The Astrophysical Journal, 639:237–245, 2006 March 1
# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
A SYSTEMATIC STUDY OF GLYCOLALDEHYDE IN SAGITTARIUS B2(N) AT 2 AND 3 mm:
CRITERIA FOR DETECTING LARGE INTERSTELLAR MOLECULES
D. T. Halfen, A. J. Apponi, N. Woolf, R. Polt, and L. M. Ziurys
Department of Chemistry, Department of Astronomy, Life and Planets Astrobiology Center, NASA Astrobiology Institute,
Arizona Radio Observatory, and Steward Observatory, University of Arizona,
933 North Cherry Avenue, Tucson, AZ 85721; [email protected]
Received 2005 July 26; accepted 2005 October 28
ABSTRACT
A comprehensive study of glycolaldehyde (CH2OHCHO) has been conducted at 2 and 3 mm toward Sgr B2( N)
using the Arizona Radio Observatory 12 m telescope. Forty favorable transitions of this species were observed in
the range 68–169 GHz. Emission on the 20–70 mK level was detected at frequencies of 38 of these lines, including
all transitions arising from the Ka ¼ 0, 1, and 2 ladders. The two transitions not detected were weak and originate
in the less populated Ka ¼ 3 levels. Twenty-one percent of the detected lines are distinct, individual features. The
remaining transitions are either contaminated by emission from abundant molecules or blended with equivalently
weak features. The unblended transitions indicate VLSR ¼ 62:3 2:4 km s1 and V1/2 ¼ 8:3 3:4 km s1, line
parameters characteristic of organic species in Sgr B2( N). A rotational diagram yields a column density of
5:9 ; 1013 cm2 for glycolaldehyde, suggesting a fractional abundance of f (H2 ) ¼ 5:9 ; 1011 . Observations of
formaldehyde toward Sgr B2( N) suggest that H2CO and CH2OHCHO arise from the same gas with an abundance
ratio of 1/27. H2CO may function as the precursor to glycolaldehyde in a gas-phase ‘‘formose’’ reaction. These
observations, combined with past results of Hollis et al., provide convincing evidence for the presence of glycolaldehyde in the ISM. This study suggests that an extensive, self-consistent data set is necessary to identify large
organic species in interstellar gas.
Subject headings: astrobiology — astrochemistry — ISM: abundances — ISM: molecules —
molecular processes — radio lines: ISM
1. INTRODUCTION
lems caused by extrapolation of molecular constants to higher
frequency.
Because glycolaldehyde is fundamental to ribose synthesis,
and hence the origin of life, we began a systematic study of all
favorable 2 and 3 mm transitions of this molecule using the
Arizona Radio Observatory (ARO) 12 m telescope in order to
accurately establish its identification and abundance in interstellar gas. This investigation involved observations of 40 transitions
of glycolaldehyde toward Sgr B2(N), covering the frequency
range 68–169 GHz and ranging from J ¼ 6 ! 5 to J ¼ 16 !
15 lines (Eground 11 71 K). Part of the motivation behind this
project was to establish criteria for the detection of large organic
compounds in molecule-rich clouds. A large amount of telescope
time was required for this project over three observing seasons,
including repeated checks of weak features to test for consistency.
In the meantime, Hollis et al. (2004a) conducted additional observations offour more transitions of glycolaldehyde at centimeter
wavelengths (13–22 GHz) using the Green Bank Telescope (GBT).
Spectral features were detected at these frequencies, but three of
the four features were primarily observed in absorption and at velocities not characteristic of the millimeter observations of Hollis
et al. (2000): 80 and 63 km s1 as opposed to 71–75 km s1.
Hollis et al. (2004a) explained these discrepancies with a threesource model for Sgr B2(N).
In this paper, we present definitive evidence of the presence
of interstellar glycolaldehyde in Sgr B2( N). Of 40 viable transitions observed, there was evidence of emission at 38 of these
frequencies. The remaining two transitions sought originate in the
Ka ¼ 3 ladder and hence may not be strong enough for detection,
given their line strength and cross-ladder excitation conditions.
(All Ka ¼ 0, 1, and 2 transitions in this frequency range were
observed.) Here we describe our observations, compare them to
One important premise of the ‘‘RNA world’’ is a large prebiotic source of ribose (e.g., Larralde et al. 1995). The accepted
source of this sugar is the so-called formose reaction. The starting material in this process is formaldehyde, H2CO, which proceeds through a 1 þ 1 carbon addition to the diose glycolaldehyde,
CH2OHCHO. Although the exact mechanism of this reaction step
is still a mystery (Breslow 1959), successive additions of H2CO
to CH2OHCHO lead facilely to ribose (e.g., Ricardo et al. 2004).
