Explaining the [C II]157.7 μm Deficit in Luminous Infrared Galaxies—First Results from a Herschel/PACS Study of the GOALS Sample

TL;DR: In this article, the authors present the first results of a survey of the [C II]157.7 {$μ$}m emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky LIRG Survey (GOALS) sample, obtained with the PACS instrument on board the Herschel Space Observatory.

Abstract: We present the first results of a survey of the [C II]157.7 {$μ$}m emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky LIRG Survey (GOALS) sample, obtained with the PACS instrument on board the Herschel Space Observatory. The [C II] luminosities, L $_{[C II]}$, of the LIRGs in GOALS range from ~{}10$^{7}$ to 2 { imes} 10$^{9}$ L $_{⊙}$. We find that LIRGs show a tight correlation of [C II]/FIR with far-IR (FIR) 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 [C II]/FIR ratio and the strength of the 9.7 {$μ$}m 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 [C II]/FIR ratios. Pure star-forming LIRGs have a mean [C II]/FIR ~{} 4 { imes} 10$^{-3}$, while galaxies with low polycyclic aromatic hydrocarbon (PAH) equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGNs), span the full range in [C II]/FIR. However, we show that even when only pure star-forming galaxies are considered, the [C II]/FIR ratio still drops by an order of magnitude, from 10$^{-2}$ to 10$^{-3}$, with {$Sigma$}$_{MIR}$ and {$Sigma$}$_{IR}$, implying that the [C II]157.7 {$μ$}m luminosity is not a good indicator of the star formation rate (SFR) for most local LIRGs, for it does not scale linearly with the warm dust emission most likely associated to the youngest stars. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies show [C II]/FIR {gt}= 10$^{-3}$ typical of high 6.2 {$μ$}m PAH EW sources, suggesting that most AGNs do not contribute significantly to the FIR emission. We provide an empirical relation between the [C II]/FIR and the specific SFR for star-forming LIRGs. Finally, we present predictions for the starburst size based on the observed [C II] and FIR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.

Summary

1. INTRODUCTION

• Systematic spectroscopic observations of far-infrared (FIR) cooling lines in large samples of local star-forming galaxies and active galactic nuclei (AGNs) were first carried out with the Infrared Space Observatory (ISO; e.g., Malhotra et al.
• The underlying causes for these trends are still debated.
• This reduces both the amount of photo-electrons released from dust grains that indirectly collisionally excite the gas, as well as the energy that they carry along after they are freed, since they are more strongly bounded.

2.1. The GOALS Sample

• The RBGS, and therefore the GOALS sample, were defined based on IRAS observations.
• From the 291 individual galaxies in GOALS, not all have Herschel observations.
• In systems with two or more galactic nuclei, minor companions with MIPS 24 μm flux density ratios smaller than 1:5 with respect to the brightest galaxy were not requested since their contribution to the total IR luminosity of the system is small.
• Because the angular resolution of Spitzer decreases with wavelength, it was not possible to obtain individual MIPS 24, 70 and 160 μm measurements for all GOALS galaxies, and therefore to derive uniform IR luminosities for them using Spitzer data only.

2.2. Herschel/PACS Observations

• The authors have obtained FIR spectroscopic observations for 153 LIRG systems of the GOALS sample using the Integral Field Spectrometer (IFS) of the PACS instrument on board Herschel.
• The high sampling density mode scan, useful to have sub-spectral resolution information of the lines (see below), was employed.
• While the authors requested line maps for some LIRGs of the sample (from two to a few raster positions depending on the target), pointed (one single raster) chop-nod observations were taken for the majority of galaxies.

3.1. Data Processing

• The Herschel Interactive Processing Environment (HIPE; ver. 8.0) application was used to retrieve the raw data from the Herschel Science Archive 26 as well as to process them.
• 0 of HIPE and later versions, and helps to improve the accuracy of the continuum level).
• Flag and reject remaining outliers, rebin all selected cubes on consistent wavelength grids and, finally, average the nod-A and nod-B rebinned cubes (all cubes at the same raster position are averaged), also known as The final steps are.
• This is the final sciencegrade product currently possible for single raster observations.

3.2. Data Analysis

• To obtain the [C ii] flux of a particular source the authors use an iterative procedure to find the line and measure its basic parameters.
• Absolute photometric uncertainties due to changes in the PACS calibration products are not taken into account (the version used in this work was PACS_CAL_32_0).
• In some occasions the pointing of Herschel is not accurate enough to achieve this and the target can be slightly misplaced 3 (up to 1/3 of a spaxel) from the center.

4.1. The [C ii]/FIR Ratio: Dust Heating and Cooling

• The FIR fine-structure line emission in normal star-forming galaxies as well as in the extreme environments hosted by ULIRGs has been extensively studied for the past two decades.
• The solid line in the upper panel corresponds to a linear fit of the data in log-log space.
• GOALS densely populates this critical part of phase-space providing a large sample of galaxies with which to explore the physical conditions behind the drop in [C ii] emission among LIRGs.
• At the same time, dust grains would be on average at higher temperatures due to the larger number of ionizing photons per dust particle available in the outer layers of the H ii regions, close to the PDRs.
• Both effects combined can explain the wide range of [C ii]157.7 μm/ FIR ratios and FIR colors the authors observe in the most warm LIRGs.

4.1.2. The Link between Mid-IR Dust Obscuration and FIR Re-emission

• P being the unobscured and observed continuum flux density measured in the mid-IR IRS spectra of their LIRGs and evaluated at the peak of the feature, λ P , normally at 9.7 μm (see Stierwalt et al. 2013 for details on how it was calculated in their sample).
• If at the same time this source is deeply buried (optically thick) and embedded in layers of progressively colder dust (geometrically thick), it could produce a cumulative absorption that the authors would measure via the strength of the silicate feature while still contributing to the emission outside of it (see Levenson et al. 2007; Sirocky et al. 2008) .
• While both explanations are plausible, the second is favored by the fact that these extremely obscured galaxies show MIPS 24/70 μm ratios very similar, or even slightly higher than those found for the rest of the LIRGs in the sample.

