An extremely low gas-to-dust ratio in the dust-lane lenticular galaxy NGC 5485

TL;DR

This study reveals that the galaxy NGC 5485 contains a surprisingly low amount of gas relative to dust, challenging typical galaxy models and suggesting a unique evolutionary history involving a possible merger with a metal-poor galaxy.

Contribution

First direct measurement of extremely low gas-to-dust ratio in an early-type galaxy with a dust lane, highlighting a potential new understanding of galaxy evolution and interstellar medium composition.

Findings

Detected 3.8 million solar masses of dust in NGC 5485.

Gas-to-dust ratio upper limit is less than 14.5, much lower than the Milky Way.

Standard uncertainties do not explain the low gas-to-dust ratio.

Abstract

Evidence is mounting that a significant fraction of the early-type galaxy population contains substantial reservoirs of cold interstellar gas and dust. We investigate the gas and dust in NGC 5485, an early-type galaxy with a prominent minor-axis dust lane. Using new Herschel PACS and SPIRE imaging data, we detect 3.8 x 10^6 Msun of cool interstellar dust in NGC 5485, which is in stark contrast with the non-detection of the galaxy in sensitive HI and CO observations from the ATLAS3D consortium. The resulting gas-to-dust ratio upper limit is Mgas/Md < 14.5, almost an order of magnitude lower than the canonical value for the Milky Way. We scrutinize the reliability of the dust, atomic gas and molecular gas mass estimates, but these do not show systematic uncertainties that can explain the extreme gas-to-dust ratio. Also a warm or hot ionized gas medium does not offer an explanation. A possible scenario could be that NGC 5485 merged with an SMC-type metal-poor galaxy with a substantial CO-dark molecular gas component and that the bulk of atomic gas was lost during the interaction, but it remains to be investigated whether such a scenario is possible.