Glycolaldehyde therefore is a key intermediate in the formation
of three-, four-, and five-carbon sugars.
Recently, the detection of glycolaldehyde was reported by
Hollis et al. (2000), who observed this molecule toward Sgr B2(N)
using the 12 m telescope. These authors observed six b-type
transitions of this species at 3 mm (JKa ; Kc ¼ 70;7 ! 61;6 , 81;7 !
72;6 , 80;8 ! 71;7 , 90;9 ! 81;8 , 100;10 ! 91;9 , and 101;9 ! 92;8 ); five
of these lines were blended with other molecules, as evidenced by
the unusually large (V1= 2 22 40 km s1) line widths. Based
on these six features, Hollis et al. (2000) determined a column density of (1:0 1:9) ; 1015 cm2 for glycolaldehyde, assuming Trot 200 K. However, detection of six features is not sufficient evidence for an identification of a large organic species in a source
such as Sgr B2(N), particularly since five were blended. Blended
lines offer little statistically in the determination of line positions, which are essential for an unambiguous assignment. For
example, although interstellar glycine was reported based on the
observation of 13 features in Sgr B2( N) and 15 lines in Orion
KL, its supposed detection was incorrect (Snyder et al. 2005).
On closer inspection, key transitions of the species were not
present in either source, and discrepancies were present among
LSR velocities of observed lines. There were additional prob237
238
HALFEN ET AL.
Vol. 639
TABLE 1
Observations of Glycolaldehyde at 2 and 3 mm toward Sgr B2( N )
Transition
! JK00a ; Kc
JK0 a ; Kc
61;6 ! 50;5 ...............
70;7 ! 61;6 ...............
81;7 ! 72;6 ...............
71;7 ! 60;6 ...............
80;8 ! 71;7 ...............
81;8 ! 70;7 ...............
91;8 ! 82;7 ...............
90;9 ! 81;8 ...............
91;9 ! 80;8 ...............
72;6 ! 61;5 ...............
100;10 ! 91;9 ...........
101;9 ! 92;8 .............
101;10 ! 90;9 ...........
82;7 ! 71;6 ...............
122;10 ! 113;9 .........
110;11 ! 101;10 ........
111;11 ! 100;10 ........
130;13 ! 121;12 ........
131;13 ! 120;12 ........
122;11 ! 111;10 ........
73;5 ! 62;4 ...............
131;12 ! 122;11 ........
140;14 ! 131;13 ........
141;14 ! 130;13 ........
142;12 ! 133;11 ........
132;12 ! 121;11 ........
83;6 ! 72;5 ...............
141;13 ! 132;12 ........
93;7 ! 82;6 ...............
150;15 ! 141;14 ........
151;15 ! 140;14 ........
142;13 ! 131;12 ........
103;8 ! 92;7 .............
152;13 ! 143;12 ........
151;14 ! 142;13 ........
160;16 ! 151;15 ........
161;16 ! 150;15 ........
152;14 ! 141;13 ........
113;9 ! 102;8 ...........
123;10 ! 112;9 .........
a
b
c
d
( MHz)
68114.935
71542.200b
75347.389b,c
76790.594c
82470.670b,c
85782.242c
89868.630c
93052.672b,c
95070.096c
102549.722c
103391.283b,c
103667.907b,c
104587.701c
109280.007c
113326.897c
113569.529c
114264.436c
133662.112d
133886.093d
137677.039d
138436.807d
140280.293d
143640.936d
143765.650d
145774.840d
146019.311d
146201.513d
151243.012d
152975.220d
153597.993
153666.947
154847.682
158909.896d
160493.712d
161834.480d
163542.283
163580.081
164047.117d
164209.569d
169125.240d
b
(arcsec)
92
88
83
82
76
73
70
68
66
61
61
61
60
58
55
55
55
47
47
46
45
45
44
44
43
43
43
42
41
41
41
41
40
39
39
38
38
38
38
37
c
0.95
0.94
0.93
0.93
0.91
0.90
0.89
0.89
0.88
0.86
0.86
0.86
0.85
0.84
0.83
0.83
0.83
0.78
0.78
0.77
0.76
0.76
0.75
0.75
0.74
0.74
0.74
0.73
0.73
0.72
0.72
0.72
0.71
0.71
0.70
0.70
0.70
0.70
0.70
0.68
TR
( K)
0.05
0.040 0.015
0.02
0.04
0.030 0.010
...
0.030 0.020
0.02
...
...
0.04
0.040 0.020
...
0.02
...
...
...
