The Astrophysical Journal, 743:94 (19pp), 2011 December 10 doi:10.1088/0004-637X/743/1/94
C
2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
OBSERVATIONS OF Arp 220 USING HERSCHEL-SPIRE: AN UNPRECEDENTED VIEW OF THE
MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT
Naseem Rangwala
1
, Philip R. Maloney
1
, Jason Glenn
1
, Christine D. Wilson
2
, Adam Rykala
3
, Kate Isaak
4
,
Maarten Baes
5
, George J. Bendo
6
, Alessandro Boselli
7
, Charles M. Bradford
8
, D. L. Clements
9
, Asantha Cooray
10
,
Trevor Fulton
11
, Peter Imhof
11
, Julia Kamenetzky
1
, Suzanne C. Madden
12
, Erin Mentuch
2
, Nicola Sacchi
13
,
Marc Sauvage
12
, Maximilien R. P. Schirm
2
,M.W.L.Smith
3
, Luigi Spinoglio
13
, and Mark Wolfire
14
1
Center for Astrophysics and Space Astronomy, University of Colorado, 1255 38th street, Boulder, CO 80303, USA
2
Department of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
3
School of Physics & Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
4
ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
5
Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
6
UK ALMA Regional Centre Node, Jordell Bank Center for Astrophysics, School of Physics and Astronomy,
University of Manchester, Oxford Road, Manchester M13 9PL, UK
7
Laboratoire d’Astrophysique de Marseille, UMR6110 CNRS, 38 rue F. Joliot-Curie, F-13388 Marseille, France
8
JPL, Pasadena, CA 91109, USA
9
Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
10
Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
11
Blue Sky Spectroscopy Inc, Suite 9-740 4th Avenue South, Lethbridge, Alberta T1J 0N9, Canada
12
CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, F-91191 Gif-sur-Yvette, France
13
Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
14
Department of Astronomy, University of Maryland, College Park, MD 20742, USA
Received 2011 June 9; accepted 2011 September 1; published 2011 November 23
ABSTRACT
We present Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (Herschel
SPIRE-FTS) observations of Arp 220, a nearby ultra-luminous infrared galaxy. The FTS provides continuous
spectral coverage from 190 to 670 μm, a wavelength region that is either very difficult to observe or completely
inaccessible from the ground. The spectrum provides a good measurement of the continuum and detection of several
molecular and atomic species. We detect luminous CO (J = 4–3 to 13–12) and water rotational transitions with
comparable total luminosity ∼2 × 10
8
L
; very high-J transitions of HCN (J = 12–11 to 17–16) in absorption;
strong absorption features of rare species such as OH
+
,H
2
O
+
, and HF; and atomic lines of [C i] and [N ii]. The
modeling of the continuum shows that the dust is warm, with T = 66 K, and has an unusually large optical depth,
with τ
dust
∼ 5 at 100 μm. The total far-infrared luminosity of Arp 220 is L
FIR
∼ 2 × 10
12
L
. Non-LTE modeling
of the extinction corrected CO rotational transitions shows that the spectral line energy distribution of CO is fit well
by two temperature components: cold molecular gas at T ∼ 50 K and warm molecular gas at T ∼ 1350
+280
−100
K
(the inferred temperatures are much lower if CO line fluxes are not corrected for dust extinction). These two
components are not in pressure equilibrium. The mass of the warm gas is 10% of the cold gas, but it dominates the
CO luminosity. The ratio of total CO luminosity to the total FIR luminosity is L
CO
/L
FIR
∼ 10
−4
(the most luminous
lines, such as J = 6–5, have L
CO,J =6–5
/L
FIR
∼ 10
−5
). The temperature of the warm gas is in excellent agreement
with the observations of H
2
rotational lines. At 1350 K, H
2
dominates the cooling (∼20 L
M
−1
) in the interstellar
medium compared to CO (∼0.4 L
M
−1
). We have ruled out photodissociation regions, X-ray-dominated regions,
and cosmic rays as likely sources of excitation of this warm molecular gas, and found that only a non-ionizing
source can heat this gas; the mechanical energy from supernovae and stellar winds is able to satisfy the large energy
budget of ∼20 L
M
−1
. Analysis of the very high-J lines of HCN strongly indicates that they are solely populated
by infrared pumping of photons at 14 μm. This mechanism requires an intense radiation field with T>350 K. We
detect a massive molecular outflow in Arp 220 from the analysis of strong P Cygni line profiles observed in OH
+
,
H
2
O
+
, and H
2
O. The outflow has a mass 10
7
M
and is bound to the nuclei with velocity 250 km s
−1
.The
large column densities observed for these molecular ions strongly favor the existence of an X-ray luminous AGN
(10
44
erg s
−1
) in Arp 220.