4.1.3. The Compactness of the Mid-IR Emitting Region

• All ULIRGs in the GOALS sample have very small mid-IR emitting regions, with sizes (measured FWHMs) <1.5 kpc (Díaz-Santos et al. 2010) .
• In Section 4.1.1 the authors found that the [C ii]/FIR ratio is related (top) , and the fraction of extended emission at 13.2 μm, FEE 13.2 μm , for individual galaxies in the GOALS sample.
• The dotted lines are the ±1σ uncertainty.
• The authors note that the 15 μm luminosities are measured within the Spitzer/IRS LL slit while the mid-IR sizes were obtained from the SL module at 13.2 μm (Díaz-Santos et al. 2010).
• Thus, the authors should expect to see a correlation between the [C ii] deficit and the luminosity surface density and compactness of LIRGs in the mid-IR.

4.2. The Role of Active Galactic Nuclei

• The EW of mid-IR PAH features is a simple diagnostic that has been widely used for the detection of AGN activity in galaxies at low and high redshifts (Genzel et al.
• Sources with intermediate values are considered composite galaxies, in which both starburst and AGN may contribute significantly to the mid-IR emission.

4.2.1. [C ii]157.7 μm Deficit in Pure Star-Forming LIRGs

• If this information is not available, galaxies are shown as small black circles.
• The solid line represents the range in [C ii]/FIR and 6.2 μm PAH with increasing contribution from an AGN (see text for details).
• Stacey et al. (2010) also find that the AGN-powered sources in their high-redshift galaxy sample display small [C ii]/FIR ratios.
• The result obtained above also implies that the [C ii]157.7 μm line alone is not a good tracer of the SFR in most local LIRGs since it does not account for the increase of warm dust emission seen in the most compact galaxies that is usually associated with the most recent starburst.
• The slope and intercept of this trend are indistinguishable (within the uncertainties) from those obtained in Equation ( 3), which was derived by fitting all data-points including low 6.2 μm PAH EW sources with measured mid-IR sizes.

4.2.2. The Influence of AGNs in the [C ii] Deficit

• As the 6.2 μm PAH EW becomes smaller the dispersion increases and the authors find galaxies with both very small ratios as well as sources with normal values (or slightly lower than those) typical of purely star-forming sources (see also Sargsyan et al. 2012) .
• The authors note that this excludes sources with low 6.2 μm PAH EWs and no other AGN signatures, and is a more conservative cut than applied in Petric et al. (2011) to identify potential AGNs.
• While ∼18% of their sample appears to have significant AGN contribution to the mid-IR emission (Petric et al. 2011) , the fraction in which the AGN dominates the bolometric luminosity of the galaxy is much smaller.
• The authors argued in Section 4.1.2 that these galaxies are probably hosting an extremely warm and compact source, optically and geometrically thick, not associated with the star-forming regions producing the bulk of the [C ii] and FIR.

5. IMPLICATIONS FOR INTERMEDIATE-AND HIGH-REDSHIFT GALAXY SURVEYS

• A surprising discovery came from the most luminous systems, and the fact that many of them show values of this ratio similar to those found in local, lower luminosity galaxies (e.g., Maiolino et al.
• The authors have used this normalization factor to derive the excess of SSFR in their galaxies (also called "starburstiness:" SSFR/SSFR MS ).
• The correlation coefficient derived from the robust fit is −0.76.
• In particular, one of them display a [C ii]/FIR more than an order of magnitude lower than the value predicted by the fit to their local galaxy sample.
• In these cases, the predicted mid-IR size of galaxies could be compared with direct measurements of the size of their FIR emitting region as observed with ALMA on physical scales similar to those the authors are probing in their GOALS LIRGs with PACS.

6. CONCLUSIONS

• The authors combined this information together with Spitzer/IRS spectroscopic data to provide the context in which the observed [C ii] luminosities and [C ii]/FIR ratios are best explained.
• There are a small number of LIRGs that have a larger [C ii]/FIR ratio than suggested by their deep S 9.7 μm and warm dust emission.
• Moreover, above this ratio the AGN fraction is expected to be 20%-25%.
• The authors also thank David Elbaz, Alexander Karim, J. D. Smith, Moshe Elitzur, and J. Graciá-Carpio for very fruitful discussions.
• This work is based on observations made with the Herschel Space Observatory, an European Space Agency Cornerstone Mission with science instruments provided by European-led Principal Investigator consortia and significant participation from NASA.

Figures

Table 1 Herschel/PACS Measurements for the GOALS Sample

Table 2 Fraction of AGN

Figure 7. [C ii]157.7μm/FIR ratio vs. integrated SSFR normalized to the SSFRMS(z ∼ 0) for individual galaxies in the GOALS sample. Galaxies are color-coded as a function of their 6.2μm PAH EW. Colored circles indicate sources for which an EW is available. Squares indicate lower limits. Small black circles are sources for which there is no information. The solid line is a fit to pure star-forming LIRGs only, excluding those sources that may harbor an AGN that could dominate their mid-IR emission (6.2μm PAH EWs < 0.5μm; see Figure 6). The dotted lines are the ±1σ uncertainty. The dashed line indicates the SSFR of MS galaxies at any redshift or M . The dotted-dashed line represents 3 × SSFRMS, the limit above which galaxies are considered to be starbursting. The open squares represent two intermediate-redshift galaxies at z ∼ 1.2, while the open diamonds represent three high-redshift sub-millimeter galaxies at z ∼ 4.5 (see text for details).

Figure 3. [C ii]157.7μm/FIR ratio for individual galaxies in the GOALS sample as a function of the strength of the 9.7μm silicate absorption feature, S9.7μm, measured with Spitzer/IRS. Galaxies are color-coded as a function of the Sν 63μm/Sν 158μm ratio, with values ranging from 0.10 to 4.15 (see Figure 2). The solid line represents an outlier-resistant fit to the bulk of the star-forming LIRG population with S9.7μm −2. The dotted lines are the ±1σ uncertainty. (A color version of this figure is available in the online journal.)

Figure 8. PACS-based Sν 63μm/Sν 158μm ratio as a function of IRAS-based Sν 60μm/Sν 100μm ratio for individual galaxies in the GOALS sample. In LIRG systems with two or more galaxies, the same IRAS color is plotted for each individual galaxy. The calculation of the LFIR and the symbols are as in Figure 2.

