionization at 5 ≲z≲ 6

We present a new method to study the contribution of faint sources to the UV background using the 1D correlation of metal absorbers with the intergalactic medium (IGM) transmission in a Quasi Stellar Object (QSO) sightline. We take advantage of a sample of $25$ high signal-to-noise ratio QSO spectra to retrieve $150$ triply-ionised carbon (\cfour) absorbers at $4.5\lesssim z\lesssim 6.2$, of which $37$ systems whose expected H{~\small I} absorption lie in the Lyman-$\alpha$ forest. We derive improved constraints on the cosmic density of \cfour \,at $4.3< z < 6.2$ and infer from abundance-matching that \cfour \,absorbers trace $M_{\text{UV}}\lesssim -16$ galaxies. Correlation with the Lyman-$\alpha$ forest of the QSOs indicates that these objects are surrounded by a highly opaque region at $r\lesssim 5 $ cMpc/h followed by an excess of transmission at $r\gtrsim 10$ cMpc/h detected at $2.7\sigma$. This is in contrast to equivalent measurements at lower redshifts where only the opaque trough is detected. We interpret this excess as a statistical enhancement of the local photoionisation rate due to clustered faint galaxies around the \cfour \,absorbers. Using the analytical framework described in Paper I of this series, we derive a constraint on the average product of the escape fraction and the Lyman continuum photon production efficiency of the galaxy population clustered around the \cfour \,absorbers, $\log \langle f_{\text{esc}}\xi_{\text{ion}}\rangle /[{\rm erg^{-1}~Hz}] = 25.01^{+0.30}_{-0.19}$. This implies that faint galaxies beyond the reach of current facilities may have harder radiation fields and/or larger escape fractions than currently detected objects at the end of the reionisation epoch.

DISSECTION OF Hα EMITTERS : LOW-zANALOGS OFz&gt; 4 STAR-FORMING GALAXIES

Strong Halpha Emitters (HAEs) dominate the z~4 Lyman-break galaxy population. We have identified local analogs of these HAEs using the Sloan Digital Sky Survey (SDSS). At z<0.4, only 0.04% of galaxies are classified as HAEs with Halpha equivalent widths (>500A) comparable to that of z~4 HAEs. Local HAEs have lower stellar mass and lower ultraviolet (UV) luminosity than z~4 HAEs, yet the Halpha-to-UV luminosity ratio as well as their specific star-formation rate is consistent with that of z~4 HAEs indicating that they are scaled down versions of high-z star-forming galaxies. Compared to the previously studied local analogs of z~2 Lyman break galaxies selected using rest-frame UV, local HAEs show similar UV luminosity surface density, weaker Dn(4000) breaks, lower metallicity and lower stellar mass. This supports the idea that local HAEs are less evolved galaxies than the traditional Lyman break analogs. We are not able to constrain if the star-formation history in local HAEs is powered by mergers or by cosmological cold flow accretion. However, in the stacked spectrum, local HAEs show a strong HeII4686 emission line suggesting a population of young (<10Myr), hot, massive stars similar to that seen in some Wolf-Rayet galaxies. Low [NII]/[OIII] line flux ratios imply that local HAEs are inconsistent with being systems that host bright AGN. Instead, it is highly likely that local HAEs are galaxies with an elevated ionization parameter, either due to a high electron density or large escape fraction of hydrogen ionizing photons as in the case for Wolf-Rayet galaxies.

VERY BLUE UV-CONTINUUM SLOPE β OF LOW LUMINOSITYz∼ 7 GALAXIES FROM WFC3/IR: EVIDENCE FOR EXTREMELY LOW METALLICITIES?

We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope beta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to reduce the red nebular emission are likely required. For example, a large escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z~7 galaxies is >~0.3, it may help to explain how galaxies reionize the universe.

