On the Ionization of Luminous WMAP Sources in the Galaxy : Constraints from He Recombination Line Observations with the GBT

TL;DR

This study uses radio recombination line observations to constrain the ionization sources of luminous WMAP sources in the Galaxy, revealing that simple models involving massive clusters or leaky HII regions cannot fully explain the observed ionization, especially for helium.

Contribution

It provides new observational constraints on the ionizing spectrum of luminous Galactic sources, challenging existing models of ionization by massive clusters or leaky HII regions.

Findings

Hydrogen RRL detected, helium line not detected, constraining ionizing spectrum.

Total ionizing luminosity exceeds that of known radio HII regions, indicating additional ionization sources.

Helium ionizing photons are significantly attenuated, likely by dust, affecting ionization models.

Abstract

The Wilkinson Microwave Anisotropy Probe (WMAP) free-free foreground emission map is used to identify diffuse ionized regions (DIR) in the Galaxy (Rahman & Murray 2010). It has been found that the 18 most luminous WMAP sources produce more than half of the total ionizing luminosity of the Galaxy. We observed radio recombination lines (RRLs) toward the luminous WMAP source G49.75-0.45 with the Green Bank Telescope near 1.4 GHz. Hydrogen RRL is detected toward the source but no helium line is detected, implying that n_He+/n_H+ < 0.024. This limit puts severe constraint on the ionizing spectrum. The total ionizing luminosity of G49 (3.05 x 10^51 s^-1) is ~ 2.8 times the luminosity of all radio HII regions within this DIR and this is generally the case for other WMAP sources. Murray & Rahman (2010) propose that the additional ionization is due to massive clusters (~ 7.5 x10^3 Msun for G49) embedded in the WMAP sources. Such clusters should produce enough photons with energy \geq 24.6 eV to fully ionize helium in the DIR. Our observations rule out a simple model with G49 ionized by a massive cluster. We also considered 'leaky' HII region models for the ionization of the DIR, suggested by Lockman and Anantharamaiah, but these models also cannot explain our observations. We estimate that the helium ionizing photons need to be attenuated by > ~10 times to explain the observations. If selective absorption of He- ionizing photons by dust is causing this additional attenuation, then the ratio of dust absorption cross sections for He- and H- ionizing photons should be > ~6.