S. Mühle

University of Bonn, Bonn, North Rhine-Westphalia, Germany

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Publications (38)75.94 Total impact

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    ABSTRACT: We present high resolution (0."4) IRAM PdBI and ALMA mm and submm observations of the (Ultra) Luminous Infrared Galaxies ((U)LIRGs) IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission from vibrationally excited (v2=1) J=3-2 and 4-3 HCN. The emission is emerging from buried, compact (r<17-70 pc) nuclei that have very high implied mid-infrared surface brightness >5e13 Lsun/kpc2. These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200 K) extreme starbursts. Vibrational, v2=1, lines of HCN are excited by intense 14 micron mid-infrared emission and are excellent probes of the dynamics, masses and physical conditions of (U)LIRG nuclei when H2 column densities exceed 1e24 cm-2. It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, the ground vibrational state (v=0), J=3-2 and 4-3 rotational lines of HCN and HCO+ fail to probe the highly enshrouded, compact nuclear regions due to strong self- and continuum absorption. The HCN and HCO+ line profiles are double-peaked because of the absorption and show evidence of non-circular motions - possibly in the form of in- or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early type spiral LIRGs and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.
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    ABSTRACT: We present 12CO J = 3→2 maps of NGC 2976 and NGC 3351 obtained with the James Clerk Maxwell Telescope (JCMT), both early targets of the JCMT Nearby Galaxy Legacy Survey (NGLS). We combine the present observations with 12CO J = 1→0 data and find that the computed 12CO J = 3→2 to 12CO J = 1→0 line ratio (R31) agrees with values measured in other NGLS field galaxies. We compute the MH2 value and find that it is robust against the value of R31 used. Using H I data from The H I Nearby Galaxy Survey, we find a tight correlation between the surface density of H2 and star formation rate density for NGC 3351 when 12CO J = 3→2 data are used. Finally, we compare the 12CO J = 3→2 intensity with the polycyclic aromatic hydrocarbon (PAH) 8 μm surface brightness and find a good correlation in the high surface brightness regions. We extend this study to include all 25 Spitzer Infrared Nearby Galaxies Survey galaxies within the NGLS sample and find a tight correlation at large spatial scales. We suggest that both PAH 8 μm and 12CO J = 3→2 are likely to originate in regions of active star formation.
    Monthly Notices of the Royal Astronomical Society 11/2013; 436(1):921-933. DOI:10.1093/mnras/stt1625 · 5.23 Impact Factor
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    ABSTRACT: 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).
    Astronomy and Astrophysics 11/2012; 550. DOI:10.1051/0004-6361/201220096 · 4.48 Impact Factor
  • Monthly Notices of the Royal Astronomical Society 08/2012; 424:3050-3080. DOI:10.1111/j.1365-2966.2012.21453.x · 5.23 Impact Factor
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    ABSTRACT: 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.
    Journal of Physics Conference Series 07/2012; 372(1). DOI:10.1088/1742-6596/372/1/012052
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    ABSTRACT: We report results from a large molecular line survey of Luminous Infrared Galaxies (L_{IR} >= 10^{11} L_sol) in the local Universe (z<=0.1), conducted during the last decade with the James Clerk Maxwell Telescope (JCMT) and the IRAM 30-m telescope. This work presents the CO and {13}CO line data for 36 galaxies, further augmented by multi-J total CO luminosities available for other IR-bright galaxies from the literature. This yields a sample of N=70 galaxies with the star-formation (SF) powered fraction of their IR luminosities spanning L_{IR} (10^{10}-2x10^{12}) L_sol and a wide range of morphologies. Simple comparisons of their available CO Spectral Line Energy Distributions (SLEDs) with local ones, as well as radiative transfer models discern a surprisingly wide range of average ISM conditions, with most of the surprises found in the high-excitation regime. These take the form of global CO SLEDs dominated by a very warm (T_{kin}>=100 K) and dense (n>=10^4 cm^{-3}) gas phase, involving galaxy-sized (~(few)x10^9 M_sol) gas mass reservoirs under conditions that would otherwise amount only ~1% of mass per typical SF molecular cloud in the Galaxy. Some of the highest excitation CO SLEDs are found in the so-called Ultra Luminous Infrared Galaxies and seem irreducible to ensembles of ordinary SF-powered regions. Highly supersonic turbulence and high cosmic ray (CR) energy densities rather than far-UV/optical photons or SNR-induced shocks from individual SF sites can globally warm the large amounts of dense gas found in these merger-driven starbursts and easily power their extraordinary CO line excitation.....
    Monthly Notices of the Royal Astronomical Society 09/2011; 426(4). DOI:10.1111/j.1365-2966.2012.21001.x · 5.23 Impact Factor
