arXiv subjects

Laura Colzi

Publications and source records attributed to Laura Colzi.

53 records · Page 3Linked to original sources

arXiv2023

Volume density structure of the NGC253 CMZ through ALCHEMI excitation analysis

We present a spatially-resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the ALMA Large program ALCHEMI, 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 ($N_\mathrm{H_2}$), volume density ($n_\mathrm{H_2}$), and temperature ($T_\mathrm{kin}$) at 27 pc resolution. Two distinct components are detected: a low-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim(10^{3.3}\ \mathrm{cm}^{-3}, 85 K)$ and a high-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim (10^{4.4}\ \mathrm{cm}^{-3}, 110\ \mathrm{K})$, separated at $n_\mathrm{H_2}\sim10^{3.8}\ \mathrm{cm}^{-3}$. NGC 253 has $\sim10$ times the high-density gas mass and $\sim3$ 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 $\sim30$ difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The $n_\mathrm{H_2}$ histogram toward NGC 253 exhibits a shallow declining slope up to $n_\mathrm{H_2}\sim10^6\ \mathrm{cm}^{-3}$, while that of the GC steeply drops in $n_\mathrm{H_2}\gtrsim10^{4.5}\ \mathrm{cm}^{-3}$ and vanishes at $10^5\ \mathrm{cm}^{-3}$. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold $n_\mathrm{H_2}$ for the dense gas is taken at $\sim 10^{4.2\mbox{-}4.6}\ \mathrm{cm}^{-3}$. 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.

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arXiv2023

First glycine isomer detected in the interstellar medium: glycolamide (NH$_2$C(O)CH$_2$OH)

We report the first detection in the interstellar medium of a C$_2$H$_5$O$_2$N isomer: $syn$-glycolamide (NH$_2$C(O)CH$_2$OH). The exquisite sensitivity at sub-mK levels of an ultra-deep spectral survey carried out with the Yebes 40m and IRAM 30m telescopes towards the G+0.693-0.027 molecular cloud have allowed us to unambiguously identify multiple transitions of this species. We derived a column density of (7.4 $\pm$ 0.7)$\times$10$^{12}$ cm$^{-2}$, which implies a molecular abundance with respect to H$_2$ of 5.5$\times$10$^{-11}$. The other C$_2$H$_5$O$_2$N 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 C$_2$H$_5$O$_2$N isomers cannot be explained in terms of thermodynamic equilibrium, and 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 favour 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 towards this molecular cloud, it stands out as the best target to discover new species with carbon, oxygen and nitrogen with increasing chemical complexity.

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arXiv2023

Kinematics of Galactic Centre clouds shaped by shear-seeded solenoidal turbulence

The Central Molecular Zone (CMZ; the central ~ 500 pc of the Galaxy) is a kinematically unusual environment relative to the Galactic disc, with high velocity dispersions and a steep size-linewidth relation of the molecular clouds. In addition, the CMZ region has a significantly lower star formation rate (SFR) than expected by its large amount of dense gas. An important factor in explaining the low SFR is the turbulent state of the star-forming gas, which seems to be dominated by rotational modes. However, the turbulence driving mechanism remains unclear. In this work, we investigate how the Galactic gravitational potential affects the turbulence in CMZ clouds. We focus on the CMZ cloud G0.253+0.016 (`the Brick'), which is very quiescent and unlikely to be kinematically dominated by stellar feedback. We demonstrate that several kinematic properties of the Brick arise naturally in a cloud-scale hydrodynamics simulation that takes into account the Galactic gravitational potential. These properties include the line-of-sight velocity distribution, the steepened size-linewidth relation, and the predominantly solenoidal nature of the turbulence. Within the simulation, these properties result from the Galactic shear in combination with the cloud's gravitational collapse. This is a strong indication that the Galactic gravitational potential plays a crucial role in shaping the CMZ gas kinematics, and is a major contributor to suppressing the SFR by inducing predominantly solenoidal turbulent modes.

