A Herschel/PACS Far Infrared Line Emission Survey of Local Luminous Infrared Galaxies (1705.04326v2)

Abstract: We present an analysis of [OI]63, [OIII]88, [NII]122 and [CII]158 far-infrared (FIR) fine-structure line observations obtained with Herschel/PACS, for ~240 local luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey (GOALS). We find pronounced declines -deficits- of line-to-FIR-continuum emission for [NII]122, [OI]63 and [CII]158 as a function of FIR color and infrared luminosity surface density, $\Sigma_{\rm IR}$. The median electron density of the ionized gas in LIRGs, based on the [NII]122/[NII]205 ratio, is $n_{\rm e}$ = 41 cm$^{-3}$. We find that the dispersion in the [CII]158 deficit of LIRGs is attributed to a varying fractional contribution of photo-dissociation-regions (PDRs) to the observed [CII]158 emission, f([CII]PDR) = [CII]PDR/[CII], which increases from ~60% to ~95% in the warmest LIRGs. The [OI]63/[CII]158PDR ratio is tightly correlated with the PDR gas kinetic temperature in sources where [OI]63 is not optically-thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, $n_{\rm H}$, and intensity of the interstellar radiation field, in units of G$0$, and find G$_0$/$n{\rm H}$ ratios ~0.1-50 cm$^3$, with ULIRGs populating the upper end of the distribution. There is a relation between G$0$/$n{\rm H}$ and $\Sigma_{\rm IR}$, showing a critical break at $\Sigma_{\rm IR}^{\star}$ ~ 5 x 10$^{10}$ Lsun/kpc$^2$. Below $\Sigma_{\rm IR}^{\star}$, G$0$/$n{\rm H}$ remains constant, ~0.32 cm$^3$, and variations in $\Sigma_{\rm IR}$ are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above $\Sigma_{\rm IR}^{\star}$, G$0$/$n{\rm H}$ increases rapidly with $\Sigma_{\rm IR}$, signaling a departure from the typical PDR conditions found in normal star-forming galaxies towards more intense/harder radiation fields and compact geometries typical of starbursting sources.

Analysis of Far Infrared Line Emission in Local LIRGs

The paper "A Herschel/PACS Far Infrared Line Emission Survey of Local Luminous Infrared Galaxies" provides an extensive analysis of far-infrared (FIR) fine-structure line observations in local luminous infrared galaxies (LIRGs). These observations were part of the Great Observatories All-sky LIRG Survey (GOALS), with data obtained through Herschel/PACS instrumentation. The aim was to understand the cooling processes of interstellar gas and the conditions within star-forming regions in LIRGs.

Key Findings

1. FIR Line Deficits: The paper identified pronounced deficits in the flux of FIR lines ([N II], [O I], [C II]) relative to the FIR continuum emission. These deficits correlate strongly with FIR color and infrared luminosity surface density. The [C II] deficit was particularly significant, decreasing by a factor of approximately 50 in the warmest LIRGs.
2. Electron Densities: The electron densities in LIRGs were estimated based on the [N II] emission line ratios. LIRGs typically exhibit median electron densities around 41 cm^-3, which is consistent with other star-forming galaxies.
3. PDR Contributions: The paper differentiated between the contributions of ionized gas and photo-dissociation regions (PDRs) to the [C II] emission. It found that PDRs contribute increasingly to [C II] emission as FIR color warms, ranging from 60% in cooler galaxies up to 95% in the warmest LIRGs.
4. Spectral Models: The [O I]/[C II] line ratio was modeled as a function of gas kinetic temperature using radiative transfer codes, showing a tight correlation with the average dust temperature in galaxies. Some galaxies showed deviations due to factors like optical thickness in the [O I] line.
5. Physical Parameters: Using PDR models, the paper inferred key physical parameters such as the intensity of the UV radiation field (G_0) and the density of the neutral gas (n_H). The findings pointed to G_0 values ranging from 10^1 to 10^3.5 Habing units, distinctly increasing with the infrared luminosity of galaxies.
6. Transition in Star-Forming Regions: A critical transition was noted in the PDR conditions related to the luminosity surface density (Σ_IR). Below a certain threshold, variations in luminosity surface density were mainly due to changes in the number densities of star-forming regions. Beyond this threshold, there was a marked departure indicating more intense radiation fields typical of starburst environments.

Implications

The implications of this research extend to the understanding of galaxy formation and evolution. By dissecting the cooling channels and ISM conditions in LIRGs, the paper provides insights into the physical processes governing starburst regions, especially in the context of galaxy mergers. The dynamics of these processes are reflected in the FIR line emission characteristics and hold significance for theoretical models of galaxy evolution.

Future Outlook

Further exploration could involve high-resolution observations and modeling to elucidate the nuances of PDR structures and their interactions with other ISM phases. Also, extending this research to higher redshift populations can reveal the cosmic evolution of star-forming galaxies and their environments in the early universe. The paper's findings underscore the need to refine models of star formation in merging galaxies, considering variations in ionization fields and the compactness of starburst regions.

In conclusion, the comprehensive analysis presented in this paper enriches the understanding of FIR emission processes in LIRGs and sets a foundation for subsequent studies aimed at deciphering the complex interplay of ISM components in dynamically evolving galaxies.