0.04
0.050 0.010
0.03
0.020 0.010
0.020 0.010
0.070 0.010
0.040 0.020
0.040 0.020
...
<0.02
0.060 0.020
0.045 0.025
0.070 0.020
0.040 0.015
0.035 0.015
0.02
0.045 0.020
0.04
0.05
0.020 0.015
0.030 0.020
<0.02
...
VLSR
( km s1)
a
V1/2
( km s1)
62
62.0 4.2
62a
62a
63.0 3.6
...
59.9 3.3
62a
...
...
62a
62.3 2.9
...
62a
...
...
...
62a
59.0 2.2
62a
62.0 2.2
60.5 2.1
62.4 2.1
63.2 2.1
62a
...
62a
62.0 2.0
61.5 2.0
62.5 2.0
63.2 2.0
63.6 2.0
62a
62.6 1.9
62a
62a
63.5 1.8
62.0 1.8
62a
...
a
8
8a
8a
8a
8a
...
11.7 3.3
8a
...
...
8a
10.4 2.9
...
8a
...
...
...
8a
7.8 2.2
8a
6.0 2.2
6.5 2.1
6.7 2.1
6.8 2.1
8a
...
...
8.8 2.0
7.5 2.0
7.3 2.0
7.9 2.0
6.4 2.0
8a
6.3 1.9
8a
8a
7.0 1.8
5.9 1.8
...
...
Eu
( K)
20 S
( D2)
Comments
11.32
14.75
21.39
14.85
18.80
18.88
26.42
23.34
23.37
17.78
28.33
31.93
28.36
22.11
47.58
33.81
33.81
46.17
46.17
44.52
21.02
51.25
53.06
53.08
62.95
51.35
25.48
58.62
30.49
60.44
60.44
58.67
36.04
71.31
66.44
68.30
68.30
66.49
42.14
48.77
23.84
28.43
15.51
29.08
34.19
34.53
20.57
39.88
40.05
16.62
45.49
26.22
45.58
19.82
18.24
51.05
51.10
62.09
62.09
38.92
15.12
44.13
67.56
67.57
28.81
44.53
15.96
49.98
17.14
73.05
73.05
50.20
18.77
34.89
55.71
78.51
78.51
55.84
20.97
23.85
Totally blended, 40 km s1 asymmetric line
Partially blended, V1/2 uncertain
Totally blended
Totally blended
Partially blended, V1/2 uncertain
Contaminated by HCOOCH3
Clean detection
Totally blended with U line
Contaminated by C2H3CN
Contaminated by CH3CCH
Totally blended
Clean detection
Contaminated by C2H5CN
Totally blended
Contaminated by C2H5OH
Near Galactic CN transition
Contaminated by HCOOCH3
Totally blended
Partially blended
Totally blended
Partially blended
Partially blended
Clean detection
Partially blended
Possibly coincident with CH3OCH3
Contaminated by HCOOCH3 + C2H5OH
Not detected
Clean detection
Partially blended
Clean detection
Clean detection
Partially blended
Totally blended
Clean detection
Totally blended
Totally blended
Clean detection
Partially blended
Not detected
Contaminated by H2CS
Assumed value.
Lines previously detected by Hollis et al. (2000).
Transition frequency measured in laboratory by Widicus Weaver et al. (2005).
Transition frequency measured in laboratory by Butler et al. (2001).
formaldehyde spectra, and discuss their relevance for prebiotic
chemistry. We also set criteria for detection of ‘‘large’’ biomolecules in the interstellar medium via millimeter astronomy.
2. OBSERVATIONS
The data were taken over several years from 2002 September
to 2005 April using the Arizona Radio Observatory 12 m telescope1 at Kitt Peak, Arizona. The receivers used were dualchannel, cooled SIS mixers covering the 2 and 3 mm band. Each
mixer was operated in single-sideband mode with image rejection of at least 16 dB. The back ends used for the observations
were 256 channel filter banks of 500 kHz and 1 MHz resolution
1
The 12 m telescope is operated by the Arizona Radio Observatory (ARO),
Steward Observatory, University of Arizona, with partial support from the Research Corporation.
operating in parallel mode (2 ; 128). The temperature scale was
determined by the chopper wheel method, corrected for forward
spillover losses, and is given as TR . The radiation temperature
TR , assuming the source fills only the main beam, is then TR ¼
TR /c . The data were taken in position-switching mode with the
off position 300 west in azimuth. The beam sizes b and efficiencies c of the 12 m telescope at the observing frequencies are
listed in Table 1. The beam size varied from 9200 to 3700 over the
observing range. A 10 MHz local oscillator shift was conducted
for all observations to ensure identification of any image contamination. Pointing was established by observations of planets, 3C 273, and 3C 279. All observations were taken toward
Sgr B2( N) ( ¼ 17h 44m 19:s8, ¼ 28 22 0 17 00 [J2000.0]).