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    ABSTRACT: We present the detection of the ammonia (NH3) (J,K) = (1,1) to (4,4) and (6,6) inversion lines toward the prototypical Seyfert 2 galaxy NGC 1068, made with the Green Bank Telescope (GBT). This is the first detection of ammonia in a Seyfert galaxy. The ortho-to-para-NH3 abundance ratio suggests that the molecule was formed in a warm medium of at least 20 K. For the NH3 column density and fractional abundance, we find (1.09\pm0.23)\times10^14 cm^-2 and (2.9\pm0.6)\times10^-8, respectively, from the inner 1.2 kpc of NGC 1068. The kinetic temperature can be constrained to 80\pm20 K for the bulk of the molecular gas, while some fraction has an even higher temperature of 140\pm30 K.
    Astronomy and Astrophysics 04/2011; 529. DOI:10.1051/0004-6361/201116595 · 4.48 Impact Factor
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    ABSTRACT: The question whether or not the initial mass function is universal, i.e. the same in all kinds of environments, is of critical importance for the theory of star formation and still intensely debated. A top-heavy initial mass function may be the result of star formation out of dense molecular clouds with a temperature of ~ 100 K. Such a molecular gas phase is not commonly found in the Galactic plane, but may be present in active environments like cores of starburst galaxies or AGN. Unfortunately, the kinetic temperature of the molecular gas in external galaxies is often not well constrained. Having proven the diagnostic power of selected formaldehyde lines as tracers of the properties of the molecular gas in external galaxies, we have engaged in observing these diagnostic lines in a number of starburst galaxies or near AGN. This contribution presents the latest results of these studies.
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    ABSTRACT: An analysis of large-area CO J=3-2 maps from the James Clerk Maxwell Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the molecular component of the interstellar medium. The three lowest luminosity galaxies show a relatively flat velocity dispersion as a function of radius while the remaining nine galaxies show a central peak with a radial fall-off within 0.2-0.4 r(25). Correcting for the average contribution due to the internal velocitydispersions of a population of giant molecular clouds, the average cloud-cloud velocity dispersion across the galactic disks is 6.1 +/- 1.0 km/s (standard deviation 2.9 km/s), in reasonable agreement with previous measurements for the Galaxy andM33. The cloud-cloud velocity dispersion derived from the CO data is on average two times smaller than the HI velocity dispersion measured in the same galaxies. The low cloud-cloudvelocity dispersion implies that the molecular gas is the critical component determining the stability of the galactic disk against gravitational collapse, especially in those regions of the disk which are H2 dominated. The cloud-cloud velocity dispersion shows a significant positivecorrelation with both the far-infrared luminosity, which traces the star formation activity, and the K-band absolute magnitude, which traces the total stellar mass. For three galaxies in the Virgo cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical resolution of high redshift CO observations) increases the measured velocity dispersion by roughly a factor of two, comparable to the dispersion measured recently in a normal galaxy at z=1. This comparison suggests that the mass and star formation rate surface densities may be similar in galaxies from z=0-1 and that the high star formation rates seen at z=1 may be partly due to the presence of physically larger molecular gas disks. Comment: 16 pages, 23 figures. Accepted to MNRAS
    Monthly Notices of the Royal Astronomical Society 09/2010; DOI:10.1111/j.1365-2966.2010.17646.x · 5.23 Impact Factor
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    ABSTRACT: We present the results of large-area 12CO 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 12CO 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 multiwavelength 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 time 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 H I Nearby Galaxy 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 12CO J = 3-2 is potentially flat or slightly declining as a function of molecular gas density, the 12CO 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.
    The Astrophysical Journal 04/2010; 714(1):571. DOI:10.1088/0004-637X/714/1/571 · 6.28 Impact Factor
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    ABSTRACT: (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 Astrophysical Journal 02/2010; 712(1). DOI:10.1088/0004-637X/712/1/536 · 6.28 Impact Factor
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    ABSTRACT: We used 3.6, 8.0, 70, 160 micron Spitzer Space Telescope data, James Clerk Maxwell Telescope HARP-B CO J=(3-2) data, National Radio Astronomy Observatory 12 meter telescope CO J=(1-0) data, and Very Large Array HI data to investigate the relations among PAHs, cold (~20 K) dust, molecular gas, and atomic gas within NGC 2403, an SABcd galaxy at a distance of 3.13 Mpc. The dust surface density is mainly a function of the total (atomic and molecular) gas surface density and galactocentric radius. The gas-to-dust ratio monotonically increases with radius, varying from ~100 in the nucleus to ~400 at 5.5 kpc. The slope of the gas-to-dust ratio is close to that of the oxygen abundance, suggesting that metallicity strongly affects the gas-to-dust ratio within this galaxy. The exponential scale length of the radial profile for the CO J=(3-2) emission is statistically identical to the scale length for the stellar continuum-subtracted 8 micron (PAH 8 micron) emission. However, CO J=(3-2) and PAH 8 micron surface brightnesses appear uncorrelated when examining sub-kpc sized regions. Comment: Accepted to MNRAS