Key words: galaxies: ISM – galaxies: starburst – ISM: molecules – line: identification – molecular processes –
techniques: spectroscopic
Online-only material: color figures
1. INTRODUCTION
Molecular gas is the raw material for the formation of
stars within galaxies. However, the star formation process
inputs energy and momentum back into this gas—and into the
interstellar medium (ISM) in general—through radiation, stellar
winds, and supernovae. The impact of the feedback from both
star formation and AGN activity on the ISM can be profound
(Maloney 1999), and the resulting change in the physical state
of the gas must in turn affect the star formation process. In the
general framework of galaxy evolution, disentangling the rela-
tive contributions of these processes for high-redshift objects,
however, is an extremely difficult undertaking, due to their small
angular size and faintness; in addition, many luminous high-z
sources are heavily obscured by dust, ruling out most of the
traditional approaches for studies of galaxy energetics. Nearby
1
The Astrophysical Journal, 743:94 (19pp), 2011 December 10 Rangwala et al.
galaxies such as M82, Arp 220, and NGC 1068, for which deep
and relatively high resolution observations can be obtained at
multiple wavelengths, are commonly used templates for cali-
brating the observations of high-z luminous dusty galaxies.
The study of the star formation process and of the effect
of AGN activity in nearby galaxies can be done in the far-
infrared (FIR) and submillimeter (submm) spectral range, where
a large variety of high-J rotational molecular transitions occur
(CO, HCN, HCO
+
, etc.). These transitions have a large span in
critical densities making them excellent tracers of the physical
conditions of gas over a wide range in temperatures (10–1000 K)
and densities (10
3
–10
8
cm
−3
). Also, their line ratios can be
used as diagnostics for distinguishing between different energy
sources responsible for the excitation of the gas, e.g., starbursts
or AGN. The low-J transitions (up to J = 4–3) of these molecules
trace the cooler gas and have been observed from ground-based
radio and submm telescopes. These low-J transitions cannot
distinguish between the AGN and starburst systems. However,
the mid-J to higher-J transitions, which can trace warmer and
denser gas and be used to test models that distinguish between
AGN and starbursts (e.g., Spaans & Meijerink 2008), are either
difficult to observe or completely inaccessible from the ground.
Until the advent of Herschel (Pilbratt et al. 2010) they had only
been observed in a handful of extragalactic objects.
The Very Nearby Galaxy Survey (VNGS; PI: C. Wilson)
is one of the guaranteed-time key projects of Herschel that
is using both the Spectral and Photometric Imaging Receiver
(SPIRE; Griffin et al. 2010) and the Photodetector Array
Camera and Spectrometer (PACS; Poglitsch et al. 2010)in-
struments. This paper focuses only on the data from the SPIRE
instrument, which is being used to obtain high-signal-to-noise
(S/N) photometric and spectroscopic maps of a sample of
13 nearby archetypical galaxies. The imaging spectrometer on
SPIRE is a Fourier Transform Spectrometer (FTS; Naylor et al.
2010), which provides continuous spectral coverage from 190
to 670 μm(∼450–1550 GHz). This region covers the spec-
tral features of several key molecular and atomic species that
are powerful diagnostic of the physics and chemistry of the
ISM. All the galaxies in this survey will have complementary
PACS observations either as a part of this program or another
key project, allowing exploration of a full spectral view from
50 to 700 μm. In this paper, we present the SPIRE observations
of Arp 220. Until now only two nearby galaxies observed with
SPIRE-FTS have been published. They are M82 (Panuzzo et al.
2010) and Mrk 231 (van der Werf et al. 2010). In both cases the
mid-J to high-J CO ladder was detected. Modeling of this CO
ladder showed that in M82 the excitation of molecular gas was
governed by the starburst, whereas in Mrk 231 the excitation
was dominated by the central luminous AGN.
