Glycolaldehyde in the ISM

There had been two initial reports in the literature on the detection of hydroxyethanal, aka glycolaldehyde, in Sgr B2(N):
J. M. Hollis, F. J. Lovas, and P. R. Jewell,
Interstellar Glycolaldehyde: The First Sugar
Astrophys. J. 540, L107–L110 (2000); and
J. M. Hollis, S. N. Vogel, L. E. Snyder, P. R. Jewell, and F. J. Lovas,
The Spatial Scale of Glycolaldehyde in the Galactic Center
Astrophys. J. 554, L81–L85 (2001).
6 b-type rotational transitions in the 70 to 105 GHz region were searched for with the NRAO 12 m radio telescope in the former work. Two transitions were severely blended, the remaining four appeared to be free of contamination. No rotational temeperature was derived, but 200 K were assumed. Because of the small number of lines and possible line overlap this detection had to be viewed with some caution until further observations.
The 808 – 717 transition near 82.471 GHz was studied with the BIMA array in the latter work. The molecule was found to be distributed over a large area of more than 60" with no pronounced peak at the Large Molecule Heimat hot core, in contrast to its isomers acetic acid and methyl formate, which are concentrated on a much smaller scale of less than 5". This indicates a much lower rotational temperature than the 200 K assumed earlier.

A report on interstellar glycolaldehyde had been presented at the 58th International Symposium on Molecular Spectroscopy; Columbus, OH, June 16–20, 2003:
D. T. Halfen, A. J. Apponi, and L. M. Ziurys,
Glycolaldehyde Revisited: Can Large Organic Molecules be Accurately Identified in the Interstellar Medium ?.
This contribution appeared to question the detection.

Recent observations of the four lowest rQ0 transitions in the microwave region toward Sgr B2(N) secure the glycolaldehyde detection:
J. M. Hollis, P. R. Jewell, F. J. Lovas, and A. Remijan,
Green Bank Telescope Observations of Interstellar Glycolaldehyde: Low-Temperature Sugar
Astrophys. J. 613, L45–L48 (2004).
Interestingly, these transitions point at temperatures of around 8 K, whereas a reanalysis of the millimeter wave transitions, observed by the same group and mentioned above, were reinterpreted in terms of a temperature of about 50 K.

The observations from the Ziurys group have appeared eventually:
D. T. Halfen, A. J. Apponi, N. Woolf, R. Polt, and L. M. Ziurys,
A Systematic Study of Glycolaldehyde in Sagittarius B2(N) at 2 and 3 mm:
Criteria for Detecting Large Interstellar Molecules

Astrophys. J. 639, 237–245 (2006).
Even though the article mentioned above already has secured the glycolaldehyde detection, this work is still worth mentioning because it shows how difficult it is to obtain definitive results for a moderately complex molecule. 40 (!) transitions have been searched for in the upper 4 mm region to the 2 mm region (68–169 GHz). No significant signals were detected for only two transitions which are fairly high in Ka. Only 8 of the remaining 38 transitions appeared to be unaffected by features of other species. All others were overlapped to a varying amount. A sizeable number of features remained unassigned (U-lines). Thus, this work does not only emphasize the importance of searching for a sufficiently large number of spectral features of a desired moderately large molecule in order to prove its presence in certain media in space, but it also indicates the importance of knowing the identity of the U-lines in order to identify further molecules in space.
The rotational temperature derived from "clean" lines only is about 25 K; inclusion of some slightly blended lines yields a value of roughly 35 K.
Some of the observed U-lines may be caused by presently not identified species. However, a considerable number of these U-lines are caused most definitively by known species in higher rotationally or vibrationally excited states or by minor isotopic species. It is also obvious that the unambiguous identification of molecules more complex than glycolaldehyde will be even more challenging.

The molecule was also detected in three Galactic center molecular clouds which have low dust temperatures (10–20 K), even lower rotational temperatures (around 10 K), which are moderately dense (a few tenthousand molecules per cubic centimeter), but which are kinetically moderately warm (about 100 K or more). Three or four transitions were detected with the 100 m GBT telescope between 13 and 23 GHz by
M. A. Requena-Torres, J. Martín-Pintado, S. Martín, and M. R. Morris,
The Galactic Center: The Largest Oxygen-bearing Organic Molecule Repository
Astrophys. J. 672, 352–360 (2008).

A report on the
First Detection of Glycolaldehyde Outside the Galactic Center
by M. T. Beltrán, C. Codella, S. Viti, R. Neri, and R. Cesaroni,
Astrophys. J. 690, L93–L96 (2009),
with the IRAM Plateau de Bure Interferometer toward the high mass star-forming region G31.41+0.31 should be viewed with great caution because only three transitions were searched for (near 220.5, 143.6 and 103.7 GHz), and for at least one (near 220.5 GHz) there is a severe problem with the line position. Finally, the assumed temperature of 300 K is at odds with the observations carried out toward Sgr B2(N). Thus, further confirming transitions are needed to establish the first detection of glycolaldehyde outside the Galactic center.

The situation is more favorable in
Detection of the Simplest Sugar, Glycolaldehyde, in a Solar-type Protostar with ALMA
by J. K. Jørgensen, C. Favre, S. E. Bisschop, T. L. Bourke, E. F. van Dishoeck, and Markus Schmalzl,
Astrophys. J. 757, Art. No. L4 (2012).
6 emission features were observed in a small frequency region near 200 GHz toward source B of IRAS 16293-2422. Though two of the featueres are very weak and at least two are blended, the situation is much more favorable than the one described in the previous paragraph – even if one takes into account that several other features remained unassigned. A rotational temperature of about 300 K was estimated for glycolaldehyde as well as for its isomer methyl formate, which has a column density slightly more than one order of magnitude higher than that of glycolaldehyde. The two molecules were also seen toward source A with similar column densities as in source B, but with a rotational temperature estimated at about 200 K. Moreover, several additional features were seen near 700 GHz red-shifted and in absorption toward source B; slightly off source, the same lines were seen unshifted in emission. This was interpreted as a sign of infall.

J. K. Jørgensen, M. H. D. van der Wiel, A. Coutens, J. M. Lykke, H. S. P. Müller, E. F. van Dishoeck, H. Calcutt, P. Bjerkeli, T. L. Bourke, M. N. Drozdovskaya, C. Favre, E. C. Fayolle, R. T. Garrod, S. K. Jacobsen, K. I. Öberg, M. V. Persson, and S. F. Wampfler
reported on
The ALMA Protostellar Interferometric Line Survey (PILS): First Results from an Unbiased Submillimeter Wavelength Line Survey of the Class 0 Protostellar Binary IRAS 16293–2422 with ALMA,
Astron. Astrophys. 595, Art. No. A117 (2016).
Emission of deuterated isotopologs was detected as well as that of 13C species. The D to H ratio was about 0.05 per H atom, irrespective of the functional group. The 12C to 13C ratio was about 30 for each C atom, lower than the canonical value of 67 expected for the local ISM.

Contributor(s): H. S. P. Müller; 09, 2003; 09, 2004; 02, 2006; 04, 2012; 07, 2012; 09, 2012; 11, 2016