arXiv subjects

R. Mauersberger

Publications and source records attributed to R. Mauersberger.

At least 19 recordsLinked to original sources

arXiv2022

Methanol masers in NGC 253 with ALCHEMI

Context: Methanol masers of Class I (collisionally-pumped) and Class II (radiatively-pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy. Aims: Our main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253. Methods: Covering a frequency range between 84 and 373 GHz ($λ$ = 3.6 to 0.8 mm) at high angular (1.6"$\sim$27 pc) and spectral ($\sim$8--9 km s$^{-1}$) resolution with the ALMA large program ALCHEMI, we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models. Results: We detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series (at 84, 132, 229, and 278 GHz) and the $J_{0}\rightarrow(J-$ 1)$_{1}-A$ series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, already reported but now being detected in more than one location. For the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series, we observe a lack of Class I maser candidates in the central star-forming disk. Conclusions: The physical conditions for maser excitation in the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock/active regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.

astro-ph.GA↗

arXiv2021

Observations of multiple NH$_3$ transitions in W33

At a distance of 2.4kpc, W33 is an outstanding massive and luminous 10pc sized star forming complex containing quiescent infrared dark clouds as well as highly active infrared bright cloud cores heated by young massive stars. We report measurements of ammonia (NH$_3$) inversion lines in the frequency range 18--26GHz, obtained with the 40" resolution of the 100 m Effelsberg telescope. We have detected the ($J$, $K$)=(1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (2,1) and (3,2) transitions. There is a maser line in the (3,3) transition towards W33 Main. Brightness temperature and line shape indicate no significant variation during the last $\sim$36yr. We have determined kinetic temperatures, column densities and other physical properties of NH$_3$ and the molecular clouds in W33. For the total-NH$_3$ column density, we find for 40"(0.5pc) sized regions 6.0($\pm$2.1)$\times$10$^{14}$, 3.5($\pm$0.1)$\times$10$^{15}$, 3.4($\pm$0.2)$\times$10$^{15}$, 3.1($\pm$0.2)$\times$10$^{15}$, 2.8($\pm$0.2)$\times$10$^{15}$ and 2.0($\pm$0.2)$\times$10$^{15}$cm$^{-2}$ at the peak positions of W33 Main, W33 A, W33 B, W33 Main1, W33 A1 and W33 B1, respectively. W33 Main has a total-NH$_3$ fractional abundance of 1.3($\pm$0.1)$\times$10$^{-9}$ at the peak position. High values of 1.4($\pm$0.3)$\times$10$^{-8}$, 1.6($\pm$0.3)$\times$10$^{-8}$, 3.4($\pm$0.5)$\times$10$^{-8}$, 1.6($\pm$0.5)$\times$10$^{-8}$ and 4.0($\pm$1.2)$\times$10$^{-8}$ are obtained at the central positions of W33 A, W33 B, W33 Main1, W33 A1, and W33 B1. From this, we confirm the already previously proposed different evolutionary stages of the six W33 clumps and find that there is no hot core in the region approaching the extreme conditions encountered in W51-IRS2 or Sgr B2. The ortho-to-para-NH$_3$ abundance ratios suggest that ammonia should have been formed in the gas phase or on dust grain mantles at kinetic temperatures of $\gtrsim$20K.

