We describe an on-going Herschel 200–700 μm spectroscopic survey of a flux-limited sample of 125 local luminous infrared galaxies (LIRGs), targeting primarily at CO line emission from warm and dense molecular gas. The program will provide important statistical data on the interplay between warm molecular gas, IR luminosity, star formation rate and efficiency, AGN, and the diverse properties of LIRGs. Of the 30 sample galaxies observed so far (18 by us; 12 taken from Herschel archive), about 15% show a dominant or significant hot CO gas component emitting at J > 10, that is likely heated by AGN. The other 85% is dominated by a warm gas component with CO line emission peaking at J ≤ 8, likely powered by starburst. While the spectral shapes of the warm gas component show little overall dependence on the total IR luminosity L_(IR), the relative contribution from the hot component appears to correlate positively with L_(IR). The tightest one-to-one correlation between CO line luminosity and L_(IR) seems to be for CO(7-6), implying that the bulk of L_(IR) should be correlated with warm molecular gas of such density and temperature that its CO line emission peaks around J ∼ 7

Appleton, P.

Armus, L.

Charmandaris, V.

Díaz-Santos, T.

Evans, A.

Gao, Y.

Howell, J.

Isaak, K.

Iwasawa, K.

Leech, J.

Lord, S.

Lu, Nanyao

Mazzarella, J.

Petric, A.

Sanders, D.

Schulz, B.

Surace, J.

van der Werf, P.

Xu, C. K.

Zhao, Y.

