Lyman Continuum Photon Production Research Articles

High Equivalent Width of Hα+[N ii] Emission in z ∼ 8 Lyman-break Galaxies from IRAC 5.8 μm Observations: Evidence for Efficient Lyman-continuum Photon Production in the Epoch of Reionization

We measure, for the first time, the median equivalent width (EW) of Hα+[N ii] in star-forming galaxies at z ∼ 8. Our estimate leverages the unique photometric depth of the Spitzer/IRAC 5.8 μm band mosaics (probing ≈5500–7100 Å at z ∼ 8) of the GOODS Reionization Era Wide Area Treasury from Spitzer (GREATS) program. We median-stacked the stamps of 102 Lyman-break galaxies in the 3.6, 4.5, 5.8, and 8.0 μm bands, after carefully removing potential contamination from neighboring sources. We infer an extreme rest-frame EW0(Hα+[N ii]) = Å from the measured red [3.6] − [5.8] = 0.82 ± 0.27 mag, consistent with young (≲107 yr) average stellar population ages at z ∼ 8. This implies an ionizing photon production efficiency of . Such a high value for photoproduction, similar to the highest values found at z ≲ 4, indicates that only modest escape fractions f esc ≲ 0.3 (at 2σ) are sufficient for galaxies brighter than M UV < −18 mag to reionize the neutral hydrogen at z ∼ 8. This requirement is relaxed even more to f esc ≤ 0.1 when considering galaxies brighter than M UV ≈ −13 mag, consistent with recent luminosity functions and as typically assumed in studies addressing reionization. These exceptional results clearly indicate that galaxies can be the dominant source of reionizing photons, and provide us with an exciting glimpse into what we might soon learn about the early universe, and particularly about the reionization epoch, from forthcoming JWST/MIRI and NIRCam programs.

Definitive upper bound on the negligible contribution of quasars to cosmic reionization

Cosmic (hydrogen) reionization marks one of the major phase transitions of the universe at redshift z >= 6. During this epoch, hydrogen atoms in the intergalactic medium (IGM) were ionized by Lyman continuum (LyC) photons. However, it remains challenging to identify the major sources of the LyC photons responsible for reionization. In particular, individual contributions of quasars (or active galactic nuclei, AGNs) and galaxies are still under debate. Here we construct the far-ultraviolet (far-UV) luminosity function for type 1 quasars at z >= 6 that spans 10 magnitudes (-19 < M_UV < -29), conclusively showing that quasars made a negligible contribution to reionization. We mainly search for quasars in the low-luminosity range of M_UV > -23 mag that is critical to determine quasars' total LyC photon production but has been barely explored previously. We find that the quasar population can only provide less than 7% (95% confidence level) of the total photons needed to keep the universe ionized at z = 6.0 - 6.6. Our result suggests that galaxies, presumably low-luminosity star-forming systems, are the major sources of hydrogen reionization.

The mean HαEW and Lyman-continuum photon production efficiency for faintz≈ 4−5 galaxies

We present the first measurements of the Lyman-continuum photon production efficiencyξion, 0atz ∼ 4–5 for galaxies fainter than 0.2L*(−19 mag).ξion, 0quantifies the production rate of ionizing photons with respect to the UV luminosity density assuming a fiducial escape fraction of zero. Extending previous measurements ofξion, 0to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probeξion, 0to such faint magnitudes by taking advantage of 200-h depthSpitzer/IRAC observations from the GREATS program and ≈300 3 &lt; z &lt; 6 galaxies with spectroscopic redshifts from the MUSE GTO Deep + Wide programs. Stacked IRAC [3.6]−[4.5] colors are derived and used to infer the Hαrest-frame equivalent widths, which range from 403 Å to 2818 Å. The derivedξion, 0is log10(ξion,0/Hz erg−1) = 25.36 ± 0.08 over −20.5 &lt; MUV &lt; −17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting thatξionshows no strong dependence onMUV. Theξion, 0values found in our sample imply that the Lyman-continuum escape fraction forMUV ≈ −19 star-forming galaxies cannot exceed ≈8–20% in the reionization era.

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.