astro-ph.GA↗

arXiv2020

Sulphur and carbon isotopes towards Galactic centre clouds

Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution. We present measurements of the carbon and sulphur abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50km/s Cloud and several l.o.s. clouds towards Sgr B2(N). Towards the +50km/s Cloud, we observed the J=2-1 rotational transitions of CS, C34S, 13CS, C33S, and 13C34S, and the J=3-2 transitions of CS and C34S with the IRAM-30m telescope, as well as the J=6-5 transitions of C34S and 13CS with the APEX 12m telescope, all in emission. The J=2-1 rotational transitions of CS, C34S, 13CS, and 13C34S were observed with ALMA in the envelope of Sgr B2(N), with those of CS and C34S also observed in the l.o.s. clouds towards Sgr B2(N), all in absorption. In the +50km/s Cloud we derive a 12C13C isotopic ratio of ~22.1, that leads, with the measured 13CS/C34S line intensity ratio, to a 32S/34S ratio of 16.3+3.0-2.4. We also derive the 32S/34S isotopic ratio more directly from the two isotopologues 13CS and 13C34S, which leads to an independent 32S/34S estimation of 16.3+2.1-1.7 and 17.9+-5.0 for the +50km/s Cloud and Sgr B2(N), respectively. We also obtain a 34S/33S ratio of ~4.3 in the +50 km/s Cloud. Previous studies observed a decreasing trend in the 32S/34S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130-30+60 pc. This is at variance with findings based on 12C/13C, 14N/15N and 18O/17O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient studies, and opens the work towards a comparison with Galactic and stellar evolution models.

astro-ph.GA↗

arXiv2020

The galaxy population within the virial radius of the Perseus cluster

We investigate the galaxy population in a field of the Perseus cluster that roughly covers the virial radius of the cluster. The galaxies were selected on Schmidt CCD images in B and H alpha in combination with SDSS images. We present a catalogue of 1294 galaxies. Morphological information was obtained for 90% of the galaxies from the `eyeball' inspection, partly supported by the surface brightness profile analysis. Redshifts were taken from SDSS, literature sources, and own spectroscopic observations and are available for 24% of the catalogues galaxies. The galaxy catalogue is used to derive cluster properties, such as radial profiles, indications of sub-structure, virial mass, and viral radius and to study the cluster galaxy population with regard to morphological types and peculiarities, star formation rates and active galactic nuclei. In addition to the statistical approach, we present brief individual descriptions of 18 cluster galaxies with conspicuous morphological peculiarities. (Abstract modified to match the arXiv format.)

astro-ph.GA↗

arXiv2019

36 GHz methanol lines from nearby galaxies: maser or quasi-thermal emission?

Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. While the strongest Galactic CH3OH lines, the so-called class II masers, show no indications for the presence of superluminous counterparts in external galaxies, the less luminous Galactic class I sources appear to be different. Here we report class I 36GHz CH3OH 4(-1) - 3(0) E line emission from the nearby galaxies Maffei2 and IC342, measured with the 100-m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36GHz methanol line of Maffei2 is the second most luminous among the sources detected with certainty outside the Local Group of galaxies. This is not matched by the moderate infrared luminosity of Maffei2. Higher resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M82, NGC4388, NGC5728 and Arp220 are also presented. The previously reported detection of 36GHz maser emission in Arp220 is not confirmed. Non-detections are reported from the related class I 44GHz methanol transition towards Maffei2 and IC342, indicating that this line is not stronger than its 36GHz counterpart. In contrast to the previously detected 36GHz CH3OH emission in NGC253 and NGC4945, our 36GHz profiles towards Maffei2 and IC342 are similar to those of previously detected non-masing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36GHz methanol lines in Maffei~2 and IC~342 are composed of a large number of faint and narrow maser features that remain spatially unresolved. In view of this, a search for a weak broad 36GHz line component would also be desirable in NGC253 and NGC4945.

astro-ph.GA↗

arXiv2018

Molecular line emission in NGC 4945, imaged with ALMA

NGC 4945 is one of the nearest (~3.8 Mpc; 1" ~ 19 pc) starburst galaxies. ALMA band 3 (3--4\,mm) observations of HCN, HCO+, CS, C3H2, SiO, HCO, and CH3C2H were carried out with ~2" resolution. The lines reveal a rotating nuclear disk of projected size 10" x 2" with position angle ~45 deg, inclination ~75 deg and an unresolved bright central core of size <2.5". The continuum source (mostly free-free radiation) is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0.39" +_ 0.14" northeast of the nuclear accretion disk defined by H2O maser emission. Outside the nuclear disk, both HCN and CS delineate molecular arms on opposite sides of the dynamical center. These are connected by a (deprojected) 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside <100 pc) with tilted rings indicates a coplanar outflow reaching a characteristic deprojectd velocity of ~50 km/s. All our molecular lines, with the notable exception of CH3C2H, show significant absorption near the systemic velocity (~571 km/s), within a range of ~500-660 km/s. Apparently, only molecular transitions with low critical H2-density do not show absorption. The velocity field of the nuclear disk, derived from CH3C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M = (2.1+_0.2) x 10^8 Mo inside a galactocentric radius of 2.45", with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to 1.5" southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk.

