High-n Hydrogen Recombination Lines from the First Galaxies

TL;DR

This study assesses the feasibility of detecting high-n hydrogen recombination lines from the first galaxies across radio frequencies, highlighting the challenges and potential strategies for successful observations with current and future telescopes.

Contribution

It provides a detailed analysis of the detectability of hydrogen recombination lines from early galaxies, including blind and targeted search strategies, and evaluates observational prospects with facilities like ALMA and SKA.

Findings

Detection is most promising in mm and submm wavelengths.

Blind searches face significant challenges, especially at high redshifts.

Targeted searches of lensed Lyman-break galaxies increase detection probability.

Abstract

We investigate the prospects of blind and targeted searches in the radio domain (10 MHz to 1 THz) for high-n hydrogen recombination lines from the first generation of galaxies, at z < 10. The expected optically thin spontaneous alpha-line luminosities are calculated as a function of the absolute AB magnitude of a galaxy at 1500 angstrom. For a blind search, semi-empirical luminosity functions are used to calculate the number of galaxies whose expected flux densities exceed an assumed detectability threshold. Plots of the minimum sky area, within which at least one detectable galaxy is expected at a given observing frequency, in the fiducial instantaneous passband of 10^4 km/s, allow to assess the blind search time necessary for detection by a given facility. We show that the chances for detection are the highest in the mm and submm domains, but finding spontaneous emission in a blind search, especially from redshifts z >> 1, is a challenge even with powerful facilities, such as ALMA and SKA. The probability of success is higher for a targeted search of lines with principal quantum number n ~ 10 in Lyman-break galaxies amplified by gravitational lensing. Detection of more than one hydrogen line in such a galaxy will allow for line identification and a precise determination of the galaxy's redshift.