Arp 220 is the nearest Ultra Luminous Infrared galaxy
(ULIRG) at a distance of about 77 Mpc and z ∼ 0.0181. It
has L
FIR
∼ 10
12
L
, and is one of the most popular templates
for studies of high-z dusty galaxies. It has two merging nuclei
separated by ∼1
(∼360 pc at D
Arp 220
= 77 Mpc; Scoville et al.
1997, SC97 hereafter) and together they have a large reservoir
of molecular gas (∼10
10
M
). Arp 220 has been observed
extensively over the years across the electromagnetic spectrum.
The extreme star formation environment in this galaxy provides
an excellent laboratory to understand the processes affecting
star formation and possibly AGN feedback.
In this paper, we describe the state of the molecular gas
by analyzing and modeling the new detections of high-J CO
and HCN lines. We discuss sources of energy that could be
responsible for the excitation of various molecular species
detected in this spectrum. The issue of a hidden AGN has
long been debated for Arp 220. We address this issue using the
detection of rare molecular species that can only be detected
by Herschel. We also present results from dust modeling
by using the continuum measurements from SPIRE-FTS and
SPIRE-photometer that adds data at wavelengths not accessible
before. We summarize these results and discuss (in light of the
new results) Arp 220 as a template for broadband spectral energy
distributions for high-z submm galaxies.
2. OBSERVATIONS AND DATA REDUCTION
Arp 220 was observed with the SPIRE-FTS in a high spec-
tral resolution with FWHM ∼ 1.44 GHz (or ranging from
950 km s
−1
to 280 km s
−1
across the spectral range), sin-
gle pointing and sparse image sampling mode on 2010 Febru-
ary 13. The total on-source integration time was 10,445 s. We
used a deep dark sky observation with total integration time
of 13,320 s taken on 2010 November 21 to account for the
emission from the telescope. The FTS has two detector ar-
rays, called Spectrometer Long Wave (SLW) and Spectrom-
eter Short Wave (SSW), to cover the long and short wave-
length ranges, respectively, with a small overlap in wavelength.
Arp 220 was also observed with the SPIRE photometer on 2009
December 28 with a total on-source integration time of 214 s at
250 μm, 350 μm, and 500 μm. The calibration uncertainties in
SPIRE-FTS are about 10% at the lower end of the frequency
range but are as good as 5% across the rest of the spectrum.
For the SPIRE photometer, the systematic uncertainty, which
is related to the model for Neptune, is 5%. This is correlated
among the three SPIRE bands. The random uncertainty, which
is related to the ability of the instrument to repeat measurements
of Neptune, is ∼2%. This is independent for each of the SPIRE
bands.
We processed the SPIRE FTS data using the Herschel data
processing pipeline (Fulton et al. 2010), HIPE version 6.0. The
spectrum is shown in Figure 1 and displays both emission
and absorption features. Arp 220 is a point source for the
diffraction limited SPIRE beam size,
15
which ranges between
FWHM of 17
–40
. The SLW and SSW continuum match
perfectly across the overlap region. The flux densities from the
SPIRE photometer were measured by fitting a two-dimensional
elliptical Gaussian function to the source in the timeline data. In
this fit, the background was treated as a free parameter in the fit.
The fit was performed to an inner region with radii of 22
,30
,
and 42
in the 250, 350, and 500 μm bands, respectively (which
were selected because they contain the part of the beam profile
that appears Gaussian and exclude the Airy rings) and using
a background annulus with radii between 300
and 350
.The
peak of the best-fitting Gaussian function in each wave band
corresponds to the flux density at that wavelength. The three
SPIRE photometer measurements are overplotted in Figure 1 as
red solid circles. They are in very good agreement with the FTS
continuum within the statistical and calibration uncertainties of
SPIRE-FTS and SPIRE-photometer.
The spectral line shape of the FTS can be modeled to a
good accuracy by a sinc function with a FWHM of about
1.44 GHz. The ground-based observations measure large line
velocity widths, of the order of 500 km s
−1
in CO and HCN in
Arp 220 (SC97; also see Greve et al. 2009). Therefore, in our
15
The SPIRE beam shapes are not Gaussian; the effective beam solid angle
can be found in the Herschel Observer’s manual.