LARGE AREA SURVEY FOR z = 7 GALAXIES IN SDF AND GOODS-N: IMPLICATIONS FOR GALAXY FORMATION AND COSMIC REIONIZATION*

We present results of our large-area survey for z'-band dropout galaxies at z=7 in a 1568 arcmin^2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z'- and y-band (lambda_eff=1um) images with legacy data of Subaru and HST, we have identified 22 bright z-dropout galaxies down to y=26, one of which has a spectroscopic redshift of z=6.96 determined from Lya emission. The z=7 luminosity function (LF) yields the best-fit Schechter parameters of phi*=0.69 +2.62/-0.55 x10^(-3) Mpc^(-3), Muv*=-20.10 +/-0.76 mag, and alpha=-1.72 +/-0.65, and indicates a decrease from z=6 at a >95% confidence level. This decrease is beyond the cosmic variance in our two fields, which is estimated to be a factor of <~2. We have found that the cosmic star formation rate density drops from the peak at z=2-3 to z=7 roughly by a factor of ~10 but not larger than ~100. A comparison with the reionization models suggests either that the Universe could not be totally ionized by only galaxies at z=7, or more likely that properties of galaxies at z=7 are different from those at low redshifts having, e.g., a larger escape fraction (>~0.2), and/or a flatter IMF. Our SDF z-dropout galaxies appear to form 60-Mpc long filamentary structures, and the z=6.96 galaxy with Lya emission is located at the center of an overdense region consisting of four UV bright dropout candidates, which might suggest an existence of a well-developed ionized bubble at z=7.

Early reionization by the first galaxies

Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, tau_e ~ 0.17. The reionization of the universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results. Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with a standard stellar IMF, with a moderate photon escape fraction from galaxies and with ionizing photon production as expected for zero metallicity stars produces tau_e = 0.104, which is within 1.0 to 1.5-sigma of the ``best'' WMAP value. Values up to 0.16 can be produced by taking larger escape fractions or a top heavy IMF. The data do not require a separate populations of ``miniquasars'' or of stars forming in objects with total masses below 10^9 M_sun. Reconciling such early reionization with the observed Gunn-Peterson troughs in z>6 quasars may be challenging. Possible resolutions of this problem are discussed.

Connections between the Cosmic Baryon Fraction, the Extragalactic Ionizing Background, and Lyman Break Galaxies

We reassess constraints on the cosmological baryon density from observations of the mean decrement and power spectrum of the Ly? forest, taking into account uncertainties in all free parameters in the simplest gravitational instability model. The uncertainty is dominated by that of the photoionizing background, but incomplete knowledge of the thermal state of the intergalactic medium also contributes significantly to the error budget. While current estimates of the baryon fraction from the forest do prefer values that are somewhat higher than the big bang nucleosynthesis value of ?b h2 = 0.02 ? 0.001, the discrepancy is at most about 3 ?. For instance, assuming the highest estimate of the ionizing background, as indicated by recent measurements of a large escape fraction from Lyman break galaxies by Steidel, Pettini, & Adelberger, we find ?bh2 = 0.045 ? 0.008. A recent measurement of the ionizing background from the proximity effect by Scott et al., on the other hand, implies ?bh2 = 0.03 ? 0.01. We provide an expression from which future likelihoods for ?b h2 can be derived as measurements of the ionizing background improve?consistency among constraints from the forest, nucleosynthesis, and the microwave background will provide a powerful test of the gravitational instability model for the forest and for large-scale structure in general. We also develop a formalism that treats lower bounds on the baryon density in a statistical manner, which is appropriate if only a lower bound on the ionizing background is known. Finally, we discuss the implications of the escape fraction measurement for the age, structure, and stellar content of Lyman break galaxies. We show that the observed hard spectrum from Lyman break galaxies requires a very young age (less than about 1 million years) and/or a top-heavy initial mass function. We also build a model in which an extended (non-disk-like) gas distribution allows a large escape fraction.

The Escape of Ionizing Photons from the Galaxy

The Magellanic Stream and several high velocity clouds have now been detected in optical line emission. The observed emission measures and kinematics are most plausibly explained by photoionization due to hot, young stars in the Galactic disk. The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons, F_esc, which escape the disk. We have modelled the production and transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires F_esc = 6%, in reasonable agreement with the flux required to ionize the Reynolds layer. Neither shock heating nor emission within a hot Galactic corona can be important in producing the observed H-alpha emission. If such a large escape fraction is typical of L_* galaxies, star-forming systems dominate the extragalactic ionizing background. Within the context of this model, both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive H-alpha observations.