[Paper Review] Dense gas in nearby galaxies XVI. The nuclear starburst environment in NGC4945

M. Wang, C. Henkel+5 more|ArXiv.org|May 18, 2004

Astrophysics and Star Formation StudiesPhysics and Astronomy69 references73 citations

TL;DR

This study presents a multi-line millimeter-wave survey of dense molecular gas in the nuclear starburst region of NGC 4945 using the SEST telescope, detecting 80 transitions of 19 molecules. It reveals a high-density gas component (n(H₂) ~ 10⁵ cm⁻³), intermediate evolutionary stage between NGC 253 and M 82, with overabundant HNC and isotopic ratios (e.g., ¹⁸O/¹⁷O = 6.4 ± 0.3) indicating significant nucleosynthetic processing by massive stars.

ABSTRACT

A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945 has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are also presented. A large number of molecular species indicate the presence of a prominent high density interstellar gas component characterized by $n_{ m H_2}\sim10^5$ cm$^{-3}$. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an `overabundance' of HNC in the nuclear environment of NGC 4945. While the HNC/HCN $J$=1--0 line intensity ratio is $\sim$0.5, the HNC/HCN abundance ratio is $\sim$1. While HCN is subthermally excited ($T_{ m ex}\sim$8 K), CN is even less excited ($T_{ m ex}\sim$3--4 K), indicating that it arises from a less dense gas component and that its $N$=2--1 line can be optically thin even though its $N$=1--0 emission is moderately optically thick. Overall, fractional abundances of NGC 4945 suggest that the starburst has reached a stage of evolution that is intermediate between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. Within the limits of uncertainty, carbon and oxygen isotope ratios appear to be the same in the nuclear regions of NGC 4945 and NGC 253. High $^{18}$O/$^{17}$O, low $^{16}$O/$^{18}$O and $^{14}$N/$^{15}$N and perhaps also low $^{32}$S/$^{34}$S ratios appear to be characteristic properties of a starburst environment in which massive stars have had sufficient time to affect the isotopic composition of the surrounding interstellar medium.

Motivation & Objective

• To characterize the dense molecular gas environment in the nuclear region of NGC 4945, a nearby starburst galaxy.
• To determine molecular abundances and isotopic ratios to probe chemical and physical conditions in extreme starburst environments.
• To compare the starburst evolution stage of NGC 4945 with other well-studied starburst galaxies like NGC 253 and M 82.
• To investigate the role of massive stars in altering the isotopic composition of the interstellar medium through nucleosynthesis.
• To identify observational gaps and guide future high-resolution interferometric studies of molecular gas in NGC 4945.

Proposed method

• Conducted a wideband millimeter-wave line survey from 82 to 354 GHz using the SEST telescope, covering 80 transitions of 19 molecules.
• Measured CO J=1–0 and 1.3 mm continuum fluxes to derive the H₂ column density to CO line luminosity conversion factor.
• Used radiative transfer models (e.g., LVG) to estimate excitation temperatures (Tex) and optical depths from line intensity ratios of HCN, CN, HNC, and HNCO.
• Calculated fractional abundances relative to H₂ and compared them with values in NGC 253, M 82, and galactic sources.
• Determined isotope ratios (¹²C/¹³C, ¹⁶O/¹⁸O, ¹⁴N/¹⁵N, ³²S/³⁴S) from line intensity measurements of isotopologues.
• Used Kₐ = 0 and 1 HNCO line intensities to derive an upper limit of 30 K for the background radiation field.

Experimental results

Research questions

• RQ1 What is the density and excitation state of molecular gas in the nuclear starburst region of NGC 4945?
• RQ2 How do the molecular abundances and isotopic ratios in NGC 4945 compare to those in NGC 253 and M 82?
• RQ3 What causes the observed HNC/HCN abundance ratio of ~1, despite a line intensity ratio of ~0.5?
• RQ4 What is the role of massive stars in altering the isotopic composition of the interstellar medium in NGC 4945?
• RQ5 Why do some molecular lines show double-peak velocity components at ~450 km s⁻¹ and ~710 km s⁻¹?

Key findings

• The H₂ column density to CO J=1–0 luminosity conversion factor is 5–10 times smaller than in the Galactic disk, indicating a more efficient CO emission per H₂ molecule.
• A high-density gas component with n(H₂) ~ 10⁵ cm⁻³ is confirmed by multiple molecular transitions, including HCN, HCO⁺, and CN.
• The HNC/HCN abundance ratio is ~1, significantly higher than the observed line intensity ratio of ~0.5, indicating HNC overabundance likely due to ion chemistry.
• The ¹⁸O/¹⁷O ratio is 6.4 ± 0.3, the ¹⁶O/¹⁸O ratio is 195 ± 45, and the ¹⁴N/¹⁵N ratio is 105 ± 25, all indicating significant isotopic processing by massive stars.
• The ³²S/³⁴S ratio is 13.5 ± 2.5, suggesting a possible depletion or low abundance of ³⁴S, possibly due to nucleosynthetic effects.
• The ¹²C/¹³C ratio is about twice that of the Galactic center, implying inflow of relatively pristine gas into the nuclear region.