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arXiv2023

Discovery of the elusive carbonic acid (HOCOOH) in space

After a quarter century since 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 also the third carboxylic acid detected so far in the interstellar medium. Albeit the limited available laboratory measurements (up to 65 GHz), we have also identified several pairs of unblended lines directly in the astronomical data (between 75-120 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 $\pm$ 0.4) $\times$ 10$^{12}$ cm$^{-2}$, which yields an abundance with respect to molecular H$_2$ of 4.7 $\times$ 10$^{-11}$. Meanwhile, the extremely low dipole moment (about fifteen 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 $\leq$ 1.2 $\times$10$^{-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 $\leq$ 25, consistent with the smaller energy difference between both conformers compared with the relative stability of trans- and cis-formic acid (HCOOH). 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.

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arXiv2023

H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium

The ${\rm H_2NC}$ radical is the high-energy metastable isomer of ${\rm H_2CN}$ radical, which has been recently detected for the first time in the interstellar medium towards a handful of cold galactic sources, besides a warm galaxy in front of the PKS 1830-211 quasar. These detections have shown that the ${\rm H_2CN}$/${\rm H_2NC}$ isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature ($T_{\rm kin}$), but the shortage of them in warm sources still prevents us to confirm this hypothesis and shed light about their chemistry. In this work, we present the first detection of ${\rm H_2CN}$ and ${\rm H_2NC}$ towards a warm galactic source, the G+0.693-0.027 molecular cloud (with $T_{\rm kin} > 70 \, {\rm K}$), using IRAM 30m observations. We have detected multiple hyperfine components of the $N_{K_\text{a}K_\text{c}} = 1_{01} - 0_{00}$ and $2_{02} - 1_{01}$ transitions. We derived molecular abundances with respect to ${\rm H_2}$ of (6.8$\pm$1.3)$\times 10^{-11}$ for ${\rm H_2CN}$ and of (3.1$\pm$0.7)$\times 10^{-11}$ for ${\rm H_2NC}$, and a ${\rm H_2CN}$/${\rm H_2NC}$ abundance ratio of 2.2$\pm$0.5. These detections confirm that the ${\rm H_2CN}$/${\rm H_2NC}$ ratio is $\gtrsim$2 for sources with $T_{\rm kin} > 70 \, {\rm K}$, larger than the $\sim$1 ratios previously found in colder cores ($T_{\rm kin}\sim10 \, {\rm K}$). This isomeric ratio dependence with temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the ${\rm H_2NC} \rightarrow {\rm H_2CN}$ isomerization, deserve consideration to explain the higher isomeric ratios and ${\rm H_2CN}$ abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.

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arXiv2023

A sequential acid-base (SAB) mechanism in the interstellar medium: The emergence of cis formic acid in dark molecular clouds

The abundance ratios between isomers of a COM observed in the ISM provides valuable information about the chemistry and physics of the gas and eventually, the history of molecular clouds. In this context, the origin of an abundance of c-HCOOH acid of only 6% the isomer c-HCOOH abundance in cold cores, remains unknown. Herein, we explain the presence of c-HCOOH in dark molecular clouds through the destruction and back formation of c-HCOOH and t-HCOOH in a cyclic process that involves HCOOH and highly abundant molecules such as HCO+ and NH3. We use high-level ab initio methods to compute the potential energy profiles for the cyclic destruction/formation routes of c-HCOOH and t-HCOOH. Global rate constants and branching ratios were calculated based on the transition state theory and the master equation formalism under the typical conditions of the ISM. The destruction of HCOOH by reaction with HCO+ in the gas phase leads to three isomers of the cation HC(OH)2+. The most abundant cation can react in a second step with other abundant molecules of the ISM like NH3 to form back c-HCOOH and t-HCOOH. This mechanism explains the formation of c-HCOOH in dark molecular clouds. Considering this mechanism, the fraction of c-HCOOH with respect t-HCOOH is 25.7%. To explain the 6% reported by the observations we propose that further destruction mechanisms of the cations of HCOOH should be taken into account. The sequential acid-base (SAB) mechanism proposed in this work involves fast processes with very abundant molecules in the ISM. Thus, HCOOH very likely suffers our proposed transformations in the conditions of dark molecular clouds. This is a new approach in the framework of the isomerism of organic molecules in the ISM which has the potential to try to explain the ratio between isomers of organic molecules detected in the ISM.

