Context. The interstellar medium is the locus of physical processes affecting the evolution of galaxies which drive or are the result of star formation activity, supermassive black hole growth, and feedback. The resulting physical conditions determine the observable chemical abundances that can be explored through molecular emission observations at millimeter and submillimeter wavelengths. Aims. Our goal is to unveiling the molecular richness of the central region of the prototypical nearby starburst galaxy NGC 253 at an unprecedented combination of sensitivity, spatial resolution, and frequency coverage. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA), covering a nearly contiguous 289 GHz frequency range between 84.2 and 373.2 GHz, to image the continuum and spectral line emission at 1.6″(∼28 pc) resolution down to a sensitivity of 30 − 50 mK. This article describes the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) large program. We focus on the analysis of the spectra extracted from the 15″ (∼255 pc) resolution ALMA Compact Array data. Results. We modeled the molecular emission assuming local thermodynamic equilibrium with 78 species being detected. Additionally, multiple hydrogen and helium recombination lines are identified. Spectral lines contribute 5 to 36% of the total emission in frequency bins of 50 GHz. We report the first extragalactic detections of C2H5OH, HOCN, HC3HO, and several rare isotopologues. Isotopic ratios of carbon, oxygen, sulfur, nitrogen, and silicon were measured with multiple species. Concluison. Infrared pumped vibrationaly excited HCN, HNC, and HC3N emission, originating in massive star formation locations, is clearly detected at low resolution, while we do not detect it for HCO+. We suggest high temperature conditions in these regions driving a seemingly “carbon-rich” chemistry which may also explain the observed high abundance of organic species close to those in Galactic hot cores. The Lvib/LIR ratio was used as a proxy to estimate a 3% contribution from the proto super star cluster to the global infrared emission. Measured isotopic ratios with high dipole moment species agree with those within the central kiloparsec of the Galaxy, while those derived from 13C/18O are a factor of five larger, confirming the existence of multiple interstellar medium components within NGC 253 with different degrees of nucleosynthesis enrichment. The ALCHEMI data set provides a unique template for studies of star-forming galaxies in the early Universe.
We present observations of the C-band 1 10 − 1 11 (4.8 GHz) and Ku-band 2 11 − 2 12 (14.5 GHz) K-doublet lines of H 2 CO and the C-band 1 10 − 1 11 (4.6 GHz) line of H 2 13 CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT). Our sample with 112 sources includes strong H 2 CO sources from the TMRT molecular line survey at C-band and other known H 2 CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C-and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H 2 12 CO/H 2 13 CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue's rotational mm-wave lines connecting our measured K-doublets, and to obtain 12 C/ 13 C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure (2013). Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12 C/ 13 C = (5.08±1.10)D GC + (11.86±6.60), with a correlation coefficient of 0.58. D GC refers to Galactocentric distances. Our 12 C/ 13 C ratios agree very well with the ones deduced from CN and C 18 O but are lower than those previously reported on the basis of H 2 CO, tending to suggest that the bulk of the H 2 CO in our sources was formed on dust grain mantles and not in the gas phase.
We present observations of 12C32S, 12C34S, 13C32S, and 12C33S J = 2−1 lines toward a large sample of massive star-forming regions by using the Arizona Radio Observatory 12 m telescope and the IRAM 30 m. Taking new measurements of the carbon 12C/13C ratio, the 32S/34S isotope ratio was determined from the integrated 13C32S/12C34S line intensity ratios for our sample. Our analysis shows a 32S/34S gradient from the inner Galaxy out to a galactocentric distance of 12 kpc. An unweighted least-squares fit to our data yields 32S/34S = (1.56 ± 0.17)D GC + (6.75 ± 1.22) with a correlation coefficient of 0.77. Errors represent 1σ standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C34S opacities >0.25, (c) combining our data and old data from previous study, and (d) using different sets of carbon isotope ratios leads to the conclusion that the observed 32S/34S gradient is not an artifact but persists irrespective of the choice of sample and carbon isotope data. A gradient with rising 32S/34S values as a function of galactocentric radius implies that the solar system ratio should be larger than that of the local interstellar medium. With the new carbon isotope ratios, we indeed obtain a local 32S/34S isotope ratio about 10% below the solar system one, as expected in the case of decreasing 32S/34S ratios with time and increased amounts of stellar processing. However, taking older carbon isotope ratios based on a lesser amount of data, such a decrease is not seen. No systematic variation of 34S/33S ratios along galactocentric distance was found. The average value is 5.9 ± 1.5, the error denoting the standard deviation of an individual measurement.
