The Effects of Stellar Population and Gas Covering Fraction on the Emergent Lyα Emission of High-redshift Galaxies*
Abstract We perform joint modeling of the composite rest-frame far-UV and optical spectra of redshift 1.85 ≤ z ≤ 3.49 star-forming galaxies to deduce key properties of the massive stars, ionized interstellar medium (ISM), and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Ly α photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Ly β through C iii ] λλ 1907, 1909 and [O ii ], [Ne iii ], H β , [O iii ], H α , [N ii ], and [S ii ]. Spectral and photoionization modeling indicates that the galaxies are uniformly consistent with stellar population synthesis models that include the effects of stellar binarity. Over the dynamic range of our sample, there is little variation in stellar and nebular abundance with Ly α equivalent width, W λ (Ly α ), and only a marginal anticorrelation between age and W λ (Ly α ). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W λ (Ly α ); rather, the covering fraction of optically thick H i appears to be the principal factor modulating the more »
Authors:
; ; ; ; ; ; ; ; ;
Award ID(s):
Publication Date:
NSF-PAR ID:
10336789
Journal Name:
The Astrophysical Journal
Volume:
926
Issue:
1
Page Range or eLocation-ID:
31
ISSN:
0004-637X
The ionizing photon escape fraction [Lyman continuum (LyC) fesc] of star-forming galaxies is the single greatest unknown in the reionization budget. Stochastic sightline effects prohibit the direct separation of LyC leakers from non-leakers at significant redshifts. Here we circumvent this uncertainty by inferring fesc using resolved (R > 4000) Lyman α (Lyα) profiles from the X-SHOOTER Lyα survey at z = 2 (XLS-z2). With empirically motivated criteria, we use Lyα profiles to select leakers ($f_{\mathrm{ esc}} > 20{{\ \rm per\ cent}}$) and non-leakers ($f_{\mathrm{ esc}} < 5{{\ \rm per\ cent}}$) from a representative sample of >0.2L* Lyman α emitters (LAEs). We use median stacked spectra of these subsets over λrest ≈ 1000–8000 Å to investigate the conditions for LyC fesc. Our stacks show similar mass, metallicity, MUV, and βUV. We find the following differences between leakers versus non-leakers: (i) strong nebular C iv and He ii emission versus non-detections; (ii) [O iii]/[O ii] ≈ 8.5 versus ≈3; (iii) Hα/Hβ indicating no dust versus E(B − V) ≈ 0.3; (iv) Mg ii emission close to the systemic velocity versus redshifted, optically thick Mg ii; and (v) Lyα fesc of ${\approx} 50{{\ \rm per\ cent}}$ versus ${\approx} 10{{\ \rm per\ cent}}$. The extreme equivalent widths (EWs) in leakers ([O iii]+$\mathrm{ H}\beta \approx 1100$ Å rest frame)more »
3. ABSTRACT We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model, the new model provides a better match to a large number of up-to-date empirical constraints, including: (1) new galaxy and AGN luminosity functions; (2) stellar spectra including binary stars; (3) obscured and unobscured AGN; (4) a measurement of the non-ionizing UV background; (5) measurements of the intergalactic H i and He ii photoionization rates at z ∼ 0−6; (6) the local X-ray background; and (7) improved measurements of the intergalactic opacity. In this model, AGN dominate the H i ionizing background at z ≲ 3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z ∼ 2, the slow evolution of the H i ionization rate inferred from the high-redshift H i Ly α forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of $\approx 1{{\ \rm per\ cent}}$ suffices to ionize the intergalactic medium at z = 3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the He ii Ly α forestmore »