astro-ph.GA↗

arXiv2017

The foot points of the Giant Molecular Loops in the Galactic center region

Aims: To reveal the morphology, chemical composition, kinematics and to establish the main processes prevalent in the gas at the foot points of the giant molecular loops (GMLs) in the Galactic center region Methods: Using the 22-m Mopra telescope, we mapped the M$-3.8+0.9$ molecular cloud, placed at the foot points of a giant molecular loop, in 3-mm range molecular lines. To derive the molecular hydrogen column density, we also observed the $^{13}$CO $(2-1)$ line at 1 mm using the 12-m APEX telescope. From the 3 mm observations 12 molecular species were detected, namely HCO$^+$, HCN, H$^{13}$CN, HNC, SiO, CS, CH$_3$OH, N$_2$H$^+$, SO, HNCO, OCS, and HC$_3$N. Results: Maps revealing the morphology and kinematics of the M$-3.8+0.9$ molecular cloud in different molecules are presented. We identified six main molecular complexes. We derive fractional abundances in 11 selected positions of the different molecules assuming local thermodynamical equilibrium. Conclusions: Most of the fractional abundances derived for the M$-3.8+0.9$ molecular cloud are very similar over the whole cloud. However, the fractional abundances of some molecules show significant difference with respect to those measured in the central molecular zone (CMZ). The abundances of the shock tracer SiO are very similar between the GMLs and the CMZ. The methanol emission is the most abundant specie in the GMLs. This indicates that the gas is likely affected by moderate $\sim $ 30 km s$^{-1}$ or even high velocity (50 km s$^{-1}$) shocks, consistent with the line profile observed toward one of the studied position. The origin of the shocks is likely related to the flow of the gas throughout the GMLs towards the foot points.

astro-ph.GA↗

arXiv2017

The diffuse molecular component in the nuclear bulge of the Milky Way

Context: The bulk of the Molecular gas in the Central Molecular Zone (CMZ) of the Galactic center region shows warm kinetic temperatures, ranging from $>20$ K in the coldest and densest regions (n$\sim 10^{4-5}$ cm$^{-3}$) up to more than 100 K for densities of about n$\sim 10^3$ cm$^{-3}$. Recently, a more diffuse, hotter ($n \sim 100$ cm$^{-3}$, $T\sim 250$ K) gas component was discovered through absorption observations of H$_3^+$. This component may be widespread in the Galactic center, and low density gas detectable in absorption may be present even outside the CMZ along sightlines crossing the extended bulge of the Galaxy. Aims: We aim to observe and characterize diffuse and low density gas using observations of 3-mm molecular transitions seen in absorption. Methods: Using the Atacama Large (sub)Millimeter Array (ALMA) we observed the absorption against the quasar J1744-312, which is located towards the Galactic bulge region at $(l,b)=(-2{^\circ}.13,-1{^\circ}.0)$, but outside the main molecular complexes. Results: ALMA observations in absorption against the J1744-312 quasar reveal a rich and complex chemistry in low density molecular and presumably diffuse clouds. We detected three velocity components, at $\sim $ 0, $-153$, and $-192$ km s$^{-1}$. The component at $\sim 0$ km s$^{-1}$ could represent gas in the Galactic disk while the velocity components at $-153$, and $-192$ km s$^{-1}$ likely originate from the Galactic bulge. We detected 12 molecules in the survey, but only 7 in the Galactic bulge gas.

astro-ph.GA↗

arXiv2014

On carbon and oxygen isotope ratios in starburst galaxies: New data from NGC253 and Mrk231 and their implications

