Explaining the [CII]158um Deficit in Luminous Infrared Galaxies - First Results from a Herschel/PACS Study of the GOALS Sample

Abstract: We present the first results of a survey of the [CII]158um emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky Survey (GOALS) sample, obtained with the PACS instrument on board Herschel. The [CII] luminosities of the LIRGs in GOALS range from ~10^7 to 2x10^9 Lsun. We find that LIRGs show a tight correlation of [CII]/FIR with far-IR flux density ratios, with a strong negative trend spanning from ~10^-2 to 10^-4, as the average temperature of dust increases. We find correlations between the [CII]/FIR ratio and the strength of the 9.7um silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region (Sigma_MIR), suggesting that warmer, more compact starbursts have substantially smaller [CII]/FIR ratios. Pure star-forming (SF) LIRGs have a mean [CII]/FIR ~ 4x10^-3, while galaxies with low 6.2um PAH equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGN), span the full range in [CII]/FIR. However, we show that even when only pure SF galaxies are considered, the [CII]/FIR ratio drops by an order of magnitude, from 10^-2 to 10^-3, with Sigma_MIR and Sigma_IR, implying that the [CII] luminosity is not a good indicator of the star formation rate (SFR) for most LIRGs, for it does not scale linearly with the warm dust emission. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies show [CII]/FIR >= 10^-3 typical of high 6.2um PAH EW sources, suggesting that most AGNs do not contribute significantly to the far-IR emission. We provide an empirical relation between the [CII]/FIR and the specific SFR (SSFR) for SF LIRGs. Finally, we present predictions for the starburst size based on the observed [CII] and far-IR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.

Correlations in Luminous Infrared Galaxies: Analyzing [C II] Line Emission

The paper presents an extensive study of the [C II] emission line across a sample of 241 Luminous Infrared Galaxies (LIRGs), utilizing data from the Herschel/PACS instrument. Such a study is crucial for understanding the intricate balance between gas cooling and dust heating within these galaxies, and potentially offers insights into their star formation processes and the role of active galactic nuclei (AGN).

Key Observations and Results

The study identifies several correlations of the [C II]/FIR (Far-Infrared) ratio with different galactic features, emphasizing how the [C II] deficit is not solely influenced by AGN activity but also by the compactness and temperature of star-forming regions. Notably:

• Dust Temperature and [C II] Deficit: A strong negative trend exists between the [C II]/FIR ratio and far-IR flux density ratios, indicating that galaxies with warmer dust show a decrease in [C II] relative to their FIR emission. This suggests that higher dust temperatures, driven by ionization parameters, reduce the gas heating efficiency relative to dust emission.

• Silicate Absorption and Compactness: The strength of the 9.7 µm silicate absorption feature correlates with the [C II]/FIR ratio, supporting the hypothesis that dust responsible for mid-IR absorption contributes significantly to far-IR emission. Moreover, mid-IR compactness, shown by higher surface densities at 15 µm, correlates with [C II] deficit, highlighting that more compact starbursts tend to have lower [C II]/FIR ratios.

The Role of AGN

Interestingly, the study finds that a significant portion of galaxies with low [C II]/FIR ratios do not exclusively host AGN. Among identified AGN, the majority do not significantly affect the far-IR emission, suggesting that the star-forming processes predominantly drive the observed [C II]/FIR trend. Only in extreme low [C II]/FIR cases might AGN play a prominent role, which prompts reevaluation of AGN's contribution to such deficits in high-redshift galaxies.

Implications and Future Directions

This paper outlines the predictive utility of [C II], mid-IR compactness, and AGN contamination in assessing star formation and infrared properties of high-redshift galaxies. The defined correlations provide a framework to estimate starburst size and efficiency in upcoming surveys with instruments like ALMA and CCAT, which will further explore galaxy evolution and star formation in different cosmic epochs.

The results encourage future studies to consider both intrinsic galactic characteristics and the influence of AGN when interpreting [C II] emission data both in the local universe and at cosmological distances. As instruments progress, comparing these luminosities across different wavelengths and epochs will be critical for a comprehensive understanding of galaxy formation and evolution mechanisms.

In conclusion, these findings elucidate the nuanced interplays between star formation, AGN activity, and galactic structure in determining the emission profiles and evolutionary paths of LIRGs. The insights provided will undoubtedly inform subsequent research endeavors in extragalactic astrophysics.