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arXiv2022

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

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 ALMA 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 $n_{\text{H}_{2}}$ 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 GMCs, where observed HCN/HNC abundance ratios are lowest, $n \sim 10^{5.5}$ cm$^{-3}$ and $ζ\sim 10^{-12}$ 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, HII 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.

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arXiv2022

ALCHEMI finds a "shocking" carbon footprint in the starburst galaxy NGC~253

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 analyze abundances of $CO_2$, a reliable tracer of ice chemistry, from data collected as part of the ALMA 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 $(1-2)\times10^{-9}$ at the outer part of the central molecular zone (CMZ), while they are lower ($\sim10^{-10}$) 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 $(1-20)\times10^{-7}$ 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 $(2-5)\times10^{-6}$ 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.

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arXiv2022

Molecular precursors of the RNA-world in space: new nitriles in the G+0.693-0.027 molecular cloud

Nitriles play a key role as molecular precursors in prebiotic experiments based on the RNA-world scenario for the origin of life. These chemical compounds could have been partially delivered to the young Earth from extraterrestrial objects, stressing the importance of establishing the reservoir of nitriles in the interstellar medium. We report here the detection towards the molecular cloud G+0.693-0.027 of several nitriles, including cyanic acid (HOCN), and three C$_4$H$_3$N isomers (cyanoallene, CH$_2$CCHCN; propargyl cyanide, HCCCH$_2$CN; and cyanopropyne (CH$_3$CCCN), and the tentative detections of cyanoformaldehyde (HCOCN), and glycolonitrile (HOCH$_2$CN). We have also performed the first interstellar search of cyanoacetaldehyde (HCOCH$_2$CN), which was not detected. Based on the derived molecular abundances of the different nitriles in G+0.693-0.027 and other interstellar sources, we have discussed their formation mechanisms in the ISM. We propose that the observed HOCN abundance in G+0.693-0.027 is mainly due to surface chemistry and subsequent shock-induced desorption, while HCOCN might be mainly formed through gas-phase chemistry. In the case of HOCH$_2$CN, several grain-surface routes from abundant precursors could produce it. The derived abundances of the three C$_4$H$_3$N isomers in G+0.693-0.027 are very similar, and also similar to those previously reported in the dark cold cloud TMC-1. This suggests that the three isomers are likely formed through gas-phase chemistry from common precursors, possibly unsaturated hydrocarbons (CH$_3$CCH and CH$_2$CCH$_2$) that react with the cyanide radical (CN). The rich nitrile feedstock found towards G+0.693-0.027 confirms that interstellar chemistry is able to synthesize in space molecular species that could drive the prebiotic chemistry of the RNA-world.

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arXiv2022

Energizing Star Formation: The Cosmic Ray Ionization Rate in NGC 253 Derived From ALCHEMI Measurements of H$_3$O$^+$ and SO

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, which impacts the physical and chemical processes which drive the formation of stars. Previous studies of the circum-molecular zone (CMZ) of the starburst galaxy NGC 253 have found evidence for a high CRIR value; $10^3-10^6$ times the average cosmic ray ionization rate 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 towards the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H$_3$O$^+$ 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 due to emission from their transitions. We find that each of the four regions modelled has a CRIR in the range $(1-80)\times10^{-14}$ s$^{-1}$ and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays including C$_2$H, HCO$^+$, HOC$^+$, and CO. From shock and PDR/XDR models, we further find that neither UV/X-ray driven nor shock dominated chemistry are a viable single alternative as none of these processes can adequately fit the abundances of all of these species.

