Parabolic-like Trend in SiO Ratios throughout the Central Molecular Zone: Possible Signature of a Past Nuclear Activity in the Galactic Center

TL;DR

This study reveals a parabolic trend in SiO emission ratios across the Galactic Center's molecular zone, suggesting a past nuclear outburst or high-energy photon activity affecting the region's molecular environment.

Contribution

First large-scale imaging of SiO ratios in the CMZ, indicating a possible signature of historical nuclear activity in the Galactic Center.

Findings

Identified a parabolic-like trend in SiO emission ratios across the CMZ.

Suggests a past nuclear outburst within the last 10^5 years.

Proposes high-energy photon activity as an alternative explanation.

Abstract

We report the discovery of a characteristic trend in the intensity ratios of SiO emissions across the Central Molecular Zone (CMZ) of our Galaxy. Using the Nobeyama Radio Observatory 45-m telescope, we conducted large-scale, high-sensitivity imaging observations in molecular lines including SiO $J$=2$-$1 and CS $J$=2$-$1. By identifying SiO-emitting clouds and examining their intensity ratios relative to the other molecular lines, we unveiled a parabolic-like trend showing lower ratios near the Galactic nucleus, Sgr A$^*$, with gradual increases toward the edges of the CMZ. This pattern suggests a possible outburst of the nucleus within the last $\sim 10^5$ yr, which may have propagated through the entire CMZ with strong shocks. Alternatively, the observed trend may also be attributed to the destruction of small dust grains by high-energy photons. Our results can potentially lead to a new perspective on the history of nuclear activity and its impact on the surrounding molecular environment.