Accurately predicting the escape fraction of ionizing photons
For galaxies to reionize the early universe, a fraction of the ionizing photons produced by high-mass stars must escape the galaxy and find their way into the inter-galactic medium (IGM). The photons that escape are thought to bring about the last phase change of the universe at redshifts between 6-10. However, ionizing photons do not easily escape galaxies because dust and neutral gas easily absorb them. There have only been about a dozen galaxies, at any redshift, observed to emit ionizing photons. Consequently, it is unsettled how ionizing photons escape galaxies.
In theory, this is an easily observable question because the Lyman continuum (blueward of 912 Å) constrains the number of ionizing photons emitted by a galaxy. Unfortunately, the IGM at these times is still mainly neutral, and the IGM absorbs the ionizing photons. This means that it is unlikely to directly observe the Lyman continuum of galaxies reionizing the universe. Indirect methods are required to measure the escape fractions and determine what reionized the universe.
In a series of recently submitted papers, a masters student (Simon Gazagnes) and I studied the H I absorption lines in the far ultraviolet to explore the neutral gas properties of 9 local emitters of ionizing photons and 9 galaxies with unknown Lyman continuum emission. We also measured the dust attenuation from the ultraviolet stellar continuum, which constrains how dust impacts ionizing photons. By studying the neutral gas and dust properties of these high-redshift analogs, we determine how ionizing photons escape galaxies and new methods to measure the escape fraction of ionizing photons at high-redshifts.
In theory, this is an easily observable question because the Lyman continuum (blueward of 912 Å) constrains the number of ionizing photons emitted by a galaxy. Unfortunately, the IGM at these times is still mainly neutral, and the IGM absorbs the ionizing photons. This means that it is unlikely to directly observe the Lyman continuum of galaxies reionizing the universe. Indirect methods are required to measure the escape fractions and determine what reionized the universe.
In a series of recently submitted papers, a masters student (Simon Gazagnes) and I studied the H I absorption lines in the far ultraviolet to explore the neutral gas properties of 9 local emitters of ionizing photons and 9 galaxies with unknown Lyman continuum emission. We also measured the dust attenuation from the ultraviolet stellar continuum, which constrains how dust impacts ionizing photons. By studying the neutral gas and dust properties of these high-redshift analogs, we determine how ionizing photons escape galaxies and new methods to measure the escape fraction of ionizing photons at high-redshifts.
IOnizing photons escape through low(er)-density holes
|
We first explored the H I column density and H I covering fraction of these galaxies. The column density determines how much neutral gas is along the line of sight. Low column densities imply that ionizing photons escape their galaxies because there is not sufficient neutral gas to absorb all of the ionizing photons. Meanwhile, the covering fraction describes the porosity of the neutral gas. A covering fraction below 1 implies that there are holes in the neutral gas for the ionizing photons to escape.
Since the Lyman series has multiple H I transitions, we can determine both the covering fraction and column density of these galaxies. We find that the Lyman series is saturated. This implies that the H I column density is too large to allow ionizing photons to escape. Contrarily, we find that galaxies that emit ionizing photons have covering fractions less than one and lower covering fractions than galaxies that do not emit ionizing photons (see the figure to the right). These low covering fractions imply that ionizing photons escape galaxies through holes in the neutral gas. At first glance this may seem counter to my previous results with the Si II lines. This suggests a more complicated picture for how ionizing photons escape galaxies. This is the first observational evidence for how ionizing photons escape galaxies. |
Histogram of the H I covering fraction for the 18 galaxies with Lyman series observations from the Cosmic Origins Spectrograph or from the Megasaura sample. Galaxies that emit ionizing photons (blue bars) have systematically lower H I covering fractions than the galaxies that are not observed to emit ionizing photons (black bars; Gazagnes et al. submitted). This illustrates that galaxies that emit ionizing photons have holes in their neutral gas that ionizing photons can escape through.
|
Ways to predict the escape fraction of ionizing photons
The escape fraction predicted by the H I covering fraction and the measured dust attenuation versus the observed escape fraction for the nine galaxies that are observed to emit ionizing photons (Chisholm et al. submitted). The H I properties accurately reproduce the observed escape fractions.
|
H I gas and dust are the major sinks of ionizing photons. Once these are constrained, the radiative transfer equation predicts the expected escape fraction of ionizing photons. We can then compare the predicted escape fractions to the observed escape fractions to determine whether the predictions accurately match observations.
The figure to the left compares the predicted and the observed escape fractions. We find that the dust and H I properties accurately (within 1.4sigma) reproduce the observed values. Neutral gas and dust determine the escape fractions of ionizing photons. However, the Lyman series is not available at high-redshifts because the IGM opacity is too large and the Lyman forest absorbs the Lyman continuum photons. Consequently, we tested whether the Si II absorption lines and the Lyman alpha escape fractions predict the escape fraction of ionizing photons (plots to the right). These plots demonstrate that the escape fraction of ionizing photons is accurately predicted by these two tracers, which are available for galaxies within the era of reionization. |
Top Panel: The escape fraction of ionizing photons predicted using the Si II absorption line. JWST can observe this absorption line from galaxies reionizing the universe.
Bottom Panel: The escape fraction of ionizing photons predicted using the Lyman alpha escape fraction.
Both methods accurately reproduce the observed escape fraction of ionizing photons (Chisholm et al. submitted).
|
These observations demonstrate, for the first time, that ionizing photons escape through low(er) holes in the neutral gas. They also show that the escape fraction of high-redshift galaxies can be estimated using indirect methods. This means that future observations with JWST will constrain whether star-forming galaxies produced sufficient ionizing photons to reionize the universe.