Ionizing Sources

In the above, we have assumed that galaxies drove reionization. And, indeed, that is the general consensus (see, for example, Faucher-Giguère et al. (2008); Becker and Bolton (2013)). Historically, both galaxies and quasars were considered good candidates for fueling reionization. However, high redshift galaxy observations suggest that galaxies alone may reionize the Universe, provided that the escape fraction, f_esc is sufficiently

Figure 1.2: Schematic of the history of the Universe, from the Big Bang to the present day, including reionization and the end of the cosmic dark ages. (Image from NAOJ)

Figure 1.3: Figures from McQuinn (2016). (Left)Originally published in Bouwens et al. (2015). The luminosity functions of LBGs (for more information on this selection tech- nique see §1.2.4.1) observed by the Hubble Space Telescope, for a range of redshifts covering much of reionization. (Right) Based off of luminosity functions, an estimate of whether galaxies can provide enough ionizing photons to maintain reionization. This figure shows an estimate of the emissivity of ionizing photons atz∼6, based both on the observed galaxy population and the integration of luminosity functions to include fainter galaxies, as a function of fesc. These values are compared with the emissivities needed to maintain reionization using model values according to Kuhlen and Faucher-Giguère (2012). Becker and Bolton (2013) have concluded that emissivities several times those shown above are allowed.

large (Robertson et al. 2010; Finkelstein et al. 2012a). The escape fraction quantifies the average fraction of ionizing photons that escape each galaxy’s host halo and are available to ionize atoms in the IGM. Further, we have a growing number of luminosity function measurements – the average abundance of galaxies as a function of their luminosity – for dropout-selected galaxies atz>6 from the Hubble Space Telescope (HST) (Finkelstein et al. 2012b; Bouwens et al. 2012; Ellis et al. 2013; McLure et al. 2013) (for a more detailed discussion of luminosity functions, see §1.2.4.3). The left panel of Fig. 1.3 shows some current luminosity function measurements for a range of redshifts. As can be seen, the abundance of galaxies declines with increasing redshift. This behavior is expected since it parallels a drop in the halo mass function.

In order to estimate the ionizing flux provided by galaxies from the observed luminos- ity functions, one must make assumptions about the escape fraction, fesc. If fesc&0.2, observed galaxies can maintain reionization atz∼6. However, if f_esc.0.2, then fainter, and currently unobserved, galaxies are required to contribute to the ionizing budget. In- deed, measurements of the high redshift galaxy luminosity function indicate a steep faint end slope (Kuhlen and Faucher-Giguère 2012; McLure et al. 2013; Bouwens et al. 2015), implying that faint galaxies emit roughly twice as many ionizing photons as those emit- ted from currently observed galaxies. Of course, fescmay scale with luminosity and there is a limit below which halos are not massive enough to host galaxies (McQuinn 2016). Better knowledge of fescwill be necessary to more fully understand which galaxies drove reionization.

While we can estimate the number of ionizing photons based on the number of sources, we can also infer the total emissivity of ionizing photons – that is, the number of ionizing photons per volume per units time – based on quasar absorption lines. Miralda-Escude (2003) find that atz=4, the ionizing emissivity is only slightly greater that that required to ionize hyrdrogen. To push this measurement to higher redshifts, we can turn to Bolton

and Haehnelt (2007) who estimate that at z∼6, 1−3 ionizing photons per hydrogen

are just barely enough photons to ionize the Universe by this redshift. This model is called “photon-starved reionization." Of course, in this calculation, they have made as- sumptions about the evolution of the ionizing emissivity; a higher ionizing emissivity in the past would soften this conclusion. Becker and Bolton (2013) revisit their work with higher redshift data and more rigorous calculations and find that atz≈5, there can be up to 3-10 photons per hydrogen atom per Gyr. These higher limits would result in a less photon-starved reionization process. All of this is shown in the right panel of Fig. 1.3.

While galaxies are the generally assumed source of reionization, this still remains something of an open question. For example, Giallongo et al. (2015) claim to have de- tected enough quasars atz≈6 to reionize the Universe. Relying on quasars as the only source of reionization, however, is not without its own complications. Quasars provide a

hard ionizing spectrum which would doubly-ionize helium byz=4 (Madau and Haardt

2015). This conflicts with observations that support a later date for He II reionization; for instance, patchy absorption in the He II Lyαforest supports He II reionization completing

only atz∼2.7 (Shull et al. 2010). Nonetheless, quasars certainly ionized their local envi- ronment. While galaxies are most likely the dominant source of reionization, a complete picture would have to include the effects of other ionizing sources, such as quasars.

In any case, once the Universe is ionized, a meta-galactic ionizing background main- tains reionization. At high redshifts, this background is most likely maintained by those

stars and galaxies which fueled reionization (McQuinn 2016). However, by z∼2−3,

there are enough quasars to maintain it (Haardt and Madau 1996; Faucher-Giguère et al. 2008; Haardt and Madau 2012).