Figure 4. [C ii]157.7μm/FIR ratio as a function of the luminosity surface density at 15μm,Σ15μm (top), and the fraction of extended emission at 13.2μm, FEE13.2μm (bottom), for individual galaxies in the GOALS sample. Galaxies are color-coded as a function of the 63/158μm ratio (top) and the strength of the silicate feature, S9.7μm (bottom). A linear fit to the datapoints without limits is shown in the top panel as a solid line. The dotted lines are the ±1σ uncertainty. We note that the 15μm luminosities are measured within the Spitzer/IRS LL slit while the mid-IR sizes were obtained from the SL module at 13.2μm (Dı́az-Santos et al. 2010). For very extended sources, the MIPS 24μm images were used instead to measure the size of the starburst region. The intrinsic sizes (FWHMint) of the mid-IR emission were obtained after subtracting, in quadrature, the contribution of the instrumental profile (FWHMPSF) from the measured FWHM.

Figure 5. [C ii]157.7μm/FIR ratio as a function of the nuclear LIR divided by the area of the mid-IR emitting region (ΣIR = LIR/πr2mid-IR) for individual galaxies in the GOALS sample. This figure is the same as Figure 4 but using the nuclear LIR of galaxies (scaled as the FIR flux; see Section 3.2) instead of their 15μm monochromatic luminosity, and it is color-coded as a function of the 6.2μm PAH EW. Only pure star-forming LIRGs, defined as to have 6.2μm PAH 0.5μm, are shown. The solid line is a fit to the data. See Equation (4).

Figure 6. [C ii]157.7μm/FIR ratio for individual galaxies in the GOALS sample as a function of the 6.2μm PAH EW measured with Spitzer/IRS. Galaxies are color-coded as a function of their νLν 63/15μm ratio. If this information is not available, galaxies are shown as small black circles. The solid line represents the range in [C ii]/FIR and 6.2μm PAH with increasing contribution from an AGN (see text for details). The dotted lines are×2 and×0.5 the predicted trend, which accounts for variations in the [C ii]/6.2μm PAH ratio and the LFIR/νLν 6μm relation for star-forming galaxies. The dashed line assumes a decreasing of the [C ii]/6.2μm PAH ratio proportional to the 6.2μm PAH EW due to pure PAH destruction from an AGN.

Figure 1. Ratio of [C ii]157.7μm to FIR flux as a function of the FIR luminosity for individual galaxies in the GOALS sample (green circles) and for unresolved galaxies observed with ISO (gray circles and limits) obtained from the compilation of Brauher et al. (2008) located at z > 0.003, similar to the distance range covered by our LIRGs. The LFIR of galaxies was calculated as explained at the end of Section 3.2 and covers the 42.5–122.5μm wavelength range as defined in Helou et al. (1988).

Figure 2. Ratio of [C ii]157.7μm to FIR flux (upper panel) and [C ii]157.7μm EW (bottom panel) as a function of the Sν 63μm/Sν 158μm continuum flux density ratio for individual galaxies in the GOALS sample. Circles of different colors indicate the LFIR of galaxies (see color bar), which is defined as explained in Section 3.2. Red diamonds mark galaxies with LIR 1012 L , ULIRGs. These two plots show that the decrease of the [C ii]/FIR ratio with warmer FIR colors seen in our LIRGs is primarily caused by a significant decrease of gas heating efficiency and an increase of warm dust emission. The solid line in the upper panel corresponds to a linear fit of the data in log–log space. The parameters of the fit are given in Equation (1). The dotted lines are the ±1σ uncertainty.

Full text

The Astrophysical Journal, 774:68 (13pp), 2013 September 1 doi:10.1088/0004-637X/774/1/68
C

2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
EXPLAINING THE [C ii]157.7 μm DEFICIT IN LUMINOUS INFRARED GALAXIES—
FIRST RESULTS FROM A HERSCHEL/PACS STUDY OF THE GOALS SAMPLE
T. D
´
ıaz-Santos
1
, L. Armus
1
, V. Charmandaris
2,3,4
, S. Stierwalt
5
,E.J.Murphy
6
, S. Haan
7
,H.Inami
8
, S. Malhotra
9
,
R. Meijerink
10
, G. Stacey
11
, A. O. Petric
12
,A.S.Evans
5,13
, S. Veilleux
14,15
, P. P. van der Werf
16
, S. Lord
17
,N.Lu
12,18
,
J. H. Howell
1
, P. Appleton
17
, J. M. Mazzarella
5
, J. A. Surace
1
,C.K.Xu
17
,B.Schulz
12,18
, D. B. Sanders
19
, C. Bridge
12
,
B. H. P. Chan
12
, D. T. Frayer
20
, K. Iwasawa
21
, J. Melbourne
22
, and E. Sturm
23
1
Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125, USA; tanio@ipac.caltech.edu
2
IESL/Foundation for Research and Technology-Hellas, GR-71110, Heraklion, Greece
3
Chercheur Associ
´
e, Observatoire de Paris, F-75014 Paris, France
4
Department of Physics, University of Crete, GR-71003, Heraklion, Greece
5
Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA
6
Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
7
CSIRO Astronomy and Space Science, Marsfield NSW 2122, Australia
8
National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
9
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
10
Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, NL-9700 AV Groningen, The Netherlands
11
Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
12
Astronomy Department, California Institute of Technology, Pasadena, CA 91125, USA
13
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
14
Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
15
Department of Astronomy, University of Maryland, College Park, MD 20742, USA
16
Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands
17
NASA Herschel Science Center, IPAC, California Institute of Technology, MS 100-22, Cech, Pasadena, CA 91125, USA
18
Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125, USA
19
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
20
National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944, USA
21
ICREA and Institut de Cincies del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Marti i Franques 1, E-08028 Barcelona, Spain
22
Caltech Optical Observatories, Division of Physics, Mathematics and Astronomy, MS 301-17, California Institute of Technology, Pasadena, CA 91125, USA
23
Max-Planck-Institut f
¨
ur extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany
Received 2013 February 15; accepted 2013 July 9; published 2013 August 19
ABSTRACT
We present the first results of a survey of the [C ii]157.7 μm emission line in 241 luminous infrared galaxies (LIRGs)
comprising the Great Observatories All-sky LIRG Survey (GOALS) sample, obtained with the PACS instrument
on board the Herschel Space Observatory.The[Cii] luminosities, L
[C ii]
, of the LIRGs in GOALS range from ∼10
7
to 2 × 10
9
L