Using the IRAM 30-m telescope, CN and CO isotopologues have been measured toward the central regions of the nearby starburst galaxy NGC253 and the prototypical ultraluminous infrared galaxy Mrk231. In NGC253, the 12C/13C ratio is 40+-10. Assuming that the ratio also holds for the CO emitting gas, this yields 16O/18O = 145+-36 and 16O/17O = 1290+-365 and a 32S/34S ratio close to that measured for the local interstellar medium (20-25). No indication for vibrationally excited CN is found. Peak line intensity ratios between NGC253 and Mrk231 are ~100 for 12C16O and 12C18O J=1-0, while the ratio for 13C16O J=1-0 is ~250. This and similar 13CO and C18O line intensities in the J=1-0 and 2-1 transitions of Mrk231 suggest 12C/13C ~ 100 and 16O/18O ~ 100, in agreement with values obtained for the less evolved ultraluminous merger Arp220. Also accounting for other extragalactic data, 12C/13C ratios appear to vary over a full order of magnitude, from >100 in ultraluminous high redshift galaxies to ~100 in more local such galaxies to ~40 in weaker starbursts not undergoing a large scale merger to 25 in the Central Molecular Zone of the Milky Way. With 12C being predominantly synthesized in massive stars, while 13C is mostly ejected by longer lived lower mass stars at later times, this is qualitatively consistent with our results of decreasing carbon isotope ratios with time and rising metallicity. It is emphasized, however, that both infall of poorly processed material, initiating a nuclear starburst, as well as the ejecta from newly formed massive stars (in particular in case of a top-heavy stellar initial mass function) can raise the carbon isotope ratio for a limited amount of time.

astro-ph.GA↗

arXiv2013

Ammonia in the hot core W51-IRS2: 12 new maser lines and a maser component with a velocity drift

With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines have been detected toward the prominent massive star forming region W51-IRS2. Eleven of these inversion lines, the (J,K) = (6,2), (5,3), (7,4), (8,5), (7,6), (7,7), (9,7), (10,7), (9,9), (10,9), and (12,12) transitions, are classified as masers for the first time in outer space. All detected masers are related to highly excited inversion doublets. The (5,4) maser originates from an inversion doublet 340 K above the ground state, while the (12,12) transition, at 1450 K, is the most highly excited NH3 maser line so far known. Strong variability is seen not only in ortho- but also in para-NH3 transitions. Bright narrow emission features are observed, for the first time, in (mostly) ortho-ammonia transitions, at V ~ 45 km/s, well separated from the quasi-thermal emission near 60 km/s. These features were absent 25 years ago and show a velocity drift of about +0.2 km/s/yr. The component is likely related to the SiO maser source in W51-IRS2 and a possible scenario explaining the velocity drift is outlined. The 57 km/s component of the (9,6) maser line is found to be strongly linearly polarized. Maser emission in the (J,K) to (J+1,K) inversion doublets is strictly forbidden by selection rules for electric dipole transitions in the ground vibrational state. However, such pairs (and even triplets with (J+2,K)) are common toward W51-IRS2. Similarities in line widths and velocities indicate that such groups of maser lines arise from the same regions, which can be explained by pumping through vibrational excitation. The large number of NH3 maser lines in W51-IRS2 is most likely related to the exceptionally high kinetic temperature and NH3 column density of this young massive star forming region.

astro-ph.GA↗

arXiv2012

The thermal state of molecular clouds in the Galactic Center: evidence for non-photon-driven heating

We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the J_KaKc=3_03-2_02, 3_22-2_21, and 3_21-2_20 transitions of para-H_2CO at 218 GHz simultaneously to determine kinetic temperatures of the dense gas in the central molecular zone (CMZ) of our Galaxy. The map extends over approximately 40 arcmin x 8 arcmin (~100x20 pc^2) along the Galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H_2CO (3_03-2_02) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H_2CO is 0.5-1.2 10^{-9}, which is consistent with results from lower frequency H_2CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50 K to values in excess of 100 K. While a systematic trend toward (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature (T_kin > 100 K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65+/-10 K. The high temperatures of molecular clouds on large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon-driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs).

