Search

[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 they 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 = 3.6–0.8 mm) at high angular (1.″6 ∼ 27 pc) and spectral (∼8–9 km s−1) resolution with ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory), 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 → (J − 1)0 − E line series (at 84, 132, 229, and 278 GHz) and the J0 → (J − 1)1 − A series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, which was already reported, but now being detected in more than one location. For the J−1 → (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 → (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 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., PH is a member of and received financial support for this research from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. PH is grateful to Arnaud Belloche, Dario Colombo, Yu Gao, and Sudeep Neupane, for their constructive advice and fruitful discussions along a variety of aspects covered in this work. VMR is funded by the Comunidad de Madrid through the Atracción de Talento Investigador (Doctores con experiencia) Grant (COOL: Cosmic Origins Of Life; 2019–T1/TIC–15379), and from the Agencia Estatal de Investigación (AEI) through the Ramón y Cajal programme (grant RYC2020-029387-I). L.C. has received partial support from the Spanish State Research Agency (AEI; project number PID2019-105552RB-C41). KS thanks MOST grant 109-2112-M-001-020. NH acknowledges support from JSPS KAKENHI Grant Number JP21K03634. This work makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.00161.L and ADS/JAO.ALMA#2018.1.00162.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

Molecular astronomy is a field that is blooming in the era of large observatories such as the Atacama Large Millimeter/Submillimeter Array (ALMA). With modern, sensitive, and high spectral resolution radio telescopes like ALMA and the Square Kilometer Array, the size of the data cubes is rapidly escalating, generating a need for powerful automatic analysis tools. This work introduces MolPred, a pilot study to perform predictions of molecular parameters such as excitation temperature (T) and column density (log(N)) from input spectra by the use of neural networks. We used as test cases the spectra of CO, HCO, SiO and CHCN between 80 and 400 GHz. Training spectra were generated with MADCUBA, a state-of-the-art spectral analysis tool. Our algorithm was designed to allow the generation of predictions for multiple molecules in parallel. Using neural networks, we can predict the column density and excitation temperature of these molecules with a mean absolute error of 8.5% for CO, 4.1% for HCO, 1.5% for SiO and 1.6% for CHCN. The prediction accuracy depends on the noise level, line saturation, and number of transitions. We performed predictions upon real ALMA data. The values predicted by our neural network for this real data differ by 13% from the MADCUBA values on average. Current limitations of our tool include not considering linewidth, source size, multiple velocity components, and line blending., A.B. wishes to thank Dr. Diego Mardones for his contribution to the early stages of this work. Also, to acknowledge support from the Federico Santa María Technical University General Directorate for Research and Postgraduate Studies (DGIP). JH and SV are funded by the European Research Council (ERC) Advanced Grant MOPPEX 833460. V.M.R. acknowledges support from the Comunidad de Madrid through the Atracción de Talento Investigador Modalidad 1 (Doctores con experiencia) Grant (COOL: Cosmic Origins Of Life; 2019-T1/TIC-15379; PI: V.M. Rivilla).

Martín, S., et al., [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.

