LIMFAST
5.3 Results
5.3.1 Redshift Evolution
5.3.1.4 Line Emission
To qualitatively illustrate the output of LIMFAST, Figure 5.5 displays LIMFAST light cones for various quantities, where the redshift-slice maps are interpolated to show a continuous evolution of such quantities with redshift. In this case, the light cones cover the redshift range 5 ≤ 𝑧 ≤ 15. From top to bottom, the signals correspond to the hydrogen neutral fraction, the brightness temperature of the 21cm radiation, the star formation rate density, the metallicity of collapsed structure, and the intensity of Ly𝛼 emission, arising from star formation and recombination in the intergalactic medium, as well as the intensity of the O ii 3727Å line emission.
Figure 5.5 allows to easily visualize qualitative correlations between quantities. For example, beyond the well-studied correlation of the 21cm signal with the hydrogen neutral fraction, Figure 5.5 illustrates the increase of metallicity and Ly𝛼emission from star formation with the corresponding evolution of the star formation rate density. Very similar evolutions not displayed here are also found for the H𝛼and H𝛽 signals. This similarity arises from the fact that all these lines mostly depend on the number of ionizing photons that is produced, which in turn is proportional to
Figure 5.4: IGM neutral fraction evolution as computed by LIMFAST. Evolution of the gas neutral fraction with redshift as computed by LIMFAST with an 8% ionizing escape fraction value (black solid line). For comparison, the colored data show constraints from the literature. Furthermore, the solid lines represent the evolution resulting from other works using 21cmFAST and that match the concurrent average electron optical depth inferred from the cosmic microwave background.
the star formation, and that is also dependent on the specific SED via the metallicity value. However, over the redshift range here considered, the metallicity varies across roughly one order of magnitude, while the star formation rate changes by around six decades, thus making the later quantity the major driver of the signal evolution.
The bottom two panels, on the other hand, highlight the strong dependence of the intergalactic Ly𝛼emission on the ionization state of the gas and less on the specific star formation value. For the case of the O ii 3727Å signal, its panel shows a steeper evolution of the intensity with redshift compared to that of Ly𝛼from star formation, indicated by the different color gradient in the respective panels. This difference in evolution is driven by the higher sensitivity of the O ii 3727Å emission to metallicity compared to Ly𝛼from star formation (we discuss this point further below).
We turn now to a more quantitative analysis of the intensity results and perform comparisons with other works. The solid black lines in Figure 5.6 display the redshift evolution of the line emission brightness computed by LIMFAST. From top to bottom and left to right, the panels show the differential brightness temperature of the 21cm line emission, where negative values denote absorption, the intensity values of Ly𝛼 from star formation, Ly𝛼 from recombination in the IGM, the Ly𝛼
Figure 5.5: LIMFAST light cones covering the redshift range 5≤ 𝑧 ≤ 15. From top to bottom, the signals correspond to the hydrogen neutral fraction, the brightness temperature of the 21cm radiation, the star formation rate density, the metallicity of collapsed structure, and the intensity of Ly𝛼emission, arising from star formation and recombination in the intergalactic medium, as well as the intensity of the O ii at 3727 Å line emission.
Figure 5.6: Redshift evolution of the emission line brightness from LIMFAST.
The colored solid lines represent results from the literature. For the oxygen lines, the dash-dotted black lines denote the LIMFAST results considering an ionization parameter value of log𝑈 = −4, instead of the fiducial value of log𝑈 = −2 to highlight the sensitivity of the line emission to this parameter value. The gray dashed lines show calculations assuming (local) relations between star formation and luminosity often used in other works.
background, H𝛼, H𝛽, [O ii] at 3727 Å, and [O iii] at 5007 Å rest frame. The colored solid lines represent results from the works by (Mesinger et al. 2016, their faint galaxies case), Silva et al. (2013), Heneka et al. (2017), Comaschi & Ferrara (2016), Pullen et al. (2014), and the THESAN1 and 2 simulations by Kannan et al.
(2022a). For the oxygen lines, the dash-dotted black lines denote the LIMFAST results considering an ionization parameter value of log𝑈 = −4, instead of the fiducial value of log𝑈 = −2. The gray dashed lines denote calculations assuming relations between star formation and luminosity that are sometimes considered in other intensity mapping works for comparison. These relations, however, are usually derived from local observations and they may not be valid to represent high-redshift galaxies. Considering a relation between luminosity and star formation rate of the form 𝐿 = 𝐶𝑀¤★, the gray dashed lines represent the cases adopting the values of 𝐶 = 1.26×1041erg s−1𝑀¤−1
★ for H𝛼(Kennicutt 1998),𝐶 =1.3×1042erg s−1𝑀¤−1
★
for Ly𝛼, assuming a factor of 8.7 times more emission for Ly𝛼 than that of H𝛼 (Osterbrock 1989), 𝐶 = 4.41×1040erg s−1𝑀¤−1
★ for H𝛽, arising from the relation H𝛼/H𝛽 =2.86,𝐶 =7.14×1040erg s−1𝑀¤−1
★ for [O ii] from the relation [O ii]/H𝛼= 0.57 (Kennicutt 1998), and𝐶 =1.32×1041erg s−1𝑀¤−1
★ for [O iii] (Ly et al. 2007).
For the case of emission from star formation, the fiducial LIMFAST signal is generally higher than that of other works, driven by the higher average star formation rate density shown in Figure 5.2, and by the fact that we have not applied any attenuation to the emission, contrary to most results from the literature. For the hydrogen lines, this signal is also higher than that derived with local relations because the lower metallicity of high redshift galaxies compared to the local ones results in higher emissivities of ionizing photons and, in turn, recombination emission.
For the oxygen lines, however, the emission is very sensitive to the metallicity and ionization parameter values, and it can thus appear to be higher or lower than that from local relations (e.g., Kewley et al. 2019). This highlights the sensitivity of the brightness of the oxygen lines to the value of the ionization parameter, which is of notable importance although typically not taken into account in the intensity mapping modeling literature (but see a discussion on the relevance of this parameter when accounted for in Silva et al. 2017). Figure 5.9 in Section 5.6 shows the relation between luminosity and metallicity at various ionization parameter values for the star-formation emission lines discussed in this work for completeness.
Figure 5.6 overall highlights the plethora of results from various works, where differences between models sometimes exceed one order of magnitude or more.
These differences further increase by their squared power when considering their power spectra, which implies the same level of differences in detectability estimates.
This comparison therefore emphasizes the dramatic dependence of detectability estimates and constraints on the models and assumptions, most times disregarded in the literature. We discuss differences between models further in Section 7.5.