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arXiv2022

Precursors of fatty alcohols in the ISM: Discovery of n-propanol

Theories on the origins of life propose that early cell membranes were synthesized from amphiphilic molecules simpler than phospholipids such as fatty alcohols. The discovery in the interstellar medium (ISM) of ethanolamine, the simplest phospholipid head group, raises the question whether simple amphiphilic molecules are also synthesized in space. We investigate whether precursors of fatty alcohols are present in the ISM. For this, we have carried out a spectral survey at 7, 3, 2 and 1 mm toward the Giant Molecular Cloud G+0.693-0.027 located in the Galactic Center using the IRAM 30m and Yebes 40m telescopes. Here, we report the detection in the ISM of the primary alcohol n-propanol (in both conformers Ga-n-C3H7OH and Aa-n-C3H7OH), a precursor of fatty alcohols. The derived column densities of n-propanol are (5.5+-0.4)x10^13 cm^-2 for the Ga conformer and (3.4+-0.3)x10^13 cm^-2 for the Aa conformer, which imply molecular abundances of (4.1+-0.3)x10^-10 for Ga-n-C3H7OH and of (2.5+-0.2)x10^-10 for Aa-n-C3H7OH. We also searched for the AGa conformer of n-butanol (AGa-n-C4H9OH) without success yielding an upper limit to its abundance of <4.1x10^-11. The inferred CH3OH:C2H5OH:C3H7OH:C4H9OH abundance ratios go as 1:0.04:0.006:<0.0004 toward G+0.693-0.027, i.e. they decrease roughly by one order of magnitude for increasing complexity. We also report the detection of both syn and anti conformers of vinyl alcohol, with column densities of (1.11+-0.08)x10^14 cm^-2 and (1.3+-0.4)x10^13 cm^-2, and abundances of (8.2+-0.6)x10^-10 and (9.6+-3.0)x10^-11, respectively. The detection of n-propanol, together with the recent discovery of ethanolamine in the ISM, opens the possibility that precursors of lipids according to theories of the origin of life, could have been brought to Earth from outer space.

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arXiv2022

Precursors of the RNA-world in space: Detection of ($Z$)-1,2-ethenediol in the interstellar medium, a key intermediate in sugar formation

We present the first detection of ($Z$)-1,2-ethenediol, (CHOH)$_2$, the enol form of glycolaldehyde, in the interstellar medium towards the G+0.693-0.027 molecular cloud located in the Galactic Center. We have derived a column density of (1.8$\pm$0.1)$\times$10$^{13}$ cm$^{-2}$, which translates into a molecular abundance with respect to molecular hydrogen of 1.3$\times$10$^{-10}$. The abundance ratio between glycolaldehyde and ($Z$)-1,2-ethenediol is $\sim$5.2. We discuss several viable formation routes through chemical reactions from precursors such as HCO, H$_2$CO, CHOH or CH$_2$CHOH. We also propose that this species might be an important precursor in the formation of glyceraldehyde (HOCH$_2$CHOHCHO) in the interstellar medium through combination with the hydroxymethylene (CHOH) radical.

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arXiv2022

Ionise hard: interstellar PO$^{+}$ detection

We report the first detection of the phosphorus monoxide ion (PO$^{+}$) in the interstellar medium. Our unbiased and very sensitive spectral survey towards the G+0.693$-$0.027 molecular cloud covers four different rotational transitions of this molecule, two of which ($J$=1$-$0 and $J$=2$-$1) appear free of contamination from other species. The fit performed, assuming Local Thermodynamic Equilibrium conditions, yields a column density of $N$=(6.0$\pm$0.7)$\times$10$^{11}$ cm$^{-2}$. The resulting molecular abundance with respect to molecular hydrogen is 4.5$\times$10$^{-12}$. The column density of PO$^{+}$ normalised by the cosmic abundance of P is larger than those of NO$^{+}$ and SO$^{+}$, normalised by N and S, by factors of 3.6 and 2.3, respectively. The $N$(PO$^{+}$)/$N$(PO) ratio is 0.12$\pm$0.03, more than one order of magnitude higher than those of $N$(SO$^{+}$)/$N$(SO) and $N$(NO$^{+}$)/$N$(NO). These results indicate that P is more efficiently ionised in the ISM than N and S. We have performed new chemical models that confirm that the PO$^+$ abundance is strongly enhanced in shocked regions with high values of cosmic-ray ionisation rates (10$^{-15}-$10$^{-14}$ s$^{-1}$), as occurs in the G+0.693$-$0.027 molecular cloud. The shocks sputter the interstellar icy grain mantles, releasing into the gas phase most of their P content, mainly in the form of PH$_3$, which is converted into atomic P, and then ionised efficiently by cosmic rays, forming P$^+$. Further reactions with O$_2$ and OH produce PO$^{+}$. The cosmic-ray ionisation of PO might also contribute significantly, which would explain the high $N$(PO$^{+}$)/$N$(PO) observed. The relatively high gas-phase abundance of PO$^{+}$ with respect to other P-bearing species stresses the relevance of this species in the interstellar chemistry of P.