Full list of authors: Falstad, N.; Aalto, S.; König, S.; Onishi, K.; Muller, S.; Gorski, M.; Sato, M.; Stanley, F.; Combes, F.; González-Alfonso, E.; Mangum, J. G.; Evans, A. S.; Barcos-Muñoz, L.; Privon, G. C.; Linden, S. T.; Díaz-Santos, T.; Martín, S.; Sakamoto, K.; Harada, N.; Fuller, G. A.; Gallagher, J. S.; van der Werf, P. P.; Viti, S.; Greve, T. R.; García-Burillo, S.; Henkel, C.; Imanishi, M.; Izumi, T.; Nishimura, Y.; Ricci, C.; Mühle, S., Context. Some luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) host extremely compact (r < 100 pc) and dusty nuclei. The high extinction associated with large column densities of gas and dust toward these objects render them hard to detect at many wavelengths. The intense infrared radiation arising from warm dust in these sources can provide a significant fraction of the bolometric luminosity of the galaxy and is prone to excite vibrational levels of molecules such as HCN. This results in emission from the rotational transitions of vibrationally excited HCN (HCN-vib); the brightest emission is found in compact obscured nuclei (CONs; ςHCN-vib > 1 L⊙ pc-2 in the J = 3-2 transition). However, there have been no systematic searches for CONs, and it is unknown how common they are. Aims. We aim to establish how common CONs are in the local Universe (z < 0.08), and whether their prevalence depends on the luminosity or other properties of the host galaxy. Methods. We conducted an Atacama Large Millimeter/submillimeter Array survey of the rotational J = 3-2 transition of HCN-vib in a volume-limited sample of 46 far-infrared luminous galaxies. Results. Compact obscured nuclei are identified in 38-13+18% of the ULIRGs, 21-6+12% of the LIRGs, and 0-0+9% of the lower luminosity galaxies. We find no dependence on the inclination of the host galaxy, but strong evidence of lower IRAS 25 μm to 60 μm flux density ratios (f25/f60) in CONs (with the exception of one galaxy, NGC 4418) compared to the rest of the sample. Furthermore, we find that CONs have stronger silicate features (s9.7 μm), but similar polycyclic aromatic hydrocarbon equivalent widths (EQW6.2 μm) compared to other galaxies. Along with signatures of molecular inflows seen in the far-infrared in most CONs, submillimeter observations also reveal compact, often collimated, outflows. Conclusions. In the local Universe, CONs are primarily found in (U)LIRGs, in which they are remarkably common. As such systems are often highly disturbed, inclinations are difficult to estimate, and high-resolution continuum observations of the individual nuclei are required to determine if the CON phenomenon is related to the inclinations of the nuclear disks. Further studies of the in- A nd outflow properties of CONs should also be conducted to investigate how these are connected to each other and to the CON phenomenon. The lower f25/f60 ratios in CONs as well as the results for the mid-infrared diagnostics investigated (EQW6.2 μm and s9.7 μm) are consistent with the notion that large dust columns gradually shift the radiation from the hot nucleus to longer wavelengths, making the mid- A nd far-infrared "photospheres"significantly cooler than the interior regions. Finally, to assess the importance of CONs in the context of galaxy evolution, it is necessary to extend this study to higher redshifts where (U)LIRGs are more common. © 2021 ESO., This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.01344.S., ADS/JAO.ALMA#2015.1.00404.S, ADS/ JAO.ALMA#2017.1.00082.S, ADS/JAO.ALMA#2017.1.00057.S, ADS/JAO. ALMA#2013.1.00032.S, ADS/JAO.ALMA#2017.1.00598.S, ADS/JAO.ALMA#2015.1.00412.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Repub lic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We acknowledge support from the Nordic ALMA Regional Centre (ARC) node based at Onsala Space Observatory. The Nordic ARC node is funded through Swedish Research Council grant No 2017-00648. S.A. gratefully acknowledges support from an ERC AdvancedGrant 789410 and from the Swedish Research Council. K.S. S.G-B. acknowledges support from the research projects PGC2018-094671-B-I00 (MCIU/AEI/FEDER, UE) and PID2019-106027GA C44 from the Spanish Ministerio de Ciencia e Innovación T.D-S. acknowledges support from the CASSACA and CONICYT fund CAS-CONICYT Call 2018. G.A.F acknowledges financial support from the State Agency for Research of the Spanish MCIU through the AYA2017-84390-C2-1-R grant (co-funded by FEDER) and through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucia (SEV-2017-0709). T.R.G. acknowledges the Cosmic Dawn Center of Excellence funded by the Danish National Research Foundation under grant No. 140. E.G-A. is a Research Associate at the Harvard Smithsonian Center for Astrophysics, and thanks the Spanish Ministerio de Economía y Competitividad for support under projects ESP2017-86582-C4-1-R and PID2019-105552RB-C41. This research has made use of NASA’s Astrophysics Data System. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Labora tory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

