Probing Cold Gas with the Resonance Doublet of Singly Ionized Magnesium

April 01, 2024

Traditional studies of the gas around galaxies rely in particular on absorption and emission features of neutral hydrogen, the simplest and most abundant element in the universe. MPA researchers have now investigated alternative tracers, in particular the resonance doublet of singly ionized magnesium and found that analyzing this emission can lead to significant advances in studying the circum-galactic medium. They showed the potential of the magnesium doublet as an alternative to Lyman-alpha emission through a new radiative transfer code and suggest that the magnesium doublet ratio could even be used as a tracer of the Lyman-continuum escape.

Light, and in particular how it interacts with the atoms of various elements, plays a pivotal role in unveiling the secrets of the universe. At the heart of this interaction lies the resonance line, the transition between the ground state and the first excited state in an atom. This transition is significant and interesting for atoms and ions with just one electron in their outermost shell. Considering the fine structure of these atoms, the resonance line manifests as a doublet, the K and H lines. The most renowned example of such a resonance doublet is the hydrogen Lyman-alpha (Lyα) line at 1216 Å. Although the two lines of Lyman-alpha are not distinguishable due to a too small energy gap, other metal doublets are observed as separated doublets.

In astrophysical spectra, the resonance doublets stand out as prominent absorption lines since the abundant electrons in the ground state easily interact with photons near the line center. In addition to absorption features in astrophysical spectra, the doublets also appear as emission lines, acting as one of the main coolants of shock and ionized gas. The atomic physics of the resonance doublet dictates that the scattering cross-section of the K line is always two times higher than that of the H line. This also translates into the ratio of K and H emission from collisional excitation and recombination, which is generally two. Furthermore, due to their resonant nature, photons of the doublet emission suffer scattering with the electrons in the ground state, by which the physical properties of the gas are imprinted on their emission features. The study of resonance doublets, therefore, opens a window into the complexities of astrophysical environments, making it a cornerstone of astrophysical spectroscopy.

Studying CGM with Resonance Doublets

The circumgalactic medium (CGM), the diffuse halo of multiphase gas that envelopes galaxies, is a key to understanding the mysteries of galaxy formation, evolution, and the flow of matter around galaxies. While traditional studies of the CGM have relied on analyzing the absorption features of the resonance lines observed in quasar spectra, this approach offers a view constrained by singular lines of sight. The evolution of observational technology, with instruments like MUSE on the VLT, KCWI on Keck, and HETDEX, has opened new windows into the CGM through the direct observation of spatially extended emissions such as Lyman-alpha and resonance doublets of heavier elements.

The Lyman-alpha emission is central to these advances and a powerful tool for probing the cold CGM (with temperatures up to 10000K) and studying the early universe (at redshift z from 2 to 5). However, observing Lyman-alpha faces significant challenges: it is obscured by Earth’s atmosphere at z < 2 and becomes difficult to detect beyond z > 6 due to the optically thick universe in the epoch of reionization. These limitations highlight the necessity for alternative tracers of cold gas within the CGM across different cosmic epochs.

Resonance Doublet of Singly Ionized Magnesium as a New Tracer of Cold Gas

The resonance doublet of singly ionized magnesium (Mg II) at 2796 Å and 2803 Å presents such an alternative. Due to its resonance nature and with a similar ionization energy to atomic hydrogen, the Mg II emission can trace cold gas properties through scattering processes like Lyman-alpha. In this work, we developed a new 3D Monte Carlo radiative transfer code to investigate the escape of both emission lines through homogeneous and clumpy multiphase media. Our new code allows for exploring gas in arbitrary 3D geometries via both Lyman-alpha and metal resonance doublets, significantly enhancing our understanding of the cold gas environment surrounding galaxies.

One of the key findings of this research is the distinct behavior of Mg II emissions compared to Lyman-alpha, despite similarities in atomic physics. Lyman-alpha emission is more spatially extended via scattering than Mg II due to a small fraction of magnesium in the gas. Furthermore, the Mg II escape fraction generally exceeds that of Lyman-alpha, offering a clearer view through the cosmic dust that often obscures Lyman-alpha emissions. This makes Mg II an invaluable alternative for tracing cold gas, particularly in environments where Lyman-alpha emission is weak or unobservable.

Magnesium Doublet Ratio

The escape of the Lyman continuum (LyC) or its leakage is particularly significant for understanding the mechanisms behind galaxy evolution and the reionization of the universe. The Mg II doublet ratio, which is the flux ratio of the two doublet lines, is one of the new promising indicators of the LyC leakage.

Our study investigates the Mg II doublet ratio in various environments. We found that the doublet ratio indicates a strong outflow/inflow. The double ratio of Mg II from stellar continuum becomes ~ 1 for high column densities of Mg II. In addition, we tested the doublet ratio as a leakage indicator of LyC and tracer of the LyC escape fraction.

The Mg II spectrum in the halo is composed of only scattered photons, and the physical properties of cold gas are clearly imprinted on it. We explored this and derived the analytic solution of LyC escape using the halo doublet ratio. These insights not only expand our methodologies in studying the CGM but also pave new pathways for future observation and theory.

Potential of Metal Resonance Doublets

Moreover, our research opens the door to exploring the emission features of other metal resonance doublets as tracers of the CGM. The success of the Mg II doublet in providing new insights into cold gas properties and the escape of ionizing photons suggests the potential for similar analyses for other elements. For example, the C IV doublet can be a good indicator of the galactic wind. Other metal doublets, such as O VI, N V, and Si IV, share similar atomic physics and could trace the CGM at different temperatures. This avenue of research holds promise for broadening our understanding of the multiphase CGM, offering a richer, more nuanced view of the processes that govern galaxy evolution and the cosmic web.


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