2
The Astrophysical Journal, 743:94 (19pp), 2011 December 10 Rangwala et al.
(a)
(b)
(c)
Figure 1. Herschel SPIRE-FTS spectrum of Arp 220. The spectrum shows the FIR continuum between 190 and 670 μm in (a). Red solid points in (a) are the continuum
measurements from the SPIRE photometer and the dotted curves show the photometer bandpasses with arbitrary normalization. Line identifications are shown for
several molecular and atomic species in (b) and (c) with like colors for like species.
(A color version of this figure is available in the online journal.)
FTS spectrum we start resolving the broad spectral lines towards
the higher frequency end. We use a sinc function of variable
FWHM to fit the emission and absorption lines using the MPFIT
package in IDL. In Table 1 we report the integrated line fluxes
in Jy km s
−1
for emission lines and equivalent widths in μmfor
the absorption lines along with their 1σ statistical uncertainties.
The observed spectral line shape suffers from small asymmetries
from instrumental effects, and the line shapes could be affected
by blending from weak lines. The effect on the line fluxes from
asymmetries is expected to be 2%.
3. LINE IDENTIFICATIONS
The spectrum (Figure 1) shows the FIR continuum and the
detection of several key molecular and atomic species and
Figure 2 shows zoom-in views of some of the spectral lines
discussed in this section. In Table 1, we list their transitions,
rest frequencies, and integrated fluxes. The FWHMs mentioned
in this section are from our sinc line fitting.
We detect a luminous CO emission ladder from J = 4–3 to J =
13–12 and several water transitions with total water luminosity
comparable to CO. Compared to the velocity widths measured
in CO from the ground-based observations (e.g., SC97; Greve
et al. 2009), the higher-J CO lines in our spectrum are much
narrower. These high-J CO lines are not resolved and follows
the resolution of FTS, which at the frequency of J = 12–11 line is
∼310 km s
−1
. This suggests that either the high-J CO emission
is coming from only one of the nuclei or that it is a dynamically
separate component from the low-J lines. On the other hand,
the water lines are much broader with a velocity FWHM of
∼500 km s
−1
implying that this emission could be coming from
both nuclei. The observed velocity separation between the two
nuclei in Arp 220 varies considerably from 50 km s
−1
(Downes
& Solomon 1998; Sakamoto et al. 2008) to 250 km s
−1
(Scoville
3
The Astrophysical Journal, 743:94 (19pp), 2011 December 10 Rangwala et al.
Tabl e 1
Line Strengths of Emission and Absorption Lines in Arp 220
Emission Lines
ID Transition Rest Frequency Flux
a
Source
b
Comments
(GHz) (Jy km s
−1
)
12
CO J = 4–3 461.04077 4550 ± 330 JPL/CDMS ···
12
CO J = 5–4 576.26793 3660 ± 170 JPL/CDMS ···
12
CO J = 6–5 691.47308 4070 ± 80 JPL/CDMS ···
12
CO J = 7–6 806.65181 3460 ± 170 JPL/CDMS ···
12
CO J = 8–7 921.79970 3260 ± 200 JPL/CDMS ···
12
CO J = 9–8 1036.91239 2920 ± 270 JPL/CDMS ···
12
CO J = 10–9 1151.98545 ··· JPL/CDMS Blended with H
2
O3
12
–2
21
12
CO J = 11–10 1267.01449 1280 ± 90 JPL/CDMS ···
12
CO J = 12–11 1381.99510 840 ± 60 JPL/CDMS ···
12
CO J = 13–12 1496.92291 450 ± 100 JPL/CDMS ···
C i J = 1–0 492.16065 1840 ± 70 CDMS ···
C i J = 2–1 809.34197 2130 ± 400 CDMS ···
H
2
O2
11
–2
02
752.0332 2970 ± 110 LOVAS ···
H
2
O2
02
–1
11
987.92676 3440 ± 450 JPL ···
H
2
O3
12
–3
03
1097.36479 3080 ± 160 JPL ···
H
2
O3
12
–2
21
1153.12682 3460 ± 330 JPL Blended with CO J = 10–9 line
H
2
O3
21
–3
12
1162.91159 4930 ± 300 JPL ···
H
2
O4
22
–4
13
1207.63871 1750 ± 170 JPL ···
H
2
O2
20
–2
11
1228.78877 1980 ± 300 JPL ···
H
2
O5
23
–5
14
1410.61807 940 ± 110 JPL ···
H
2
O
+
2
02
–1
11
, J
5/3–3/2
742.0332 840 ± 90 BR10
c
···
H
2
O
+
2
02
–1
11
, J
3/2–3/2
746.1938 980 ± 130 BR10/JPL ···
HCN J = 6–5 531.71635 1700 ± 120 CDMS (?)
