Molecular line emission in NGC 4945, imaged with ALMA

Abstract

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.

Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array

“…Also in the other two Seyfert galaxies detected in the gamma-ray band, high efficiencies of gamma-ray production are predicted by the starburst scenario: F cal ≃1 for NGC 4945, and F cal ≫1 for Circinus [26,10,60,61]. Similarly to NGC 1068, AGN-driven molecular winds and weak radio jets have been observed in NGC 4945 and Circinus [62,63,64,65]. These galaxies also show similarity of their Eddingtonscaled X-ray and gamma-ray luminosity with L 0.1−100GeV /L Edd ≃10 −4 and L 2−10keV /L Edd ≃10 −2 in all three sources [66,60], corroborating a correlation between the AGN activity and gamma-ray emission.…”

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

“…NGC 253 and NGC 4945 are both highly inclined giant spiral galaxies (i ≈ 75 • ; e.g., Iodice et al 2014;Henkel et al 2018b). Furthermore, with outflowing gas in their nuclear regions (Turner 1985;Bolatto et al 2013;Henkel et al 2018b), they also provide an environment in which shocks potentially leading to class I methanol maser emission may be ubiquitous. It is remarkable, that the various maser components in NGC 253 occur not right at the center but at offsets of a few hundred parsecs from the nucleus .…”

Variability in extragalactic class I methanol masers: new maser components towards NGC 4945 and NGC 253

“…NGC 253 and NGC 4945 are both highly inclined giant spiral galaxies (i ≈ 75 • ; e.g., Iodice et al 2014;Henkel et al 2018b). Furthermore, with outflowing gas in their nuclear regions (Turner 1985;Bolatto et al 2013;Henkel et al 2018b), they also provide an environment in which shocks potentially leading to class I methanol maser emission may be ubiquitous. It is remarkable, that the various maser components in NGC 253 occur not right at the center but at offsets of a few hundred parsecs from the nucleus (Ellingsen et al 2017).…”