Caltech Authors - Main

Galaxy Mergers In An Evolving Universe
ASP Conference Series, Vol. 477
Wei-Hsin Sun, Kevin Xu, Nick Scoville, and David Sanders eds.
c©2013 Astronomical Society of the Pacific
A Herschel Spectroscopic Survey of Warm Molecular Gas in Local
Luminous Infrared Galaxies
Nanyao Lu,1 C. K. Xu,1 Y. Gao,2 K. Isaak,3 L. Armus,4 P. Appleton,1 V.
Charmandaris,5 T. Diaz-Santos,4 A. Evans,6 J. Howell,7 K. Iwasawa,8
J. Leech,9 S. Lord,1 J. Mazzarella,7 A. Petric,10 D. Sanders,11 B. Schulz,1
J. Surace,4 P. van der Werf, 12 and Y. Zhao2,7
1NHSC/IPAC, MS 100-22, Caltech, Pasadena, CA 91125, USA; email:
lu@ipac.caltech.edu
2Purple Mountain Observatory, China
3ESA Astrophysical Missions Division, ESTEC, The Netherlands
4SSC/IPAC, Caltech, Pasadena, CA, 91125, USA
5University of Crete, Greece
6University of Virginia, USA
7IPAC, Caltech, CA, 91125, USA
8University of Barcelona, Spain
9University of Oxford; UK
10Caltech, Pasadena, CA 91125, USA
11University of Hawaii, USA
12Leiden Observatory, Leiden University, The Netherlands
Abstract. We describe an on-going Herschel 200-700 µm spectroscopic survey of a
flux-limited sample of 125 local luminous infrared galaxies (LIRGs), targeting primar-
ily at CO line emission from warm and dense molecular gas. The program will provide
important statistical data on the interplay between warm molecular gas, IR luminosity,
star formation rate and efficiency, AGN, and the diverse properties of LIRGs. Of the 30
sample galaxies observed so far (18 by us; 12 taken from Herschel archive), about 15%
show a dominant or significant hot CO gas component emitting at J > 10, that is likely
heated by AGN. The other 85% is dominated by a warm gas component with CO line
emission peaking at J ∼< 8, likely powered by starburst. While the spectral shapes of the
warm gas component show little overall dependence on the total IR luminosity LIR, the
relative contribution from the hot component appears to correlate positively with LIR.
The tightest one-to-one correlation between CO line luminosity and LIR seems to be for
CO(7-6), implying that the bulk of LIR should be correlated with warm molecular gas
of such density and temperature that its CO line emission peaks around J ∼ 7.
31
32 Lu et al.
1. Introduction
We have started a campaign to obtain 200-700 µm spectra for an IR flux-limited sam-
ple of 125 local luminous infrared galaxies (LIRGs: with an 8-1000 µm LIR > 1011L)
with the SPIRE Fourier Transform Spectrometer (FTS) on board Herschel (Pilbratt
et al. 2010), with the main goal of obtaining CO spectral line energy distribution
(SLED) to beyond mid J levels to probe warm and dense molecular gas that should
be more directly related to star formation rate than cold molecular gas traced by CO(1-
0) (e.g., Iono et al. 2009). Unlike ultra-LIRGs (ULIRGs; LIR > 1012L), which are
almost exclusively found in merging systems of two gas-rich disk galaxies (Sanders &
Mirabel 1996), LIRGs form a morphological diverse group of galaxies, ranging from
close galaxy-galaxy interactions, to widely separated pairs, and to relatively isolated
spirals. Contrary to other Herschel programs, ours puts an emphasis on typical nearby
luminous LIRGs to obtain a more complete understanding of molecular gas properties,
star formation activity and AGN phenomenon in LIRGs.
2. The Program
As Fig. 1 shows, our sample is a subset of the one used in the Great Observatories All-
Sky LIRGs Survey (GOALS; Armus et al. 2009), which is complete to a flux density
of 5.24 Jy at 60 µm, and consists of all the 125 GOALS individual galaxies with LIR >
1011L and a total IR flux FIR > 6.5 × 10−13 W m−2. Our own observations cover
93 galaxies with the remaining 32 having similar FTS spectra from existing Herschel
programs.
In order to cover a large sample, our observations use the staring mode and an
integration time to detect CO(6-5) at S/N > 5. The total telescope time per target ranges
from 1,764 to 8,524 seconds, depending on the anticipated CO(6-5) fluxes. Since the
SPIRE FTS sensitivity is more or less flat for higher J CO lines, this design still allows
us to have some confidence in differentiating a flat-to-rising SLED, which calls for a
strong gas heating by AGN (e.g., Mrk 231; van der Werf et al. 2010), from a declining
one beyond J ∼ 7, which indicates a starburst dominated galaxy nucleus (e.g., M 82;
Panuzzo et al. 2010).
At the time of this report, eighteen targets have been observed by us. These data,
together with additional 12 sample galaxies with public FTS data from Herschel data
archive, are used here. The data reduction was done in a homogeneous way, using Her-
schel HIPE version 7. For the majority of the galaxies considered here, their warm CO
emission should resemble a point source for the SPIRE beams. Therefore, each galaxy
spectrum is simply taken from the central SPIRE detectors (i.e., SSWD4 & SLWC3).
As an example, Fig. 2 shows the spectrum of a relatively faint galaxy, NGC 0695. In
almost every case, clearly detected lines are CO lines up to some J level, one or both of