Observations of several isotopomers of formaldehyde, H2CO,
were also conducted toward Sgr B2( N) using the 12 m telescope. The observing setup was identical to that of glycolaldehyde. The JKa ; Kc ¼ 10;1 ! 00;0 transition at 71–72 GHz and the
No. 1, 2006
SYSTEMATIC STUDY OF GLYCOLALDEHYDE IN SGR B2( N)
239
TABLE 2
Observations of Formaldehyde Isotopomers at 2 and 3 mm toward Sgr B2( N )
Transition
! JK00a ; Kc
JK0 a ; Kc
Molecule
H2CO.......................
H213CO....................
H2C18O....................
10;1
21;2
20;2
21;1
10;1
21;2
20;2
21;1
21;2
20;2
21;1
! 00;0
! 11;1
! 10;1
! 11;0
! 00;0
! 11;1
! 10;1
! 11;0
! 11;1
! 10;1
! 11;0
( MHz)
b
(arcsec)
72837.948
140839.502
145602.949
150498.334
71024.788
137449.950
141983.740
146635.672
134435.920
138770.861
143213.068
86
45
43
42
89
46
44
43
47
45
44
c
TR
( K)
VLSR
( km s1)
V1/2
( km s1)
Eu
( K)
S
Comments
0.94
0.76
0.75
0.73
0.94
0.77
0.76
0.74
0.78
0.76
0.75
...
...
...
...
...
...
...
...
0.050 0.020
0.045 0.015
0.055 0.020
...
...
...
...
...
...
...
...
61.6 2.2
62.9 2.2
63.8 3.2
...
...
...
...
...
...
...
...
7.0 2.2
7.3 2.2
7.6 3.2
3.50
21.92
10.48
22.62
3.41
21.72
10.22
22.38
21.54
9.99
22.18
1.0
1.5
2.0
1.5
1.0
1.5
2.0
1.5
1.5
2.0
1.5
Optically thick, self-reversed
Optically thick, self-reversed
Optically thick, self-reversed
Optically thick, self-reversed
Self-reversed at 63
Self-reversed at 63
Self-reversed at 63
Self-reversed at 63
Partially blended
Clean detection
Partially blended
JKa ; Kc ¼ 20; 2 ! 10;1 , 21; 2 ! 11;1 , and 21;1 ! 11;0 transitions
at 137–150 GHz were observed for H212CO and H213CO; the
JKa ; Kc ¼ 20; 2 ! 10;1 , 21; 2 ! 11;1 , and 21;1 ! 11;0 transitions
were measured for H2 C18O at 134, 139, and 143 GHz. Rest
frequencies and line parameters for formaldehyde are given in
Table 2.
3. RESULTS AND ANALYSIS
The transitions of glycolaldehyde studied in this work are
listed in Table 1, along with their rest frequency, energy above
ground state Eu , and the product of the square of the dipole
moment 0 with the line strength (20 S ). These transitions were
chosen because they are the strongest lines of glycolaldehyde at
2 mm (130–170 GHz) and 3 mm (65–115 GHz). All of the transitions are R-branch b-type lines, involve Ka ¼ 0, 1, 2, and 3, and
have 20 S 15 D2. The b-dipole moment (2.33 D) for glycolaldehyde is significantly larger than the a-dipole of a ¼ 0:26 D
( Marstokk & Møllendal 1973). The transitions involve levels
that range in energy from 5 cm1 (8 K) to 50 cm1 (70 K), as
shown in the diagram in Figure 1. Here the energy levels are sorted
as a function of Ka quantum number, and each level is labeled
as JKa ; Kc . The transitions observed in this study are indicated by
arrows.
All together, 40 separate transitions of glycolaldehyde were
searched for toward Sgr B2( N), and the complete data set is
presented in Figure 2 (spectral resolution: 500 kHz). The glycolaldehyde spectra are ordered by increasing frequency starting
with the top of the first panel of Figure 2, which displays the lowest frequency transition at 68114.9 MHz. The positions of the respective lines are indicated and assume VLSR ¼ 62:0 km s1. As
demonstrated by this figure, almost all spectra measured had emission at the given rest frequency, with the exception of two transitions at 146201.5 and 164209.6 MHz. There is clearly no emission
at the first frequency, while the other is located in the line wing of a
methyl formate transition. These transitions are marked by an ‘‘X’’
on the energy level diagram of Figure 1. Many features are blended
completely with other lines, such that individual features are not
discernible (see 76790.6 or 103391.3 MHz, for example), while
some are partially blended and an estimate of line parameters can
be adequately made (see 138436.8 or 143765.6 MHz). Other transitions appear as distinct, separate features; there are eight of
these in all, labeled as ‘‘clean detections’’ in Table 1 (89868.6 MHz,
103667.9 MHz, etc.). Where possible, Gaussian profiles were fit to
the glycolaldehyde features, and the resulting line parameters (intensity in TR , VLSR , and the line width V1/2 ) are listed in Table 1.
