Significance
The detection of ethanolamine (
N
H
2
C
H
2
C
H
2
OH) in a molecular cloud in the interstellar medium confirms that a precursor of phospholipids is efficiently formed by interstellar ...chemistry. Hence, ethanolamine could have been transferred from the proto-Solar nebula to planetesimals and minor bodies of the Solar System and thereafter to our planet. The prebiotic availability of ethanolamine on early Earth could have triggered the formation of efficient and permeable amphiphilic molecules such as phospholipids, thus playing a relevant role in the evolution of the first cellular membranes needed for the emergence of life.
Cell membranes are a key element of life because they keep the genetic material and metabolic machinery together. All present cell membranes are made of phospholipids, yet the nature of the first membranes and the origin of phospholipids are still under debate. We report here the presence of ethanolamine in space,
N
H
2
C
H
2
C
H
2
OH, which forms the hydrophilic head of the simplest and second-most-abundant phospholipid in membranes. The molecular column density of ethanolamine in interstellar space is
N
= (1.51
±
0.07)
×
10
13
c
m
−
2
, implying a molecular abundance with respect to
H
2
of
(
0.9
−
1.4
)
×
10
−
10
. Previous studies reported its presence in meteoritic material, but they suggested that it is synthesized in the meteorite itself by decomposition of amino acids. However, we find that the proportion of the molecule with respect to water in the interstellar medium is similar to the one found in the meteorite (
10
−
6
). These results indicate that ethanolamine forms efficiently in space and, if delivered onto early Earth, could have contributed to the assembling and early evolution of primitive membranes.
The chemical compounds carrying the thiol group (-SH) have been considered essential in recent prebiotic studies regarding the polymerization of amino acids. We have searched for this kind of ...compound toward the Galactic Center quiescent cloud G+0.693–0.027. We report the first detection in the interstellar space of the trans-isomer of monothioformic acid (t-HC(O)SH) with an abundance of ∼1 נ10−10. Additionally, we provide a solid confirmation of the gauche isomer of ethyl mercaptan (g-C2H5SH) with an abundance of ∼3 נ10−10, and we also detect methyl mercaptan (CH3SH) with an abundance of ∼5 נ10−9. Abundance ratios were calculated for the three SH-bearing species and their OH analogs, revealing similar trends between alcohols and thiols with increasing complexity. Possible chemical routes for the interstellar synthesis of t-HC(O)SH, CH3SH, and C2H5SH are discussed, as well as the relevance of these compounds in the synthesis of prebiotic proteins in the primitive Earth.
Abstract
The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in ...a peculiar observable composition. We analyse the abundances of CO
2
, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO
2
abundances in the gas phase using its protonated form HOCO
+
. The distribution of HOCO
+
is similar to that of methanol, which suggests that HOCO
+
is indeed produced from the protonation of CO
2
sublimated from ice. The HOCO
+
fractional abundances are found to be (1–2) × 10
−9
at the outer part of the central molecular zone (CMZ), while they are lower (∼10
−10
) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO
+
/CO
2
ratios suggested from chemical models, the gas-phase CO
2
fractional abundance is estimated to be (1–20) × 10
−7
at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO
2
ice fractional abundances at the outer CMZ to be (2–5) × 10
−6
from the literature. A comparison between the ice and gas CO
2
abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.
Abstract
We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple ...HCN and HNC rotational transitions observed using the Atacama Large Millimeter/submillimeter Array via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models, which are constrained using our HCN and HNC measurements. We find that the volume density
n
H
2
and cosmic-ray ionization rate (CRIR)
ζ
are enhanced by about an order of magnitude in the galaxy’s central regions as compared to those further from the nucleus. In NGC 253's central giant molecular clouds (GMCs), where observed HCN/HNC abundance ratios are the lowest,
n
∼ 10
5.5
cm
−3
and
ζ
∼ 10
−12
s
−1
(greater than 10
4
times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, H
ii
regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where
ζ
is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic-ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.
Abstract
A quarter century after the detection of the last interstellar carboxylic acid, acetic acid (CH
3
COOH), we report the discovery of a new one, the
cis-trans
form of carbonic acid (HOCOOH), ...toward the Galactic center molecular cloud G+0.693–0.027. HOCOOH stands as the first interstellar molecule containing three oxygen atoms and the third carboxylic acid detected so far in the interstellar medium. Albeit the limited available laboratory measurements (up to 65 GHz), we have also directly identified several pairs of unblended lines in the astronomical data (between 75 and 120 GHz), which allowed us to slightly improve the set of spectroscopic constants. We derive a column density for
cis-trans
HOCOOH of
N
= (6.4 ± 0.4) × 10
12
cm
−2
, which yields an abundance with respect to molecular H
2
of 4.7 × 10
−11
. Meanwhile, the extremely low dipole moment (about 15 times lower) of the lower-energy conformer,
cis-cis
HOCOOH, precludes its detection. We obtain an upper limit to its abundance with respect to H
2
of ≤1.2 × 10
−9
, which suggests that
cis-cis
HOCOOH might be fairly abundant in interstellar space, although it is nearly undetectable by radio astronomical observations. We derive a
cis-cis
/
cis-trans
ratio of ≤25, consistent with the smaller energy difference between both conformers compared with the relative stability of
trans-
and
cis
-formic acid. Finally, we compare the abundance of these acids in different astronomical environments, further suggesting a relationship between the chemical content found in the interstellar medium and the chemical composition of the minor bodies of the solar system, which could be inherited during the star formation process.