the [CI] lines at 369 and 609 µm, respectively, and the [NII] 205µm line. Our detected
CO lines are mostly unresolved. We have fit the spectral lines into SINC profiles and
the underlying continuum into a polynomial. The SINC line profile fitting is quite satis-
factory, except for the [NII] line, which may be partially resolved in some cases. In the
remainder of this report, we present some first results based on the detected CO lines,
referring to a formal paper (Lu et al. 2012; in preparation) for details on the sample,
observations and a full data reduction and spectral analysis.
Warm Molecular Gas in Luminous Infrared Galaxies 33
Figure 1. Left. Plot of the total IR flux as a function of the total IR luminosity
for all the individual galaxies (including some minor galaxy pair members) in the
GOALS sample. The two dotted lines indicate our sample selection as described in
the text. The filled squares are 32 galaxies observed in other SPIRE FTS programs.
Right. The observed spectrum of NGC 0695 (in blue) and an illustration of line and
continuum fits (in red). The relative flat spectrum (in green) is the residual after the
fits. Also marked are potential CO lnes, the two [CI] lines and the [NII] 205µm line.
In this case, CO lines up to J=8-7 are clearly detected.
3. First Results
As examples, we show in Fig. 3 the observed CO SLEDs for two relatively IR compact
galaxies, NGC 0695 and CGCG 436-030. These plots are constructed using detected
CO lines at S/N > 3, where the line flux uncertainty N is based on a simple r.m.s. value
(including any fringes) measured in the residual spectrum. NGC 0695 is a starburst
with an IRAS color of fν(60)/ fν(100) = 0.55. By contrast, CGCG 436-030 may harbor
a LINER and has a blue IRAS color of 1.11, which may be correlated with its flatter CO
SLED. Of the 30 galaxies studied here, about 15% (4 galaxies) of the sample galaxies
have a SLED that clearly peaks beyond J = 8. The other 85% (26 galaxies) have their
SLEDs peaking at J ≤ 8. The 4 galaxies in the former group are all known AGNs.
But not all the known AGNs fall into this group. The SPIRE FTS sensitivity ensures
that none detections of CO lines at J > 8 correspond to a declining CO SLED above
that J level. Since an AGN related X-ray gas heating produces a CO SLED that peaks
well beyond J = 8, a reasonable conclusion from this statistical result is that X-ray gas
heating is not the dominant mechanism in the majority of LIRGs.
Fig. 3 shows a few plots of the line flux ratio of CO(J-[J-1])/CO(5-4) as a function
of LIR, for J between 6 and 12. Up to J ∼ 8, the ratios show little overall dependence on
LIR for LIR < 1012L. By contrast, at J ∼> 11, there appears to be a clear trend over the
entire luminosity range. A plausible, simplified model for this consists of two CO gas
components: a warm component that dominates mid-J CO lines and a hot component
that dominates high-J CO lines. The latter component becomes progressively important
as LIR increases, to the point that its significance is noticeable for the 3 ULIRGs in all
the plots in Fig. 4.
We show luminosity correlations between various CO lines and LIR in Fig. 5. In
producing these plots, we have applied a crude aperture correction to each CO line
luminosity using the fraction of Spitzer 24 µm flux within a fixed aperture comparable
34 Lu et al.
Figure 2. CO SLEDs, normalized at CO(5-4), for two sample galaxies of compa-
rable LIR.
Figure 3. Left. Plots of CO line ratios as a function of the IR luminosity. The
J levels of the CO lines used in the ratio are labelled in each plot. Right. Plots of
selected CO line luminosity as a function of the total IR luminosity. In each plot, the
dotted line is a best fit to the data at a fixed slope of unity.
to the largest SPIRE FTS beam (42′′ FWHM). Except for a few cases, this correction
factor is less than 15%. The dotted line in each plot is a best fit to the data points with
a fixed slope of unity. The best overall fit seems to be around J = 7. For J < 6, the
data distribution seems to be slightly shallower than the fit. By contrast, at J = 12, the
opposite seems to be true. These plots imply that LIR is mostly associated with warm
molecular gas of such density and temperature that the CO line emissions peak around
Warm Molecular Gas in Luminous Infrared Galaxies 35
J ∼ 7. This analysis needs to be repeated with more galaxies and a more rigorous
treatment of the aperture correction and calibration errors.
Acknowledgments. This work is based (in part) on observations made with Her-
schel, a European Space Agency Cornerstone Mission with significant participation
by NASA. Support for this work was provided by NASA through an award issued by
JPL/Caltech.
References
Armus, L., et al. 2009, PASP, 121, 559
Iono, D., et al. 2009, ApJ, 695, 1537
Panuzzo, P., et al. 2010, A&A, 518, 37
Pilbratt, G. L., et al. 2010, A&A, 518, 1
Sanders, D. B., & Mirabel, I. F. 1996, ARA&A, 34, 749
van der Werf, P., et al. 2010, A&A, 518, 42


