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Register a new userThe ionizing photon production efficiency of compact z~0.3 Lyman continuum leakers and comparison with high redshift galaxies
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D. Schaerer
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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. After correction for extinction this value is close to the canonical value. The properties of our five Lyman continuum leakers are found to be very similar to those of the confirmed z=3.218 leaker Ion2 from de Barros et al. (2016) and very similar to those of typical star-forming galaxies at z>~6. Our results suggest that UV bright galaxies at high-z such as Lyman break galaxies can be Lyman continuum leakers and that their contribution to cosmic reionization may be underestimated.
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The Lyman Continuum photon production efficiency ($xi_{rm ion}$) is a critical ingredient for inferring the number of photons available to reionise 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 $log_{10}(xi_{rm ion}/{rm erg^{-1}, Hz})approx 25.1-25.5$ 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
We measure the ionizing photon production efficiency ($xi_{ion}$) of low-mass galaxies ($10^{7.8}$-$10^{9.8}$ $M_{odot}$) at $1.4<z<2.7$, allowing us to better understand the contribution of dwarf galaxies to the ionizing background and cosmic reionization. We target galaxies that are magnified by the strong lensing galaxy clusters Abell 1689, MACS J0717, and MACS J1149. We utilize Keck/MOSFIRE spectra to measure optical nebular emission line fluxes and HST imaging to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log(L$_{Halpha}$ /L$_{UV}$) of galaxies in our sample to determine the typical log($xi_{ion}$). Second, we take the logarithm of the total L$_{Halpha}$ over the total L$_{UV}$. We prefer the latter as it provides the total ionizing UV luminosity density of galaxies when multiplied by the non-ionizing UV luminosity density from the UV luminosity function. log($xi_{ion}$) calculated from the second method is $sim$ 0.2 dex higher than the first method. We do not find any strong dependence between log($xi_{ion}$) and stellar mass, M$_{UV}$ or UV spectral slope ($beta$). We report a value of log($xi_{ion}$) $sim25.47pm 0.09$ for our UV-complete sample ($-22<M_{UV}<-17.3$) and $sim25.37pm0.11$ for our mass-complete sample ($7.8<log(M_*)<9.8)$. These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log($xi_{ion}$) is $0.2-0.3$ dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity, age, or the prevalence of binary stars. We also find a correlation between log($xi_{ion}$) and the equivalent widths of H$alpha$ and [OIII]$lambda$5007 fluxes, confirming that these equivalent widths can be used to estimate $xi_{ion}$.
An analysis of the dynamics of a star formation event is performed. It is shown that galaxies able to drive leftover gas to sufficient altitudes in a few million years are characterized by two basic properties: small sizes (<1kpc) and high star formation rate surface densities (Sigma_SFR > 10 Msun/yr/kpc2). For the parameter space of relevance, the outflow is primarily driven by supernovae with radiation pressure being significant but subdominant. Our analysis provides the unifying physical origin for a diverse set of observed LyC leakers, including the green-peas galaxies, [SII]-weak galaxies, Lyman-alpha emitters, with these two characteristics as the common denominator. Among verifiable physical properties of LyC leakers, we predict that (1) the newly formed stellar masses are are typically in the range of 1e8-1e10 Msun, except perhaps ULIRGs, (2) the outflow velocities are typically in the range typically of 100-600km/s, but may exceed 1e3 km/s in ULIRGs, with a strong positive correlation between the stellar masses formed and the outflow velocities, (3) the overall escape fraction of galaxies is expected to increase with increasing redshift, given the cosmological trend that galaxies become denser and more compact with increasing redshift. In addition, two interesting by-product predictions are also borne out. First, ULIRGs appear to be in a parameter region where they should be prodigious LyC leakers, unless there is a large ram-pressure. Second, Lyman break galaxies (LBGs) are not supposed to be prodigious LyC leakers in our model, given their claimed effective radii exceeding 1kpc.
We present the first measurements of the Lyman-continuum photon production efficiency $xi_{textrm{ion,0}}$ at $zsim4$-5 for galaxies fainter than 0.2 $L^*$ ($-$19 mag). $xi_{textrm{ion,0}}$ 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 $xi_{textrm{ion,0}}$ to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe $xi_{textrm{ion,0}}$ to such faint magnitudes by taking advantage of 200-hour depth Spitzer/IRAC observations from the GREATS program and $approx$300 3<$z$<6 galaxies with spectroscopic redshifts from the MUSE GTO Deep + Wide programs. Stacked IRAC [3.6]$-$[4.5] colors are derived and used to infer the H$alpha$ rest-frame equivalent widths, which range from 403r{A} to 2818r{A}. The derived $xi_{textrm{ion,0}}$ is $log_{10}(xi_{textrm{ion,0}} / textrm{Hz erg}^{-1}) = 25.36 pm 0.08$ over $-$20.5 < M$_{textrm{UV}}$ < $-$17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that $xi_{textrm{ion}}$ shows no strong dependence on $M_{UV}$. The $xi_{textrm{ion,0}}$ values found in our sample imply that the Lyman-continuum escape fraction for $M_{textrm{UV}} approx -19$ star-forming galaxies cannot exceed $approx$8-20% in the reionization era.
Following our first detection reported in Izotov et al. (2016), we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727 > 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ~6%-13%, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%, among the highest known for Lyalpha emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low metallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star formation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35 Msun/yr/kpc^2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al. (2016), reveal that a selection for compact star-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.
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