High-resolution (0."03 - 0."09) (9 - 26 pc)) ALMA (100 - 350 GHz (3 - 0.8 mm)) and VLA 45 GHz measurements have been used to image continuum and spectral line emission from the inner region of the nearby infrared luminous galaxy IC 860. We detect compact (r=8-10 pc), luminous, mm continuum emission in the core of IC 860, with brightness temperatures $T_{\rm B}>$ 280 K. The 45 GHz continuum is equally compact, but fainter in flux. We suggest that the 3 to 0.8 mm continuum is opaque at mm-wavelengths, implying a very large H$_2$ column density $N$(H$_2$) of 1e26 cm-2, and that it emerges from hot dust with temperature $T_{\rm d} \sim 280$ K. Vibrationally excited lines of HCN $��_2$=1f J=4-3 and 3-2 (HCN-VIB) are seen in emission and resolved on scales of 40-50 pc. The HCN-VIB emission reveals a north-south nuclear velocity gradient with projected rotation velocities of v=100 kms$^{-1}$ at r=10 pc. The brightest HCN-VIB emission is oriented perpendicular to the velocity gradient. Ground state lines of HCN 3-2, 4-3, HC$^{15}$N 4-3, HCO$^+$ 3-2, 4-3 and CS 7-6, show complex line absorption and emission features. HCN and HCO$^+$ have red-shifted, reversed P-Cygni profiles consistent with gas inflows of $\sim$50 kms$^{-1}$. The absorption features can be traced from the north-east into the nucleus. In contrast, CS 7-6 shows blue-shifted line wings extending to -180 kms$^{-1}$. We suggest that a dense and slow outflow is hidden behind a foreground layer of inflowing gas. It appears that the centre of IC 860 is in a phase of rapid evolution where an inflow is building up the nuclear column density of gas. A slow, dense outflow may be signalling the onset of feedback. The inner, r=10 pc, IR luminosity can be powered by an accreting black hole and/or a compact starburst with a top-heavy initial mass function., 13 pages, 9 figures, Accepted for publication in Astronomy and Astrophysics; Language edited

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, 31 pages, 29 figures, 6 tables, accepted for publication in A&A; for the draft showing figures with full resolution, see https://gongyan2444.github.io/pdf/Henkel-NGC4945.pdf

When comet nuclei approach the Sun, the increasing energy flux through the surface layers leads to sublimation of the underlying ices and subsequent outgassing that promotes the observed emission of gas and dust. While the release of gas can be straightforwardly understood by solving the heat-transport equation and taking into account the finite permeability of the ice-free dust layer close to the surface of the comet nucleus, the ejection of dust additionally requires that the forces binding the dust particles to the comet nucleus must be overcome by the forces caused by the sublimation process. This relates to the question of how large the tensile strength of the overlying dust layer is. Homogeneous layers of micrometer-sized dust particles reach tensile strengths of typically 10 3 to 10 4 Pa . This exceeds by far the maximum sublimation pressure of water ice in comets. It is therefore unclear how cometary dust activity is driven. To solve this paradox, we used the model by Skorov and Blum (Skorov, Y.V., Blum, J. 2012. Icarus 221, 361–11), who assumed that cometesimals formed by gravitational instability of a cloud of dust and ice aggregates and calculated for the corresponding structure of comet nuclei tensile strength of the dust-aggregate layers on the order of 1 Pa. Here we present evidence that the emitted cometary dust particles are indeed aggregates with the right properties to fit the model by Skorov and Blum. Then we experimentally measure the tensile strengths of layers of laboratory dust aggregates and confirm the values derived by the model. To explain the comet activity driven by the evaporation of water ice, we derive a minimum size for the dust aggregates of ∼1 mm, in agreement with meteoroid observations and dust-agglomeration models in the solar nebula. Finally we conclude that cometesimals must have formed by gravitational instability, because all alternative formation models lead to higher tensile strengths of the surface layers.

We derive molecular-gas-phase $^{12}$C/$^{13}$C isotope ratios for the central few 100 pc of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of the $\lambda$ $\sim$ 3 mm $^{12}$CN and $^{13}$CN $N$ = 1--0 lines in the ALMA Band 3. The $^{12}$C/$^{13}$C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945. The highest $^{12}$C/$^{13}$C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. In case of NGC 1068, the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar. Low $^{12}$C/$^{13}$C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material., Comment: Accepted for publication in A&A

