NGC 4945 is one of the nearest (D≈3.8 Mpc; 1 ′′ ≈ 19 pc) starburst galaxies. To investigate structure, dynamics, and composition of its dense nuclear gas, ALMA band 3 (λ≈3-4 mm) observations were carried out with ≈2 ′′ resolution. Measured were three HCN and two HCO + isotopologues, CS, C3H2, SiO, HCO, and CH3C2H. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10 ′′ ×2 ′′ reaching out to a galactocentric radius of r≈100 pc with position angle PA = 45 • ±2 • , inclination i = 75 • ±2 • and an unresolved bright central core of size < ∼ 2 ′′ . The continuum source, representing mostly free-free radiation from star forming regions, 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. Near the systemic velocity but outside the nuclear disk, both HCN J=1→0 and CS J=2→1 delineate molecular arms of length > ∼ 15 ′′ ( > ∼ 285 pc) 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 (r < ∼ 100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojectd velocity of ≈50 km s −1 . All our molecular lines, with the notable exception of CH3C2H, show significant absorption near the systemic velocity (≈571 km s −1 ), within a range of ≈500 -660 km s −1 . Apparently, only molecular transitions with low critical H2-density (ncrit < ∼ 10 4 cm −3 ) 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 Mtot = (2.1±0.2) × 10 8 M⊙ inside a galactocentric radius of 2 .′′ 45 (≈45 pc), 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 (≈30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of 14 N/ 15 N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, with 15 N being less abundant than expected, the question for strong 15 N enrichment by massive star ejecta in starbursts still remains to be settled.
Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. However, 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 36 GHz (λ ≈ 0.8 cm) CH3OH 4−1 → 30 E line emission from the nearby galaxies Maffei 2 (D ≈ 6 Mpc) and IC 342 (D ≈ 3.5 Mpc), measured with the 100m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36 GHz methanol line of Maffei 2 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 Maffei 2. Higher-resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M 82, NGC 4388, NGC 5728 and Arp 220 are also presented. The previously reported detection of 36 GHz maser emission in Arp 220 is not confirmed. Nondetections are reported from the related class I 44 GHz (λ ≈ 0.7 cm) methanol transition towards Maffei 2 and IC 342, indicating that this line is not stronger than its 36 GHz counterpart. In contrast to the previously detected 36 GHz CH3OH emission in NGC 253 and NGC 4945, our 36 GHz profiles towards Maffei 2 and IC 342 are similar to those of previously detected nonmasing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36 GHz 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 36 GHz line component would also be desirable in NGC 253 and NGC 4945.
At a distance of 2.4 kpc, W33 is an outstanding massive and luminous 10 pc-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 (NH3) inversion lines in the frequency range 18–26 GHz obtained with the 40′′ resolution of the 100 m Effelsberg telescope. We detect 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 ~36 yr. We determined kinetic temperatures, column densities, and other physical properties of NH3 and the molecular clouds in W33. For the total-NH3 column density inside 40′′ (0.5 pc) regions, we find 6.0 (±2.1) × 1014, 3.5 (±0.1) × 1015, 3.4 (±0.2) × 1015, 3.1 (±0.2) × 1015, 2.8 (±0.2) × 1015, and 2.0 (±0.2) × 1015 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-NH3 fractional abundance of 1.3 (±0.1) × 10−9 at the peak position. High values of 1.4 (±0.3) × 10−8, 1.6 (±0.3) × 10−8, 3.4 (±0.5) × 10−8, 1.6 (±0.5) × 10−8, and 4.0 (±1.2) × 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 previously proposed variation in the 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-NH3 abundance ratios suggest that ammonia should have been formed in the gas phase or on dust grain mantles at kinetic temperatures of ≳20 K. We determine kinetic temperatures only using NH3 (1,1) and (2,2), and from this we provide gas volume densities for the six main sources in the W33 region. With our new Tkin values, we find that our volume densities are similar to those estimated by Immer et al. (2014, A&A, 572, A63), suggesting that ammonia beam-filling factors are close to unity.
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