THE LYMAN-CONTINUUM PHOTON PRODUCTION EFFICIENCY ξION OF z ∼ 4–5 GALAXIES FROM IRAC-BASED Hα MEASUREMENTS: IMPLICATIONS FOR THE ESCAPE FRACTION AND COSMIC REIONIZATION

ABSTRACT Galaxies represent one of the preferred candidate sources to drive the reionization of the universe. Even as gains are made in mapping the galaxy UV luminosity density to , significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency and the escape fraction f esc. As we show here, the first of these unknowns is directly measurable in z = 4–5 galaxies based on the impact the Hα line has on the observed IRAC fluxes. By computing a LyC photon production rate from the implied Hα luminosities for a broad selection of z = 4–5 galaxies and comparing this against the dust-corrected UV-continuum luminosities, we provide the first-ever direct estimates of the LyC photon production efficiency for the galaxy population. We find to have a mean value of and for sub-L* z = 4–5 galaxies adopting Calzetti and SMC dust laws, respectively. Reassuringly, both derived values are consistent with the standard assumed 's in reionization models, with a slight preference for higher 's (by ∼0.1 dex) adopting the SMC dust law. High values of (∼25.5–25.8 dex) are derived for the bluest galaxies ( ) in our samples, independent of dust law and consistent with results for a z = 7.045 galaxy. Such elevated values of would have important consequences, indicating that f esc cannot be in excess of 13% for standard assumptions about the faint-end cut-off to the LF and the clumping factor.

The photometric properties of galaxies in the early Universe

We use the large cosmological hydro-dynamic simulation BlueTides to predict the photometric properties of galaxies during the epoch of reionisation ($z=8-15$). These properties include the rest-frame UV to near-IR broadband spectral energy distributions, the Lyman continuum photon production, the UV star formation rate calibration, and intrinsic UV continuum slope. In particular we focus on exploring the effect of various modelling assumptions, including the assumed choice of stellar population synthesis model, initial mass function, and the escape fraction of Lyman continuum photons, upon these quantities. We find that these modelling assumptions can have a dramatic effect on photometric properties leading to consequences for the accurate determination of physical properties from observations. For example, at $z=8$ we predict that nebular emission can account for up-to $50\%$ of the rest-frame $R$-band luminosity, while the choice of stellar population synthesis model can change the Lyman continuum production rate up to a factor of $\times 2$.

The Lyman-continuum photon production efficiency in the high-redshift Universe

Abstract The Lyman-continuum photon production efficiency (ξion) is a critical ingredient for inferring the number of photons available to reionize the intergalactic medium. To estimate the theoretical production efficiency in the high-redshift Universe we couple the BlueTides cosmological hydrodynamical simulation with a range of stellar population synthesis models. We find Lyman-continuum photon production efficiencies of log10(ξion/erg−1 Hz) ≈ 25.1–25.5 depending on the choice of stellar population synthesis model. These results are broadly consistent with recent observational constraints at high-redshift though favour a model incorporating the effects of binary evolution.

The epoch of reionization in theRh = ctuniverse

The measured properties of the epoch of reionization (EoR) show that reionization probably began around z ~ 12-15 and ended by z=6. In addition, a careful analysis of the fluctuations in the cosmic microwave background indicate a scattering optical depth tau ~ 0.066+/-0.012 through the EoR. In the context of LCDM, galaxies at intermediate redshifts and dwarf galaxies at higher redshifts now appear to be the principal sources of UV ionizing radiation, but only for an inferred (ionizing) escape fraction f_ion ~ 0.2, which is in tension with other observations that suggest a value as small as ~ 0.05. In this paper, we examine how reionization might have progressed in the alternative Friedmann-Robertson Walker cosmology known as the R_h=ct Universe, and determine the value of f_ion required with this different rate of expansion. We find that R_h=ct accounts quite well for the currently known properties of the EoR, as long as its fractional baryon density falls within the reasonable range 0.026 < Omega_b < 0.037. This model can also fit the EoR data with f_ion ~ 0.05, but only if the Lyman continuum photon production is highly efficient and Omega_b ~ 0.037. These results are still preliminary, however, given their reliance on a particular form of the star-formation rate density, which is still uncertain at very high redshifts. It will also be helpful to reconsider the EoR in R_h=ct when complete structure formation models become available.

REIONIZATION AFTERPLANCK: THE DERIVED GROWTH OF THE COSMIC IONIZING EMISSIVITY NOW MATCHES THE GROWTH OF THE GALAXY UV LUMINOSITY DENSITY