We present the results of large-area CO J=3-2 emission mapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627, completed at the James Clerk Maxwell Telescope as part of the Nearby Galaxies Legacy Survey. These galaxies all have moderate to strong CO J=3-2 detections over large areas of the fields observed by the survey, showing resolved structure and dynamics in their warm/dense molecular gas disks. All three galaxies were part of the Spitzer Infrared Nearby Galaxies Survey sample, and as such have excellent published multi-wavelength ancillary data. These data sets allow us to examine the star formation properties, gas content, and dynamics of these galaxies on sub-kiloparsec scales. We find that the global gas depletion times for dense/warm molecular gas in these galaxies is consistent with other results for nearby spiral galaxies, indicating this may be independent of galaxy properties such as structures, gas compositions, and environments. Similar to the results from the THINGS HI survey, we do not see a correlation of the star formation efficiency with the gas surface density consistent with the Schmidt-Kennicutt law. Finally, we find that the star formation efficiency of the dense molecular gas traced by CO J=3-2 is potentially flat or slightly declining as a function of molecular gas density, the CO J=3-2/J=1-0 ratio (in contrast to the correlation found in a previous study into the starburst galaxy M83), and the fraction of total gas in molecular form.

(abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. We find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium., Comment: ApJ accepted. 56 pages, 14 figures (low resolution), 8 tables. Version with high resolution figures at http://www.astro.virginia.edu/~aak8t/data/n1569/ms.pdf

The radical CN, through the CN/HCN ratio, has proven to be an exceptionally good tool to study the photodissociation regions (PDRs) dominated by FUV radiation. Our region of interest, the circumnuclear disk (CND) surrounding Sgr A*, is exposed to the strong UV radiation field originating from the central stellar cluster. It contains potentially star forming cores recently elucidated by interferometric HCN observations with OVRO by Christopher et al. (2005). We present the results of preliminary observations of CN (N = 2-l) transitions with the IRAM 30-m telescope and report, for the first time, detection of CN emission in the GC region indicative of PDR, activity. This work motivates the further investigation of the role of PDRs in the clumpy medium of the Galactic center (GC) environment in global photodissociation and in promoting or inhibiting star formation in the CND.

We have used the Atacama Pathfinder Experiment (APEX) 12 m telescope at 218 GHz to observe molecular clouds simultaneously in the JKaKc = 303→202, 322→221, and 321→220 transitions of para-H2CO to determine kinetic temperatures of the dense gas in the central molecular zone of the Galaxy. Gas kinetic temperatures for individual molecular clouds range from 55 to 125 K or even higher. The molecular clouds at high temperatures may be heated by turbulent dissipation and/or cosmic-rays.

We explore the potential of imaging vibrationally excited molecular emission at high angular resolution to better understand the morphology and physical structure of the dense gas in Arp~220 and to gain insight into the nature of the nuclear powering sources. Vibrationally excited emission of HCN is detected in both nuclei with a very high ratio relative to the total $L_{FIR}$, higher than in any other observed galaxy and well above what is observed in Galactic hot cores. HCN $v_2=1f$ is observed to be marginally resolved in $\sim60\times50$~pc regions inside the dusty $\sim100$~pc sized nuclear cores. Its emission is centered on our derived individual nuclear velocities based on HCO$^+$ emission ($V_{WN}=5342\pm4$ and $V_{EN}=5454\pm8$~\kms, for the western and eastern nucleus, respectively). With virial masses within $r\sim25-30$~pc based on the HCN~$v_2=1f$ line widths, we estimate gas surface densities (gas fraction $f_g=0.1$) of $3\pm0.3\times10^4~M_\odot~\rm pc^{-2}$ (WN) and $1.1\pm0.1\times10^4~M_\odot~\rm pc^{-2}$ (EN). The $4-3/3-2$ flux density ratio could be consistent with optically thick emission, which would further constrain the size of the emitting region to $>15$~pc (EN) and $>22$~pc (WN). The absorption systems that may hide up to $70\%$ of the HCN and HCO$^+$ emission are found at velocities of $-50$~\kms~(EN) and $6$, $-140$, and $-575$~\kms (WN) relative to velocities of the nuclei. Blueshifted absorptions are the evidence of outflowing motions from both nuclei. The bright vibrational emission implies the existence of a hot dust region radiatively pumping these transitions. We find evidence of a strong temperature gradient that would be responsible for both the HCN $v_2$ pumping and the absorbed profiles from the vibrational ground state as a result of both continuum and self-absorption by cooler foreground gas., 14 pages accepted for publication in A&A

