The centres of M83 and the Milky Way: opposite extremes of a common star formation cycle

Abstract

In the centres of the Milky Way and M83, the global environmental properties thought to control star formation are very similar. However, M83’s nuclear star formation rate (SFR), as estimated by synchrotron and Hα emission, is an order of magnitude higher than the Milky Way’s. To understand the origin of this difference we use ALMA observations of HCN (1 − 0) and HCO+ (1 − 0) to trace the dense gas at the size scale of individual molecular clouds (0.54″, 12pc) in the inner ∼500 pc of M83, and compare this to gas clouds at similar resolution and galactocentric radius in the Milky Way. We find that both the overall gas distribution and the properties of individual clouds are very similar in the two galaxies, and that a common mechanism may be responsible for instigating star formation in both circumnuclear rings. Given the considerable similarity in gas properties, the most likely explanation for the order of magnitude difference in SFR is time variability, with the Central Molecular Zone (CMZ) currently being at a more quiescent phase of its star formation cycle. We show M83’s SFR must have been an order of magnitude higher 5 − 7 Myr ago. M83’s ‘starburst’ phase was highly localised, both spatially and temporally, greatly increasing the feedback efficiency and ability to drive galactic-scale outflows. This highly dynamic nature of star formation and feedback cycles in galaxy centres means (i) modeling and interpreting observations must avoid averaging over large spatial areas or timescales, and (ii) understanding the multi-scale processes controlling these cycles requires comparing snapshots of a statistical sample of galaxies in different evolutionary stages.

Effects of Varying Mass Inflows on Star Formation in Nuclear Rings of Barred Galaxies

“…Lopsided Star Formation in Nuclear Rings -It has long been known that the star formation in the CMZ is asymmetric, such that most star formation occurs in positive longitudes, notably at Sgr B1 and B2 complexes (Bally et al 2010). Similar asymmetry has also been noted for the nuclear ring in M83 (Harris et al 2001;Callanan et al 2021). Our second series (models asym, off, and boost) offers a possible explanation for lopsided star formation.…”

“…Although a majority of nuclear rings appear more or less symmetric in the distributions of star-forming regions, some rings show clear asymmetric star formation around their circumference (Comerón et al 2010;Ma et al 2018). Notable examples of asymmetric star formation include the CMZ (Bally et al 1988;Henshaw et al 2016) and the nuclear ring of M83 (Harris et al 2001;Callanan et al 2021), in which star formation is not uniformly distributed but concentrated roughly in a quarter-to-half portion of the ring. Possible causes for such asymmetry include a recent minor merger potentially responsible for an offset between the photometric and kinematic nucleus (e.g., Sakamoto et al 2004;Knapen et al 2010) and asymmetric mass inflow along the two dust lanes, the latter of which can readily be checked by direct numerical simulations using the framework presented in Paper I.…”

WISDOM Project -- X. The morphology of the molecular ISM in galaxy centres and its dependence on galaxy structure

“…One environment which has received comparatively less attention is the centres of galaxies, where gas properties can be strongly affected by both the deep potential well (creating high pressures, and high shear), and by the presence of bars and/or active galactic nuclei (AGN). The nuclei of spiral galaxies have been revealed to be dynamic environments, where gas streams, resonances and warps are common (García-Burillo et al 2003;Krips et al 2005;Longmore et al 2013;Audibert et al 2019;Sun et al 2020;Callanan et al 2021;Garcia-Burillo et al 2021). However, disentangling which effects are due to the AGN/bars, and which are due to the galaxy potential can be difficult.…”

A wind-blown bubble in the Central Molecular Zone cloud G0.253+0.016

“…To better understand the morphology and dynamics of the arc we construct a simple model of a tilted ring projected on the plane of the sky (cf. López-Calderón et al 2016;Callanan et al 2021). The model is described by five free-parameters: i & ii) the coordinates of the ring centre on the plane of the sky, {l 0 , b 0 }, iii) the radius of the ring, R arc ; and iv & v) two angles, β, γ, that describe the orientation of the ring relative to the plane of the sky (inclination and position angle, see Callanan et al 2021).…”

“…López-Calderón et al 2016;Callanan et al 2021). The model is described by five free-parameters: i & ii) the coordinates of the ring centre on the plane of the sky, {l 0 , b 0 }, iii) the radius of the ring, R arc ; and iv & v) two angles, β, γ, that describe the orientation of the ring relative to the plane of the sky (inclination and position angle, see Callanan et al 2021). Formally, we describe the shape of the ring by constructing a local Cartesian coordinate system centred on the ring, with x along the line of sight, and ŷ and ẑ aligned with Galactic longitude and latitude.…”