In some instances, only estimates of these parameters could be
given, while for others, contamination made even rough approximations impossible. It should be noted that all reported features
were checked for contamination by other spectral features from
known spectral-line databases (Lovas 1992; Pickett et al. 1998;
Müller et al. 2005).
Detailed study of the complete frequency range measured was
crucial in determining the degree of contamination of the glycolaldehyde transitions. Emission from 28 individual molecules
was identified in these data, accounting for over 150 molecular
transitions of predictable intensity. These intensities were individually estimated for a given species using rotational temperatures extracted from the literature. Most of these temperatures
Fig. 1.—Energy level diagram of glycolaldehyde showing the transitions
observed in this study, indicated by arrows. The levels are separated into Ka
ladders for the b-type rotational transitions in this molecule. Each level is labeled by the quantum numbers JKa ; Kc . An ‘‘X’’ over an arrow indicates the two
transitions out of 40 that were not detected in this study.
240
HALFEN ET AL.
Vol. 639
Fig. 2.— Observed spectra of glycolaldehyde at 2 and 3 mm obtained at the ARO Kitt Peak 12 m telescope toward Sgr B2( N ). Each spectrum is labeled by the
glycolaldehyde transition with a line pointing to the corresponding rest frequency, assuming VLSR ¼ 62:0 km s1. The spectral resolution is 500 kHz or 2.2–0.89 km s1
over the range 68–169 GHz.
were taken from Nummelin et al. (2000), but a few were from
specific detection papers; for example, in the case of ethylene
glycol, Hollis et al. (2002) was used. An additional 61 distinct
U lines were found and identified in the data. A larger number of
blended U lines are also present. Details of this analysis will be
presented in a later paper.
As shown in Table 1, the LSR velocities determined for the glycolaldehyde features lie in the range VLSR 59:9 63:5 km s1,
with the line widths spanning V1/2 6:3 11:7 km s1 for
‘‘clean detection’’ features. These line parameters agree with other
organic species observed toward Sgr B2(N) such as ethanol, acetaldehyde, and formic acid, which have VLSR 62 km s1 and
V1/2 10 15 km s1, or methyl mercaptan, where VLSR ¼
64 km s1 and V1/2 ¼ 10 km s1 (Nummelin et al. 2000). However, they do not agree well with those established by Hollis et al.
(2000) for glycolaldehyde, who found a center LSR velocity of
VLSR 71 km s1 and V1/2 22 40 km s1. This discrepancy is not surprising, because most of the lines published in their
work were blended with other emission features. However, one
of the absorption features attributed to CH2OHCHO in Hollis
et al. (2004a) lies at VLSR 63 km s1 with V1/2 10 km s1,
in good agreement with our results. There is some evidence
of emission in the millimeter glycolaldehyde data near VLSR 70 km s1 for the lines below 89 GHz in frequency, which could
perhaps correspond to the 73 km s1 component described by
Hollis et al. (2004a). This possible component does not appear to
be present in the 89868.6 MHz data (91;8 ! 82;7 ; see Fig. 2) and is
clearly absent in the 103667.9 MHz spectrum (101;9 ! 92;8 ; see
Fig. 2). The existence of this second, lower energy velocity feature
warrants additional investigation.
The line intensities for CH2OHCHO fall in the range 20–
70 mK, which is consistent with the intrinsic line strengths and
energies above the ground state of the observed transitions. There
are no unusually intense lines that indicate a wrong identification,
and the transitions with the strongest line strengths were always
detected. The consistency is supported by the rotational diagram
presented in Figure 3. Here 18 data points, corresponding to the
‘‘clean’’ (open diamonds) and ‘‘partially blended’’ ( filled diamonds) features, are plotted over the energy range 10–75 K. It
was assumed that the glycolaldehyde source filled the telescope
beam, i.e., s 92 00 . There is some scatter in this diagram, but
no large deviations from the mean slope. The rotational diagram,
based on all data points, indicates a total column density of
Ntot 6 ; 1013 cm2 and Trot ¼ 35 K. A fit to only the ‘‘clean’’
transitions yields Ntot 9 ; 1013 cm2 and Trot ¼ 25 K. These
values are close to those found by Hollis et al. (2004a), who derived 3 ; 1014 cm2 and Trot 50 K. The rotational temperature
of glycolaldehyde is also comparable to those of methyl mercaptan
No. 1, 2006
SYSTEMATIC STUDY OF GLYCOLALDEHYDE IN SGR B2( N)
241
Fig. 2.— Continued
(Trot 35 K), formic acid (Trot 74 K), ethanol (Trot 73 K),
and acetaldehyde, which has Trot 59 K (Nummelin et al. 2000).