Context. Molecular maser lines are signposts of high-mass star formation, probing the excitation and kinematics of very compact regions in the close environment of young stellar objects and providing useful targets for trigonometric parallax measurements. Aims. Only a few NH3 (9,6) masers are known so far, and their origin is still poorly understood. Here we aim to find new NH3 (9,6) masers to provide a better observational basis for studying their role in high-mass star-forming regions. Methods. We carried out NH3 (9,6) observations toward Cepheus A and G34.26+0.15 with the Effelsberg 100-meter telescope (beam size 49′′) and the Karl G. Jansky Very Large Array (JVLA; beam size about 1′′.2). Results. We discovered new NH3 (9,6) masers in Cep A and G34.26+0.25, which increases the number of known high-mass star-forming regions hosting NH3 (9,6) masers from five to seven. Long-term monitoring (20 months) at Effelsberg shows that the intensity of the (9,6) maser in G34.26+0.25 is decreasing, while the Cep A maser remains stable. Compared to the Effelsberg data and assuming linear variations between the epochs of observation, the JVLA data indicate no missing flux. This suggests that the NH3 (9,6) emission arises from single compact emission regions that are not resolved by the interferometric measurements. As JVLA imaging shows, the NH3 (9,6) emission in Cep A originates from a sub-arcsecond-sized region, slightly to the west (0′′.28 ± 0′′.10) of the peak position of the 1.36 cm continuum object, HW2. In G34.26+0.25, three NH3 (9,6) maser spots are observed: one is close to the head of the cometary ultracompact H II region C, and the other two are emitted from a compact region to the west of the hypercompact H II region A. Conclusions. The newly found (9,6) masers appear to be related to outflows. The higher angular resolution of JVLA and very long baseline interferometry observations are needed to provide more accurate positions and constraints for pumping scenarios.
Our aim is to measure the interstellar 14N/15N ratio across the Galaxy, to establish a standard data set on interstellar ammonia isotope ratios, and to provide new constraints on the Galactic chemical evolution. The (J, K) = (1, 1), (2, 2), and (3, 3) lines of 14NH3 and 15NH3 were observed with the Shanghai Tianma 65 m radio telescope (TMRT) and the Effelsberg 100 m telescope toward a large sample of 210 sources. One hundred fourty-one of these sources were detected by the TMRT in 14NH3. Eight of them were also detected in 15NH3. For 10 of the 36 sources with strong NH3 emission, the Effelsberg 100 m telescope successfully detected their 15NH3(1, 1) lines, including 3 sources (G081.7522, W51D, and Orion-KL) with detections by the TMRT telescope. Thus, a total of 15 sources are detected in both the 14NH3 and 15NH3 lines. Line and physical parameters for these 15 sources are derived, including optical depths, rotation and kinetic temperatures, and total column densities. 14N/15N isotope ratios were determined from the 14NH3/15NH3 abundance ratios. The isotope ratios obtained from both telescopes agree for a given source within the uncertainties, and no dependence on heliocentric distance and kinetic temperature is seen. 14N/15N ratios tend to increase with galactocentric distance, confirming a radial nitrogen isotope gradient. This is consistent with results from recent Galactic chemical model calculations, including the impact of superasymptotic giant branch stars and novae.
Context. Isotope abundance ratios provide a powerful tool for tracing stellar nucleosynthesis, evaluating the composition of stellar ejecta, and constraining the chemical evolution of the Milky Way. Aims. We aim to measure the 12C/13C, 32S/34S, 32S/33S, 32S/36S, 34S/33S, 34S/36S, and 33S/36S isotope ratios across the Milky Way. Methods. With the IRAM 30 meter telescope, we performed observations of the J = 2−1 transitions of CS, C33S, C34S, C36S, 13CS, 13C33S, and 13C34S as well as the J = 3−2 transitions of C33S, C34S, C36S, and 13CS toward a large sample of 110 high-mass star-forming regions. Results. We measured the 12C/13C, 32S/34S, 32S/33S, 32S/36S, 34S/33S, 34S/36S, and 33S/36S abundance ratios with rare isotopologs of CS, thus avoiding significant saturation effects. With accurate distances obtained from parallax data, we confirm previously identified 12C/13C and 32S/34S gradients as a function of galactocentric distance. In the central molecular zone, 12C/13C ratios are higher than suggested by a linear fit to the disk values as a function of galactocentric radius. While 32S/34S ratios near the Galactic center and in the inner disk are similar, this is not the case for 12C/13C, when comparing central values with those near galactocentric radii of 5 kpc. As was already known, there is no 34S/33S gradient but the average ratio of 4.35 ± 0.44 derived from the J = 2−1 transition lines of C34S and C33S is well below previously reported values. A comparison between solar and local interstellar 32S/34S and 34S/33S ratios suggests that the Solar System may have been formed from gas with a particularly high 34S abundance. For the first time, we report positive gradients of 32S/33S, 34S/36S, 33S/36S, and 32S/36S in our Galaxy. The predicted 12C/13C ratios from the latest Galactic chemical-evolution models are in good agreement with our results. While 32S/34S and 32S/36S ratios show larger differences at larger galactocentric distances, 32S/33S ratios show an offset across the entire inner 12 kpc of the Milky Way.
Progress in experiments to simulate the hydrodynamics of supernova remnants (SNRs) in the laboratory is reported. The experiment design involves shock heating of a dense material, which expands to become the ejecta that drive a blast wave through low-density foam. In the design, a variety of issues, such as radiative preheat of the unshocked matter, had to be addressed. A careful analysis of the scaling between hydrodynamic systems shows that the experiment is a good, scaled model of a local region in a young SNR. Measurements of the basic hydrodynamic behavior for two blast-wave velocities are nearly complete. Measurements of hydrodynamic instabilities at the contact surface between the ejecta and the low-density matter will commence in the near future.
We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO + J=1-0 line profiles. We used JCMT to conduct mapping observations in HCO + J=4-3 as well as single-point observations in HCO + J=3-2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5−2.7 km s −1 , with a median value of 1.0 km s −1 , and obtained mass accretion rates of 0.5-14 ×10 −3 M yr −1 . These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO + J=3-2 and HCO + J=1-0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO + J=4-3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacities. No obvious correlation for these clumps was found between infall velocity and the NH 3 /CCS ratio.
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