. We find that LIRGs show a tight correlation of [C ii]/FIR with far-IR (FIR) 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 [C ii]/FIR ratio and the strength of the 9.7 μm silicate absorption feature as well as with the
luminosity surface density of the mid-IR emitting region (Σ
MIR
), suggesting that warmer, more compact starbursts
have substantially smaller [C ii]/FIR ratios. Pure star-forming LIRGs have a mean [C ii]/FIR ∼ 4 × 10
−3
, while
galaxies with low polycyclic aromatic hydrocarbon (PAH) equivalent widths (EWs), indicative of the presence
of active galactic nuclei (AGNs), span the full range in [C ii]/FIR. However, we show that even when only pure
star-forming galaxies are considered, the [C ii]/FIR ratio still drops by an order of magnitude, from 10
−2
to 10
−3
,
with Σ
MIR
and Σ
IR
, implying that the [C ii]157.7 μm luminosity is not a good indicator of the star formation rate
(SFR) for most local LIRGs, for it does not scale linearly with the warm dust emission most likely associated to the
youngest stars. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies
show [C ii]/FIR  10
−3
typical of high 6.2 μm PAH EW sources, suggesting that most AGNs do not contribute
significantly to the FIR emission. We provide an empirical relation between the [C ii]/FIR and the specific SFR
for star-forming LIRGs. Finally, we present predictions for the starburst size based on the observed [C ii] and FIR
luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with
ALMA and CCAT.
Key words: galaxies: ISM – galaxies: nuclei – galaxies: starburst – infrared: galaxies
Online-only material: color figures, machine-readable table
1. INTRODUCTION
Systematic spectroscopic observations of far-infrared (FIR)
cooling lines in large samples of local star-forming galaxies and
active galactic nuclei (AGNs) were first carried out with the
Infrared Space Observatory (ISO; e.g., Malhotra et al. 1997,
2001; Luhman et al. 1998; Brauher et al. 2008). These studies
showed that [C ii]157.7 μm is the most intense FIR emission
line observed in normal, star-forming galaxies (Malhotra et al.
1997) and starbursts (e.g., Nikola et al. 1998; Colbert et al.
1999), dominating the gas cooling of their neutral interstellar
medium (ISM). This fine-structure line arises from the
2
P
3/2
→
2
P
1/2
transition (E
ul
/k = 92 K) of singly ionized Carbon
atoms (ionization potential = 11.26 eV and critical density,
1

The Astrophysical Journal, 774:68 (13pp), 2013 September 1 D
´
ıaz-Santos et al.
n
cr
H
 2.7×10
3
cm
−3
; n
cr
e
−
 46 cm
−3
) which are predominantly
excited by collisions with neutral hydrogen atoms; or with free
electrons and protons in regions where n
e
−
/n
H
 10
−3
(Hayes
& Nussbaumer 1984). Ultraviolet (UV) photons with energies
>6 eV emitted by newly formed stars are able to release the most
weakly bound electrons from small dust grains via photo-electric
heating (Watson 1972;Draine1978). In particular, polycyclic
aromatic hydrocarbons (PAHs) are thought to be an important
source of photo-electrons (Helou et al. 2001) that contribute,
through kinetic energy transfer, to the heating of the neutral gas
which subsequently cools down via collision with C
+
atoms and
other elements in photo-dissociation regions (PDRs; Tielens &
Hollenbach 1985; Wolfire et al. 1995).
The [C ii]157.7 μm emission accounts, in the most extreme
cases, for as much as ∼1% of the total IR luminosity of galaxies
(Stacey et al. 1991; Helou et al. 2001). However, the [C ii]/FIR
ratio is observed to decrease by more than an order of magnitude
in sources with high L
IR
and warm dust temperatures (T
dust
). The
underlying causes for these trends are still debated. The physical
arguments most often proposed to explain the decrease in
[C ii]/FIR are: (1) self-absorption of the C
+
emission, (2)
saturation of the [C ii] line flux due to high density of the neutral
gas, (3) progressive ionization of dust grains in high far-UV field
to gas density environments, and (4) high dust-to-gas opacity
caused by an increase of the average ionization parameter.
Although self-absorption has been used to explain the faint
[C ii] emission arising from warm, AGN-dominated systems
such as Mrk 231 (Fischer et al. 2010), this interpretation has
been questioned in normal star-forming galaxies due to the
requirement of extraordinarily large column densities of gas
in the PDRs (Luhman et al. 1998, Malhotra et al. 2001).
Furthermore, contrary to the [O i]or[Ci] lines, the [C ii]
emission is observed to arise from the external edges of
those molecular clouds exposed to the UV radiation originated
from starbursts, as for example in Arp 220 (Contini 2013).
Therefore, self-absorption is not the likely explanation of the low
[C ii]/FIR ratios seen in most starburst galaxies, except perhaps
in a few extreme cases, like NGC 4418 (Malhotra et al. 1997).
The [C ii] emission becomes saturated when the hydro-
gen density in the neutral medium, n
H
, increases to values
10
3
cm
−3
, provided that the far-UV (6–13.6 eV) radiation field
is not extreme (G
0
 10
4
; where G
0
is normalized to the average
local interstellar radiation field; Habing 1968). For example, for
a constant G
0
= 10
2
, an increase of the gas density from 10
4
to 10
6
cm
−3
would produce a suppression of the [C ii] emission
of almost two orders of magnitude due to the rapid recombi-
nation of C
+
into neutral carbon and then into CO (Kaufman
et al. 1999). However, PDR densities as high as 10
4
cm
−3
are
not very common. [O i]63.18 μm and [C ii]157.7 μm ISO ob-
servations of normal star-forming galaxies and some IR-bright
sources confine the physical parameters of their PDRs to a range
of G
0
 10
4.5
and 10
2
 n
H
 10
4
cm
−3
(Malhotra et al. 2001).
On the other hand, the [C ii] emission can be also saturated when
G
0
> 10
1.5
provided that n
H
 10
3
cm
−3
. In this regime, the
line is not sensitive to an increase of G
0
because the tempera-
ture of the gas is well above the excitation potential of the [C ii]
transition.
It has also been suggested that in sources where G
0
/n
H
is high
(10
2
cm
3
)the[Cii] line is a less efficient coolant of the ISM
because of the following reason. As physical conditions become
more extreme (higher G
0
/n
H
), dust particles progressively
increase their positive charge (Tielens & Hollenbach 1985;
Malhotra et al. 1997; Negishi et al. 2001). This reduces both
the amount of photo-electrons released from dust grains that
indirectly collisionally excite the gas, as well as the energy that
they carry along after they are freed, since they are more strongly
bounded. The net effect is the decreasing of the efficiency in the
transformation of incident UV radiation into gas heating without
an accompanied reduction of the dust emission (Wolfire et al.
1990; Kaufman et al. 1999; Stacey et al. 2010).
In a recent work, Graci
´
a-Carpio et al. (2011)have
shown that the deficits observed in several FIR emis-
sion lines ([C ii]157.7 μm, [O i]63.18 μm, [O i]145 μm, and
[N ii]122 μm) could be explained by an increase of the aver-
age ionization parameter of the ISM, U .
24
In “dust bounded”
star-forming regions the gas opacity is reduced within the H ii
region due to the higher U . As a consequence, a significant
fraction of the UV radiation is eventually absorbed by large dust
grains before being able to reach the neutral gas in the PDRs and
ionize the PAH molecules (Voit 1992; Gonz
´
alez-Alfonso et al.
2004; Abel et al. 2009), causing a deficit of photo-electrons and
hence the subsequent suppression of the [C ii] line with respect
to the total FIR dust emission.
Local luminous IR galaxies (LIRGs; L
IR
= 10
11−12
L

