Reconstructing the shock history in the CMZ of NGC 253 with ALCHEMI

Abstract

Context. HNCO and SiO are well-known shock tracers and have been observed in nearby galaxies, including the nearby (D = 3.5 Mpc) starburst galaxy NGC 253. The simultaneous detection of these two species in regions where the star-formation rate is high may be used to study the shock history of the gas. Aims. We perform a multi-line molecular study of NGC 253 using the shock tracers SiO and HNCO and aim to characterize its gas properties. We also explore the possibility of reconstructing the shock history in the central molecular zone (CMZ) of the galaxy. Methods. Six SiO transitions and eleven HNCO transitions were imaged at high resolution 1.″6 (28 pc) with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALCHEMI Large Programme. Both non local thermaldynamic equilibrium (non-LTE) radiative transfer analysis and chemical modeling were performed in order to characterize the gas properties and investigate the chemical origin of the emission. Results. The nonLTE radiative transfer analysis coupled with Bayesian inference shows clear evidence that the gas traced by SiO has different densities and temperatures than that traced by HNCO, with an indication that shocks are needed to produce both species. Chemical modeling further confirms such a scenario and suggests that fast and slow shocks are responsible for SiO and HNCO production, respectively, in most GMCs. We are also able to infer the physical characteristics of the shocks traced by SiO and HNCO for each GMC. Conclusions. Radiative transfer and chemical analysis of the SiO and HNCO in the CMZ of NGC 253 reveal a complex picture whereby most of the GMCs are subjected to shocks. We speculate on the possible shock scenarios responsible for the observed emission and provide potential history and timescales for each shock scenario. Observations of higher spatial resolution for these two species are required in order to quantitatively differentiate between the possible scenarios.

Tracing the chemical footprint of shocks in AGN-host and starburst galaxies with ALMA multi-line molecular studies

“…In the latter, SiO was clearly associated with strong shocks, yet the chemical origin of HNCO could either be weak shocks or thermal sublimation in denser gas (n H 2 $ 10 4 cm −3 ) environment. The chemical modeling performed by Huang et al 40 strongly supports the fast-shock origin of SiO and slow-shock origin of HNCO in the cases tailored for the high cosmic-ray ionization rate (CRIR) environment (z = 10 3-5 z 0 , z 0 = 1.3 × 10 −17 s −1 is the standard galactic CRIR) of NGC 253. Huang et al 40 shows higher CRIR not only affects both HNCO and SiO abundances in shock chemistry, but makes the thermal sublimation an unviable route in enhancing HNCO abundance.…”

“…Fig. 1 shows some of the line intensity maps from Huang et al 39 from HNCO and SiO, and their relevant layout with the outow and the host galaxy itself shown in the dust continuum map adapted from García-Burillo et al 2 In Huang et al, 40 the same pair of shock tracers, HNCO and SiO, were used to study the CMZ of NGC 253 at higher spatial resolution of ∼28 pc using ALMA observations as part of the ALCHEMI large program. Fig.…”

“…Fig. 2 shows examples of the line intensity maps from Huang et al 40 of HNCO and SiO, and their relevant layout with the outow and the host galaxy itself with maps adapted from Bolatto et al 14 We list below the main observational results and the remaining open questions from these two previous studies that are relevant to the current work:…”

“…(3) With well-constrained gas density and gas temperature inferred from the RADEX modeling coupled with Bayesian inference processes that are informed with observational data, Huang et al 40 have shown it to be possible to further infer the age since the last shock episode with each shock tracer. This is because the shock inuence timescale is inversely proportional to the gas density.…”

“…In our two recent works, we presented the observations of multiple transitions of HNCO and SiO observed with ALMA towards both the CND of NGC 1068 39 and the CMZ of NGC 253 (Huang et al, 40 ). These two studies have shown clear signatures of shocks in these two different environments under the inuence of shocks from potentially multiple origins.…”