Spitzer Infrared Spectrograph Observations of the Galactic Center: Quantifying the Extreme Ultraviolet/so ft X-ray fluxes

TL;DR

This study analyzes Spitzer infrared spectra of the Galactic Center to quantify the extreme ultraviolet and soft X-ray fluxes, revealing complex ionization conditions and estimating ages of star clusters using detailed modeling.

Contribution

It provides a comprehensive analysis of GC spectra with Cloudy models incorporating X-rays, offering new insights into ionizing sources and cluster ages in the Galactic Center.

Findings

GC spectra show unusual ionization states and diffuse emission.

Star clusters Sgr B1, Sgr C, and Arches are about 4.5 Myr old.

Quintuplet Cluster appears younger, possibly due to shocks.

Abstract

It has long been shown that the extreme ultraviolet spectrum of the ionizing stars of H II regions can be estimated by comparing the observed line emission to detailed models. In the Galactic Center (GC), however, previous observations have shown that the ionizing spectral energy distribution (SED) of the local photon field is strange, producing both very low excitation ionized gas (indicative of ionization by late O stars) and also widespread diffuse emission from atoms too highly ionized to be found in normal H II regions. This paper describes the analysis of all the GC spectra taken by Spitzer's Infrared Spectrograph and downloaded from the Spitzer Heritage Archive. In it, H II region densities and abundances are described, and serendipitously discovered candidate planetary nebulae, compact shocks, and candidate young stellar objects are tabulated. Models were computed with Cloudy, using SEDs from Starburst99 plus additional X-rays, and compared to the observed mid-infrared forbidden and recombination lines. The ages inferred from the model fits do not agree with recent proposed star formation sequences (star formation in the GC occurring along streams of gas with density enhancements caused by close encounters with the black hole, Sgr A*), with Sgr~B1, Sgr~C, and the Arches Cluster being all about the same age, around 4.5 Myr old, with similar X-ray requirements. The fits for the Quintuplet Cluster appear to give a younger age, but that could be caused by higher-energy photons from shocks from stellar winds or from a supernova.