Neon Emission in the Early Universe

The optical spectra of nearby star-forming galaxies provide astronomers with a comprehensive view of the ionized gas in their interstellar medium (ISM). Atoms in the gas, excited to varying degrees by ultraviolet radiation from hot stars or other processes, radiate in distinctive spectral features. From the intensities of these emission lines, and from the ratios of line strengths from the same species and/or different species, astronomers can deduce many key properties of the ISM including the gas density, the dust content, the star formation rate, the activity of the nuclear supermassive black hole, the atomic abundances and, from the shapes of the spectral lines, the gas motions.

With the advent of sensitive multi-object near-infrared spectrographs on large telescopes, recent progress has been made extending these diagnostics to galaxies seen in the cosmic epoch of peak star formation that took place about ten billion years ago. There is an emerging consensus among astronomers that the results reflect the fact that their stellar populations are chemically different from local populations in that these early galaxies have not had much time to accumulate and disburse the more complex elements manufactured in stars. One consequence is that these early stars tend to be hotter and their radiation is "harder" - more capable of ionizing atoms into higher states of excitation.

CfA astronomers Mojegan Azadi and Francesco Fornasini are members of the MOSDEF Survey team, a near infrared spectroscopic study of about 1500 galaxies in the cosmic epoch of very active star formation. The team used the Multi-object Spectrometer for Infrared Exploration (MOSFIRE) on the Keck-I telescope for their observations, and in their new paper, the team analyzes the emission from atomic neon. Previous studies have determined the line strength ratios between neon and oxygen (for example) are unlike that in local galaxies, but have been unable to distinguish between several possible causes: the presence of harder stellar ionizing radiation, a more active galactic nucleus (AGN) that also results in harder radiation, or perhaps different elemental abundances. The new neon results support the first hypothesis of harder stellar radiation, and that in turn suggests that a larger percentage of hot, massive stars are being produced. The new results based on infrared measurements of neon support a physical picture of the early universe in which the atomic gas in galaxies' ISM, with neon, hydrogen, oxygen, nitrogen, sulfur and other constituents, is being irradiated by hot, young, massive stars. The next frontier will be to extend these studies to even earlier cosmic epochs using the James Webb Space Telescope.

Reference(s):

"The MOSDEF Survey: Neon as a Probe of ISM Physical Conditions at High Redshift," Moon-Seong Jeong, Alice E. Shapley, Ryan L. Sanders, Jordan N. Runco, Michael W. Topping, Naveen A. Reddy, Mariska Kriek, Alison L. Coil, Bahram Mobasher, Brian Siana, Irene Shivaei, William R. Freeman, Mojegan Azadi, Sedona H. Price, Gene C. K. Leung, Tara Fetherolf, Laura de Groot, Tom Zick, Francesca M. Fornasini, and Guillermo Barro, The Astrophysical Journal Letters 902, L16, 2020.