HCO
+
/HOC
+
J = 6–5 536.82796 850 ± 160 CDMS (?)
N ii
3
P
1
–
3
P
0
1461.1319 1800 ± 120 C93
d
Very broad
Absorption Lines
ID Transition Rest Frequency W
λ
Source Comments
(GHz) (×10
−2
μm)
HCN J = 12–11 1062.980 2.5 ± 0.2 CDMS ···
HCN J = 13–12 1151.449 ··· CDMS Blended with CO (10–9) and water line
HCN J = 14–13 1239.890 1.9 ± 0.2 CDMS ···
HCN J = 15–14 1328.302 1.6 ± 0.1 CDMS ···
HCN J = 16–15 1416.683 1.2 ± 0.1 CDMS ···
HCN J = 17–16 1505.030 0.9 ± 0.1 CDMS ···
CH
+
1–0 835.07895 18.8 ± 0.3 CDMS ···
OH
+
1
01
–0
12
909.1588 11.1 ± 1.4 CDMS ···
OH
+
1
22
–0
11
971.8053 16.6 ± 2.0 CDMS ···
OH
+
1
12
–0
1,2
1033.118 16.5 ± 1.8 CDMS ···
H
2
O1
10
–0
00
1113.34296 8.4 ± 1.4 JPL ···
H
2
O
+
1
11
–0
00
, J
3/2–1/2
1115.2040 9.1 ± 1.2 G10
e
···
H
2
O
+
1
11
–0
00
, J
3=1/2–1/2
1139.5606 8.6 ± 0.5 G10 ···
HF J = 1–0 1232.47622 7.0 ± 0.7 JPL ···
Notes.
a
The line fluxes can be corrected for dust extinction using the following two relations: τ
d
= (ν/ν
0
)
β
(where β = 1.84 and ν
0
= 1270 GHz),
and the mixed dust extinction model given by I = I
0
(1 − e
−τ
d
)/τ
d
. The extinction correction factor (I
0
/I ) ranges from 1.076 at 450 GHz to
1.953 at 1600 GHz.
b
JPL: Jet Propulsion Lab; CDMS: Cologne Database for Molecular Spectroscopy; LOVAS: NIST recommended rest frequencies.
c
Bruderer et al. (2010).
d
Gupta et al. (2010).
e
Brown et al. (1994); Colgan et al. (1993).
et al. 1997; Sakamoto et al. 1999). Therefore, it is difficult to
conclude whether the line widths are affected by the velocity
shift between the two nuclei or by the dynamics of the molecular
material.
In addition to the emission line features, the spectrum
also shows several absorption features. Most surprising are
the detections of five very high-J HCN (J = 12–11 to J =
17–16) lines in absorption; their low-J transitions (J = 1–0
to J = 4–3) are detected in emission with ground-based
observatories (Greve et al. 2009 and references therein). Such
high-J transitions of HCN have never been detected before in
external galaxies, in emission or absorption.
Strong absorption lines are detected from hydrides, includ-
ing three transitions of OH
+
(see Figure 10) and one of CH
+
(see Figure 2(e)). We report four detections of H
2
O
+
transitions;
two of them (1115, 1139 GHz) show strong absorption and the
other two, at 742 and 746 GHz (rest frequencies are reported
in Bruderer 2006; Ossenkopf et al. 2010), appear in emission.
4
The Astrophysical Journal, 743:94 (19pp), 2011 December 10 Rangwala et al.
HCN/HCO
+
6−5 (?)