    Monthly Notices of the Royal Astronomical Society 11/2009; 402(3). DOI:10.1111/j.1365-2966.2009.16043.x · 5.23 Impact Factor
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    ABSTRACT: We present large-area maps of the CO J = 3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO J = 1-0, 24 μm, and Hα images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC 4321, and NGC 4569), we obtain molecular gas masses of 7 × 108 – 3 × 109 M ☉ and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO J = 3-2 line is a better tracer of the dense star-forming molecular gas than the CO J = 1-0 line, as it shows a better correlation with the star formation rate surface density both within and between galaxies. NGC 4254 appears to have a larger star formation efficiency (smaller gas depletion time), perhaps because it is on its first passage through the Virgo Cluster. NGC 4569 shows a large-scale gradient in the gas properties traced by the CO J = 3-2/J = 1-0 line ratio, which suggests that its interaction with the intracluster medium is affecting the dense star-forming portion of the interstellar medium directly. The fourth galaxy in our sample, NGC 4579, has weak CO J = 3-2 emission despite having bright 24 μm emission; however, much of the central luminosity in this galaxy may be due to the presence of a central active galactic nucleus.
    The Astrophysical Journal 03/2009; 693(2):1736. DOI:10.1088/0004-637X/693/2/1736 · 6.28 Impact Factor
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    ABSTRACT: There is growing evidence that the properties of the molecular gas in the nuclei of starburst galaxies and in AGN may be very different from those seen in Galactic star forming regions and that a high kinetic temperature in the molecular gas may lead to a non-standard initial mass function in the next generation of stars. Unfortunately, among the fundamental parameters derived from molecular line observations, the kinetic temperature of the molecular gas in external galaxies is often not well determined due to a lack of suitable tracer molecules. We discuss the diagnostic power of selected transition lines of formaldehyde (H_2CO), which can be used as a molecular thermometer as well as an excellent tracer of the molecular gas density. As a proof of concept, we present the results of our multi-transition line study of the H_2CO emission from the prototypical starburst galaxy M82. Using our large velocity gradient model, we tightly constrain the physical properties of the dense gas in the prominent molecular lobes, completely independent of the standard "cloud thermometer" ammonia (NH_3) or other molecular tracers. Our results agree well with the properties of the high-excitation molecular gas component found in the most comprehensive CO studies. Our observations also indicate that there may be an asymmetry between the two molecular lobes.
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    ABSTRACT: Methanol possesses a rich mm spectrum allowing the reliable determination of both density and kinetic temperature from radiative transfer models. It is formed on dust grains, evaporated by UV radiation and shocks, and destroyed by gas phase chemical reactions. As the nearest galactic nuclei that can be observed in great detail, the center of the Milky Way has often been regarded as a template for understanding physical processes governing the extragalactic nuclei in general. It is a dynamic environment where existing energetic processes affect the chemistry on the grain mantles, promoting the formation of complex organic molecules. However, little is known to date about their distribution within the central 30pc. We present the results of our surveys of methanol emission in the central 10'x12'( 20pcx25pc) of our Galaxy, observed at three different frequencies with the JCMT and the IRAM 30-m telescopes. We discuss the morphology of methanol emission and its correlation to other tracers, in particular to those processed on the grain mantles (e.g. NH3, SiO). The possible scenarios that could be accounted for a rather puzzling lack of methanol emission in the inner 5pc are examined. We also present preliminary results of radiative transfer analysis using the LVG code specifically tailored to methanol. Such analysis allows us to measure the distribution of density, kinetic temperature and abundance, and thus to gain insight into the processes which form and destroy gaseous methanol in the Sgr A* region. Furthermore, the understanding of the molecular clouds structure can contribute to the resolution of the longstanding problem of star formation in the central parsec, as recent studies by Wardle & Yusef-Zadeh (2008) and Hobbs & Nayakashin (2008) indicate the importance that giant molecular clouds close to the Sgr A* have in the formation of young stellar population in this region.
