The Ionizing Photon Production Efficiency (ξ_ion) of Lensed Dwarf Galaxies at z ∼ 2*

Abstract

We measure the ionizing photon production efficiency (ξ ion) of low-mass galaxies (10 7.8-10 9.8 M e) at 1.4<z<2.7 to better understand the contribution of dwarf galaxies to the ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of Hα-to-UV luminosity ratios (L Hα /L UV) of galaxies to determine the standard log(ξ ion). Second, we take the logarithm of the total L Hα over the total L UV. We prefer the latter, as it provides the total ionizing UV luminosity density of galaxies when multiplied by the nonionizing UV luminosity density. log(ξ ion) calculated from the second method is ∼0.2 dex higher than the first method. We do not find any strong dependence between log(ξ ion) and stellar mass, far-UV magnitude (M UV), or UV spectral slope (β). We report a value of log(ξ ion) ∼ 25.47±0.09 for our UV-complete sample (-< <-M 22 17.3 UV) and ∼25.37±0.11 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(ξ 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 or age. We also find a correlation between log(ξ ion) and the equivalent widths of Hα and [O III] λ5007 fluxes, confirming that these equivalent widths can be used to estimate ξ ion .

The Effects of Stellar Population and Gas Covering Fraction on the Emergent Lyman Alpha Emission of High-Redshift Galaxies

“…where N (H 0 ) is the ionizing photon rate in s −1 and L FUV is the luminosity density at 1500 Å in erg s −1 Hz −1 . ξ ion is typically constrained by combining dust-corrected Hα and FUV luminosities (e.g., Bouwens et al 2016a;Matthee et al 2017;Shivaei et al 2018;Emami et al 2020). In this work, ξ ion was computed directly from the best-fit intrinsic SPSneb model, a route that obviates the need to apply potentially uncertain dust corrections to Hα and FUV luminosities (Shivaei et al 2018), account for LyC photons that may escape the ISM or be absorbed by dust (Inoue 2001), or implement conversions between Hα luminosity and N (H 0 ) that may not apply to the galaxies in question (e.g., Reddy et al 2016b;Theios et al 2019).…”

The [OIII]$\lambda5007$ equivalent width distribution at z $\sim2$: The redshift evolution of the extreme emission line galaxies

“…In this paper, we have derived the [OIII]𝜆5007 EW distribution in z 2 UV-selected galaxies, thereby quantifying the fraction of EELGs at z 2. Locally (z ∼ 0) and at intermediate redshifts (1 < z < 3), EELGs have been shown to be extremely efficient ionisers, both due to their large ionisation production efficiencies (Chevallard et al 2018;Tang et al 2019;Nakajima et al 2020;Emami et al 2020) and their large escape fractions (e.g. Jaskot et al 2019;Nakajima et al 2020;Izotov et al 2016;Vanzella et al 2016;Fletcher et al 2019).…”

Low-redshift compact star-forming galaxies as analogues of high-redshift star-forming galaxies

“…8b, we show the relation Ne3O2 -O 32 for CSFGs (blue and red dots) (Shivaei et al 2018) for Calzetti et al (1994Calzetti et al ( , 2000 and SMC reddening laws are shown in (a) by green and magenta lines, respectively. Mean ξ ion values of z = 4 -5 galaxies (Lam et al 2019a) (Chevallard et al 2018), z ∼ 2 lensed galaxies (Emami et al 2020), and z ∼ 1.4 -3.8 galaxies (Reddy et al 2018), respectively; LAEs at z ∼ 3 (Nakajima et al 2020) and z ∼ 6.6 SFGs with Lyα emission (Endsley et al 2020) and z ∼ 2 MOSDEF SFGs (black-filled circles). Both Ne3O2 and O 32 were proposed by Levesque & Richardson (2014) as estimators for the ionisation parameter U in local SFGs.…”

“…Together with the escape fraction of ionising photons from galaxies, the ionising photon production efficiency ξ ion is one of the important parameters characterising the ability of highz SFGs to ionise the intergalactic medium during the epoch of reionisation at z > ∼ 6. The ionising photon production efficiency ξ ion has been derived in several studies for high-z SFGs, with log(ξ ion /[Hz erg −1 ]) values in the range of ∼ 25.1 -25.8 (Bouwens et al 2016;Harikane et al 2018;De Barros et al 2019;Shivaei et al 2018;Emami et al 2020). On the other hand, Maseda et al (2020) used a sample of 35 z ∼ 4 -5 continuum-faint LAEs and measured a very high log(ξ ion /[Hz erg −1 ]) = 26.28, implying a more efficient production of ionising photons in lower-luminosity Lyα selected galaxies, possibly produced by extremely low-metallicity stellar populations in very young starbursts.…”