astro-ph.GA↗

arXiv2012

Sense and sensitivity: How ALMA receivers work

In previous articles, we described how electromagnetic waves emitted from objects in the sky are collected by the ALMA antennas (Anatomy of ALMA), and how they are combined in order to produce images. Before these images can be processed, they are picked up by the antennas and concentrated by the large main mirror and a smaller secondary mirror in the so called focal point of each antenna. In order to process the data they must be first converted to electromagnetic waves of a lower frequency and amplified. This is the role of the ALMA receivers. In principle they work like a normal AM receiver, but at much higher frequencies. Here we describe how they work and what makes them special.

astro-ph.IM↗

arXiv2012

How ALMA is calibrated: I Antenna-based pointing, focus and amplitude calibration

Here we will discuss which calibrations are needed, how to perform them and how the calibration affect ALMA observations. This first part concentrates on aspects related to a single antenna, namely antenna pointing (including the antenna metrology), focusing, and the amplitude calibration, we also mention the bandpass calibration for single measurements.

astro-ph.IM↗

arXiv2012

Kinetic temperatures toward X1/X2 orbit interceptions regions and Giant Molecular Loops in the Galactic center region

Context: It is well known that the kinetic temperatures, Tkin, of the molecular clouds in the Galactic center region are higher than in typical disk clouds. However, the Tkin of the molecular complexes found at higher latitudes towards the giant molecular loops in the central region of the Galaxy is so far unknown. The gas of these high latitude molecular clouds (hereafter referred to as halo clouds) is located in a region where the gas in the disk may interact with the gas in the halo in the Galactic center region. Aims: To derive Tkin in the molecular clouds at high latitude and understand the physical process responsible for the heating of the molecular gas both in the Central Molecular Zone (the concentration of molecular gas in the inner 500 pc) and in the giant molecular loops. Methods: We measured the metastable inversion transitions of NH3 from (1,1) to (6,6) toward six positions selected throughout the Galactic central disk and halo. We used rotational diagrams and large velocity gradient modeling to estimate the kinetic temperatures toward all the sources. We also observed other molecules like SiO, HNCO, CS, C34S, C18O, and 13CO, to derive the densities and to trace different physical processes (shocks, photodissociation, dense gas) expected to dominate the heating of the molecular gas. Results: We derive for the first time Tkin of the high latitude clouds interacting with the disk in the Galactic center region. We find high rotational temperatures in all the observed positions. We derive two kinetic temperature components (150 K and 40 K) for the positions in the Central Molecular Zone, and only the warm kinetic temperature component for the clouds toward the giant molecular loops. The fractional abundances derived from the different molecules suggest that shocks provide the main heating mechanism throughout the Galactic center, also at high latitudes

astro-ph.GA↗

arXiv2011

Dense Gas in Nearby Galaxies: XVII. The Distribution of Ammonia in NGC253, Maffei2 and IC342

The central few 100 pc of galaxies often contain large amounts of molecular gas. The chemical and physical properties of these extragalactic star formation regions differ from those in galactic disks, but are poorly constrained. This study aims to develop a better knowledge of the spatial distribution and kinetic temperature of the dense neutral gas associated with the nuclear regions of three prototypical spiral galaxies, NGC253, IC342, and Maffei2. VLA CnD and D configuration measurements have been made of three ammonia (NH3) inversion transitions. The (J,K)=(1,1) and (2,2) transitions of NH3 were imaged toward IC342 and Maffei2. The (3,3) transition was imaged toward NGC253. The entire flux obtained from single-antenna measurements is recovered for all three galaxies observed. Derived lower limits to the kinetic temperatures determined for the giant molecular clouds in the centers of these galaxies are between 25 and 50K. There is good agreement between the distributions of NH3 and other H2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3 is representative of the distribution of dense gas. The "Western Peak" in IC342 is seen in the (6,6) line but not in lower transitions, suggesting maser emission in the (6,6) transition.

