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Register a new userReconstructing the observed ionizing photon production efficiency at z~2 using stellar population models
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Themiya Nanayakkara Dr
Publication date
2020
fields
Physics
and research's language is
English
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The ionizing photon production efficiency, $xi_{ion}$, is a critical parameter that provides a number of physical constraints to the nature of the early Universe, including the contribution of galaxies to the timely completion of the reionization of the Universe. Here we use KECK/MOSFIRE and ZFOURGE multi-band photometric data to explore the $xi_{ion}$ of a population of galaxies at $zsim2$ with $log_{10}(M_*/M_odot)sim9.0-11.5$. Our 130 Halpha detections show a median $log_{10}(xi_{ion}[Hz/erg])$ of $24.8pm0.5$ when dust corrected using a Calzetti et al. (2000) dust prescription. Our values are typical of mass/magnitude selected $xi_{ion}$ values observed in the $zsim2$ Universe. Using BPASSv2.2.1 and Starburst99 stellar population models with simple parametric star-formation-histories (SFH), we find that even with models that account for effects of stellar evolution with binaries/stellar rotation, model galaxies at $log_{10}(xi_{ion}[Hz/erg])lesssim25.0$ have low H$alpha$ equivalent widths (EW) and redder colors compared to our $zsim2$ observed sample. We find that introducing star-bursts to the SFHs resolve the tension with the models, however, due to the rapid time evolution of $xi_{ion}$, H$alpha$ EWs, and rest-frame optical colors, our Monte Carlo simulations of star-bursts show that random distribution of star-bursts in evolutionary time of galaxies are unlikely to explain the observed distribution. Thus, either our observed sample is specially selected based on their past SFH or stellar models require additional mechanisms to reproduce the observed high UV luminosity of galaxies for a given production rate of hydrogen ionizing photons.
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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 $xi_{ion}$.
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.
We use integral field spectroscopy to study in detail the Wolf-Rayet (WR) population in NGC 3310, spatially resolving 18 star-forming knots with typical sizes of 200-300 pc in the disc of the galaxy hosting a substantial population of WRs. The detected emission in the so-called blue bump is attributed mainly to late-type nitrogen WRs (WNL), ranging from a few dozens to several hundreds of stars per region. Our estimated WNL/(WNL+O) ratio is comparable to reported empirical relations once the extinction-corrected emission is further corrected by the presence of dust grains inside the nebula that absorb a non-negligible fraction of UV photons. Comparisons of observables with stellar population models show disagreement by factors larger than 2-3. However, if the effects of interacting binaries and/or photon leakage are taken into account, observations and predictions tend to converge. We estimate the binary fraction of the hii regions hosting WRs to be significant in order to recover the observed X-ray flux, hence proving that the binary channel can be critical when predicting observables. We also explore the connection of the environment with the current hypothesis that WRs can be progenitors to long-duration gamma-ray bursts (GRBs). Galaxy interactions, which can trigger strong episodes of star formation in the central regions, may be a plausible environment where WRs may act as progenitors of GRBs. Finally, even though the chemical abundance is generally homogeneous, we also find weak evidence for rapid N pollution by WR stellar winds at scales of ~ 200 pc.
Context. Synthetic model atmosphere calculations are still the most commonly used tool when determining precise stellar parameters and stellar chemical compositions. Besides three-dimensional models that consistently solve for hydrodynamic processes, one-dimensional models that use an approximation for convective energy transport play the major role. Aims. We use modern Balmer-line formation theory as well as spectral energy distribution (SED) measurements for the Sun and Procyon to calibrate the model parameter {alpha} that describes the efficiency of convection in our 1D models. Convection was calibrated over a significant range in parameter space, reaching from F-K along the main sequence and sampling the turnoff and giant branch over a wide range of metallicities. This calibration was compared to theoretical evaluations and allowed an accurate modeling of stellar atmospheres. Methods. We used Balmer-line fitting and SED fits to determine the convective efficiency parameter {alpha}. Both methods are sensitive to the structure and temperature stratification of the deeper photosphere. Results. While SED fits do not allow a precise determination of the convective parameter for the Sun and Procyon, they both favor values significantly higher than 1.0. Balmer-line fitting, which we find to be more sensitive, suggests that the convective efficiency parameter {alpha} is $approx$ 2.0 for the main sequence and quickly decreases to $approx$ 1.0 for evolved stars. These results are highly consistent with predictions from 3D models. While the values on the main sequence fit predictions very well, measurements suggest that the decrease of convective efficiency as stars evolve to the giant branch is more dramatic than predicted by models.
The first stars are thought to be one of the dominant sources of hydrogen reionization in the early Universe, with their high luminosities and surface temperatures expected to drive high ionizing photon production rates. In this work, we take our Geneva stellar evolution models of zero-metallicity stars and predict their production rates of photons capable to ionize H, He I and He II, based on a blackbody approximation. We present analytical fits in the range 1.7-500 solar masses. We then explore the impact of stellar initial mass, rotation, and convective overshooting for individual stars. We have found that ionizing photon production rates increase with increasing initial mass. For the rotational velocities considered we see changes of up to 25% to ionizing photons produced. This varies with initial mass and ionizing photon species and reflects changes to surface properties due to rotation. We have also found that higher convective overshooting increases ionizing photon production by approximately 20% for the change in overshooting considered here. For stellar populations, we explore how the production of ionizing photons varies as a function of the initial mass function (IMF) slope, and minimum and maximum initial masses. For a fixed population mass we have found changes of the order of 20-30% through varying the nature of the IMF. This work presents ionizing photon production predictions for the most up to date Geneva stellar evolution models of Population III stars, and provides insight into how key evolutionary parameters impact the contribution of the first stars to reionization.
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