4. DISCUSSION
4.1. Validity of the Identification
The accuracy of the rest frequencies used for any interstellar
detection is crucial. Marstokk & Møllendal (1970) investigated
the spectrum of glycolaldehyde from 12 to 18 GHz and from 22
to 26 GHz, and these authors established rotational constants for
glycolaldehyde for J 31. Butler et al. (2001) extended the range
of frequencies measured to include 7.5–12, 18–48, and 128–
354 GHz. This data set included transitions for J 66 and Ka 29 and Kc 52. The frequencies used for this work were recorded by Butler et al. (2001). Only 18 out of the 40 transitions
were directly measured by these authors; the remaining frequencies were predicted directly from their constants and associated
Hamiltonian. The overwhelming majority of the predicted frequencies used here lie in between those directly measured at lower
(Marstokk & Møllendal 1970) and higher frequencies (Butler
et al. 2001). Hence, there was no unwarranted extrapolation to
higher frequencies based on spectroscopic constants determined
for lower frequencies, a practice that is seriously inaccurate and
justifiably condemned by Snyder et al. (2005). In addition, glycolaldehyde measurements have recently been conducted in the range
72–122.5 GHz (Widicus Weaver et al. 2005). The newly measured
frequencies agree with the predicted values used here to 100 kHz.
At this point, frequencies of only seven transitions have yet to be
directly measured (see Table 1).
Another important criterion is the consistency of line shape
between detected transitions. Sgr B2 has multiple velocity components and velocity gradients, as well as cold, foreground gas
that causes self-reversal and absorption features (e.g., Miao et al.
1995); all add confusion to the evaluation of line profiles. In this
case, for transitions of glycolaldehyde that are ‘‘clean,’’ the LSR
velocities and line widths are consistent, typically falling in the
range VLSR ¼ 62:3 2:4 km s1 and V1/2 ¼ 8:3 3:4 km s1.
These line parameters are very typical for Sgr B2(N), as well
(Nummelin et al. 2000). Moreover, observed brightness temperatures are internally consistent, as previously mentioned. Fortunately, a wide frequency range was searched in this case, such that
a sufficient number of uncontaminated transitions were found
from which line parameters could be reliably determined.
Perhaps the most convincing evidence for the presence of glycolaldehyde in Sgr B2( N) is that no favorable transitions in the
complete 2 and 3 mm windows are absent. Clearly, failure to find
even one favorable transition of a given molecule is enough to
discredit an identification. In this case, emission is present at the
frequencies for all lines that originate in the Ka ¼ 0, 1, and 2 ladders in the observed frequency regions (see Fig. 1). The two transitions not observed are among the weakest transitions sought in
this study, with 20 S 21 D2. Typically, 20 S ¼ 15 75 D2 (see
242
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Vol. 639
Fig. 2.— Continued
Table 1). Their predicted intensities, based on Trot 35 K, are
0.02–0.03 K, at the limits of the observed noise levels. In addition, the transitions both originate in the Ka ¼ 3 ladder. Because
the governing dipole moment is the b-dipole, radiation connects
these levels via the Ka ¼ 1, Kc ¼ 1 selection rules. Collisions could also populate these levels. As has been found for
other molecules, there may be selection rules for collisions that
follow Ka ¼ 1 as well (Pottage et al. 2004). Hence, the population must pass from Ka ¼ 0 to Ka ¼ 1 to Ka ¼ 2 and then
to Ka ¼ 3 sequentially. Consequently, Ka ¼ 3 levels should be
somewhat less populated than those with Ka 2.
It should be noted that emission was present at four other
transitions originating in the Ka ¼ 3 levels. One line was completely contaminated by H2CS. The other three features were
partially blended, so their exact intensities are subject to some
uncertainty. It could be that all the Ka ¼ 3 to Ka ¼ 2 transitions
are missing, consistent with the above arguments.