astro-ph.GA↗

arXiv2010

A survey of the Galactic center region in HCO+, H13CO+ and SiO

Aims: A large scale survey of the Galactic center region in he 3 mm rotational transitions of SiO, HCO+ and H13CO+ (beamsize ~ 3. 6 arcmin) was conducted to provide an estimate of cloud conditions, heating mechanisms, chemistry and other properties. Methods: Using the NANTEN 4m telescope from Nagoya University, a region between -5.75<l<5.6 (degree) and -0.68<b<1.3 (degree) was mapped in the J=1-0 lines of HCO+ and H13CO+ and in the J=2-1 line of SiO with a spacing of 3.75 arcmin (HCO+) and 1.875 arcmin (SiO and H13CO+). Results: Velocity channel maps, longitude-velocity maps and latitude-velocity maps are presented. We identify 51 molecular clouds; 33 of them belong to the Galactic center, and 18 to disk gas. We derive an average of the luminosity ratio of SiO(J=2-1)/CO(J=1-0) in clouds belonging to the Galactic center of 4.9x10^{-3} and for disk clouds of 3.4x 10^{-3}. The luminosity ratio of HCO^+(J=1-0)/CO(J=1\to0) in the Galactic Center is 3.5x 10^{-2}, and for disk clouds is 1.5x 10^{-2}. We can distinguish clearly between regions where the SiO or HCO+ are dominating.

astro-ph.GA↗

arXiv2010

Tracing gas accretion in the Galactic center using isotopic ratios

Ams: We study the 12C/13C isotopic ratio in the disk of the central molecular zone and in the halo to trace gas accretion toward the Galactic center region in the Milky Way. Methods: Using the IRAM 30m telescope, we observe the J=1-0 rotational transition of HCO+, HCN, HNC and their 13C isotopic substitutions in order to measure the 12C/13C isotopic ratio. We observe 9 positions selected throughout the Galactic center region, including clouds at high latitude; locations where the X1 and X2 orbits associated with the barred potential are expected to intersect; and typical Galactic center molecular clouds. Results: We find a systematically higher 12C/13C isotopic ratio (>40) toward the halo and the X1 orbits than for the Galactic center molecular clouds (20-25). Our results point out to molecular gas which has undergone a different degree of nuclear processing than that observed in the gas towards the inner Galactic center region. Conclusions: The high isotopic ratios are consistent with the accretion of the gas from the halo and from the outskirts of the Galactic disk.

astro-ph.GA↗

arXiv2010

A large 12C/13C isotopic ratio in M82 and NGC253

To derive carbon isotopic ratios from optically thin tracers in the central regions of the starburst galaxies M82 and NGC253. We present high sensitivity observations of CCH and two of its 13C isotopologues, C13CH and 13CCH, as well as the optically thin emission from C18O and 13C18O. We assume the column density ratio between isotopologues is representative of the 12C13C isotopic ratio. From CCH, lower limits to the 12C/13C isotopic ratio of 138 in M82, and 81 in NGC253, are derived. Lower limits to the 12C/13C ratios from CO isotopologues support these. 13C18O is tentatively detected in NGC253, which is the first reported detection in the extragalactic ISM. Based on these limits, we infer ratios of 16O/18O>350 and >300 in M82 and NGC253, respectively, and 32S/34S>16 in NGC253. and the H2 column density determination through the optically thin tracers 13CO and C18O. The derived CCH fractional abundances toward these galaxies of <~1.1\times10^-8 are in good agreement with those of molecular clouds in the Galactic disk. Our lower limits to the 12C/13C ratio from CCH are a factor of 2-3 larger than previous limits. The results are discussed in the context of molecular and nucleo-chemical evolution. The large 12C/13C isotopic ratio of the molecular ISM in these starburst galaxies suggest that the gas has been recently accreted toward their nuclear regions.

astro-ph.CO↗