A Herschel Spectroscopic Survey of Warm Molecular Gas in Local Luminous Infrared Galaxies

Galaxy Mergers In An Evolving Universe
ASP Conference Series, Vol. 477
Wei-Hsin Sun, Kevin Xu, Nick Scoville, and David Sanders eds.
c©2013 Astronomical Society of the Pacific
A Herschel Spectroscopic Survey of Warm Molecular Gas in Local
Luminous Infrared Galaxies
Nanyao Lu,1 C. K. Xu,1 Y. Gao,2 K. Isaak,3 L. Armus,4 P. Appleton,1 V.
Charmandaris,5 T. Diaz-Santos,4 A. Evans,6 J. Howell,7 K. Iwasawa,8
J. Leech,9 S. Lord,1 J. Mazzarella,7 A. Petric,10 D. Sanders,11 B. Schulz,1
J. Surace,4 P. van der Werf, 12 and Y. Zhao2,7
1NHSC/IPAC, MS 100-22, Caltech, Pasadena, CA 91125, USA; email:
lu@ipac.caltech.edu
2Purple Mountain Observatory, China
3ESA Astrophysical Missions Division, ESTEC, The Netherlands
4SSC/IPAC, Caltech, Pasadena, CA, 91125, USA
5University of Crete, Greece
6University of Virginia, USA
7IPAC, Caltech, CA, 91125, USA
8University of Barcelona, Spain
9University of Oxford; UK
10Caltech, Pasadena, CA 91125, USA
11University of Hawaii, USA
12Leiden Observatory, Leiden University, The Netherlands
Abstract. We describe an on-going Herschel 200-700 µm spectroscopic survey of a
flux-limited sample of 125 local luminous infrared galaxies (LIRGs), targeting primar-
ily at CO line emission from warm and dense molecular gas. The program will provide
important statistical data on the interplay between warm molecular gas, IR luminosity,
star formation rate and efficiency, AGN, and the diverse properties of LIRGs. Of the 30
sample galaxies observed so far (18 by us; 12 taken from Herschel archive), about 15%
show a dominant or significant hot CO gas component emitting at J > 10, that is likely
heated by AGN. The other 85% is dominated by a warm gas component with CO line
emission peaking at J ∼< 8, likely powered by starburst. While the spectral shapes of the
warm gas component show little overall dependence on the total IR luminosity LIR, the
relative contribution from the hot component appears to correlate positively with LIR.
The tightest one-to-one correlation between CO line luminosity and LIR seems to be for
CO(7-6), implying that the bulk of LIR should be correlated with warm molecular gas
of such density and temperature that its CO line emission peaks around J ∼ 7.
31
32 Lu et al.
1. Introduction
We have started a campaign to obtain 200-700 µm spectra for an IR flux-limited sam-
ple of 125 local luminous infrared galaxies (LIRGs: with an 8-1000 µm LIR > 10
11L)
with the SPIRE Fourier Transform Spectrometer (FTS) on board Herschel (Pilbratt
et al. 2010), with the main goal of obtaining CO spectral line energy distribution
(SLED) to beyond mid J levels to probe warm and dense molecular gas that should
be more directly related to star formation rate than cold molecular gas traced by CO(1-
0) (e.g., Iono et al. 2009). Unlike ultra-LIRGs (ULIRGs; LIR > 10
12L), which are
almost exclusively found in merging systems of two gas-rich disk galaxies (Sanders &
Mirabel 1996), LIRGs form a morphological diverse group of galaxies, ranging from
close galaxy-galaxy interactions, to widely separated pairs, and to relatively isolated
spirals. Contrary to other Herschel programs, ours puts an emphasis on typical nearby
luminous LIRGs to obtain a more complete understanding of molecular gas properties,
star formation activity and AGN phenomenon in LIRGs.
2. The Program
As Fig. 1 shows, our sample is a subset of the one used in the Great Observatories All-
Sky LIRGs Survey (GOALS; Armus et al. 2009), which is complete to a flux density
of 5.24 Jy at 60 µm, and consists of all the 125 GOALS individual galaxies with LIR >
1011L and a total IR flux FIR > 6.5 × 10
−13Wm−2. Our own observations cover
93 galaxies with the remaining 32 having similar FTS spectra from existing Herschel
programs.
In order to cover a large sample, our observations use the staring mode and an
integration time to detect CO(6-5) at S/N > 5. The total telescope time per target ranges
from 1,764 to 8,524 seconds, depending on the anticipated CO(6-5) fluxes. Since the
SPIRE FTS sensitivity is more or less flat for higher J CO lines, this design still allows
us to have some confidence in differentiating a flat-to-rising SLED, which calls for a
strong gas heating by AGN (e.g., Mrk 231; van der Werf et al. 2010), from a declining
one beyond J ∼ 7, which indicates a starburst dominated galaxy nucleus (e.g., M 82;
Panuzzo et al. 2010).
At the time of this report, eighteen targets have been observed by us. These data,
together with additional 12 sample galaxies with public FTS data from Herschel data
archive, are used here. The data reduction was done in a homogeneous way, using Her-
schel HIPE version 7. For the majority of the galaxies considered here, their warm CO
emission should resemble a point source for the SPIRE beams. Therefore, each galaxy
spectrum is simply taken from the central SPIRE detectors (i.e., SSWD4 & SLWC3).
As an example, Fig. 2 shows the spectrum of a relatively faint galaxy, NGC0695. In
almost every case, clearly detected lines are CO lines up to some J level, one or both of
