Searching for the Precursors of Life in External Galaxies

TL;DR

This study investigates the presence of organic molecules linked to life in early-epoch galaxies by analyzing diffuse interstellar bands, revealing these molecules are less abundant in such galaxies compared to the Milky Way.

Contribution

It provides the first measurements of DIBs in extragalactic HI-rich galaxies, showing they are underabundant relative to the Milky Way, and highlights differences in dust and gas properties.

Findings

HI-rich galaxies are dust poor with lower reddening.

DIBs are significantly less abundant in extragalactic galaxies.

Extragalactic DIBs do not correlate with hydrogen as in the Milky Way.

Abstract

Are the organic molecules crucial for life on Earth abundant in early-epoch galaxies? To address this, we searched for organic molecules in extragalactic sources via their absorption features, known as diffuse interstellar bands (DIBs). There is strong evidence that DIBs are associated with polycyclic aromatic hydrocarbons (PAHs) and carbon chains. Galaxies with a preponderance of DIBs may be the most likely places in which to expect life. We use the method of quasar absorption lines to probe intervening early-epoch galaxies for the DIBs. We present the equivalent width measurements of DIBs in one neutral hydrogen (HI) abundant galaxy and limits for five DIB bands in six other HI-rich galaxies (damped Lyman-alpha systems--DLAs). Our results reveal that HI-rich galaxies are dust poor and have significantly lower reddening than known DIB-rich Milky Way environments. We find that DIBs in HI-rich galaxies do not show the same correlation with hydrogen abundance as observed in the Milky Way; the extragalactic DIBs are underabundant by as much as 10 times. The lower limit gas-to-dust ratios of four of the HI-rich early epoch galaxies are much higher than the gas-to-dust ratios found in the Milky Way. Our results suggest that the organic molecules responsible for the DIBs are underabundant in HI-rich early epoch galaxies relative to the Milky Way.