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arXiv2021

Probing the chemical complexity of amines in the ISM: detection of vinylamine (C$_2$H$_3$NH$_2$) and tentative detection of ethylamine (C$_2$H$_5$NH$_2$)

Amines, in particular primary amines (R-NH$_2$) are closely related to the primordial synthesis of amino acids since they share the same structural backbone. However, only limited number of amines has been identified in the ISM which prevents us from studying their chemistry as well as their relation to pre-biotic species that could lead to the emergence of life. In this letter, we report the first interstellar detection of vinylamine (C$_2$H$_3$NH$_2$) and tentative detection of ethylamine (C$_2$H$_5$NH$_2$) towards the Galactic Centre cloud G+0.693-0.027. The derived abundance with respect to H$_2$ is (3.3$\pm$0.4)$\times$10$^{-10}$ and (1.9$\pm$0.5)$\times$10$^{-10}$, respectively. The inferred abundance ratios of C$_2$H$_3$NH$_2$ and C$_2$H$_5$NH$_2$ with respect to methylamine (CH$_3$NH$_2$) are $\sim$0.02 and $\sim$0.008 respectively. The derived abundance of C$_2$H$_3$NH$_2$, C$_2$H$_5$NH$_2$ and several other NH$_2$-bearing species are compared to those obtained towards high-mass and low-mass star-forming regions. Based on recent chemical and laboratory studies, possible chemical routes for the interstellar synthesis of C$_2$H$_3$NH$_2$ and C$_2$H$_5$NH$_2$ are discussed.

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arXiv2021

Starburst Energy Feedback Seen Through HCO$^+$/HOC$^+$ Emission in NGC 253 from ALCHEMI

Molecular abundances are sensitive to 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 ALMA large program ALCHEMI. The distribution of the HOC$^+$(1-0) integrated intensity shows its association with "superbubbles", cavities created either by supernovae or expanding HII regions. The observed HCO$^+$/HOC$^+$ abundance ratios are $\sim 10-150$, and the fractional abundance of HOC$^+$ relative to H$_2$ is $\sim 1.5\times 10^{-11} - 6\times 10^{-10}$, 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 $G_0\gtrsim 10^3$ if the maximum visual extinction is $\gtrsim 5$, or a cosmic-ray ionization rate of $ζ\gtrsim 10^{-14}$ 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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arXiv2021

Discovery in space of ethanolamine, the simplest phospholipid head group

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 first detection in space of ethanolamine, NH$_2$CH$_2$CH$_2$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$\pm$0.07)$\times$10$^{13}$ cm$^{-2}$, implying a molecular abundance with respect to H$_2$ of (0.9-1.4)$\times$10$^{-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, it could have contributed to the assembling and early evolution of primitive membranes.

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arXiv2021

Thiols in the ISM: first detection of HC(O)SH and confirmation of C$_2$H$_5$SH

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 compounds toward the Galactic Centre 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 $\sim\,$1$\,\times\,$10$^{-10}$. Additionally, we provide a solid confirmation of the gauche isomer of ethyl mercaptan (g-C$_2$H$_5$SH) with an abundance of $\sim\,$3$\,\times\,$10$^{-10}$, and we also detect methyl mercaptan (CH$_3$SH) with an abundance of $\sim\,$5$\,\times\,$10$^{-9}$. Abundance ratios were calculated for the three SH-bearing species and their OH-analogues, revealing similar trends between alcohols and thiols with increasing complexity. Possible chemical routes for the interstellar synthesis of t-HC(O)SH, CH$_3$SH and C$_2$H$_5$SH are discussed, as well as the relevance of these compounds in the synthesis of prebiotic proteins in the primitive Earth.

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