)area
mixture of single galaxies, disk galaxy pairs, interacting systems
and advanced mergers, exhibiting enhanced star formation rates
(SFRs), and a lower fraction of AGNs compared to higher
luminous galaxies. A detailed study of the physical properties
of low-redshift LIRGs is critical for our understanding of
the cosmic evolution of galaxies and black holes since (1)
IR-luminous galaxies comprise the bulk of the cosmic IR
background and dominate star formation activity between 0.5 <
z<2 (Caputi et al. 2007; Magnelli et al. 2011; Murphy
et al. 2011; Berta et al. 2011) and (2) AGN activity may
preferentially occur during episodes of enhanced nuclear star
formation. Moreover, LIRGs are now assumed to be the local
analogs of the IR-bright galaxy population at z>1. However,
a comprehensive analysis of the most important FIR cooling
lines of the ISM in a complete sample of nearby LIRGs
has not been possible until the advent of the Herschel Space
Observatory (Herschel hereafter; Pilbratt et al. 2010) and, in
particular, its Photodetector Array Camera and Spectrometer
(PACS; Poglitsch et al. 2010).
In this work we present the first results obtained from
Herschel/PACS spectroscopic observations of a complete sam-
ple of FIR selected local LIRGs that comprise the Great Ob-
servatories All-sky LIRG Survey (GOALS; Armus et al. 2009).
Using this complete, flux-limited sample of local LIRGs, we are
able for the first time to perform a systematic, statistically sig-
nificant study of the FIR cooling lines of star-forming galaxies
covering a wide range of physical conditions: from isolated disks
where star formation is spread across kiloparsec scales to the
most extreme environments present in late stage major mergers
where most of the energy output of the system comes from its
central kiloparsec region. In particular, in this paper we focus on
the [C ii]157.7 μm line and its relation with the dust emission in
LIRGs. We make use of a broad set of mid-IR diagnostics based
on Spitzer/IRS spectroscopy, such as high ionization emission
lines, silicate dust opacities, PAH equivalent widths (EW), dust
luminosity concentrations, and mid-IR colors, to provide the
context in which the observed [C ii] emission and [C ii]/FIR
24
The ionization parameter is defined as U ≡ Q(H)/4πR
2
n
H
c,whereQ(H)
is the number of hydrogen ionizing photons, R is the distance of the ionizing
source to the PDR, n
H
is the atomic hydrogen density, and c is the speed of
light. If an average stellar population and size for the star-forming region is
assumed, then U ∝ G
0
/n
H
.
2