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an HI-selected sample of 155 galaxies spanning all morphological types with distances less than 25 Mpc. We describe the scientific goals of the survey, the sample selection, and the observing strategy. We also present an atlas and analysis of the CO J=3-2 maps for the 47 galaxies in the NGLS which are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide range of molecular gas mass fractions in the galaxies in this sample and explore the correlation of the far-infrared luminosity, which traces star formation, with the CO luminosity, which traces the molecular gas mass. By comparing the NGLS data with merging galaxies at low and high redshift which have also been observed in the CO J=3-2 line, we show that the correlation of far-infrared and CO luminosity shows a significant trend with luminosity. This trend is consistent with a molecular gas depletion time which is more than an order of magnitude faster in the merger galaxies than in nearby normal galaxies. We also find a strong correlation of the L(FIR)/L(CO3-2) ratio with the atomic to molecular gas mass ratio. This correlation suggests that some of the far-infrared emission originates from dust associated with atomic gas and that its contribution is particularly important in galaxies where most of the gas is in the atomic phase., Comment: 37 pages, 107 figures; accepted for publication in MNRAS. Data and additional information are available at http://www.physics.mcmaster.ca/~wilson/www_xfer/NGLS/

Using the IRAM 30-m telescope and the 15-m JCMT, we explore the value of para-formaldehyde (p-H_2CO) as a tracer of density and temperature of the molecular gas in external galaxies. The target of our observations are the lobes of the molecular ring around the center of the nearby prototypical starburst galaxy M 82. It is shown that p-H_2CO provides one of the rare direct molecular thermometers. Reproducing the measured line intensities with a large velocity gradient (LVG) model, we find densities of n_H2 ~ 7 10^3 cm^-3 and kinetic temperatures of T_kin ~ 200 K. The derived kinetic temperature is significantly higher than the dust temperature or the temperature deduced from ammonia (NH_3) lines, but our results agree well with the properties of the high-excitation component seen in CO. We also present the serendipitous discovery of the 4_2 - 3_1 line of methanol (CH_3OH) in the northeastern lobe, which shows -- unlike CO and H_2CO -- significantly different line intensities in the two lobes., Comment: 32 pages, 6 figures, accepted for publication by the Astrophysical Journal, for a pdf version with color figures, see http://www.astro.utoronto.ca/~muehle/papers/h2co1.pdf

The low masses of irregular galaxies change the behavior of their interstellar medium (ISM) compared to that of normal spirals, so the role of magnetic fields in the ISM in irregulars may be very different than in spirals. We present high-resolution and high-sensitivity observations of the magnetic fields of two irregular galaxies: NGC 4214 and NGC 1569.

The question whether or not the initial mass function (IMF) is universal, i.e. the same in all kinds of environments, is subject to intense debate. A number of recent observations have been interpreted as evidence for a nonstandard IMF. Hydrodynamical simulations indicate that the kinetic temperature of the collapsing molecular gas is crucial for the shape of the resulting IMF. Unfortunately, the kinetic temperature of the molecular gas in external galaxies is often not well constrained. We demonstrate the diagnostic power of a selected set of para-formaldehyde lines as tracers of the kinetic temperature as well as the gas density in external galaxies using our non-LTE radiative transfer model. With this new observational tool, we have engaged in characterizing the properties of the dense molecular gas phase in a number of nearby starburst galaxies and near AGN. Our first results suggest the existence of a dense molecular gas phase in these active environments that is significantly warmer than the dust and much warmer than dense molecular gas found in the disk of our own Galaxy.

[Context] Observations of chemical species can provide insights into the physical conditions of the emitting gas however it is important to understand how their abundances and excitation vary within different heating environments. C2H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments. [Aims] As part of the ALCHEMI ALMA large program, we map the emission of C2H in the central molecular zone of the nearby starburst galaxy NGC 253 at 1.6″ (28 pc) resolution and characterize it to understand its chemical origins. [Methods] We used spectral modeling of the N = 1-0 through N = 4-3 rotational transitions of C2H to derive the C2H column densities towards the dense clouds in NGC 253. We then use chemical modeling, including photodissociation region (PDR), dense cloud, and shock models to investigate the chemical processes and physical conditions that are producing the molecular emission. [Results] We find high C2H column densities of ∼1015 cm-2 detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced if it is assumed that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C2H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.