Making Blue Photons in Dwarf Galaxies

Title: The Ionizing Photon Production Efficiency Of Lensed Dwarf Galaxies At z∼2
Authors: Najmeh Emami, et al.
First author’s institution: University of California Riverside, Riverside, CA
Status: Open access on arXiv

How does star light escape from galaxies? It may seem like a simple question on its face, but it becomes more complicated the more you think about it. When you see a picture of a galaxy in optical wavelengths it seems to be very bright, but switching to a UV image you see less light. This decrease in the amount of light escaping for shorter wavelengths becomes worse as you progress into the far and extreme-UV (i.e. shorter wavelengths), where it becomes particularly prone to being absorbed by neutral hydrogen gas. Because of this tendency, when this short-wavelength radiation is created by stars, which are surrounded by dense clouds containing neutral hydrogen, much or even all of it gets absorbed and blocked from view.

The absorption of this light creates problems for understanding the reionization of hydrogen in the intergalactic medium (IGM) that took place more than 12 billion years ago. In short, this process consisted of ionizing radiation coming from…somewhere…and separating the IGM hydrogen atoms into individual protons and electrons. But if our observations indicate that not much hydrogen-ionizing light can escape from galaxies, then how can reionization even happen? Although there are alternative possible sources of radiation, galaxies are the current best candidate for causing reionization, and the fact that photon escape and this process can’t be reconciled would seem to indicate astronomers are missing something important about this portion of the Universe’s history.

This quandary motivates many experiments looking at how light escapes from galaxies, and today’s paper focuses on how efficiently galaxies, particularly dwarf galaxies, generate ionizing light and allow it to pass out into intergalactic space.

Galaxy traits and UV photon production

The goal of today’s featured paper is to determine the average efficiency of ionizing photon production from dwarf galaxies and to also evaluate their total photon output as an entire population. The average can be compared to that calculated for larger galaxies, which have already been studied for some time in this manner, and the total will quantify the contribution of small galaxies to reionization.

The efficiency of ionizing light is the ratio of production of ionizing UV to the non-ionizing UV luminosity density. To calculate this, the authors collect observations of a sample of dwarf galaxies from about 11 billion years ago magnified by foreground galaxy clusters via an effect called gravitational lensing to measure their brightnesses at a few wavelengths: Hα, [O III], and 1500 Angstroms (a generic ultraviolet line).

Figure 1: The efficiency of ionizing photon production (indicated with ζion on the y-axis) as a function of galaxy stellar mass (M* on the x-axis). There is no correlation with mass, but the young galaxies from this paper (red filled and empty circles) show generally higher values than those from a modern sample (green filled and empty circles). Purple circles from even earlier times also have higher efficiencies.

No relation was found between the efficiencies of individual galaxies and their stellar masses (Figure 1 above), their total UV brightness, or even their brightnesses at short wavelengths relative to long (also called their UV spectral slope), and the dwarf galaxies were found to have similar efficiencies to more massive galaxies. However, there is a correlation with the efficiency and a galaxy’s brightness in both the Hα and [O III] emission lines, and the correlation is stronger for Hα (Figure 2 below). This suggests that galaxies that are brighter in these emission lines are also more efficient at generating ionizing radiation, and this may be related to the presence of very young stars, less than 100 million years old.

Figure 2: The efficiency as a function of [O III] emission (left panel) and H α emission (right panel). There are positive correlations of the efficiency with both lines, with a fit line shown in red and the 1 sigma uncertainty in light pink shading.

When compared to similarly-sized galaxies from later in cosmic history (modern, nearby galaxies to be specific), they found that their sample seems to have a higher efficiency, indicating that there might be some evolution in time with how good dwarf galaxies are at making ionizing photons (Figure 1).

The explanation for this might lie in the amount of iron contained in stars. A neutral iron atom has many electrons, which are good at preventing photons from making it out of a star’s atmosphere. However, iron is made mostly by Type Ia supernovae which take a long time to occur, and this means there is a time-delay for iron production in galaxies. Thus, galaxies from earlier in time have a deficit in iron, so their stars may be able to let out more ionizing photons compared to modern galaxies.

The difference in efficiency with cosmic time could also be due to more bursts in the star formation rate or differences in typical stellar populations (for example, changing fractions of binary stars), but that cannot be determined without a more extensive analysis of these galaxies’ star formation behavior throughout cosmic time.

If galaxies earlier in time were better at generating ionizing photons, then perhaps this could reconcile why both (1) observations of modern galaxies don’t show much evidence of ionizing photon escape but (2) reionization still managed to complete. In the future, the authors hope to further investigate the cause of the scatter in the efficiencies of dwarf galaxies to nail down the intricacies of photon escape.

Featured image credit: The Small Magellanic Cloud, a dwarf galaxy orbiting the Milky Way. Image from NASA.

About Caitlin Doughty

Caitlin Doughty is a fifth year graduate student at New Mexico State University. They use cosmological simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the intergalactic medium.

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