4.2. Connection to the Formose Reaction Scheme
If glycolaldehyde is produced via some form of gas-phase formose reaction, then it should be present in material that also contains H2CO. To test this hypothesis, spectra of H2CO were also
measured in the 2 and 3 mm bands toward Sgr B2( N). Because
H2CO is a very abundant molecule, it is optically thick in its 2
and 3 mm transitions, such that it undergoes self-reversal near
63 km s1. This effect occurs in H213CO as well, as illustrated in
the spectra displayed in Figure 4. Consequently, the 2 mm transitions of H2C18O had to be measured. Two of the three asymmetry components at 2 mm lines are blended with other molecules,
but the JKa ; Kc ¼ 20; 2 ! 10;1 transition is relatively clean of contamination. It is also shown in Figure 4.
The H2C18O spectrum agrees very well with those of glycolaldehyde. The LSR velocity is VLSR ¼ 62:9 2:2 km s1, and the
line width is V1/2 ¼ 7:3 2:2 km s1 for this transition. Similar line parameters have been found for H2 CO observed at 6 cm
with the VLA toward the Sgr B2(N) position (Mehringer et al.
1995). It is probable that H2CO and CH2OHCHO arise at least in
part from the same gas, although mapping the emission of both
species is necessary to establish a definitive connection. H2CO
could be the major precursor to glycolaldehyde, perhaps via its
protonated form, H2COH+, as well. This ion has also been detected toward the Galactic center (Ohishi et al. 1996), but high
signal-to-noise ratio spectra in Sgr B2(N) have yet to be obtained.
Based on the optically thin H2C18O spectrum, and assuming 16 O/ 18 O ¼ 250 ( Wilson & Rood 1994), the column density
of formaldehyde in Sgr B2( N) is Ntot ¼ 1:6 ; 1015 cm2. This
calculation assumes that Trot ¼ 35 K. Therefore, the ratio of
CH2 OHCHO/ H2 CO ¼ 1/27. This ratio implies that only 4% of
formaldehyde is converted to glycolaldehyde, which is not unreasonable. If N (H2 ) ¼ 1024 cm2 (Nummelin et al. 2000), then
the fractional abundances of H2CO and CH2OHCHO relative
to H2 are 1:6 ; 109 and 5:9 ; 1011 , respectively, in Sgr B2( N).
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SYSTEMATIC STUDY OF GLYCOLALDEHYDE IN SGR B2( N)
243
Fig. 2.— Continued
How is glycolaldehyde synthesized from H2CO in the gas
phase? Very early it was noted that formaldehyde reacts under
basic conditions to form a mixture of sugars (Butlerow 1861).
This reaction became known as the formose reaction and has been
the subject of many mechanistic studies in the intervening years.
Many base-catalyzed reactions have acid-catalyzed counterparts,
and this seems more likely in the gas phase. As pointed out earlier
(Breslow 1959), ‘‘it has long been obvious that the formation of
glycolaldehyde from formaldehyde is the process to be explained.’’
A possible mechanism for the problematic glycolaldehyde formation is provided in Figure 5. The putative reaction sequence begins with the association reaction of H2C = O (species 1 in Fig. 5)
and H2C = OH + (species 1-H +), as shown in reactions (a) and
( b), creating a Nazarov-like coupling of the two carbonyls leading to an oxene species +O CH2 CH2OH in reaction (c). This
electron-deficient oxygen species could insert into the neighboring
C-H bond to produce protonated glycolaldehyde (species 2-H+),
which in turn can lose a proton to form the simple two-carbon
diose glycolaldehyde (species 2). Preliminary studies of this
reaction using proton transfer mass spectroscopy ( L. Abrell &
R. Polt 2006, in preparation) suggest that glycolaldehyde can be
synthesized in the gas phase from the acid-catalyzed reaction of
H2CO, although the intermediate species are speculative. Clearly,
further gas-phase mechanistic studies are needed to substantiate
this scheme. There has been continued interest in the formose reaction as it might apply to the interstellar medium (e.g., Ptasinska
et al. 2005).
The next step leading to ribose is the creation of the threecarbon sugar glyceraldehyde, CH2OHCHOHCHO. Rest frequencies for this molecule have been measured by Lovas et al.
(2003) in the 10–25 GHz range. An unsuccessful search has
subsequently been conducted for glyceraldehyde by Hollis et al.
(2004b) using the Green Bank Telescope. However, the upper
limits obtained by these authors were TA 5 10 mK. If the
CH2OHCHO/ H2CO ratio of 1/27 can be applied to the next step
of the interstellar formose reaction, then at least an order-ofmagnitude decrease in the abundance of glyceraldehyde is expected relative to glycolaldehyde. In addition, the larger partition
function in glyceraldehyde relative to glycolaldehyde will also
reduce the intensity in any given transition. Given these considerations, the expected antenna temperature at the GBT should
be TA < 5 mK, below the upper limit obtained by Hollis et al.