520 525 530 535 540 545
Frequency (GHz)
−0.5
0.0
0.5
1.0
1.5
2.0
Flux (Jy)
(a)
CO 6−5
680 685 690 695 700
Frequency (GHz)
−2
0
2
4
6
8
Flux (Jy)
(b)
H
2
O
+
2
02
−1
11
doublet
730 735 740 745 750
Frequency (GHz)
−0.5
0.0
0.5
1.0
1.5
2.0
2.5
Flux (Jy)
(c)
CO 7−6/CI 1−0
800 805 810 815 820
Frequency (GHz)
0
2
4
6
8
10
Flux (Jy)
(d)
CH
+
1−0
825 830 835 840 845
Frequency (GHz)
−4
−3
−2
−1
0
1
Flux (Jy)
(e)
HCN/CO/H
2
O blend
1145 1150 1155 1160
Frequency (GHz)
0
2
4
6
8
Flux (Jy)
(f)
HCN 15−14
1320 1325 1330 1335
Frequenc
y
(GHz)
−3
−2
−1
0
1
Flux (Jy)
(g)
CO 12−11
1370 1375 1380 1385 1390
Frequenc
y
(GHz)
−1
0
1
2
3
Flux (Jy)
(h)
NII
1450 1455 1460 1465 1470
Frequenc
y
(GHz)
−1
0
1
2
3
4
5
Flux (Jy)
(i)
Figure 2. Zoom-in view of some of the spectral lines detected in the Herschel-SPIRE spectrum of Arp 220.
Before Herschel,H
2
O
+
was only detected in comets. The transi-
tions at 1115 and 1139 GHz (see Figure 10) were first detected
in Galactic molecular clouds and in extragalactic objects by
HIFI (Ossenkopf et al. 2010) and SPIRE-FTS (van der Werf
et al. 2010). We have the first detection of transitions at 742 and
746 GHz (see Figure 2(c)). The detections of OH
+
and H
2
O
+
are
very important in Arp 220 for modeling the water transitions be-
cause they are the major intermediaries in the ion-neutral chem-
istry network producing water in the ISM (Gerin et al. 2010;
Neufeld et al. 2010a), and their detection is very difficult from
current ground-based observatories. Herschel HIFI and SPIRE-
FTS also made the first detection of the HF 1232 GHz line
(Neufeld et al. 2010b; van der Werf et al. 2010), and we detect
it in absorption in Arp 220. The spectrum also shows several
features of Nitrogen hydrides including NH, NH
2
, and NH
3
.
The rest frequencies of all the above detections of the
molecular species are consistent with the systemic z = 0.0181
of Arp 220. However, we detect two emission features at ∼532.6
and ∼536.8 GHz (see Figure 2 (a)) that are close in frequency
to the J = 6–5 transitions of HCN and HCO
+
; their low-J
transitions (up to J = 4–3) have been seen in emission from the
ground. These features also have larger FWHM (∼1000 km s
−1
)
compared to the spectral resolution at these frequencies. The
redshifted frequencies of both HCN and HCO
+
are offset from
the systemic velocity of Arp 220 by ∼−0.9 GHz (−500 km s
−1
)
and ∼−1.7 GHz (−1000 km s
−1
), respectively. Such large shifts
are not seen for any other detections in our spectrum. Since
we see very high-J lines of HCN in absorption it is possible
that the HCN J = 6–5 line is partially absorbed by the same
gas producing HCN in absorption at higher frequencies. This
effect could shift its apparent central frequency. The case for
HCO
+
is even more problematic. The rest frequency of the
emission feature is in perfect agreement with the rest frequency
of the HOC
+
molecule instead of HCO
+
. Since the width of
this feature is about ∼1000 km s
−1
, these two molecules could
be blended. However, identifying this feature as HOC
+
would
imply that it has comparable to or higher line strength than
HCO
+
. This would be extremely unusual and has never been
observed in other galaxies or in Galactic molecular clouds. All
observations of HOC
+
and HCO
+
show that the latter is at least
100–1000 times stronger (Mart
´
ın et al. 2009; Savage & Ziurys
2004). In Table 1 we mark these identifications with a question
mark.
Among the atomic species we detect [C i] and [N ii] lines (see
Figure 2(d) and (i)). The detection of the two [C i] lines at 492
and 809 GHz have been rare in the past. The forbidden [N ii]
line at 1462 GHz has been inaccessible from the ground and
rarely accessible from space. The Herschel FTS has been very
successful in detecting them in several Galactic and extragalactic
targets. The [C i] (1–0) 492 GHz line survey for nearby galaxies
5