the [CI] lines at 369 and 609 µm, respectively, and the [NII] 205µm line. Our detected
CO lines are mostly unresolved. We have fit the spectral lines into SINC profiles and
the underlying continuum into a polynomial. The SINC line profile fitting is quite satis-
factory, except for the [NII] line, which may be partially resolved in some cases. In the
remainder of this report, we present some first results based on the detected CO lines,
referring to a formal paper (Lu et al. 2012; in preparation) for details on the sample,
observations and a full data reduction and spectral analysis.
Warm Molecular Gas in Luminous Infrared Galaxies 33
Figure 1. Left. Plot of the total IR flux as a function of the total IR luminosity
for all the individual galaxies (including some minor galaxy pair members) in the
GOALS sample. The two dotted lines indicate our sample selection as described in
the text. The filled squares are 32 galaxies observed in other SPIRE FTS programs.
Right. The observed spectrum of NGC0695 (in blue) and an illustration of line and
continuum fits (in red). The relative flat spectrum (in green) is the residual after the
fits. Also marked are potential CO lnes, the two [CI] lines and the [NII] 205µm line.
In this case, CO lines up to J=8-7 are clearly detected.
3. First Results
As examples, we show in Fig. 3 the observed CO SLEDs for two relatively IR compact
galaxies, NGC0695 and CGCG436-030. These plots are constructed using detected
CO lines at S/N > 3, where the line flux uncertainty N is based on a simple r.m.s. value
(including any fringes) measured in the residual spectrum. NGC0695 is a starburst
with an IRAS color of fν(60)/ fν(100) = 0.55. By contrast, CGCG436-030 may harbor
a LINER and has a blue IRAS color of 1.11, which may be correlated with its flatter CO
SLED. Of the 30 galaxies studied here, about 15% (4 galaxies) of the sample galaxies
have a SLED that clearly peaks beyond J = 8. The other 85% (26 galaxies) have their
SLEDs peaking at J ≤ 8. The 4 galaxies in the former group are all known AGNs.
But not all the known AGNs fall into this group. The SPIRE FTS sensitivity ensures
that none detections of CO lines at J > 8 correspond to a declining CO SLED above
that J level. Since an AGN related X-ray gas heating produces a CO SLED that peaks
well beyond J = 8, a reasonable conclusion from this statistical result is that X-ray gas
heating is not the dominant mechanism in the majority of LIRGs.
Fig. 3 shows a few plots of the line flux ratio of CO(J-[J-1])/CO(5-4) as a function
of LIR, for J between 6 and 12. Up to J ∼ 8, the ratios show little overall dependence on
LIR for LIR < 10
12L. By contrast, at J ∼> 11, there appears to be a clear trend over the
entire luminosity range. A plausible, simplified model for this consists of two CO gas
components: a warm component that dominates mid-J CO lines and a hot component
that dominates high-J CO lines. The latter component becomes progressively important
as LIR increases, to the point that its significance is noticeable for the 3 ULIRGs in all
the plots in Fig. 4.
We show luminosity correlations between various CO lines and LIR in Fig. 5. In
producing these plots, we have applied a crude aperture correction to each CO line
luminosity using the fraction of Spitzer 24 µm flux within a fixed aperture comparable
34 Lu et al.
Figure 2. CO SLEDs, normalized at CO(5-4), for two sample galaxies of compa-
rable LIR.
Figure 3. Left. Plots of CO line ratios as a function of the IR luminosity. The
J levels of the CO lines used in the ratio are labelled in each plot. Right. Plots of
selected CO line luminosity as a function of the total IR luminosity. In each plot, the
dotted line is a best fit to the data at a fixed slope of unity.
to the largest SPIRE FTS beam (42′′ FWHM). Except for a few cases, this correction
factor is less than 15%. The dotted line in each plot is a best fit to the data points with
a fixed slope of unity. The best overall fit seems to be around J = 7. For J < 6, the
data distribution seems to be slightly shallower than the fit. By contrast, at J = 12, the
opposite seems to be true. These plots imply that LIR is mostly associated with warm
molecular gas of such density and temperature that the CO line emissions peak around
Warm Molecular Gas in Luminous Infrared Galaxies 35
J ∼ 7. This analysis needs to be repeated with more galaxies and a more rigorous
treatment of the aperture correction and calibration errors.
Acknowledgments. This work is based (in part) on observations made with Her-
schel, a European Space Agency Cornerstone Mission with significant participation
by NASA. Support for this work was provided by NASA through an award issued by
JPL/Caltech.
References
Armus, L., et al. 2009, PASP, 121, 559
Iono, D., et al. 2009, ApJ, 695, 1537
Panuzzo, P., et al. 2010, A&A, 518, 37
Pilbratt, G. L., et al. 2010, A&A, 518, 1
Sanders, D. B., & Mirabel, I. F. 1996, ARA&A, 34, 749
van der Werf, P., et al. 2010, A&A, 518, 42


English

http://authors.library.caltech.edu/43358/1/Lu_2013p31.pdf

A Herschel Spectroscopic Survey of Warm Molecular Gas in Local Luminous Infrared Galaxies

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main