We report the first detection in the interstellar medium (ISM) of a C2H5O2N isomer: syn-glycolamide (NH2C(O)CH2OH). The exquisite sensitivity at sub-mK levels of an ultradeep spectral survey carried ...out with the Yebes 40 m and IRAM 30 m telescopes toward the G+0.693–0.027 molecular cloud has allowed us to unambiguously identify multiple transitions of this species. We derived a column density of (7.4 ± 0.7) × 1012 cm−2, which implies a molecular abundance with respect to H2 of 5.5 × 10−11. The other C2H5O2N isomers, including the higher-energy anti conformer of glycolamide and two conformers of glycine, were not detected. The upper limit derived for the abundance of glycine indicates that this amino acid is surely less abundant than its isomer glycolamide in the ISM. The abundances of the C2H5O2N isomers cannot be explained in terms of thermodynamic equilibrium; thus, chemical kinetics need to be invoked. While the low abundance of glycine might not be surprising, based on the relative low abundances of acids in the ISM compared to other compounds (e.g., alcohols, aldehydes, or amines), several chemical pathways can favor the formation of its isomer glycolamide. It can be formed through radical–radical reactions on the surface of dust grains. The abundances of these radicals can be significantly boosted in an environment affected by a strong ultraviolet field induced by cosmic rays, such as that expected in G+0.693–0.027. Therefore, as shown by several recent molecular detections toward this molecular cloud, it stands out as the best target to discover new species with carbon, oxygen, and nitrogen with increasing chemical complexity.
Abstract We present the first detection in space of O-protonated carbonyl sulfide (HOCS + ), in the midst of an ultradeep molecular line survey toward the G+0.693-0.027 molecular cloud. From the ...observation of all K a = 0 transitions ranging from J lo = 2 to J lo = 13 of HOCS + covered by our survey, we derive a column density of N = (9 ± 2) × 10 12 cm −2 , translating into a fractional abundance relative to H 2 of ∼7 × 10 −11 . Conversely, the S-protonated HSCO + isomer remains undetected, and we derive an upper limit to its abundance with respect to H 2 of ≤3 × 10 −11 , a factor of ≥2.3 less abundant than HOCS + . We obtain an HOCS + /OCS ratio of ∼2.5 × 10 −3 , in good agreement with the prediction of astrochemical models. These models show that one of the main chemical routes to the interstellar formation of HOCS + is likely the protonation of OCS, which appears to be more efficient at the oxygen end. Also, we find that high values of cosmic-ray ionization rates (10 −15 –10 −14 s −1 ) are needed to reproduce the observed abundance of HOCS + . In addition, we compare the O/S ratio across different interstellar environments. G+0.693-0.027 appears as the source with the lowest O/S ratio. We find an HOCO + /HOCS + ratio of ∼31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (∼37). This fact indicates that S is not significantly depleted within this cloud due to the action of large-scale shocks, unlike in other sources where S-bearing species remain trapped on icy dust grains.
The cosmic-ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star ...formation regions, impacting the physical and chemical processes that drive the formation of stars. Previous studies of the circum-molecular zone of the starburst galaxy NGC 253 have found evidence for a high CRIR value: 103–106 times the average CRIR within the Milky Way. This is a broad constraint, and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations toward the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H3O+ and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 from emission from their transitions. We find that each of the four regions modeled has a CRIR in the range (1–80) × 10−14 s−1 and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays, including C2H, HCO+, HOC+, and CO. From shock and photon-dominated/X-ray dominated region models, we further find that neither UV-/X-ray-driven nor shock-dominated chemistry is a viable single alternative as none of these processes can adequately fit the abundances of all of these species.
Abstract
We present a spatially resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the Atacama Large Millimeter/submillimeter Array ...Comprehensive High-resolution Extragalactic Molecular Inventory, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way’s Galactic center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3–7 transitions from nine molecular species to delineate the position–position–velocity distributions of column density (
N
H
2
), volume density (
n
H
2
), and temperature (
T
kin
) at 27 pc resolution. Two distinct components are detected: a low-density component with
(
n
H
2
,
T
kin
)
∼
(
10
3.3
cm
−
3
,
85
K
)
and a high-density component with
(
n
H
2
,
T
kin
)
∼
(
10
4.4
cm
−
3
,
110
K
)
, separated at
n
H
2
∼
10
3.8
cm
−
3
. NGC 253 has ∼10 times the high-density gas mass and ∼3 times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of ∼30 difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The
n
H
2
histogram toward NGC 253 exhibits a shallow declining slope up to
n
H
2
∼
10
6
cm
−
3
, while that of the GC steeply drops in
n
H
2
≳
10
4.5
cm
−
3
and vanishes at 10
5
cm
−3
. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold
n
H
2
for the dense gas is taken at ∼10
4.2−4.6
cm
−3
. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies.
Abstract
Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be ...stronger. We examine these astrochemical signatures through multiple transitions of HCO
+
and its metastable isomer HOC
+
in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC
+
(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H
ii
regions. The observed HCO
+
/HOC
+
abundance ratios are ∼10–150, and the fractional abundance of HOC
+
relative to H
2
is ∼1.5 × 10
−11
–6 × 10
−10
, which implies that the HOC
+
abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of
G
0
≳ 10
3
if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of
ζ
≳ 10
−14
s
−1
(3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC
+
, we propose that a low-excitation line of HOC
+
traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.
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