The Astrophysical Journal, 774:68 (13pp), 2013 September 1 D
´
ıaz-Santos et al.
ratios are best explained. The paper is organized as follows: In
Section 2 we present the LIRG sample and the observations. In
Section 3 we describe the processing and analysis of the data.
The results are presented in Section 4. In Section 5 we put in
context our findings with recent results from intermediate- and
high-redshift surveys started to be carried out by ALMA and in
the future by Cornell–Caltech Atacama Telescope (CCAT). The
summary of the results is given in Section 6.
2. SAMPLE AND OBSERVATIONS
2.1. The GOALS Sample
The GOALS (Armus et al. 2009) encompasses the complete
sample of 202 LIRGs and ULIRGs contained in the IRAS
Revised Bright Galaxy Sample (RBGS; Sanders et al. 2003)
which, in turn, is also a complete sample of 629 galaxies with
IRAS S
60 μm
> 5.24 Jy and Galactic latitudes |b| > 5
◦
. There
are 180 LIRGs and 22 ULIRGs in GOALS and their median
redshift is z = 0.0215 (or ∼95.2 Mpc), with the closest galaxy
being at z = 0.0030 (15.9 Mpc; NGC 2146) and the farthest at
z = 0.0918 (400 Mpc; IRAS 07251−0248). To date, there are
many published and ongoing works that have already exploited
the potential of all the multi-wavelength data available for this
sample including, among others, Galaxy Evolution Explorer
UV (Howell et al. 2010), Hubble Space Telescope optical and
near-IR (Haan et al. 2011; Kim et al. 2013), and Chandra
X-ray (Iwasawa et al. 2011) imaging, as well as Spitzer/IRS
mid-IR spectroscopy (D
´
ıaz-Santos et al. 2010, 2011; Petric et al.
2011; Stierwalt et al. 2013; S. Stierwalt, in preparation; Inami
et al. 2013), as well as a number of ground-based observatories
(Very Large Array, CARMA, etc.) and soon ALMA.
The RBGS, and therefore the GOALS sample, were defined
based on IRAS observations. However, the higher angular res-
olution achieved by Spitzer allowed us to spatially disentangle
galaxies that belong to the same LIRG system into separate
components. From the 291 individual galaxies in GOALS, not
all have Herschel observations. In systems with two or more
galactic nuclei, minor companions with MIPS 24 μm flux den-
sity ratios smaller than 1:5 with respect to the brightest galaxy
were not requested since their contribution to the total IR lu-
minosity of the system is small. Because the angular resolution
of Spitzer decreases with wavelength, it was not possible to
obtain individual MIPS 24, 70 and 160 μm measurements for
all GOALS galaxies, and therefore to derive uniform IR lumi-
nosities for them using Spitzer data only. Instead, to calculate
the individual, spatially integrated L
IR
of LIRGs belonging to a
system of two or more galaxies, we distributed the L
8–1000 μm
IR
of
the system as measured by IRAS (using the prescription given
in Sanders & Mirabel 1996) proportionally to the individual
MIPS 70 μm flux density of each component when available, or
to their MIPS 24 μm otherwise.
25
We will use these measure-
ments of L
IR
in Section 5.
2.2. Herschel/PACS Observations
We have obtained FIR spectroscopic observations for 153
LIRG systems of the GOALS sample using the Integral
Field Spectrometer (IFS) of the PACS instrument on board
Herschel. The data were collected as part of an OT1 program
(OT1_larmus_1; P.I.: L. Armus) awarded with more than 165 hr
25
There are two systems for which no individual MIPS 24 μm fluxes could be
obtained. In these cases their IRAC 8 μm emission was used for scaling the
L
IR
. These LIRGs are MCG+02-20-003 and VV250a.
of observing time. In this work will focus mainly on the
analysis and interpretation of the [C ii] observations of our
galaxy sample. PACS range spectroscopy of the [C ii]157.7 μm
fine-structure emission line was obtained for 163 individual
sources. Our observations were complemented with the in-
clusion of the remaining LIRGs in the GOALS sample for
which [C ii] observations are publicly available in the archive
(as of 2012 October) from various Herschel projects. The
main programs from which these data were gathered are:
KPGT_esturm_1 (P.I.: E. Sturm), KPOT_pvanderw_1 (P.I.: P.
van der Werf), and OT1_dweedman_1 (P.I.: D. Weedman). The
total number of LIRG systems for which there are [C ii] data
is 200 (IRASF08339+6517 and IRASF09111−1007 were not
observed). However, because some LIRGs are actually sys-
tems of galaxies (see above), the number of observed galaxies
was 241.
The IFS on PACS is able to perform simultaneous spec-
troscopy in the 51–73 or 70–105 μm (third and second orders,
respectively; “blue” camera) and the 102–210 μm (first order;
“red” camera) ranges. The integral field unit (IFU) is composed
by a 5 × 5 array of individual detectors (spaxels) each of one
with a field of view (FoV) of ∼9.

4, for a total of 47

× 47

.
The physical size of the PACS FoV at the median distance of
our LIRG sample is ∼20 kpc on a side. The number of spectral
elements in each pixel is 16, which are rearranged together via
an image slicer over two 16 × 25 Ge:Ga detector arrays (blue
and red cameras).
Our Astronomical Observation Requests were consistently
constructed using the “Range” spectroscopy template, which
allows the user to define a specific wavelength range for the
desired observations. Our selected range was slightly larger than
that provided by default for the “Line” mode. This was necessary
(1) to obtain parallel observations of the wide OH 79.18 μm
absorption feature using the blue camera when observing the
[C ii]157.7 μm line, and (2) to ensure that the targeted emission
lines have a uniform signal-to-noise ratio (S/N) across their
spectral profiles even if they are to be broader than a few hundred
km s
−1
. The high sampling density mode scan, useful to have
sub-spectral resolution information of the lines (see below), was
employed. While we requested line maps for some LIRGs of
the sample (from two to a few raster positions depending on
the target), pointed (one single raster) chop-nod observations
were taken for the majority of galaxies. For those galaxies with
maps, only one raster position was used to obtain the line fluxes
used in this work. The chopper throw varied from small to large
depending on the source. Spectroscopy of the LIRGs included
in GOALS but observed by other programs in [C ii] was not
always obtained using the “Range” mode but some of them
were observed using “LineScan” spectroscopy. The S/Nofthe
data varies not only from galaxy to galaxy but also depending
on the emission line considered. We provide uncertainties for all
quantities used across the analysis presented here that are based
on the individual spectrum of each line, therefore reflecting the
errors associated with—and measured directly on—the data.
2.3. Spitzer/IRS Spectroscopy
As part of the Spitzer GOALS legacy, all galaxies observed
with Herschel/PACS have available Spitzer/IRS low resolution
(R ∼ 60–120) slit spectroscopy (SL module: 5.5–14.5 μm, and
LL module: 14–38 μm). The 244 IRS spectra were extracted us-
ing the standard extraction aperture and point source calibration
mode in SPICE. The projected angular sizes of the apertures on
the sky are 3.

7 × 12

at the average wavelength of 10 μmin
3

The Astrophysical Journal, 774:68 (13pp), 2013 September 1 D
´
ıaz-Santos et al.
SL and 10.