(2004b).
4.3. Feasibility of the Detection of Large Organic
Compounds in Interstellar Gas
The history of searches for ‘‘large’’ organic molecules (in the
astrochemist’s sense) has demonstrated that the identification
of such species can be very problematic, especially in the Galactic center clouds. First of all, the line density in Sgr B2( N) is
extremely high. Even with efficient single-sideband receivers
(>16 dB rejection of the image), the average line density in the
2 and 3 mm windows is 10 lines/100 MHz at sensitivity levels of 10 mK, peak to peak. At some frequencies, the confusion
244
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Vol. 639
Fig. 2.— Continued
Fig. 3.—Rotational diagram for glycolaldehyde constructed from the observed
‘‘clean’’ (open diamonds) and partially blended ( filled diamonds) transitions. The
data points have a small scatter about the fit and suggest Ntot 6 ; 1013 cm2 and
Trot 35 K. The error bars reflect the uncertainties in fitting line profiles. An alternative fit is also shown, based on the ‘‘clean’’ transitions only, which results in
Ntot 9 ; 1013 cm2 and Trot 25 K.
limit (i.e., no significant baseline regions) is actually achieved.
As our study has shown, with such a line density, only a dwindling percentage of transitions (in our case 21%) actually appear
as separate features. Observation of uncontaminated features is
critical to establish consistency of line parameters, and hence a
viable identification.
The many hazards of searching for new molecules have been
pointed out by Snyder et al. (2005). These authors spell out
some of the criteria needed for secure molecular identification,
including accurate rest frequencies and consistency among the
acquired data set. Based on our experience with glycolaldehyde,
we propose to extend their qualifications. We suggest that for an
accurate identification, there must be strong evidence for emission at all favorable, physically connected transitions over a
sufficiently large wavelength range. There cannot be ‘‘missing’’
favorable lines. ‘‘Large’’ in this case means that a significant
number of ‘‘clean’’ transitions are detected, because an identification cannot be made on the basis of only blended lines. Assuming the confusion limit, the chance of finding a line at a
particular LSR velocity (2 km s1) is 40%, assuming simple
Gaussian profiles. Using simple statistics, then the detection of
each successive line after the initial one reduces the probability
of a chance coincidence by that amount. Consequently, six clean
lines are needed for a secure detection at the 99% confidence
level, while eight such transitions means 99.8% confidence. These
criteria are particularly important for species that primarily have
No. 1, 2006
SYSTEMATIC STUDY OF GLYCOLALDEHYDE IN SGR B2( N)
245
Fig. 5.—Proposed gas-phase reaction for the synthesis of glycolaldehyde.
The first step (reaction [a]) is the formation of H2COH+, which then reacts with
H2CO to form a Nazarov-type cation (reaction [ b]). This in turn rearranges to
protonated glycolaldehyde (reactions [c] and [d]), which subsequently forms the
neutral species via dissociative electron recombination.
organic molecule based on even 10–20 transitions is subject to
question.
5. CONCLUSION
Fig. 4.—Representative spectra for three isotopomers of formaldehyde, H2CO,
observed using the ARO 12 m telescope toward Sgr B2( N ). The JKa ; Kc ¼
20; 2 10;1 transitions of H212CO, H213CO, and H2C18O are displayed in the panels
from top to bottom , respectively. The dotted vertical line indicates a VLSR of
62 km s1. The 12C and 13C isotopomers are self-reversed, but H2C18O shows
a clean emission feature.
b-dipole transitions. Here patterns such as asymmetry splittings
in a K-ladder structure are not present, as might be available for
a-type or c-type transitions.
Can larger molecules such as glyceraldehyde or even a pentose
be securely identified in the interstellar medium? As this study
suggests, such an endeavor will be a daunting task requiring observations of possibly over a hundred individual transitions. As
demonstrated by Snyder et al. (2005), any detection of a large
An intensive search to confirm the presence of glycolaldehyde
in interstellar gas has led to observations at 34 new transitions of
this molecule in the 2 and 3 mm windows in Sgr B2(N). Out of
this data set, seven new transitions were measured that appear to
be uncontaminated. These lines, along with the previous data of
Hollis et al. (2000, 2004b), provide convincing evidence that glycolaldehyde is present in at least one molecular cloud. Given the
line density in the spectra measured here, it is clear that numerous chance coincidences are extremely likely in any spectral line
search. Therefore, there must be evidence for emission at all favorable transitions of a molecule over a substantial frequency range
in order to claim any interstellar identification.
This material is based on work supported by the National
Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement CAN-02-OSS-02
issued through the Office of Space Science.
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