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    ABSTRACT: Irregular galaxies are the present day analogs of the high redshift building blocks of galaxies like the Milky Way. The shallow potential wells of irregular galaxies makes their interstellar medium a chaotic system prone to disruption by star formation, interactions, and mergers. An important, but oft-overlooked component of the interstellar medium of irregulars is their magnetic field. Previous observations suggest that irregulars have a wide range magnetic field strengths and properties. To increase the number of irregulars with detailed observations of their magnetic fields, we have observed three irregular galaxies with the VLA and WSRT: NGC 4214, NGC 1569, and NGC 1156. The magnetic field of NGC 1569 is shaped almost entirely by the outflow of gas from this galaxy, but the central magnetic field in this galaxy may be a dominant source of pressure in this galaxy. The magnetic field of NGC 4214 is mostly random and is not a dominant source of pressure. NGC 1156 is similar in size to NGC 4214, but has a much more extended radio continuum envelope. We will place these fields into the context of the interstellar medium of these galaxies. Finally, we will draw conclusions on the role of magnetic fields in irregular galaxies in general.
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    ABSTRACT: Using the IRAM 30 m telescope and the 15 m JCMT, we explore the value of paraformaldehyde (p-H2CO) 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 M82. It is shown that p-H2CO provides one of the rare direct molecular thermometers. Reproducing the measured line intensities with a large velocity gradient (LVG) model, we find densities of nH2 ~ 7 × 103 cm −3 and kinetic temperatures of Tkin ~ 200 K. The derived kinetic temperature is significantly higher than the dust temperature or the temperature deduced from ammonia (NH3) 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 42→ 31 line of methanol (CH3OH) in the northeastern lobe, which shows—unlike CO and H2CO—significantly different line intensities in the two lobes.
    The Astrophysical Journal 12/2008; 671(2):1579. DOI:10.1086/522294 · 6.28 Impact Factor
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    ABSTRACT: Accepted year month day. Received year month day; in original form year month day ABSTRACT We have made the first map of CO(J=3-2) emission covering the disk of the edge-on galaxy, NGC 4631, which is known for its spectacular gaseous halo. The strongest emission, which we model with a Gaussian ring, occurs within a radius of 5 kpc. Weaker disk emission is detected out to radii of 12 kpc, the most extensive molecular component yet seen in this galaxy. From comparisons with infrared data, we find that CO(J=3-2) emission more closely follows the hot dust component, rather than the cold dust, consistent with it being a good tracer of star formation. The first maps of R 3−2/1−0 , H 2 mass surface density and SFE have been made for the inner 2.4 kpc radius region. Only 20% of the SF occurs in this region and excitation conditions are typical of galaxy disks, rather than of central starbursts. The SFE suggests long gas consumption timescales (> 10 9 yr). The velocity field is dominated by a steeply rising rotation curve in the region of the central molecular ring followed by a flatter curve in the disk. A very steep gradient in the rotation curve is observed at the nucleus, providing the first evidence for a central concentration of mass: M dyn = 5 × 10 7 M ⊙ within a radius of 282 pc. The velocity field shows anomalous features indicating the presence of molecular outflows; one of them is associated with a previously observed CO(J=1-0) expanding shell. Consistent with these outflows is the presence of a thick (z up to 1.4 kpc) CO(J=3-2) disk. We suggest that the interaction between NGC 4631 and its companion(s) has agitated the disk and also initiated star formation which was likely higher in the past than it is now. These may be necessary conditions for seeing prominent halos.
    Monthly Notices of the Royal Astronomical Society 09/2008; 000:1-21. DOI:10.1111/j.1365-2966.2010.17510.x · 5.23 Impact Factor
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    01/2008; DOI:10.1007/978-0-387-72768-4_92
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    ABSTRACT: We demonstrate the diagnostic value of para-formaldehyde (p-H2CO) as a tracer of the density and the temperature of the molecular gas in external galaxies using the prototypical starburst galaxy M 82 as an example. We also report the discovery of the methanol line CH3OH(42 to 3_1,E) from M 82.
    EAS Publications Series 01/2008; 31:189-190. DOI:10.1051/eas:0831040

Publication Stats

278 Citations
75.94 Total Impact Points

Institutions

  • 2002–2013
    • University of Bonn
      • Argelander-Institute of Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2008–2010
    • Joint Institute for VLBI in Europe
      Hoogeveen, Drenthe, Netherlands
  • 2006–2008
    • University of Toronto
      • Department of Astronomy and Astrophysics
      Toronto, Ontario, Canada