6 × 35

at the average wavelength of 26 μm in LL.
Thus, the area covered by the SL aperture is approximately
equivalent (within a factor of ∼2) to that of an individual spaxel
of the IFS in PACS, and so is that of the LL aperture to a 3 × 3
spaxel box. The observables derived from the Infrared Spectro-
graph (IRS) data that we use in this work are the strength of the
9.7 μm silicate feature, S
9.7 μm
, and the EW of the 6.2 μm PAH,
which were presented in Stierwalt et al. (2013). We refer the
reader to this work for further details about the reduction,
extraction, calibration, and analysis of the spectra.
3. IFS/PACS DATA REDUCTION AND ANALYSIS
3.1. Data Processing
The Herschel Interactive Processing Environment (HIPE;
ver. 8.0) application was used to retrieve the raw data from
the Herschel Science Archive
26
as well as to process them.
We used the script for “LineScan” observations (also valid for
“Range” mode) included within HIPE to reduce our spectra. We
processed the data from level 0 up to level 2 using the following
steps: flag and reject saturated data, perform initial calibrations,
flag and reject “glitches,” compute the differential signal of
each on–off pair of data-points for each chopper cycle, calculate
the relative spectral response function, divide by the response,
convert frames to PACS cubes, and correct for flat-fielding (this
extra step is included in ver. 8.0 of HIPE and later versions, and
helps to improve the accuracy of the continuum level). Next, for
each camera (red or blue), HIPE builds the wavelength grid, for
which we chose a final rebinning with an oversample = 2, and
an upsample = 3 that corresponds to a Nyquist sampling. The
spectral resolution achieved at the position of the [C ii]157.7 μm
line was derived directly from the data and is ∼235 km s
−1
.
The final steps are: flag and reject remaining outliers, rebin
all selected cubes on consistent wavelength grids and, finally,
average the nod-A and nod-B rebinned cubes (all cubes at the
same raster position are averaged). This is the final science-
grade product currently possible for single raster observations.
From this point on, the analysis of the spectra was performed
using in-house developed IDL routines.
3.2. Data Analysis
To obtain the [C ii] flux of a particular source we use
an iterative procedure to find the line and measure its basic
parameters. First, we fit a linear function to the continuum
emission, which is evaluated at the edges of the spectrum,
masking the central 60% of spectral elements (where the line
is expected to be detected) and without using the first and final
10%, where the noise is large due to the poor sampling of the
scanning. Then, we fit a Gaussian function to the continuum-
subtracted spectrum and calculate its parameters. We define a
line as not detected when the peak of the Gaussian is below
2.5 times the standard deviation of the continuum, as measured
in the previous step. On the other hand, if the line is found,
we return to the original, total spectrum and fit again the
continuum using this time a wavelength range determined by
the two portions of the spectrum adjacent to the line located
beyond ±3σ from its center (where σ is the width of the
fitted Gaussian) and the following ±15% of spectral elements.
We then subtract this continuum from the total spectrum and
fit the line again. The new parameters of the Gaussian are
compared with the previous ones. This process is repeated until
26
http://herschel.esac.esa.int/Science_Archive.shtml
the location, sigma, and intensity of the line converge with an
accuracy of 1%, or when reaching 10 iterations. Due to the
merger-driven nature of many LIRGs, their gas kinematics are
extremely complicated and, as a consequence, the emission lines
of several sources present asymmetries and double peaks in their
profiles. However, despite the fact that the width determined by
the fit is not an accurate representation of the real shape of
the line, it can be used as a first order approximation for its
broadness. Therefore, instead of using the parameters of the
Gaussian to derive the flux of the line, we decided to integrate
directly over the final continuum-subtracted spectrum within
the ±3σ region around the central position of the line. The
associated uncertainty is calculated as the standard deviation of
the latest fitted continuum, integrated over the same wavelength
range as the line. Absolute photometric uncertainties due to
changes in the PACS calibration products are not taken into
account (the version used in this work was PACS_CAL_32_0).
27
We obtained the line fluxes for our LIRGs from the spectra
extracted from the spaxel at which the [C ii] line + continuum
emission of each galaxy peaks within the PACS FoV. The
Spitzer/IRS and Herschel pointings usually coincide within
2

. There are a few targets for which the IRS pointing is
located more than half a spaxel away from that of PACS. In
these cases, we decided to obtain the nuclear line flux of the
galaxy by averaging the spaxels closest to the coordinates of the
IRS pointing. These values are used only when PACS and IRS
measurements are compared directly in the same plot. There
is one additional LIRG system, IRAS 03582+6012, for which
the PACS pointing exactly felt in the middle of two galaxies
separated by only 5

. This LIRG is not used in the comparisons
of the [C ii] emission to the IRS data since the two individual
sources cannot be disentangled.
As mentioned in Section 2.3, the angular size of a PACS
spaxel is roughly similar (within a factor of two) to that of the
aperture used to extract the Spizer/IRS spectra of our galaxies.
Because the PACS beam is under-sampled at 160 μm(FWHM∼
12

compared with the 9.

6 size of the PACS spaxels), and most
of the sources in the sample are unresolved at 24 μm in our
MIPS images (which have a similar angular resolution as PACS
at ∼80 μm), an aperture correction has to be performed to the
spectra extracted from the emission-peak spaxel of each galaxy
to obtain their total nuclear fluxes. This was the same proce-
dure employed to obtain the mid-IR IRS spectra of our LIRGs.
The nominal, wavelength-dependent aperture correction func-
tion provided by HIPE ver. 8.0 works optimally when the source
is exactly positioned at the center of a given spaxel. However, in
some occasions the pointing of Herschel is not accurate enough
to achieve this and the target can be slightly misplaced 3

(up to 1/3 of a spaxel) from the center. In these cases, the
flux of the line might be underestimated. We explored whether
this effect could be corrected by measuring the position of the
source within the spaxel. However, some LIRGs in our sample
show low surface-brightness extended emission, either because
of their proximity and/or merger nature, or simply because the
gas and dust emission are spatially decoupled. This, combined
with the spatial sub-sampling of the PACS/IFS detector and
the poor S/N of some sources prevented us from obtaining an
accurate measurement of the spatial position and angular width
of the [C ii] emission and therefore from obtaining a more re-
fined aperture correction. Thus, we performed only the nominal
aperture correction provided by HIPE.
27
http://herschel.esac.esa.int/twiki/bin/view/Public/PacsCalTreeHistory
4

The Astrophysical Journal, 774:68 (13pp), 2013 September 1 D
´
ıaz-Santos et al.
Figure 1. Ratio of [C ii]157.7 μm to FIR flux as a function of the FIR
luminosity for individual galaxies in the GOALS sample (green circles) and
for unresolved galaxies observed with ISO (gray circles and limits) obtained
from the compilation of Brauher et al. (2008) located at z>0.003, similar to
the distance range covered by our LIRGs. The L
FIR
of galaxies was calculated
as explained at the end of Section 3.2 and covers the 42.5–122.5 μm wavelength
range as defined in Helou et al. (1988).
(A color version of this figure is available in the online journal.)
The IRAS FIR fluxes used throughout this paper were calcu-
lated as FIR = 1.26 × 10
−14
(2.58 S
60 μm
+ S
100 μm
)(Wm
−2
),
with S
ν
in (Jy). The FIR luminosities, L
FIR
, were defined as
4πD
2
L
FIR (L

