Far-Infrared Line Deficits in Galaxies with Extreme Lfir/MH2 Ratios

TL;DR

This study shows that the ratio of far-infrared luminosity to molecular gas mass (Lfir/MH2) better predicts far-infrared line deficits in galaxies, revealing differences in interstellar medium properties linked to star formation modes.

Contribution

It demonstrates that Lfir/MH2 is a superior proxy for line deficits and links these deficits to different star formation regimes in galaxies, supported by spectral modeling.

Findings

Line deficits occur at high Lfir/MH2 ratios.

All far-infrared lines show deficits, not just [C II].

Line deficits correlate with increased ionization parameters.

Abstract

We report initial results from the far-infrared fine structure line observations of a sample of 44 local starbursts, Seyfert galaxies and infrared luminous galaxies obtained with the PACS spectrometer on board Herschel. We show that the ratio between the far-infrared luminosity and the molecular gas mass, Lfir/MH2, is a much better proxy for the relative brightness of the far-infrared lines than Lfir alone. Galaxies with high Lfir/MH2 ratios tend to have weaker fine structure lines relative to their far-infrared continuum than galaxies with Lfir/MH2 < 80 Lsun/Msun. A deficit of the [C II] 158 micron line relative to Lfir was previously found with the ISO satellite, but now we show for the first time that this is a general aspect of all far-infrared fine structure lines, regardless of their origin in the ionized or neutral phase of the interstellar medium. The Lfir/MH2 value where these line deficits start to manifest is similar to the limit that separates between the two modes of star formation recently found in galaxies on the basis of studies of their gas-star formation relations. Our finding that the properties of the interstellar medium are also significantly different in these regimes provides independent support for the different star forming relations in normal disk galaxies and major merger systems. We use the spectral synthesis code Cloudy to model the emission of the lines. The expected increase of the ionization parameter with Lfir/MH2 can simultaneously explain the line deficits in the [C II], [N II] and [O I] lines.