BOOSTING LYα  AND He ii λ1640 LINE FLUXES FROM POPULATION III GALAXIES: STOCHASTIC IMF SAMPLING AND DEPARTURES FROM CASE-B

Abstract

We revisit calculations of nebular hydrogen Lyα and HeII λ1640 line strengths for population III galaxies, undergoing continuous and bursts of star formation. We focus on initial mass functions (IMFs) motivated by recent theoretical studies, which generally span a lower range of stellar masses than earlier works. We also account for case-B departures and the stochastic sampling of the IMF. In agreement with previous works, we find that departures from case-B can enhance the Lyα flux by a factor of a few, but we argue that this enhancement is driven mainly by collisional excitation and ionization, and not due to photoionization from the n = 2 state of atomic hydrogen. The increased sensitivity of the Lyα flux to the high-energy end of the galaxy spectrum makes it more subject to stochastic sampling of the IMF. The latter introduces a dispersion in the predicted nebular line fluxes around the deterministic value by as much as a factor of ∼ 4. In contrast, the stochastic sampling of the IMF has less impact on the emerging Lyman Werner (LW) photon flux. When case-B departures and stochasticity effects are combined, nebular line emission from population III galaxies can be up to one order of magnitude brighter than predicted by 'standard' calculations that do not include these effects. This enhances the prospects for detection with future facilities such as JWST and large, groundbased telescopes.

Candidate Population III stellar complex at z = 6.629 in the MUSE Deep Lensed Field

“…Under the assumption that N(Lyα) = N(MUV) (being Lyα and MUV referring to the same star complex), the resulting TIGM(Lyα) are 0.09, 0.11, and 0.16, respectively. These values double if the case-B is assumed, i.e., the predicted L(Lyα) is about a factor two fainter (Mas-Ribas et al 2016). With such values of TIGM(Lyα) the resulting EW0(Lyα) ranges between 4000-1500Å, for the three classes of Pop III stellar masses.…”

Lyman–Werner escape fractions from the first galaxies

“…For our lower limit SFE of 0.1% in Halo A we get a total of ≈ 10 4 M in stars. Mas-Ribas, Dijkstra & Forero-Romero (2016) show that for a total stellar mass of 10 3 M , there is a factor of few difference in the LW flux for different stochastically sampled IMFs, but this is much reduced for ≈ 10 4 M in stars.…”

“…We create a stellar population from a flat slope IMF within the mass range 9 -500 M , using the stellar SEDs and same methodology as in Mas-Ribas, Dijkstra & Forero-Romero (2016); we refer the reader to that work for a detailed description of the calculations. These authors used parameters from Schaerer (2002) and Marigo et al (2001), and assumed the stars to be metal-free and to reside on the main sequence.…”

The MUSE Hubble Ultra Deep Field Survey X. Ly$α$ Equivalent Widths at $2.9 < z < 6.6$

“…It has been shown that assuming different mostly changes the peak value of the EW 0 distribution but does not increase its width (e.g., Garel et al 2015), unless one adopts evolving or spatially varying IMFs within galaxies (e.g., Orsi et al 2012). Nevertheless, at fixed IMF, Forero-Romero & Dijkstra (2013) hinted that the stochastic sampling of the IMF can induce fluctuations in the predicted EW 0 values for a given star formation event, hence broadening the EW 0 distributions (see also Mas-Ribas et al 2016). Alternatively, bursty star formation may also help reconcile models and observations.…”