). The luminosity distances, D
L
, were taken from
Armus et al. (2009). This definition of the FIR accounts for the
flux emitted within the 42.5–122.5 μm wavelength range as
originally defined in Helou et al. (1988). The FIR fluxes and
luminosities of galaxies were then matched to the aperture with
which the nuclear [C ii] flux was extracted (see above) by scal-
ing the integrated IRAS FIR flux of the LIRG system with the
ratio of the continuum flux density of each individual galaxy
evaluated at 63 μm in the PACS spectrum (extracted at the same
position and with the same aperture as the [C ii] line) to the total
IRAS 60 μm flux density of the system.
In Table 1 we present the [C ii]157.7 μm flux, the [C ii]/FIR
ratio, and the continuum flux densities at 63 and 158 μmfor
all the galaxies in our sample. Future updates of the data in
this table processed with newer versions of HIPE and PACS
calibration files will be available at the GOALS Web site:
http://goals.ipac.caltech.edu.
4. RESULTS AND DISCUSSION
4.1. The [C
ii
]/FIR Ratio: Dust Heating and Cooling
The FIR fine-structure line emission in normal star-forming
galaxies as well as in the extreme environments hosted by
ULIRGs has been extensively studied for the past two decades. A
number of works based on ISO data already suggested that the
relative contribution of the [C ii]157.7 μm line to the cooling
of the ISM in PDRs compared to that of large dust grains,
as gauged by the FIR emission, diminishes as galaxies are
more IR luminous (Malhotra et al. 1997; Luhman et al. 1998;
Brauher et al. 2008). Figure 1 display the classical plot of the
[C ii]/FIR ratio as a function of the FIR luminosity for our LIRG
sample. In addition, we also show for reference those galaxies
observed with ISO compiled by Brauher et al. (2008) that are
classified as unresolved and located at redshifts z>0.003,
similar to the distance range covered by GOALS. As we can
Figure 2. Ratio of [C ii]157.7 μm to FIR flux (upper panel) and [C ii]157.7 μm
EW (bottom panel) as a function of the S
ν
63 μm/S
ν
158 μm contin-
uum flux density ratio for individual galaxies in the GOALS sample. Cir-
cles of different colors indicate the L
FIR
of galaxies (see color bar), which
is defined as explained in Section 3.2. Red diamonds mark galaxies with
L
IR
 10
12
L

, ULIRGs. These two plots show that the decrease of the
[C ii]/FIR ratio with warmer FIR colors seen in our LIRGs is primarily caused
by a significant decrease of gas heating efficiency and an increase of warm dust
emission. The solid line in the upper panel corresponds to a linear fit of the data
in log–log space. The parameters of the fit are given in Equation (1). The dotted
lines are the ±1σ uncertainty.
(A color version of this figure is available in the online journal.)
see, our Herschel data confirm the trend seen with ISO by
which galaxies with L
FIR
 10
11
L

show a significant decrease
of the [C ii]/FIR ratio. GOALS densely populates this critical
part of phase-space providing a large sample of galaxies with
which to explore the physical conditions behind the drop in [C ii]
emission among LIRGs. For the 32 galaxies with measurements
obtained with both telescopes, the higher angular resolution
Herschel observations of the nuclei of LIRGs are able to recover
an average of ∼87% of the total [C ii] flux measured by ISO.
4.1.1. The Average Dust Temperature of LIRGs
Figure 2 (upper panel) shows the [C ii]157.7 μm/FIR ratio for
the GOALS sample as a function of the FIR PACS S
ν
63 μm/
S
ν
158 μm continuum flux density ratio. We chose to use this
PACS-based FIR color in the x-axis instead of the more common
IRAS 60/100 μm color mainly because of two main reasons:
(1) this way we are able plot data from individual galaxies
instead of being constrained by the spatial resolution of IRAS,
5

Frequently asked questions

Q1. What are the contributions in "C: " ?

The authors present the first results of a survey of the [ C ii ] 157. The authors find that LIRGs show a tight correlation of [ C ii ] /FIR with far-IR ( FIR ) flux density ratios, with a strong negative trend spanning from ∼10−2 to 10−4, as the average temperature of dust increases. However, the authors show that even when only pure star-forming galaxies are considered, the [ C ii ] /FIR ratio still drops by an order of magnitude, from 10−2 to 10−3, with ΣMIR and ΣIR, implying that the [ C ii ] 157. Moreover, even in LIRGs in which the authors detect an AGN in the mid-IR, the majority ( 2/3 ) of galaxies show [ C ii ] /FIR 10−3 typical of high 6. 2 μm PAH EW sources, suggesting that most AGNs do not contribute significantly to the FIR emission. The authors provide an empirical relation between the [ C ii ] /FIR and the specific SFR for star-forming LIRGs. Finally, the authors present predictions for the starburst size based on the observed [ C ii ] and FIR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT. 7 μm silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region ( ΣMIR ), suggesting that warmer, more compact starbursts have substantially smaller [ C ii ] /FIR ratios. 

Q2. What is the important source of photo-electrons?

In particular, polycyclic aromatic hydrocarbons (PAHs) are thought to be an important source of photo-electrons (Helou et al. 2001) that contribute, through kinetic energy transfer, to the heating of the neutral gas which subsequently cools down via collision with C+ atoms and other elements in photo-dissociation regions (PDRs; Tielens & Hollenbach 1985; Wolfire et al. 1995). 

Q3. What is the net effect of the emission of dust particles?

The net effect is the decreasing of the efficiency in the transformation of incident UV radiation into gas heating without an accompanied reduction of the dust emission (Wolfire et al. 

Q4. What is the trend for LIRGs with deeper silicate strengths?

There is a clear trend for LIRGs with deeper 9.7 μm silicate strengths (S9.7 μm), higher mid-IR luminosity surface densities (ΣMIR), smaller fractions of extended emission (FEE13.2 μm) and higher SSFRs to display lower [C ii]/FIR ratios. 

Q5. How many spectral elements are in each pixel?

The number of spectral elements in each pixel is 16, which are rearranged together via an image slicer over two 16 × 25 Ge:Ga detector arrays (blue and red cameras). 

Q6. What is the FIR ratio for the GOALS sample?

Figure 2 (upper panel) shows the [C ii]157.7 μm/FIR ratio for the GOALS sample as a function of the FIR PACS Sν 63 μm/ Sν 158 μm continuum flux density ratio. 

Q7. Why is the [C ii] line less efficient?

It has also been suggested that in sources where G0/nH is high ( 102 cm3) the [C ii] line is a less efficient coolant of the ISM because of the following reason.