Thomson optical depth tau measurements from Planck provide new insights into the reionization of the universe. In pursuit of model-independent constraints on the properties of the ionising sources, we determine the empirical evolution of the cosmic ionizing emissivity. We use a simple two-parameter model to map out the evolution in the emissivity at z>~6 from the new Planck optical depth tau measurements, from the constraints provided by quasar absorption spectra and from the prevalence of Ly-alpha emission in z~7-8 galaxies. We find the redshift evolution in the emissivity dot{N}_{ion}(z) required by the observations to be d(log Nion)/dz=-0.15(-0.11)(+0.08), largely independent of the assumed clumping factor C_{HII} and entirely independent of the nature of the ionising sources. The trend in dot{N}_{ion}(z) is well-matched by the evolution of the galaxy UV-luminosity density (dlog_{10} rho_UV/dz=-0.11+/-0.04) to a magnitude limit >~-13 mag, suggesting that galaxies are the sources that drive the reionization of the universe. The role of galaxies is further strengthened by the conversion from the UV luminosity density rho_UV to dot(N)_{ion}(z) being possible for physically-plausible values of the escape fraction f_{esc}, the Lyman-continuum photon production efficiency xi_{ion}, and faint-end cut-off $M_{lim}$ to the luminosity function. Quasars/AGN appear to match neither the redshift evolution nor normalization of the ionizing emissivity. Based on the inferred evolution in the ionizing emissivity, we estimate that the z~10 UV-luminosity density is 8(-4)(+15)x lower than at $z~6, consistent with the observations. The present approach of contrasting the inferred evolution of the ionizing emissivity with that of the galaxy UV luminosity density adds to the growing observational evidence that faint, star-forming galaxies drive the reionization of the universe.

VLA H53α Observations of the Central Region of the Super Star Cluster Galaxy NGC 5253

We present observations in the H53α line and radio continuum at 43 GHz carried out with the VLA in the D configuration (2'' angular resolution) toward the starburst galaxy NGC 5253. VLA archival data have been reprocessed to produce a uniform set of 2, 1.3, and 0.7 cm high angular (0.2'' × 0.1'') radio continuum images. The radio recombination line (RRL) H53α, a previously reported measurement of the H92α RRL flux density, and the reprocessed high angular resolution radio continuum flux densities have been modeled using a collection of H II regions. Based on the models, the ionized gas in the nuclear source has an electron density of ~6 × 104 cm-3 and a volume filling factor of 0.05. A Lyman continuum photon production rate of 2 × 1052 s-1 is necessary to sustain the ionization in the nuclear region. The number of required O7 stars in the central 1.5 pc of the supernebula is ~2000. The H53α velocity gradient (10 km s-1 arcsec-1) implies a dynamical mass of ~3 × 105 M☉; this mass suggests that the supernebula is confined by gravity.

Starburst in the Ultraluminous Galaxy Arp 220: Constraints from Observations of Radio Recombination Lines and Continuum

We present observations of radio recombination lines (RRL) from the starburst galaxy Arp 220 at 8.1 GHz (H92a) and 1.4 GHz (H167a and H165a) and at 84 GHz (H42a), 96 GHz (H40a) and 207 GHz (H31a) using the Very Large Array and the IRAM 30 m telescope, respectively. RRLs were detected at all the frequencies except 1.4 GHz, where a sensitive upper limit was obtained. We also present contin- uum —ux measurements at these frequencies as well as at 327 MHz made with the VLA. The continuum spectrum, which has a spectral index a D (0.6 between 5 and 10 GHz, shows a break near 1.5 (S l P la) GHz, a prominent turnover below 500 MHz, and a —atter spectral index above 50 GHz. We show that a model with three components of ionized gas with diUerent densities and area covering factors can consis- tently explain both RRL and continuum data. The total mass of ionized gas in the three components is 3.2 ) 107 requiring 3 ) 105 O5 stars with a total Lyman continuum production rate M _ , N Lyc D 1.3 ) 1055 photons s~1. The ratio of the expected to observed Bra and Brc —uxes implies a dust extinction mag. The derived Lyman continuum photon production rate implies a continuous star forma- A V D 45 tion rate (SFR) averaged over the lifetime of OB stars of D240 yr~1. The Lyman continuum photon M _ production rate of D3% associated with the high-density H II regions implies a similar SFR at recent epochs (t 105 yr). An alternative model of high-density gas, which cannot be excluded on the basis of the available data, predicts 10 times higher SFR at recent epochs. If con—rmed, this model implies that star formation in Arp 220 consists of multiple starbursts of very high SFR (few times 103 yr~1) and M _ short duration (D105 yr). The similarity of IR excess, in Arp 220 to values observed in L IR /L Lya D 24, starburst galaxies shows that most of the high luminosity of Arp 220 is due to the ongoing starburst rather than to a hidden active galactic nucleus (AGN). A comparison of the IR excesses in Arp 220, the Galaxy, and M33 indicates that the starburst in Arp 220 has an initial mass function that is similar to that in normal galaxies and has a duration longer than 107 yr. If there was no infall of gas during this period, then the star formation efficiency (SFE) in Arp 220 is D50%. The high SFR and SFE in Arp 220 is consistent with their known dependences on mass and density of gas in star-forming regions of normal galaxies. Subject headings: galaxies: individual (Arp 220) ¨ galaxies: nucleigalaxies: starburst ¨ radio continuum: galaxiesradio lines: galaxies